Showing 1–50 of 82 results for author: Chattopadhyay, T

Development of a one-dimensional position sensitive detector for Compton X-ray polarimeters

Authors: Abhay Kumar, Santosh V. Vadawale, N. P. S. Mithun, Tanmoy Chattopadhyay, S. K. Goyal, A. R. Patel, M. Shanmugam

Abstract: The scientific potential of X-ray polarimetry has long been recognized, but the challenges in measuring polarization have left it largely unexplored, particularly in the hard X-ray regime. While tremendous advancement has been made in soft X-ray polarimetery, the lack of sensitive hard X-ray polarimeters and polarisation measurements continues to limit our understanding of high-energy astrophysica… ▽ More The scientific potential of X-ray polarimetry has long been recognized, but the challenges in measuring polarization have left it largely unexplored, particularly in the hard X-ray regime. While tremendous advancement has been made in soft X-ray polarimetery, the lack of sensitive hard X-ray polarimeters and polarisation measurements continues to limit our understanding of high-energy astrophysical processes. With the development of hard X-ray mirrors, it is now possible to develop a sensitive focal plane hard X-ray polarimeter. One such effort is CXPOL, a prototype developed at PRL, India, which consists of a plastic scintillator as active scatterer readout by PMT surrounded by CsI(Tl) scintillators in cylindrical array with SiPM readout from one side. First results of the prototype have been demonstrated in 20 to 80 keV energy range. The sensitivity of the instrument can be significantly enhanced using faster and better light yield scintillator like NaI as absorbers. Further, the use of a position-sensitive scatterer and absorbers, can also provide spectroscopic information by measuring the interaction position along the length and from the known energy depositions in the detectors. Position sensitive detectors are also helpful in mitigating the systematic effects introduced by the off-axis events in the polarisation measurements. Here, we demonstrate the detection sensitivity in the 100x20x5 mm^3 NaI(Tl) scintillator absorber readout on both ends by SiPM arrays operating in co-incidence. In this work, we characterize the first prototype of this detector system and investigate the variation in energy and position resolution, and light output with irradiation position along the length of the detector. The two end readout in co-incidence also reduces the overall SiPM background per absorber by an order of magnitude, further enhancing the polarimetric sensitivity of the instrument. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Comments: 18 pages, 12 figures, Accepted for publication in Experimental Astronomy (2026)

Multi-modal Imputation for Alzheimer's Disease Classification

Authors: Abhijith Shaji, Tamoghna Chattopadhyay, Sophia I. Thomopoulos, Greg Ver Steeg, Paul M. Thompson, Jose-Luis Ambite

Abstract: Deep learning has been successful in predicting neurodegenerative disorders, such as Alzheimer's disease, from magnetic resonance imaging (MRI). Combining multiple imaging modalities, such as T1-weighted (T1) and diffusion-weighted imaging (DWI) scans, can increase diagnostic performance. However, complete multimodal datasets are not always available. We use a conditional denoising diffusion proba… ▽ More Deep learning has been successful in predicting neurodegenerative disorders, such as Alzheimer's disease, from magnetic resonance imaging (MRI). Combining multiple imaging modalities, such as T1-weighted (T1) and diffusion-weighted imaging (DWI) scans, can increase diagnostic performance. However, complete multimodal datasets are not always available. We use a conditional denoising diffusion probabilistic model to impute missing DWI scans from T1 scans. We perform extensive experiments to evaluate whether such imputation improves the accuracy of uni-modal and bi-modal deep learning models for 3-way Alzheimer's disease classification-cognitively normal, mild cognitive impairment, and Alzheimer's disease. We observe improvements in several metrics, particularly those sensitive to minority classes, for several imputation configurations. △ Less

Submitted 28 January, 2026; originally announced January 2026.

ERMoE: Eigen-Reparameterized Mixture-of-Experts for Stable Routing and Interpretable Specialization

Authors: Anzhe Cheng, Shukai Duan, Shixuan Li, Chenzhong Yin, Mingxi Cheng, Heng Ping, Tamoghna Chattopadhyay, Sophia I Thomopoulos, Shahin Nazarian, Paul Thompson, Paul Bogdan

Abstract: Mixture-of-Experts (MoE) architectures expand model capacity by sparsely activating experts but face two core challenges: misalignment between router logits and each expert's internal structure leads to unstable routing and expert underutilization, and load imbalances create straggler bottlenecks. Standard solutions, such as auxiliary load-balancing losses, can reduce load disparities but often we… ▽ More Mixture-of-Experts (MoE) architectures expand model capacity by sparsely activating experts but face two core challenges: misalignment between router logits and each expert's internal structure leads to unstable routing and expert underutilization, and load imbalances create straggler bottlenecks. Standard solutions, such as auxiliary load-balancing losses, can reduce load disparities but often weaken expert specialization and hurt downstream performance. To address these issues, we propose ERMoE, a sparse MoE transformer that reparameterizes each expert in a learned orthonormal eigenbasis and replaces learned gating logits with an "Eigenbasis Score", defined as the cosine similarity between input features and an expert's basis. This content-aware routing ties token assignments directly to experts' representation spaces, stabilizing utilization and promoting interpretable specialization without sacrificing sparsity. Crucially, ERMoE removes the need for explicit balancing losses and avoids the interfering gradients they introduce. We show that ERMoE achieves state-of-the-art accuracy on ImageNet classification and cross-modal image-text retrieval benchmarks (e.g., COCO, Flickr30K), while naturally producing flatter expert load distributions. Moreover, a 3D MRI variant (ERMoE-ba) improves brain age prediction accuracy by more than 7\% and yields anatomically interpretable expert specializations. ERMoE thus introduces a new architectural principle for sparse expert models that directly addresses routing instabilities and enables improved performance with scalable, interpretable specialization. △ Less

Submitted 26 March, 2026; v1 submitted 14 November, 2025; originally announced November 2025.

Comments: Accepted in CVPR2026 Main Track

Design, development, and commissioning of a flexible test setup for the AXIS prototype detector

Authors: Abigail Y. Pan, Haley R. Stueber, Tanmoy Chattopadhyay, Steven W. Allen, Marshall W. Bautz, Kevan Donlon, Catherine E. Grant, Sven Hermann, Beverly LaMarr, Andrew Malonis, Eric D. Miller, Glenn Morris, Peter Orel, Artem Poliszczuk, Gregory Prigozhin, Dan Wilkins

Abstract: The Advanced X-ray Imaging Satellite (AXIS) is one of two candidate mission concepts selected for Phase-A study for the new NASA Astrophysics Probe Explorer (APEX) mission class, with a planned launch in 2032. The X-ray camera for AXIS is under joint development by the X-ray Astronomy and Observational Cosmology (XOC) Group at Stanford, the MIT Kavli Institute (MKI), and MIT Lincoln Laboratory (MI… ▽ More The Advanced X-ray Imaging Satellite (AXIS) is one of two candidate mission concepts selected for Phase-A study for the new NASA Astrophysics Probe Explorer (APEX) mission class, with a planned launch in 2032. The X-ray camera for AXIS is under joint development by the X-ray Astronomy and Observational Cosmology (XOC) Group at Stanford, the MIT Kavli Institute (MKI), and MIT Lincoln Laboratory (MIT-LL). To accelerate development efforts and meet the AXIS mission requirements, XOC has developed a twin beamline testing system, capable of providing the necessary performance, flexibility, and robustness. We present design details, simulations, and performance results for the newer of the two beamlines, constructed and optimized to test and characterize the first full-size MIT-LL AXIS prototype detectors, operating with the Stanford-developed Multi-Channel Readout Chip (MCRC) integrated readout electronics system. The XOC X-ray beamline design is forward-looking and flexible, with a modular structure adaptable to a wide range of detector technologies identified by the Great Observatories Maturation Program (GOMAP) that span the X-ray to near-infrared wavelengths. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: To appear in SPIE Optics + Photonics proceedings 2025

Ground calibration plans for the AXIS high speed camera

Authors: Catherine E. Grant, Eric D. Miller, Marshall W. Bautz, Jill Juneau, Beverly J. LaMarr, Andrew Malonis, Gregory Y. Prigozhin, Christopher W. Leitz, Steven W. Allen, Tanmoy Chattopadhyay, Sven Herrmann, R. Glenn Morris, Abigail Y. Pan, Artem Poliszczuk, Haley R. Stueber, Daniel R. Wilkins

Abstract: The Advanced X-ray Imaging Satellite (AXIS), an astrophysics NASA probe mission currently in phase A, will provide high-throughput, high-spatial resolution X-ray imaging in the 0.3 to 10 keV band. We report on the notional ground calibration plan for the High Speed Camera on AXIS, which is being developed at the MIT Kavli Institute for Astrophysics and Space Research using state-of-the-art CCDs pr… ▽ More The Advanced X-ray Imaging Satellite (AXIS), an astrophysics NASA probe mission currently in phase A, will provide high-throughput, high-spatial resolution X-ray imaging in the 0.3 to 10 keV band. We report on the notional ground calibration plan for the High Speed Camera on AXIS, which is being developed at the MIT Kavli Institute for Astrophysics and Space Research using state-of-the-art CCDs provided by MIT Lincoln Laboratory in combination with an integrated, high-speed ASIC readout chip from Stanford University. AXIS camera ground calibration draws on previous experience with X-ray CCD focal plans, in particular Chandra/ACIS and Suzaku/XIS, utilizing mono-energetic X-ray line sources to measure spectral resolution and quantum efficiency. Relative quantum efficiency of the CCDs will be measured against an sCMOS device, with known absolute calibration from synchrotron measurements. We walk through the envisioned CCD calibration pipeline and we discuss the observatory-level science and calibration requirements and how they inform the camera calibration. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: 12 pages, 5 figures, to appear in the proceedings of SPIE Optics + Photonics: UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXIV, paper 13625-50

Fast, low noise CCD systems for future strategic x-ray missions

Authors: Haley R. Stueber, Abigail Y. Pan, Tanmoy Chattopadhyay, Steven W. Allen, Marshall W. Bautz, Kevan Donlon, Catherine E. Grant, Sven Herrmann, Beverly J. LaMarr, Andrew Malonis, Eric D. Miller, R. Glenn Morris, Peter Orel, Artem Poliszczuk, Gregory Y. Prigozhin, Daniel R. Wilkins

Abstract: Future strategic X-ray missions, such as the Advanced X-ray Imaging Satellite (AXIS) and those targeted by the Great Observatories Maturation Program (GOMaP), require fast, low-noise X-ray imaging spectrometers. To achieve the speed and noise capabilities required by such programs, the X-ray Astronomy and Observational Cosmology (XOC) Group at Stanford, in collaboration with the MIT Kavli Institut… ▽ More Future strategic X-ray missions, such as the Advanced X-ray Imaging Satellite (AXIS) and those targeted by the Great Observatories Maturation Program (GOMaP), require fast, low-noise X-ray imaging spectrometers. To achieve the speed and noise capabilities required by such programs, the X-ray Astronomy and Observational Cosmology (XOC) Group at Stanford, in collaboration with the MIT Kavli Institute (MKI) and MIT Lincoln Laboratory (MIT-LL), is developing readout systems that leverage the high speed, low noise, and low power consumption of application-specific integrated circuit (ASIC) devices. Here, we report the energy resolution and noise performance achieved using MIT-LL AXIS prototype charge-coupled device (CCD) detectors in conjunction with Stanford-developed Multi-Channel Readout Chip (MCRC) ASICs. Additionally, we present a new sampling method for simultaneous optimization of the output gate (OG), reset gate (RG), and reset drain (RD) biases which, in combination with new integrated fast summing well (SW) and RG clock operation modes, enables the data rates required of future X-ray telescopes. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: To appear in SPIE Optics and Photonics 2025 proceedings. 16 pages, 12 figures

Development and testing of integrated readout electronics for next generation SiSeRO (Single electron Sensitive Read Out) devices

Authors: Tanmoy Chattopadhyay, Haley R. Stueber, Abigail Y. Pan, Sven Herrmann, Peter Orel, Kevan Donlon, Steven W. Allen, Marshall W. Bautz, Michael Cooper, Catherine E. Grant, Beverly LaMarr, Christopher Leitz, Andrew Malonis, Eric D. Miller, R. Glenn Morris, Gregory Prigozhin, Ilya Prigozhin, Artem Poliszczuk, Keith Warner, Daniel R. Wilkins

Abstract: The first generation of Single electron Sensitive Read Out (SiSeRO) amplifiers, employed as on-chip charge detectors for charge-coupled devices (CCDs) have demonstrated excellent noise and spectral performance: a responsivity of around 800 pA per electron, an equivalent noise charge (ENC) of 3.2 electrons root mean square (RMS), and a full width half maximum (FWHM) energy resolution of 130 eV at 5… ▽ More The first generation of Single electron Sensitive Read Out (SiSeRO) amplifiers, employed as on-chip charge detectors for charge-coupled devices (CCDs) have demonstrated excellent noise and spectral performance: a responsivity of around 800 pA per electron, an equivalent noise charge (ENC) of 3.2 electrons root mean square (RMS), and a full width half maximum (FWHM) energy resolution of 130 eV at 5.9 keV for a readout speed of 625 Kpixel/s. Repetitive Non Destructive Readout (RNDR) has also been demonstrated with these devices, achieving an improved ENC performance of 0.36 electrons RMS after 200 RNDR cycles. In order to mature this technology further, Stanford University, in collaboration with MIT Kavli Institute and MIT Lincoln Laboratory, are developing new SiSeRO detectors with improved geometries that should enable greater responsivity and improved noise performance. These include CCD devices employing arrays of SiSeRO amplifiers to optimize high speed, low noise RNDR readout and a proof-of-concept SiSeRO active pixel sensor (APS). To read out these devices, our team has developed a compact, 8-channel, fast, low noise, low power application specific integrated circuit (ASIC) denoted the Multi-Channel Readout Chip (MCRC) that includes an experimental drain current readout mode intended for SiSeRO devices. In this paper, we present results from the first tests of SiSeRO CCD devices operating with MCRC readout, and our designs for next generation SiSeRO devices. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: To appear in SPIE Optics + Photonics 2025 conference proceedings

The high-speed X-ray camera on AXIS: design and performance updates

Authors: Eric D. Miller, Catherine E. Grant, Robert Goeke, Marshall W. Bautz, Christopher Leitz, Kevan Donlon, Steven W. Allen, Sven Herrmann, Abraham D. Falcone, F. Elio Angile, Tanmoy Chattopadhyay, Michael Cooper, Mallory A. Jensen, Jill Juneau, Beverly LaMarr, Andrew Malonis, R. Glenn Morris, Peter Orel, Abigail Y. Pan, Steven Persyn, Artem Poliszczuk, Gregory Y. Prigozhin, Ilya Prigozhin, Andrew Ptak, Christopher Reynolds , et al. (3 additional authors not shown)

Abstract: AXIS, a Probe mission concept now in a Phase A study, will provide transformative studies of high-energy astrophysical phenomena thanks to its high-resolution X-ray spectral imaging. These capabilities are enabled by improvements to the mirror design that greatly increase the X-ray throughput per unit mass; and to the detector system, which operates more than an order of magnitude faster than heri… ▽ More AXIS, a Probe mission concept now in a Phase A study, will provide transformative studies of high-energy astrophysical phenomena thanks to its high-resolution X-ray spectral imaging. These capabilities are enabled by improvements to the mirror design that greatly increase the X-ray throughput per unit mass; and to the detector system, which operates more than an order of magnitude faster than heritage instruments while maintaining excellent spectral performance. We present updates to the design of the AXIS High-Speed Camera, a collaborative effort by MIT, Stanford University, the Pennsylvania State University, and the Southwest Research Institute. The camera employs large-format MIT Lincoln Laboratory CCDs that feature multiple high-speed, low-noise output amplifiers and an advanced single-layer polysilicon gate structure for fast, low-power clock transfers. A first lot of prototype CCID100 CCDs has completed fabrication and will soon begin X-ray performance testing. The CCDs are paired with high-speed, low-noise ASIC readout chips designed by Stanford to provide better performance than conventional discrete solutions at a fraction of the power consumption and footprint. Complementary Front-End Electronics employ state-of-the-art digital video waveform capture and advanced signal processing to further deliver low noise at high speed. The Back-End Electronics provide high-speed identification of candidate X-ray events and transient monitoring that relays fast alerts of changing sources to the community. We highlight updates to our parallel X-ray performance test facilities at MIT and Stanford, and review the current performance of the CCD and ASIC technology from testing of prototype devices. These measurements achieve excellent spectral response at the required readout rate, demonstrating that we will meet mission requirements and enable AXIS to achieve world-class science. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: 14 pages, 9 figures, submitted to Proceedings of SPIE Optics + Photonics 2025

One Video to Steal Them All: 3D-Printing IP Theft through Optical Side-Channels

Authors: Twisha Chattopadhyay, Fabricio Ceschin, Marco E. Garza, Dymytriy Zyunkin, Animesh Chhotaray, Aaron P. Stebner, Saman Zonouz, Raheem Beyah

Abstract: The 3D printing industry is rapidly growing and increasingly adopted across various sectors including manufacturing, healthcare, and defense. However, the operational setup often involves hazardous environments, necessitating remote monitoring through cameras and other sensors, which opens the door to cyber-based attacks. In this paper, we show that an adversary with access to video recordings of… ▽ More The 3D printing industry is rapidly growing and increasingly adopted across various sectors including manufacturing, healthcare, and defense. However, the operational setup often involves hazardous environments, necessitating remote monitoring through cameras and other sensors, which opens the door to cyber-based attacks. In this paper, we show that an adversary with access to video recordings of the 3D printing process can reverse engineer the underlying 3D print instructions. Our model tracks the printer nozzle movements during the printing process and maps the corresponding trajectory into G-code instructions. Further, it identifies the correct parameters such as feed rate and extrusion rate, enabling successful intellectual property theft. To validate this, we design an equivalence checker that quantitatively compares two sets of 3D print instructions, evaluating their similarity in producing objects alike in shape, external appearance, and internal structure. Unlike simple distance-based metrics such as normalized mean square error, our equivalence checker is both rotationally and translationally invariant, accounting for shifts in the base position of the reverse engineered instructions caused by different camera positions. Our model achieves an average accuracy of 90.87 percent and generates 30.20 percent fewer instructions compared to existing methods, which often produce faulty or inaccurate prints. Finally, we demonstrate a fully functional counterfeit object generated by reverse engineering 3D print instructions from video. △ Less

Submitted 27 June, 2025; originally announced June 2025.

Comments: 17 pages [Extended Version]

Diffusion Bridge Models for 3D Medical Image Translation

Authors: Shaorong Zhang, Tamoghna Chattopadhyay, Sophia I. Thomopoulos, Jose-Luis Ambite, Paul M. Thompson, Greg Ver Steeg

Abstract: Diffusion tensor imaging (DTI) provides crucial insights into the microstructure of the human brain, but it can be time-consuming to acquire compared to more readily available T1-weighted (T1w) magnetic resonance imaging (MRI). To address this challenge, we propose a diffusion bridge model for 3D brain image translation between T1w MRI and DTI modalities. Our model learns to generate high-quality… ▽ More Diffusion tensor imaging (DTI) provides crucial insights into the microstructure of the human brain, but it can be time-consuming to acquire compared to more readily available T1-weighted (T1w) magnetic resonance imaging (MRI). To address this challenge, we propose a diffusion bridge model for 3D brain image translation between T1w MRI and DTI modalities. Our model learns to generate high-quality DTI fractional anisotropy (FA) images from T1w images and vice versa, enabling cross-modality data augmentation and reducing the need for extensive DTI acquisition. We evaluate our approach using perceptual similarity, pixel-level agreement, and distributional consistency metrics, demonstrating strong performance in capturing anatomical structures and preserving information on white matter integrity. The practical utility of the synthetic data is validated through sex classification and Alzheimer's disease classification tasks, where the generated images achieve comparable performance to real data. Our diffusion bridge model offers a promising solution for improving neuroimaging datasets and supporting clinical decision-making, with the potential to significantly impact neuroimaging research and clinical practice. △ Less

Submitted 21 April, 2025; originally announced April 2025.

Spectro-polarimetry of GRB 180427A: evidence for distinct emission sites with varying polarisation

Authors: Rushikesh Sonawane, Shabnam Iyyani, Soumya Gupta, Tanmoy Chattopadhyay, Dipankar Bhattacharya, Varun. B. Bhalerao, Santosh V. Vadawale, G. C. Dewangan

Abstract: The dynamics of the origin of gamma-ray emissions in gamma-ray bursts (GRBs) remains an enigma. Through a joint analysis of GRB 180427A, observed by the Fermi Gamma-ray Space Telescope and AstroSat's Cadmium Zinc Telluride Imager, we identify emissions from two distinct regions with varying polarisation properties. Time-resolved polarisation analysis reveals a synchronous evolution of the polarisa… ▽ More The dynamics of the origin of gamma-ray emissions in gamma-ray bursts (GRBs) remains an enigma. Through a joint analysis of GRB 180427A, observed by the Fermi Gamma-ray Space Telescope and AstroSat's Cadmium Zinc Telluride Imager, we identify emissions from two distinct regions with varying polarisation properties. Time-resolved polarisation analysis reveals a synchronous evolution of the polarisation angle (PA) and fraction (PF) with two emission pulses, peaking with a delay of $ 5.09 \pm 0.29\, \mathrm{s}$. Spectral analysis indicates that the first pulse is characterised by a stronger blackbody component, while the second pulse exhibits a more prominent non-thermal spectrum (power law with an exponential cutoff). Using a bottom-to-top approach through simulations, we decouple the polarisation properties of the individual spectral components, revealing polarisation fractions of 25\% - 40\% for the blackbody spectrum and 30\% - 60\% for the non-thermal spectrum. At a redshift of $z \sim 0.22$, the blackbody emission originates from the jet photosphere at $\sim$ a few $10^{11}\, \mathrm{cm}$, whereas the non-thermal emission arises from an optically thin region at a few $10^{13}\, \mathrm{cm}$. The changing dominance of these emissions explains the observed PA shift of $60^\circ \pm 22^\circ$. The spectral cutoff at 1 MeV suggests pair opacity due to the jet's relatively lower bulk Lorentz factor ($Γ\sim$ a few tens). The high polarisation fraction and hard low energy spectral slopes ($α> -0.5$) imply a top-hat jet structure observed off-axis, near the jet's edge. This off-axis viewing introduces anisotropy in the observed radiation within the viewing cone ($1/Γ$), accounting for the observed polarisation. △ Less

Submitted 29 July, 2025; v1 submitted 16 January, 2025; originally announced January 2025.

Comments: 14 pages, 6 figures, 1 table. Accepted for publication in ApJ. This version includes the final accepted revisions

Journal ref: ApJ, Vol. 990, Issue 2, Article 215 (Sept 2025)

Towards efficient machine-learning-based reduction of the cosmic-ray induced background in X-ray imaging detectors: increasing context awareness

Authors: Artem Poliszczuk, Dan Wilkins, Steven W. Allen, Eric D. Miller, Tanmoy Chattopadhyay, Benjamin Schneider, Julien Eric Darve, Marshall Bautz, Abe Falcone, Richard Foster, Catherine E. Grant, Sven Herrmann, Ralph Kraft, R. Glenn Morris, Paul Nulsen, Peter Orel, Gerrit Schellenberger, Haley R. Stueber

Abstract: Traditional cosmic ray filtering algorithms used in X-ray imaging detectors aboard space telescopes perform event reconstruction based on the properties of activated pixels above a certain energy threshold, within 3x3 or 5x5 pixel sliding windows. This approach can reject up to 98% of the cosmic ray background. However, the remaining unrejected background constitutes a significant impediment to st… ▽ More Traditional cosmic ray filtering algorithms used in X-ray imaging detectors aboard space telescopes perform event reconstruction based on the properties of activated pixels above a certain energy threshold, within 3x3 or 5x5 pixel sliding windows. This approach can reject up to 98% of the cosmic ray background. However, the remaining unrejected background constitutes a significant impediment to studies of low surface brightness objects, which are especially prevalent in the high-redshift universe. The main limitation of the traditional filtering algorithms is their ignorance of the long-range contextual information present in image frames. This becomes particularly problematic when analyzing signals created by secondary particles produced during interactions of cosmic rays with body of the detector. Such signals may look identical to the energy deposition left by X-ray photons, when one considers only the properties within the small sliding window. Additional information is present, however, in the spatial and energy correlations between signals in different parts of the frame, which can be accessed by modern machine learning (ML) techniques. In this work, we continue the development of an ML-based pipeline for cosmic ray background mitigation. Our latest method consist of two stages: first, a frame classification neural network is used to create class activation maps (CAM), localizing all events within the frame; second, after event reconstruction, a random forest classifier, using features obtained from CAMs, is used to separate X-ray and cosmic ray features. The method delivers >40% relative improvement over traditional filtering in background rejection in standard 0.3-10keV energy range, at the expense of only a small (<2%) level of lost X-ray signal. Our method also provides a convenient way to tune the cosmic ray rejection threshold to adapt to a user's specific scientific needs. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: To appear in SPIE Astronomical Telescopes + Instrumentation proceedings 2024

Augmenting astronomical X-ray detectors with AI for enhanced sensitivity and reduced background

Authors: D. R. Wilkins, A. Poliszczuk, B. Schneider, E. D. Miller, S. W. Allen, M. Bautz, T. Chattopadhyay, A. D. Falcone, R. Foster, C. E. Grant, S. Herrmann, R. Kraft, R. G. Morris, P. Nulsen, P. Orel, G. Schellenberger

Abstract: Bringing artificial intelligence (AI) alongside next-generation X-ray imaging detectors, including CCDs and DEPFET sensors, enhances their sensitivity to achieve many of the flagship science cases targeted by future X-ray observatories, based upon low surface brightness and high redshift sources. Machine learning algorithms operating on the raw frame-level data provide enhanced identification of b… ▽ More Bringing artificial intelligence (AI) alongside next-generation X-ray imaging detectors, including CCDs and DEPFET sensors, enhances their sensitivity to achieve many of the flagship science cases targeted by future X-ray observatories, based upon low surface brightness and high redshift sources. Machine learning algorithms operating on the raw frame-level data provide enhanced identification of background vs. astrophysical X-ray events, by considering all of the signals in the context within which they appear within each frame. We have developed prototype machine learning algorithms to identify valid X-ray and cosmic-ray induced background events, trained and tested upon a suite of realistic end-to-end simulations that trace the interaction of cosmic ray particles and their secondaries through the spacecraft and detector. These algorithms demonstrate that AI can reduce the unrejected instrumental background by up to 41.5 per cent compared with traditional filtering methods. Alongside AI algorithms to reduce the instrumental background, next-generation event reconstruction methods, based upon fitting physically-motivated Gaussian models of the charge clouds produced by events within the detector, promise increased accuracy and spectral resolution of the lowest energy photon events. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: Proceedings of the SPIE, Astronomical Telescopes and Instrumentation, Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray

Journal ref: Proc. SPIE, 2024, 13093-65

Continued developments in X-ray speed reading: fast, low noise readout for next-generation wide-field imagers

Authors: Sven Herrmann, Peter Orel, Tanmoy Chattopadhyay, Glenn Morris, Gregory Prigozhin, Haley R. Stueber, Steven W. Allen, Marshall W. Bautz, Kevan Donlon, Beverly LaMarr, Chris Leitz, Eric Miller, Abigail Pan, Artem Poliszczuk, Daniel R. Wilkins

Abstract: Future strategic X-ray astronomy missions will require unprecedentedly sensitive wide-field imagers providing high frame rates, low readout noise and excellent soft energy response. To meet these needs, our team is employing a multi-pronged approach to advance several key areas of technology. Our first focus is on advanced readout electronics, specifically integrated electronics, where we are coll… ▽ More Future strategic X-ray astronomy missions will require unprecedentedly sensitive wide-field imagers providing high frame rates, low readout noise and excellent soft energy response. To meet these needs, our team is employing a multi-pronged approach to advance several key areas of technology. Our first focus is on advanced readout electronics, specifically integrated electronics, where we are collaborating on the VERITAS readout chip for the Athena Wide Field Imager, and have developed the Multi-Channel Readout Chip (MCRC), which enables fast readout and high frame rates for MIT-LL JFET (junction field effect transistor) CCDs. Second, we are contributing to novel detector development, specifically the SiSeRO (Single electron Sensitive Read Out) devices fabricated at MIT Lincoln Laboratory, and their advanced readout, to achieve sub-electron noise performance. Hardware components set the stage for performance, but their efficient utilization relies on software and algorithms for signal and event processing. Our group is developing digital waveform filtering and AI methods to augment detector performance, including enhanced particle background screening and improved event characterization. All of these efforts make use of an efficient, new X-ray beamline facility at Stanford, where components and concepts can be tested and characterized. △ Less

Submitted 30 July, 2024; v1 submitted 23 July, 2024; originally announced July 2024.

Comments: To appear in SPIE Astronomical Telescopes + Instrumentation proceedings 2024

X-ray speed reading with the MCRC: prototype success and next generation upgrades

Authors: Peter Orel, Abigail Y. Pan, Sven Herrmann, Tanmoy Chattopadhyay, Glenn Morris, Haley Stueber, Steven W. Allen, Daniel Wilkins, Gregory Prigozhin, Beverly LaMarr, Richard Foster, Andrew Malonis, Marshall W. Bautz, Michael J. Cooper, Kevan Donlon

Abstract: The Advanced X-ray Imaging Satellite (AXIS) is a NASA probe class mission concept designed to deliver arcsecond resolution with an effective area ten times that of Chandra (at launch). The AXIS focal plane features an MIT Lincoln Laboratory (MIT-LL) X-ray charge-coupled device (CCD) detector working in conjunction with an application specific integrated circuit (ASIC), denoted the Multi-Channel Re… ▽ More The Advanced X-ray Imaging Satellite (AXIS) is a NASA probe class mission concept designed to deliver arcsecond resolution with an effective area ten times that of Chandra (at launch). The AXIS focal plane features an MIT Lincoln Laboratory (MIT-LL) X-ray charge-coupled device (CCD) detector working in conjunction with an application specific integrated circuit (ASIC), denoted the Multi-Channel Readout Chip (MCRC). While this readout ASIC targets the AXIS mission, it is applicable to a range of potential X-ray missions with comparable readout requirements. Designed by the X-ray astronomy and Observational Cosmology (XOC) group at Stanford University, the MCRC ASIC prototype (MCRC-V1.0) uses a 350 nm technology node and provides 8 channels of high speed, low noise, low power consumption readout electronics. Each channel implements a current source to bias the detector output driver, a preamplifier to provide gain, and an output buffer to interface directly to an analog-to-digital (ADC) converter. The MCRC-V1 ASIC exhibits comparable performance to our best discrete electronics implementations, but with ten times less power consumption and a fraction of the footprint area. In a total ionizing dose (TID) test, the chip demonstrated a radiation hardness equal or greater to 25 krad, confirming the suitability of the process technology and layout techniques used in its design. The next iteration of the ASIC (MCRC-V2) will expand the channel count and extend the interfaces to external circuits, advancing its readiness as a readout-on-a-chip solution for next generation X-ray CCD-like detectors. This paper summarizes our most recent characterization efforts, including the TID radiation campaign and results from the first operation of the MCRC ASIC in combination with a representative MIT-LL CCD. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: To appear in SPIE Astronomical Telescopes + Instrumentation proceedings 2024

The XOC X-ray Beamline: Probing Colder, Quieter, and Softer

Authors: Haley R. Stueber, Tanmoy Chattopadhyay, Sven C. Herrmann, Peter Orel, Tsion Gebre, Aanand Joshi, Steven W. Allen, Glenn Morris, Artem Poliszczuk

Abstract: Future strategic X-ray satellite telescopes, such as the probe-class Advanced X-ray Imaging Satellite (AXIS), will require excellent soft energy response in their imaging detectors to enable maximum discovery potential. In order to characterize Charge-Coupled Device (CCD) and Single Electron Sensitive Read Output (SiSeRO) detectors in the soft X-ray region, the X-ray Astronomy and Observational Co… ▽ More Future strategic X-ray satellite telescopes, such as the probe-class Advanced X-ray Imaging Satellite (AXIS), will require excellent soft energy response in their imaging detectors to enable maximum discovery potential. In order to characterize Charge-Coupled Device (CCD) and Single Electron Sensitive Read Output (SiSeRO) detectors in the soft X-ray region, the X-ray Astronomy and Observational Cosmology (XOC) group at Stanford has developed, assembled, and commissioned a 2.5-meter-long X-ray beamline test system. The beamline is designed to efficiently produce monoenergetic X-ray fluorescence lines in the 0.3-10 keV energy range and achieve detector temperatures as low as 173 K. We present design and simulation details of the beamline, and discuss the vacuum, cooling, and X-ray fluorescence performance achieved. As a workhorse for future detector characterization at Stanford, the XOC beamline will support detector development for a broad range of X-ray astronomy instruments. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: To appear in SPIE Astronomical Telescopes and Instrumentation 2024 proceedings. 17 pages, 13 figures

Demonstrating sub-electron noise performance in Single electron Sensitive Readout (SiSeRO) devices

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Peter Orel, Kevan Donlon, Steven W. Allen, Marshall W. Bautz, Brianna Cantrall, Michael Cooper, Beverly LaMarr, Chris Leitz, Eric Miller, R. Glenn Morris, Abigail Y. Pan, Gregory Prigozhin, Ilya Prigozhin, Haley R. Stueber, Daniel R. Wilkins

Abstract: Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detection technology that can, in principle, provide significantly greater responsivity and improved noise performance than traditional charge coupled device (CCD) readout circuitry. The SiSeRO, developed by MIT Lincoln Laboratory, uses a p-MOSFET transistor with a depleted back-gate region under the transistor channel; as charg… ▽ More Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detection technology that can, in principle, provide significantly greater responsivity and improved noise performance than traditional charge coupled device (CCD) readout circuitry. The SiSeRO, developed by MIT Lincoln Laboratory, uses a p-MOSFET transistor with a depleted back-gate region under the transistor channel; as charge is transferred into the back gate region, the transistor current is modulated. With our first generation SiSeRO devices, we previously achieved a responsivity of around 800 pA per electron, an equivalent noise charge (ENC) of 4.5 electrons root mean square (RMS), and a full width at half maximum (FWHM) spectral resolution of 130 eV at 5.9 keV, at a readout speed of 625 Kpixel/s and for a detector temperature of 250 K. Importantly, since the charge signal remains unaffected by the SiSeRO readout process, we have also been able to implement Repetitive Non-Destructive Readout (RNDR), achieving an improved ENC performance. In this paper, we demonstrate sub-electron noise sensitivity with these devices, utilizing an enhanced test setup optimized for RNDR measurements, with excellent temperature control, improved readout circuitry, and advanced digital filtering techniques. We are currently fabricating new SiSeRO detectors with more sensitive and RNDR-optimized amplifier designs, which will help mature the SiSeRO technology in the future and eventually lead to the pathway to develop active pixel sensor (APS) arrays using sensitive SiSeRO amplifiers on each pixel. Active pixel devices with sub-electron sensitivity and fast readout present an exciting option for next generation, large area astronomical X-ray telescopes requiring fast, low-noise megapixel imagers. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: To appear in SPIE Astronomical Telescopes + Instrumentation proceedings 2024

Localisation of Gamma Ray Bursts using AstroSat Mass Model

Authors: Divita Saraogi, J Venkata Aditya, Varun Bhalerao, Suman Bala, Arvind Balasubramanian, Sujay Mate, Tanmoy Chattopadhyay, Soumya Gupta, Vipul Prasad, Gaurav Waratkar, Navaneeth P K, Rahul Gopalakrishnan, Dipankar Bhattacharya, Gulab Dewangan, Santosh Vadawale

Abstract: The Cadmium Zinc Telluride Imager (CZTI) aboard AstroSat has good sensitivity to Gamma Ray Bursts (GRBs), with close to 600 detections including about 50 discoveries undetected by other missions. However, CZTI was not designed to be a GRB monitor and lacks localisation capabilities. We introduce a new method of localising GRBs using "shadows" cast on the CZTI detector plane due to absorption and s… ▽ More The Cadmium Zinc Telluride Imager (CZTI) aboard AstroSat has good sensitivity to Gamma Ray Bursts (GRBs), with close to 600 detections including about 50 discoveries undetected by other missions. However, CZTI was not designed to be a GRB monitor and lacks localisation capabilities. We introduce a new method of localising GRBs using "shadows" cast on the CZTI detector plane due to absorption and scattering by satellite components and instruments. Comparing the observed distribution of counts on the detector plane with simulated distributions with the AstroSat Mass Model, we can localise GRBs in the sky. Our localisation uncertainty is defined by a two-component model, with a narrow Gaussian component that has close to 50% probability of containing the source, and the remaining spread over a broader Gaussian component with an 11.3 times higher $σ$. The width ($σ$) of the Gaussian components scales inversely with source counts. We test this model by applying the method to GRBs with known positions and find good agreement between the model and observations. This new ability expands the utility of CZTI in the study of GRBs and other rapid high-energy transients. △ Less

Submitted 29 January, 2024; originally announced January 2024.

Comments: 9 pages, 5 figures

The high-speed X-ray camera on AXIS

Authors: Eric D. Miller, Marshall W. Bautz, Catherine E. Grant, Richard F. Foster, Beverly LaMarr, Andrew Malonis, Gregory Prigozhin, Benjamin Schneider, Christopher Leitz, Sven Herrmann, Steven W. Allen, Tanmoy Chattopadhyay, Peter Orel, R. Glenn Morris, Haley Stueber, Abraham D. Falcone, Andrew Ptak, Christopher Reynolds

Abstract: AXIS is a Probe-class mission concept that will provide high-throughput, high-spatial-resolution X-ray spectral imaging, enabling transformative studies of high-energy astrophysical phenomena. To take advantage of the advanced optics and avoid photon pile-up, the AXIS focal plane requires detectors with readout rates at least 20 times faster than previous soft X-ray imaging spectrometers flying ab… ▽ More AXIS is a Probe-class mission concept that will provide high-throughput, high-spatial-resolution X-ray spectral imaging, enabling transformative studies of high-energy astrophysical phenomena. To take advantage of the advanced optics and avoid photon pile-up, the AXIS focal plane requires detectors with readout rates at least 20 times faster than previous soft X-ray imaging spectrometers flying aboard missions such as Chandra and Suzaku, while retaining the low noise, excellent spectral performance, and low power requirements of those instruments. We present the design of the AXIS high-speed X-ray camera, which baselines large-format MIT Lincoln Laboratory CCDs employing low-noise pJFET output amplifiers and a single-layer polysilicon gate structure that allows fast, low-power clocking. These detectors are combined with an integrated high-speed, low-noise ASIC readout chip from Stanford University that provides better performance than conventional discrete solutions at a fraction of their power consumption and footprint. Our complementary front-end electronics concept employs state of the art digital video waveform capture and advanced signal processing to deliver low noise at high speed. We review the current performance of this technology, highlighting recent improvements on prototype devices that achieve excellent noise characteristics at the required readout rate. We present measurements of the CCD spectral response across the AXIS energy band, augmenting lab measurements with detector simulations that help us understand sources of charge loss and evaluate the quality of the CCD backside passivation technique. We show that our technology is on a path that will meet our requirements and enable AXIS to achieve world-class science. △ Less

Submitted 1 September, 2023; originally announced September 2023.

Comments: 17 pages, 11 figures, submitted to Proceedings of SPIE Optics + Photonics 2023

High hard X-ray polarization in Cygnus X-1 confined to the intermediate hard state: evidence for a variable jet component

Authors: Tanmoy Chattopadhyay, Abhay Kumar, A. R. Rao, Yash Bhargava, Santosh V. Vadawale, Ajay Ratheesh, Gulab Dewangan, Dipankar Bhattacharyay, Mithun N. P. S., Varun Bhalerao

Abstract: Cygnus X-1, the well-known accreting black hole system, exhibits several observational features hinting at an intricate interplay between the accretion disk, its atmosphere known as the corona and the putative relativistic jet. It has been extensively studied using all available observational methods, including using the newly available technique of sensitive X-ray polarimetry. X-ray polarization… ▽ More Cygnus X-1, the well-known accreting black hole system, exhibits several observational features hinting at an intricate interplay between the accretion disk, its atmosphere known as the corona and the putative relativistic jet. It has been extensively studied using all available observational methods, including using the newly available technique of sensitive X-ray polarimetry. X-ray polarization characteristics are distinct for coronal and jet emissions. The low X-ray polarization measured below $\sim$100 keV is understood as arising from the corona. In contrast, the high polarization measurements reported above $\sim$400 keV required a separate jet-dominated spectral component, which spectroscopy does not demonstrate conclusively. Here we report precise polarization measurements in the 100-380 keV region made during three different sub-classes of spectral states of the source using the CZTI instrument onboard {\em AstroSat}. A high polarization (23$\pm$4 \%) is found mainly in the Intermediate Hard State of the source, and the energy-resolved measurements smoothly connect the coronal and the jet regimes. When high polarization is observed, the simultaneous spectral data hints at a separate power law component above 100 keV. We examine the possible sources of this energy-dependent high polarization in Cygnus X-1. △ Less

Submitted 30 November, 2023; v1 submitted 6 June, 2023; originally announced June 2023.

Comments: Accepted for publication in ApJ Letters (ApJL)

Journal ref: Astrophysical Journal Letters, Volume 960, Number 1, L2, 2023 December 29

Demonstrating repetitive non-destructive readout (RNDR) with SiSeRO devices

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Peter Orel, Kevan Donlon, Gregory Prigozhin, R. Glenn Morris, Michael Cooper, Beverly LaMarr, Andrew Malonis, Steven W. Allen, Marshall W. Bautz, Chris Leitz

Abstract: We demonstrate so-called repetitive non-destructive readout (RNDR) for the first time on a Single electron Sensitive Readout (SiSeRO) device. SiSeRO is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors, developed at MIT Lincoln Laboratory. This technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor… ▽ More We demonstrate so-called repetitive non-destructive readout (RNDR) for the first time on a Single electron Sensitive Readout (SiSeRO) device. SiSeRO is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors, developed at MIT Lincoln Laboratory. This technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. RNDR was realized by transferring the signal charge non-destructively between the internal gate and the summing well (SW), which is the last serial register. The advantage of the non-destructive charge transfer is that the signal charge for each pixel can be measured at the end of each transfer cycle and by averaging for a large number of measurements ($\mathrm{N_{cycle}}$), the total noise can be reduced by a factor of 1/$\mathrm{\sqrt{N_{cycle}}}$. In our experiments with a prototype SiSeRO device, we implemented nine ($\mathrm{N_{cycle}}$ = 9) RNDR cycles, achieving around 2 electron readout noise (equivalent noise charge or ENC) with spectral resolution close to the fano limit for silicon at 5.9 keV. These first results are extremely encouraging, demonstrating successful implementation of the RNDR technique in SiSeROs. They also lay foundation for future experiments with more optimized test stands (better temperature control, larger number of RNDR cycles, RNDR-optimized SiSeRO devices) which should be capable of achieving sub-electron noise sensitivities. This new device class presents an exciting technology for next generation astronomical X-ray telescopes requiring very low-noise spectroscopic imagers. The sub-electron sensitivity also adds the capability to conduct in-situ absolute calibration, enabling unprecedented characterization of the low energy instrument response. △ Less

Submitted 12 December, 2023; v1 submitted 3 May, 2023; originally announced May 2023.

Comments: Accepted for publication in Journal of Astronomical Telescopes, Instruments, and Systems (JATIS). arXiv admin note: text overlap with arXiv:2208.01082

Journal ref: Journal of Astronomical Telescopes, Instruments, and Systems, Vol. 10, Issue 1, 016004 (January 2024)

Transferring Models Trained on Natural Images to 3D MRI via Position Encoded Slice Models

Authors: Umang Gupta, Tamoghna Chattopadhyay, Nikhil Dhinagar, Paul M. Thompson, Greg Ver Steeg, The Alzheimer's Disease Neuroimaging Initiative

Abstract: Transfer learning has remarkably improved computer vision. These advances also promise improvements in neuroimaging, where training set sizes are often small. However, various difficulties arise in directly applying models pretrained on natural images to radiologic images, such as MRIs. In particular, a mismatch in the input space (2D images vs. 3D MRIs) restricts the direct transfer of models, of… ▽ More Transfer learning has remarkably improved computer vision. These advances also promise improvements in neuroimaging, where training set sizes are often small. However, various difficulties arise in directly applying models pretrained on natural images to radiologic images, such as MRIs. In particular, a mismatch in the input space (2D images vs. 3D MRIs) restricts the direct transfer of models, often forcing us to consider only a few MRI slices as input. To this end, we leverage the 2D-Slice-CNN architecture of Gupta et al. (2021), which embeds all the MRI slices with 2D encoders (neural networks that take 2D image input) and combines them via permutation-invariant layers. With the insight that the pretrained model can serve as the 2D encoder, we initialize the 2D encoder with ImageNet pretrained weights that outperform those initialized and trained from scratch on two neuroimaging tasks -- brain age prediction on the UK Biobank dataset and Alzheimer's disease detection on the ADNI dataset. Further, we improve the modeling capabilities of 2D-Slice models by incorporating spatial information through position embeddings, which can improve the performance in some cases. △ Less

Submitted 2 March, 2023; originally announced March 2023.

Comments: To appear at IEEE International Symposium on Biomedical Imaging 2023 (ISBI 2023). Code is available at https://github.com/umgupta/2d-slice-set-networks

Improved noise performance from the next-generation buried-channel p-Mosfet SiSeROs

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Matthew Kaplan, Peter Orel, Kevan Donlon, Gregory Prigozhin, R. Glenn Morris, Michael Cooper, Andrew Malonis, Steven W. Allen, Marshall W. Bautz, Chris Leitz

Abstract: The Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors. Developed at MIT Lincoln Laboratory, this technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. At Stanford, we… ▽ More The Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors. Developed at MIT Lincoln Laboratory, this technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. At Stanford, we have developed a readout module based on the drain current of the on-chip transistor to characterize the device. In our earlier work, we characterized a number of first prototype SiSeROs with the MOSFET transistor channels at the surface layer. An equivalent noise charge (ENC) of around 15 electrons root mean square (RMS) was obtained. In this work, we examine the first buried-channel SiSeRO. We have achieved substantially improved noise performance of around 4.5 electrons root mean square (RMS) and a full width half maximum (FWHM) energy resolution of 132 eV at 5.9 keV, for a readout speed of 625 kpixel/s. We also discuss how digital filtering techniques can be used to further improve the SiSeRO noise performance. Additional measurements and device simulations will be essential to further mature the SiSeRO technology. This new device class presents an exciting new technology for the next-generation astronomical X-ray telescopes requiring fast, low-noise, radiation-hard megapixel imagers with moderate spectroscopic resolution. △ Less

Submitted 27 April, 2023; v1 submitted 11 February, 2023; originally announced February 2023.

Comments: Accepted in Journal of Astronomical Telescopes, Instruments, and Systems (JATIS). arXiv admin note: text overlap with arXiv:2208.01082

Journal ref: Journal of Astronomical Telescopes, Instruments, and Systems, Vol. 9, Issue 2, 026001 (May 2023)

Experimental verification of off-axis polarimetry with Cadmium Zinc Telluride detectors of AstroSat-CZT Imager

Authors: C. S. Vaishnava, N. P. S. Mithun, Santosh V. Vadawale, Esakkiappan Aarthy, Arpit R. Patel, Hiteshkumar L. Adalja, Neeraj Kumar Tiwari, Tinkal Ladiya, Nilam Navale, Tanmoy Chattopadhyay, A. R. Rao, Varun Bhalerao, Dipankar Bhattacharya

Abstract: The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat consists of an array of a large number of pixellated CZT detectors capable of measuring the polarization of incident hard X-rays. The polarization measurement capability of CZTI for on-axis sources was experimentally confirmed before the launch. CZTI has yielded tantalizing results on the X-ray polarization of the Crab nebula and pulsar in… ▽ More The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat consists of an array of a large number of pixellated CZT detectors capable of measuring the polarization of incident hard X-rays. The polarization measurement capability of CZTI for on-axis sources was experimentally confirmed before the launch. CZTI has yielded tantalizing results on the X-ray polarization of the Crab nebula and pulsar in the energy range of 100 - 380 keV. CZTI has also contributed to the measurement of prompt emission polarization for several Gamma-Ray Bursts (GRBs). However, polarization measurements of off-axis sources like GRBs are challenging. It is vital to experimentally calibrate the CZTI sensitivity to off-axis sources to enhance the credence of the measurements. In this context, we report the verification of the off-axis polarimetric capability of pixellated CZT detectors through the controlled experiments carried out with a CZT detector similar to that used in CZTI and extensive Geant4 simulations of the experimental set-up. Our current results show that the CZT detectors can be used to measure the polarization of bright GRBs up to off-axis angles of ~60 degrees. However, at incidence angles between 45-60 degrees, there might be some systematic effects which needs to be taken into account while interpreting the measured polarisation fraction. △ Less

Submitted 1 September, 2022; originally announced September 2022.

Comments: Accepted for publication in JATIS

Extending the energy range of AstroSat-CZTI up to 380 keV with Compton Spectroscopy

Authors: Abhay Kumar, Tanmoy Chattopadhyay, Santosh V. Vadawale, A. R. Rao, Mithun N. P. S., Varun Bhalerao, Dipankar Bhattacharya

Abstract: The CZTI (Cadmium Zinc Telluride Imager) onboard AstroSat is a high energy coded mask imager and spectrometer in the energy range of 20 - 100 keV. Above 100 keV, the dominance of Compton scattering cross-section in CZTI results in a significant number of 2-pixel Compton events and these have been successfully utilized for polarization analysis of Crab pulsar and nebula (and transients like Gamma-r… ▽ More The CZTI (Cadmium Zinc Telluride Imager) onboard AstroSat is a high energy coded mask imager and spectrometer in the energy range of 20 - 100 keV. Above 100 keV, the dominance of Compton scattering cross-section in CZTI results in a significant number of 2-pixel Compton events and these have been successfully utilized for polarization analysis of Crab pulsar and nebula (and transients like Gamma-ray bursts) in 100 - 380 keV. These 2-pixel Compton events can also be used to extend the spectroscopic energy range of CZTI up to 380 keV for bright sources. However, unlike the spectroscopy in primary energy range, where simultaneous background measurement is available from masked pixels, Compton spectroscopy requires blank sky observation for background measurement. Background subtraction, in this case, is non-trivial because of the presence of both short-term and long-term temporal variations in the data, which depend on multiple factors like earth rotation and the effect of South Atlantic Anomaly (SAA) regions etc. We have developed a methodology of background selection and subtraction that takes into account for these effects. Here, we describe these background selection and subtraction techniques and validate them using spectroscopy of Crab in the extended energy range of 30 - 380 keV region, and compare the obtained spectral parameters with the INTEGRAL results. This new capability allows for the extension of the energy range of AstroSat spectroscopy and will also enable the simultaneous spectro-polarimetric study of other bright sources like Cygnus X-1. △ Less

Submitted 24 August, 2022; originally announced August 2022.

Comments: 11 pages, 15 figures, Accepted for publication in MNRAS Journal

Reducing the background in X-ray imaging detectors via machine learning

Authors: D. R. Wilkins, S. W. Allen, E. D. Miller, M. Bautz, T. Chattopadhyay, R. Foster, C. E. Grant, S. Hermann, R. Kraft, R. G. Morris, P. Nulsen, G. Schellenberger

Abstract: The sensitivity of astronomical X-ray detectors is limited by the instrumental background. The background is especially important when observing low surface brightness sources that are critical for many of the science cases targeted by future X-ray observatories, including Athena and future US-led flagship or probe-class X-ray missions. Above 2keV, the background is dominated by signals induced by… ▽ More The sensitivity of astronomical X-ray detectors is limited by the instrumental background. The background is especially important when observing low surface brightness sources that are critical for many of the science cases targeted by future X-ray observatories, including Athena and future US-led flagship or probe-class X-ray missions. Above 2keV, the background is dominated by signals induced by cosmic rays interacting with the spacecraft and detector. We develop novel machine learning algorithms to identify events in next-generation X-ray imaging detectors and to predict the probability that an event is induced by a cosmic ray vs. an astrophysical X-ray photon, enabling enhanced filtering of the cosmic ray-induced background. We find that by learning the typical correlations between the secondary events that arise from a single primary, machine learning algorithms are able to successfully identify cosmic ray-induced background events that are missed by traditional filtering methods employed on current-generation X-ray missions, reducing the unrejected background by as much as 30 per cent. △ Less

Submitted 16 August, 2022; originally announced August 2022.

Comments: Proceedings of the SPIE, Astronomical Telescopes and Instrumentation, Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray

Journal ref: Proc. SPIE, 2022, 12181, 155

X-ray speed reading: enabling fast, low noise readout for next-generation CCDs

Authors: S. Herrmann, P. Orel, T. Chattopadhyay, R. G. Morris, G. Prigozhin, K. Donlon, R. Foster, M. Bautz, S. Allen, C. Leitz

Abstract: Current, state-of-the-art CCDs are close to being able to deliver all key performance figures for future strategic X-ray missions except for the required frame rates. Our Stanford group is seeking to close this technology gap through a multi-pronged approach of microelectronics, signal processing and novel detector devices, developed in collaboration with the Massachusetts Institute of Technology… ▽ More Current, state-of-the-art CCDs are close to being able to deliver all key performance figures for future strategic X-ray missions except for the required frame rates. Our Stanford group is seeking to close this technology gap through a multi-pronged approach of microelectronics, signal processing and novel detector devices, developed in collaboration with the Massachusetts Institute of Technology (MIT) and MIT Lincoln Laboratory (MIT-LL). Here we report results from our (integrated) readout electronics development, digital signal processing and novel SiSeRO (Single electron Sensitive Read Out) device characterization. △ Less

Submitted 2 August, 2022; originally announced August 2022.

Comments: To appear in SPIE Proceeding of Astronomical Telescopes + Instrumentation, 2022

Single electron Sensitive Readout (SiSeRO) X-ray detectors: Technological progress and characterization

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Peter Orel, R. G. Morris, Daniel R. Wilkins, Steven W. Allen, Gregory Prigozhin, Beverly LaMarr, Andrew Malonis, Richard Foster, Marshall W. Bautz, Kevan Donlon, Michael Cooper, Christopher Leitz

Abstract: Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors. Developed at MIT Lincoln Laboratory, this technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. At Stanford, we have… ▽ More Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors. Developed at MIT Lincoln Laboratory, this technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. At Stanford, we have developed a readout module based on the drain current of the on-chip transistor to characterize the device. Characterization was performed for a number of prototype sensors with different device architectures, e.g. location of the internal gate, MOSFET polysilicon gate structure, and location of the trough in the internal gate with respect to the source and drain of the MOSFET (the trough is introduced to confine the charge in the internal gate). Using a buried-channel SiSeRO, we have achieved a charge/current conversion gain of >700 pA per electron, an equivalent noise charge (ENC) of around 6 electrons root mean square (RMS), and a full width half maximum (FWHM) of approximately 140 eV at 5.9 keV at a readout speed of 625 Kpixel/s. In this paper, we discuss the SiSeRO working principle, the readout module developed at Stanford, and the characterization test results of the SiSeRO prototypes. We also discuss the potential to implement Repetitive Non-Destructive Readout (RNDR) with these devices and the preliminary results which can in principle yield sub-electron ENC performance. Additional measurements and detailed device simulations will be essential to mature the SiSeRO technology. However, this new device class presents an exciting technology for next generation astronomical X-ray telescopes requiring fast, low-noise, radiation hard megapixel imagers with moderate spectroscopic resolution. △ Less

Submitted 1 August, 2022; originally announced August 2022.

Comments: To appear in SPIE Proceedings of Astronomical Telescopes + Instrumentation, 2022

Hard X-ray polarization catalog for a 5-year sample of Gamma-Ray Bursts using AstroSat CZT-Imager

Authors: Tanmoy Chattopadhyay, Soumya Gupta, Shabnam Iyyani, Divita Saraogi, Vidushi Sharma, Anastasia Tsvetkova, Ajay Ratheesh, Rahul Gupta, N. P. S. Mithun, C. S. Vaishnava, Vipul Prasad, E. Aarthy, Abhay Kumar, A. R. Rao, Santosh Vadawale, Varun Bhalerao, Dipankar Bhattacharya, Ajay Vibhute, Dmitry Frederiks

Abstract: Cadmium Zinc Telluride Imager (CZTI) aboard AstroSat has been regularly detecting Gamma-Ray Bursts (GRBs) since its launch in 2015. Its sensitivity to polarization measurements at energies above 100 keV allows CZTI to attempt spectro-polarimetric studies of GRBs. Here, we present the first catalog of GRB polarization measurements made by CZTI during its first five years of operation. This presents… ▽ More Cadmium Zinc Telluride Imager (CZTI) aboard AstroSat has been regularly detecting Gamma-Ray Bursts (GRBs) since its launch in 2015. Its sensitivity to polarization measurements at energies above 100 keV allows CZTI to attempt spectro-polarimetric studies of GRBs. Here, we present the first catalog of GRB polarization measurements made by CZTI during its first five years of operation. This presents the time integrated polarization measurements of the prompt emission of 20 GRBs in the energy range 100-600 keV. The sample includes the bright GRBs which were detected within an angle range of 0-60 degree and 120-180 degree where the instrument has useful polarization sensitivity and is less prone to systematics. We implement a few new modifications in the analysis to enhance polarimetric sensitivity of the instrument. Majority of the GRBs in the sample are found to possess less / null polarization across the total bursts' duration in contrast to a small fraction of five GRBs exhibiting high polarization. The low polarization across the bursts can be speculated to be either due to the burst being intrinsically weakly polarized or due to varying polarization angle within the burst even when it is highly polarized. In comparison to POLAR measurements, CZTI has detected a larger number of cases with high polarization. This may be a consequence of the higher energy window of CZTI observations which results in the sampling of smaller duration of burst emissions in contrast to POLAR, thereby, probing emissions of less temporal variations of polarization properties. △ Less

Submitted 19 July, 2022; originally announced July 2022.

Comments: Accepted for publication in ApJ

Journal ref: The Astrophysical Journal, volume - 936, number -1, pages - 12, year - 2022

Gamma-Ray Polarimetry

Authors: Denis Bernard, Tanmoy Chattopadhyay, Fabian Kislat, Nicolas Produit

Abstract: While the scientific potential of high-energy X-ray and gamma-ray polarimetry has long been recognized, measuring the polarization of high-energy photons is challenging. To date, there has been very few significant detections from an astrophysical source. However, recent technological developments raise the possibility that this may change in the not-too-distant future. Significant progress has be… ▽ More While the scientific potential of high-energy X-ray and gamma-ray polarimetry has long been recognized, measuring the polarization of high-energy photons is challenging. To date, there has been very few significant detections from an astrophysical source. However, recent technological developments raise the possibility that this may change in the not-too-distant future. Significant progress has been made in the development of Gamma-ray Burst (GRB) polarimeters and polarization sensitive Compton telescopes. A second-generation dedicated GRB polarimeter, POLAR-2, is under development for launch in 2024, and COSI a second-generation polarization sensitive Compton Telescope has been selected by NASA for launch in 2025. This chapter reviews basic concepts and experimental approaches of scattering polarimetry of hard X-rays to MeV γ-rays, and pair production polarimetry of higher-energy photons △ Less

Submitted 4 May, 2022; originally announced May 2022.

Comments: 44 pages

Clustering of eclipsing binary light curves through functional principal component analysis

Authors: Soumita Modak, Tanuka Chattopadhyay, Asis Kumar Chattopadhyay

Abstract: In this paper, we revisit the problem of clustering 1318 new variable stars found in the Milky way. Our recent work distinguishes these stars based on their light curves which are univariate series of brightness from the stars observed at discrete time points. This work proposes a new approach to look at these discrete series as continuous curves over time by transforming them into functional data… ▽ More In this paper, we revisit the problem of clustering 1318 new variable stars found in the Milky way. Our recent work distinguishes these stars based on their light curves which are univariate series of brightness from the stars observed at discrete time points. This work proposes a new approach to look at these discrete series as continuous curves over time by transforming them into functional data. Then, functional principal component analysis is performed using these functional light curves. Clustering based on the significant functional principal components reveals two distinct groups of eclipsing binaries with consistency and superiority compared to our previous results. This method is established as a new powerful light curve-based classifier, where implementation of a simple clustering algorithm is effective enough to uncover the true clusters based merely on the first few relevant functional principal components. Simultaneously we discard the noise from the data study involving the higher order functional principal components. Thus the suggested method is very useful for clustering big light curve data sets which is also verified by our simulation study. △ Less

Submitted 8 February, 2022; originally announced February 2022.

Comments: 32 pages, 12 figures; Accepted for publication in the journal 'Astrophysics & Space Science'

MSC Class: 62H30 ACM Class: I.5.3

Journal ref: Astrophysics and Space Science (2022), Volume 367, Article id 19

Development and characterization of a fast and low noise readout for the next generation X-ray CCDs

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Peter Orel, R. Glenn Morris, Gregory Prigozhin, Andrew Malonis, Richard Foster, David Craig, Barry E. Burke, Steven W. Allen, Marshall Bautz

Abstract: The broad energy response, low electronic read noise, and good energy resolution have made X-ray Charge-Coupled Devices (CCDs) an obvious choice for developing soft X-ray astronomical instruments over the last half century. They also come in large array formats with small pixel sizes which make them a potential candidate for the next generation astronomical X-ray missions. However, the next genera… ▽ More The broad energy response, low electronic read noise, and good energy resolution have made X-ray Charge-Coupled Devices (CCDs) an obvious choice for developing soft X-ray astronomical instruments over the last half century. They also come in large array formats with small pixel sizes which make them a potential candidate for the next generation astronomical X-ray missions. However, the next generation X-ray telescopic experiments propose for significantly larger collecting area compared to the existing observatories in order to explore the low luminosity and high redshift X-ray universe which requires these detectors to have an order of magnitude faster readout. In this context, the Stanford University (SU) in collaboration with the Massachusetts Institute of Technology (MIT) has initiated the development of fast readout electronics for X-ray CCDs. At SU, we have designed and developed a fast and low noise readout module with the goal of achieving a readout speed of 5 Mpixel/s. We successfully ran a prototype CCD matrix of 512 $\times$ 512 pixels at 4 Mpixels/s. In this paper, we describe the details of the readout electronics and report the performance of the detectors at these readout speeds in terms of read noise and energy resolution. In the future, we plan to continue to improve performance of the readout module and eventually converge to a dedicated ASIC based readout system to enable parallel read out of large array multi-node CCD devices. △ Less

Submitted 21 January, 2022; originally announced January 2022.

Comments: Submitted in Journal of Astronomical Telescopes, Instruments, and Systems (JATIS)

Journal ref: Journal of Astronomical Telescopes, Instruments, and Systems 8(2), 026005 (24 May 2022)

Structural properties and classification of variable stars: A study through unsupervised machine learning techniques

Authors: Suman Paul, Tanuka Chattopadhyay

Abstract: The advancement in the field of data science especially in machine learning along with vast databases of variable star projects like the Optical Gravitational Lensing Experiment (OGLE) encourages researchers to analyse as well as classify light curves of different variable stars automatically with efficiency. In the present work, we have demonstrated the relative performances of principal componen… ▽ More The advancement in the field of data science especially in machine learning along with vast databases of variable star projects like the Optical Gravitational Lensing Experiment (OGLE) encourages researchers to analyse as well as classify light curves of different variable stars automatically with efficiency. In the present work, we have demonstrated the relative performances of principal component analysis (PCA) and independent component analysis (ICA) applying to huge databases of OGLE variable star light curves after obtaining 1000 magnitudes between phase 0 to 1 with step length 0.001 for each light curves in identifying resonances for fundamental mode (FU) and first overtone (FO) Cepheids and in the classification of variable stars for Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC) as well as Milky Way (MW). We have seen that the performance of ICA is better for finding resonances for Cepheid variables as well as for accurately classifying large data sets of light curves than PCA. Using K-means clustering algorithm (CA) with respect to independent components (ICs), we have plotted period-luminosity diagrams and colour-magnitude diagrams separately for LMC, SMC and MW and found that ICA along with K-means CA is a very robust tool for classification as well as future prediction on the nature of light curves of variable stars. △ Less

Submitted 21 January, 2022; originally announced January 2022.

Comments: 13 pages, 13 figures

Effect of dark matter halos on the orbital and escape dynamics of barred galaxies

Authors: Debasish Mondal, Tanuka Chattopadhyay

Abstract: This study examines the effect of dark matter halos on the orbital and escape dynamics of stars in the central region of barred galaxies. A three-dimensional gravitational model with a central bulge, bar, disc, and dark matter halo (or simply dark halo) has been set up and analyzed from the viewpoint of escape in open Hamiltonian systems for this purpose. Additionally, this model has been examined… ▽ More This study examines the effect of dark matter halos on the orbital and escape dynamics of stars in the central region of barred galaxies. A three-dimensional gravitational model with a central bulge, bar, disc, and dark matter halo (or simply dark halo) has been set up and analyzed from the viewpoint of escape in open Hamiltonian systems for this purpose. Additionally, this model has been examined separately for the dark halo profiles: oblate and NFW. In both circumstances, an escape mechanism has been identified near the saddle points of the phase space, which correspond to the bar ends. The escaping motion of stars is seen using orbital maps and Poincaré surface section maps generated in various phase planes. Finally, the relationship between chaos and dark halo parameters such as mass, size, circular velocity, and nature has been studied. Our findings suggest that oblate dark halos are preferred over NFW dark halos for justifying the formation of full-fledged spiral arms and extended distribution of dark halos in giant spiral galaxies with supermassive black holes (SMBHs) at their centers. Again, the oblate dark halos well justify the emergence of less prominent or poor spiral arms and the core-dominated distribution of dark halos in dwarf and LSB galaxies in the absence of central SMBHs. On the other hand, extreme central baryonic feedback is required for the NFW halos to generate spiral patterns, and such dark halos should be preferred for galaxies with extremely energetic centers. △ Less

Submitted 16 June, 2022; v1 submitted 19 January, 2022; originally announced January 2022.

Comments: 29 pages, 7 figures, 3 tables, comments are welcome

Probing into emission mechanisms of GRB 190530A using time-resolved spectra and polarization studies: Synchrotron Origin?

Authors: Rahul Gupta, S. Gupta, T. Chattopadhyay, V. Lipunov, A. J. Castro-Tirado, D. Bhattacharya, S. B. Pandey, S. R. Oates, Amit Kumar, Y. -D. Hu, A. F. Valeev, P. Yu. Minaev, H. Kumar, J. Vinko, Dimple, V. Sharma, A. Aryan, A. Castellón, A. Gabovich, A. Moskvitin, A. Ordasi, A. Pál, A. Pozanenko, B. -B. Zhang, B. Kumar , et al. (25 additional authors not shown)

Abstract: Multi-pulsed GRB 190530A, detected by the GBM and LAT onboard \fermi, is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using \AstroSat and \fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy… ▽ More Multi-pulsed GRB 190530A, detected by the GBM and LAT onboard \fermi, is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using \AstroSat and \fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time-integrated (55.43 $\pm$ 21.30 \%) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard \AstroSat, show a hint of high degree of polarization. The presence of a hint of high degree of polarization and the values of low energy spectral index ($α_{\rm pt}$) do not run over the synchrotron limit for the first two pulses, supporting the synchrotron origin in an ordered magnetic field. However, during the third pulse, $α_{\rm pt}$ exceeds the synchrotron line of death in few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore, we also report the earliest optical observations constraining afterglow polarization using the MASTER (P $<$ 1.3 \%) and the redshift measurement ($z$= 0.9386) obtained with the 10.4m GTC telescopes. The broadband afterglow can be described with a forward shock model for an ISM-like medium with a wide jet opening angle. We determine a circumburst density of $n_{0} \sim$ 7.41, kinetic energy $E_{\rm K} \sim$ 7.24 $\times 10^{54}$ erg, and radiated $γ$-ray energy $E_{\rm γ, iso} \sim$ 6.05 $\times 10^{54}$ erg, respectively. △ Less

Submitted 4 January, 2022; originally announced January 2022.

Comments: 28 pages, 17 figures, accepted for publication in MNRAS

First results on SiSeRO (Single electron Sensitive Read Out) devices -- a new X-ray detector for scientific instrumentation

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Barry Burke, Kevan Donlon, Gregory Prigozhin, R. Glenn Morris, Peter Orel, Michael Cooper, Andrew Malonis, Dan Wilkins, Vyshnavi Suntharalingam, Steven W. Allen, Marshall Bautz, Chris Leitz

Abstract: We present an evaluation of a novel on-chip charge detector, called the Single electron Sensitive Read Out (SiSeRO), for charge-coupled device (CCD) image sensor applications. It uses a p-MOSFET transistor at the output stage with a depleted internal gate beneath the p-MOSFET. Charge transferred to the internal gate modulates the source-drain current of the transistor. We have developed a drain cu… ▽ More We present an evaluation of a novel on-chip charge detector, called the Single electron Sensitive Read Out (SiSeRO), for charge-coupled device (CCD) image sensor applications. It uses a p-MOSFET transistor at the output stage with a depleted internal gate beneath the p-MOSFET. Charge transferred to the internal gate modulates the source-drain current of the transistor. We have developed a drain current readout module to characterize the detector. The prototype sensor achieves a charge/current conversion gain of 700 pA per electron, an equivalent noise charge (ENC) of 15 electrons (e-) root mean square (RMS), and a full width half maximum (FWHM) of 230 eV at 5.9 keV. In this paper, we discuss the SiSeRO working principle, the readout module developed at Stanford, and the first characterization test results of the SiSeRO prototypes. While at present only a proof-of-concept experiment, in the near future we plan to use next generation sensors with improved noise performance and an enhanced readout module. In particular, we are developing a readout module enabling Repetitive Non-Destructive Readout (RNDR) of the charge, which can in principle yield sub-electron ENC performance. With these developments, we eventually plan to build a matrix of SiSeRO amplifiers to develop an active pixel sensor with an on-chip ASIC-based readout system. Such a system, with fast readout speeds and sub-electron noise, could be effectively utilized in scientific applications requiring fast and low-noise spectro-imagers. △ Less

Submitted 9 December, 2021; originally announced December 2021.

Comments: Submitted to Journal of Astronomical Telescopes, Instruments, and Systems (JATIS)

Journal ref: Journal of Astronomical Telescopes, Instruments, and Systems, 8(2), 026006 (26 May 2022)

Qualitative Analysis for Human Centered AI

Authors: Orestis Papakyriakopoulos, Elizabeth Anne Watkins, Amy Winecoff, Klaudia Jaźwińska, Tithi Chattopadhyay

Abstract: Human-centered artificial intelligence (AI) posits that machine learning and AI should be developed and applied in a socially aware way. In this article, we argue that qualitative analysis (QA) can be a valuable tool in this process, supplementing, informing, and extending the possibilities of AI models. We show this by describing how QA can be integrated in the current prediction paradigm of AI,… ▽ More Human-centered artificial intelligence (AI) posits that machine learning and AI should be developed and applied in a socially aware way. In this article, we argue that qualitative analysis (QA) can be a valuable tool in this process, supplementing, informing, and extending the possibilities of AI models. We show this by describing how QA can be integrated in the current prediction paradigm of AI, assisting scientists in the process of selecting data, variables, and model architectures. Furthermore, we argue that QA can be a part of novel paradigms towards Human Centered AI. QA can support scientists and practitioners in practical problem solving and situated model development. It can also promote participatory design approaches, reveal understudied and emerging issues in AI systems, and assist policy making. △ Less

Submitted 7 December, 2021; originally announced December 2021.

Journal ref: HCAI:Human Centered AI workshop at Neural Information Processing Systems 2021

The AstroSat Mass Model: Imaging and Flux studies of off-axis sources with CZTI

Authors: Sujay Mate, Tanmoy Chattopadhyay, Varun Bhalerao, E. Aarthy, Arvind Balasubramanian, Dipankar Bhattacharya, Soumya Gupta, Krishnan Kutty, N. P. S. Mithun, Sourav Palit, A. R. Rao, Divita Saraogi, Santosh Vadawale, Ajay Vibhute

Abstract: The Cadmium Zinc Telluride Imager (CZTI) on AstroSat is a hard X-ray coded-aperture mask instrument with a primary field of view of 4.6 x 4.6 degrees (FWHM). The instrument collimators become increasingly transparent at energies above $\sim$100 keV, making CZTI sensitive to radiation from the entire sky. While this has enabled CZTI to detect a large number of off-axis transient sources, calculatin… ▽ More The Cadmium Zinc Telluride Imager (CZTI) on AstroSat is a hard X-ray coded-aperture mask instrument with a primary field of view of 4.6 x 4.6 degrees (FWHM). The instrument collimators become increasingly transparent at energies above $\sim$100 keV, making CZTI sensitive to radiation from the entire sky. While this has enabled CZTI to detect a large number of off-axis transient sources, calculating the source flux or spectrum requires knowledge of the direction and energy dependent attenuation of the radiation incident upon the detector. Here, we present a GEANT4-based mass model of CZTI and AstroSat that can be used to simulate the satellite response to the incident radiation, and to calculate an effective "response file" for converting the source counts into fluxes and spectra. We provide details of the geometry and interaction physics, and validate the model by comparing the simulations of imaging and flux studies with observations. Spectroscopic validation of the mass model is discussed in a companion paper, Chattopadhyay 2021. △ Less

Submitted 20 May, 2021; originally announced May 2021.

Comments: Accepted in the "AstroSat - 5 years" special issue of the Journal of Astrophysics and Astronomy

Hard X-ray Polarimetry -- An overview of the method, science drivers and recent findings

Authors: Tanmoy Chattopadhyay

Abstract: The last decade has seen a leapfrog in the interest in X-ray polarimetry with a number of new polarization measurements in hard X-rays from AstroSat, POLAR, GAP, and PoGO+. The measurements provide some interesting insights into various astrophysical phenomena such as coronal geometry and disk-jet connection in black hole X-ray binaries, hard X-ray emission mechanism in pulsars and Gamma Ray Burst… ▽ More The last decade has seen a leapfrog in the interest in X-ray polarimetry with a number of new polarization measurements in hard X-rays from AstroSat, POLAR, GAP, and PoGO+. The measurements provide some interesting insights into various astrophysical phenomena such as coronal geometry and disk-jet connection in black hole X-ray binaries, hard X-ray emission mechanism in pulsars and Gamma Ray Bursts (GRB). They also highlight an increase in polarization with energy which makes hard X-ray polarimetry extremely appealing. There is a number of confirmed hard X-ray polarimetry experiments which along with the existing instruments (AstroSat and INTEGRAL) makes this field further exciting. Polarization experiments may also see a significant progress in sensitivity with new developments in scintillator readouts, active pixel sensors, CZT detectors. In particular, the advent of hard X-ray focusing optics, will enable designing of compact focal plane polarimeters with a multifold enhancement in sensitivity. In this review, we will focus on the recent polarimetry findings, science potential of hard X-ray polarimetry along with possible improvements in the measurement techniques. △ Less

Submitted 12 April, 2021; originally announced April 2021.

Comments: Review article (accepted for publication in JAA)

Journal ref: Journal of Astrophysics and Astronomy, Volume 42, Issue 2, article id.82, Page 106, year 2021

Investigation of the Effect of Bars on the Properties of Spiral Galaxies: A Multivariate Statistical Study

Authors: Prasenjit Banerjee, Tanuka Chattopadhyay, Asis Kumar Chattopadhyay

Abstract: Subjective classification of spiral galaxies is not sufficient for studying the effect of bars on their physical characteristics. In reality the problem is to comprehend the complex correlations in a multivariate parametric space. Multivariate tools are the best ones for understanding this complex correlation. In this work an objective classification of a large set (26,089) of spiral galaxies was… ▽ More Subjective classification of spiral galaxies is not sufficient for studying the effect of bars on their physical characteristics. In reality the problem is to comprehend the complex correlations in a multivariate parametric space. Multivariate tools are the best ones for understanding this complex correlation. In this work an objective classification of a large set (26,089) of spiral galaxies was compiled as a value added galaxy catalogue from sdss DR 15 virtual data archive. Initially for dimensionality reduction, Independent Component Analysis is performed to determine a set of Independent Components that are linear combinations of 48 observed features (namely ionised lines, Lick indices, photometric and morphological properties). Subsequently a K-means cluster analysis is carried out on the basis of the 14 best chosen Independent Components to obtain 12 distinct homogeneous groups of spiral galaxies. Amongst these, 3 groups are the oldest ones (1.6 Gyr - 5.9 Gyr), while 5 groups fall in the medium aged category (1.4 Gyr - 1.6 Gyr), 2 groups consist of only unbarred spirals, 1 group is the youngest one and the remaining one is an outlier. In many groups there are clear indication of recurrent bar formation phenomena which is consistent with few previous simulation works. In order to study the robustness a second method of clustering by Gaussian Mixture Modeling Method (GMMBC) is applied. △ Less

Submitted 11 April, 2021; originally announced April 2021.

Sub-MeV spectroscopy with AstroSat-CZT Imager for Gamma Ray Bursts

Authors: Tanmoy Chattopadhyay, Soumya Gupta, Vidushi Sharma, Shabnam Iyyani, Ajay Ratheesh, N. P. S. Mithun, E. Aarthy, Sourav Palit, Abhay Kumar, Santosh V Vadawale, A. R. Rao, Varun Bhalerao, Dipankar Bhattacharya

Abstract: Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100 - 300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (around 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature al… ▽ More Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100 - 300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (around 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100 - 600 keV for the GRBs detected in the first year of operation. CZTI in last five years has detected around 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism. △ Less

Submitted 26 February, 2021; originally announced February 2021.

Comments: Accepted for publication in Journal of Astrophysics and Astronomy, 5 years of AstroSat special issue

Journal ref: Journal of Astrophysics and Astronomy, Volume 42, Issue 2, article id.82, year 2021

Exploring Sub-MeV Sensitivity of AstroSat-CZTI for ON-axis Bright Sources

Authors: Abhay Kumar, Tanmoy Chattopadhyay, Santosh V Vadawale, A. R. Rao, Soumya Gupta, Mithun N. P. S., Varun Bhalerao, Dipankar Bhattacharya

Abstract: The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat is designed for hard X-ray imaging and spectroscopy in the energy range of 20 - 100 keV. The CZT detectors are of 5 mm thickness and hence have good efficiency for Compton interactions beyond 100 keV. The polarisation analysis using CZTI relies on such Compton events and have been verified experimentally. The same Compton events can also be… ▽ More The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat is designed for hard X-ray imaging and spectroscopy in the energy range of 20 - 100 keV. The CZT detectors are of 5 mm thickness and hence have good efficiency for Compton interactions beyond 100 keV. The polarisation analysis using CZTI relies on such Compton events and have been verified experimentally. The same Compton events can also be used to extend the spectroscopy up to 380 keV. Further, it has been observed that about 20% pixels of the CZTI detector plane have low gain, and they are excluded from the primary spectroscopy. If these pixels are included, then the spectroscopic capability of CZTI can be extended up to 500 keV and further up to 700 keV with a better gain calibration in the future. Here we explore the possibility of using the Compton events as well as the low gain pixels to extend the spectroscopic energy range of CZTI for ON-axis bright X-ray sources. We demonstrate this technique using Crab observations and explore its sensitivity. △ Less

Submitted 26 February, 2021; originally announced February 2021.

Comments: 10 pages, 8 figures

Role of galactic bars in the formation of spiral arms: A study through orbital and escape dynamics -- I

Authors: Debasish Mondal, Tanuka Chattopadhyay

Abstract: In the present work we have developed a three-dimensional gravitational model of barred galaxies, in order to study orbital and escape dynamics of the stars inside their central barred region. Our gravitational model is composed of four components, central nucleus, bar, disc and dark matter halo. Furthermore we have analysed the model for two different types of bar potentials. The study has been c… ▽ More In the present work we have developed a three-dimensional gravitational model of barred galaxies, in order to study orbital and escape dynamics of the stars inside their central barred region. Our gravitational model is composed of four components, central nucleus, bar, disc and dark matter halo. Furthermore we have analysed the model for two different types of bar potentials. The study has been carried out for a Hamiltonian system and thorough numerical studies have been done in order to categorize regular and chaotic motions of stars. We have seen that escape mechanism has only seen near saddle points ($L_2$, $L_4$ and $L_2^{'}$, $L_4^{'}$) of the Hamiltonian system. Orbital structures in $x$ - $y$ plane indicate that this escaping motion corresponds to the two ends of the bar. Classifications of orbits are found by calculating maximal Lyapunov exponent of the stellar trajectories corresponding to a specific initial condition vector. Poincaré surface section maps are studied in both $x$ - $y$ and $x$ - $p_x$ ($p_x$ is the momentum along $x$ - direction) plane to get a complete view of the escape properties of the system in the phase space. Also we studied in detail how the chaotic dynamics varies with mass, length and nature of the bar. We found that under suitable physical conditions the chaos plays a pivotal role behind the formation of grand design or poor spiral pattern for stronger bars and ring structures for weaker bars. △ Less

Submitted 25 February, 2021; originally announced February 2021.

Comments: 13 pages, 19 figures, 10 tables, comments are welcome

Tiny-box: A tool for the versatile development and characterization of low noise fast X-ray imaging detectors

Authors: Tanmoy Chattopadhyay, Sven Herrmann, Steven Allen, Jack Hirschman, Glenn Morris, Marshall Bautz, Andrew Malonis, Richard Foster, Gregory Prigozhin, Dave Craig, Barry Burke

Abstract: X-ray Charge Coupled Devices (CCDs) have been the workhorse for soft X-ray astronomical instruments for the past quarter century. They provide broad energy response, extremely low electronic read noise, and good energy resolution in soft X-rays. These properties, along with the large arrays and small pixel sizes available with modern-day CCDs, make them a potential candidate for next generation as… ▽ More X-ray Charge Coupled Devices (CCDs) have been the workhorse for soft X-ray astronomical instruments for the past quarter century. They provide broad energy response, extremely low electronic read noise, and good energy resolution in soft X-rays. These properties, along with the large arrays and small pixel sizes available with modern-day CCDs, make them a potential candidate for next generation astronomical X-ray missions equipped with large collecting areas, high angular resolutions and wide fields of view, enabling observation of the faint, diffuse and high redshift X-ray universe. However, such high collecting area (about 30 times Chandra) requires these detectors to have an order of magnitude faster readout than current CCDs to avoid saturation and pile up effects. In this context, Stanford University and MIT have initiated the development of fast readout X-ray cameras. As a tool for this development, we have designed a fast readout, low noise electronics board (intended to work at a 5 Megapixel per second data rate) coupled with an STA Archon controller to readout a 512 x 512 CCD (from MIT Lincoln Laboratory). This versatile setup allows us to study a number of parameters and operation conditions including the option for digital shaping. In this paper, we describe the characterization test stand, the concept and development of the readout electronics, and simulation results. We also report the first measurements of read noise, energy resolution and other parameters from this set up. While this is very much a prototype, we plan to use larger, multi-node CCD devices in the future with dedicated ASIC readout systems to enable faster, parallel readout of the CCDs. △ Less

Submitted 13 December, 2020; originally announced December 2020.

Comments: To appear in SPIE Astronomical Telescopes + Instrumentation, 2020, Paper No. 11454-80

Journal ref: Proc. SPIE 11454, X-Ray, Optical, and Infrared Detectors for Astronomy IX, 1145423, 2020

Identifying charged particle background events in X-ray imaging detectors with novel machine learning algorithms

Authors: D. R. Wilkins, S. W. Allen, E. D. Miller, M. Bautz, T. Chattopadhyay, S. Fort, C. E. Grant, S. Herrmann, R. Kraft, R. G. Morris, P. Nulsen

Abstract: Space-based X-ray detectors are subject to significant fluxes of charged particles in orbit, notably energetic cosmic ray protons, contributing a significant background. We develop novel machine learning algorithms to detect charged particle events in next-generation X-ray CCDs and DEPFET detectors, with initial studies focusing on the Athena Wide Field Imager (WFI) DEPFET detector. We train and t… ▽ More Space-based X-ray detectors are subject to significant fluxes of charged particles in orbit, notably energetic cosmic ray protons, contributing a significant background. We develop novel machine learning algorithms to detect charged particle events in next-generation X-ray CCDs and DEPFET detectors, with initial studies focusing on the Athena Wide Field Imager (WFI) DEPFET detector. We train and test a prototype convolutional neural network algorithm and find that charged particle and X-ray events are identified with a high degree of accuracy, exploiting correlations between pixels to improve performance over existing event detection algorithms. 99 per cent of frames containing a cosmic ray are identified and the neural network is able to correctly identify up to 40 per cent of the cosmic rays that are missed by current event classification criteria, showing potential to significantly reduce the instrumental background, and unlock the full scientific potential of future X-ray missions such as Athena, Lynx and AXIS. △ Less

Submitted 2 December, 2020; originally announced December 2020.

Comments: Proceedings of the SPIE, Astronomical Telescopes and Instrumentation, Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray

Journal ref: Proc. SPIE, 2020, 11444, 308

Exploration about the origin of galactic and extragalactic star clusters through simulated H-R diagrams

Authors: Tanuka Chattopadhyay, Sreerup Mondal, Suman Paul, Subhadip Maji, Asis Kumar Chattopadhyay

Abstract: The present work explores the origin of the formation of star clusters in our Galaxy and in Small Magellanic Cloud (SMC) through simulated H-R diagrams and compare those with observed star clusters. The simulation study produces synthetic H-R diagrams by Markov Chain Monte Carlo (MCMC) technique using star formation history (SFH), luminosity function (LF), abundance of heavy metal (Z) and a big li… ▽ More The present work explores the origin of the formation of star clusters in our Galaxy and in Small Magellanic Cloud (SMC) through simulated H-R diagrams and compare those with observed star clusters. The simulation study produces synthetic H-R diagrams by Markov Chain Monte Carlo (MCMC) technique using star formation history (SFH), luminosity function (LF), abundance of heavy metal (Z) and a big library of isochrones as basic inputs and compares them with observed H-R diagrams of various star clusters. The distance based comparison between those two diagrams is carried out through two dimensional matching of points in Color-Magnitude Diagram (CMD) after optimal choice of bin size and appropriate distance function. It is found that a poor medium of heavy elements (Z = 0.0004), Gaia LF along with mixture of multiple Gaussian distributions of SFH may be the origin of formation of globular clusters (GCs). On the contrary, enriched medium (Z = 0.019) is favoured with Gaia LF along with double power law (i.e. unimodal) SFH. For SMC clusters, the choice of exponential LF and exponential SFH is a proper combination for poor medium whereas Gaia LF with Beta type SFH is preferred in an enriched medium for the formation of star clusters. △ Less

Submitted 30 October, 2020; v1 submitted 29 October, 2020; originally announced October 2020.

Comments: 29 pages, 7 figures

Spectro-polarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Authors: Vidushi Sharma, Shabnam Iyyani, Dipankar Bhattacharya, Tanmoy Chattopadhyay, Santosh V. Vadawale, Varun. B. Bhalerao

Abstract: GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The firs… ▽ More GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The first emission episode shows a thermal component as well as a low polarisation fraction of $PF < 37\, \%$ at $1.5\, σ$ confidence level. On the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarised with $PF > 43\, \%$ at $1.5 σ$ confidence level. We also study the afterglow properties of the jet using {\it Swift}/XRT data. The observed jet break suggests that the jet is pointed towards the observer and has an opening angle of $1.2^{\circ}$ for an assumed redshift, $z = 2$. With composite modelling of polarisation, spectrum of the prompt emission and the afterglow, we infer that the first episode of emission originates from the photosphere with localised dissipation happening below it, and the second from the optically thin region above the photosphere. The photospheric emission is generated mainly by inverse Compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. The low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant Poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet. △ Less

Submitted 4 March, 2020; originally announced March 2020.

Comments: 15 pages, 13 figures, accepted for publication in MNRAS

Random fragmentation of turbulent molecular clouds lying in the central region of giant galaxies

Authors: Suman Paul, Tanuka Chattopadhyay

Abstract: A stochastic model of fragmentation of molecular clouds has been developed for studying the resulting Initial Mass Function (IMF) where the number of fragments, inter-occurrence time of fragmentation, masses and velocities of the fragments are random variables. Here two turbulent patterns of the velocities of the fragments have been considered, namely, Gaussian and Gamma distributions. It is found… ▽ More A stochastic model of fragmentation of molecular clouds has been developed for studying the resulting Initial Mass Function (IMF) where the number of fragments, inter-occurrence time of fragmentation, masses and velocities of the fragments are random variables. Here two turbulent patterns of the velocities of the fragments have been considered, namely, Gaussian and Gamma distributions. It is found that for Gaussian distribution of the turbulent velocity, the IMFs are shallower in general compared to Salpeter mass function. On the contrary, a skewed distribution for turbulent velocity leads to an IMF which is much closer to Salpeter mass function. The above result might be due to the fact that strong driving mechanisms e.g. shocks, arising out of a big explosion occurring at the centre of the galaxy or due to big number of supernova explosions occurring simultaneously in massive parent clouds during the evolution of star clusters embedded into them are responsible for stripping out most of the gas from the clouds. This inhibits formation of massive stars in large numbers making the mass function a steeper one. △ Less

Submitted 10 June, 2020; v1 submitted 22 October, 2019; originally announced October 2019.

Comments: 26 pages, 4 figures

Journal ref: Journal: New Astronomy, Volume: 80, Year: 2020, Issue ID: 101423

Time varying polarized gamma-rays from GRB 160821A: evidence for ordered magnetic fields

Authors: Vidushi Sharma, Shabnam Iyyani, Dipankar Bhattacharya, Tanmoy Chattopadhyay, A. R. Rao, E. Aarthy, Santosh V. Vadawale, N. P. S. Mithun, Varun. B. Bhalerao, Felix Ryde, Asaf Peer

Abstract: GRB 160821A is the third most energetic gamma ray burst observed by the {\it Fermi} gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27} \%$; $5.3 σ$ detection), and (ii) variation of its polarization angle with time happening tw… ▽ More GRB 160821A is the third most energetic gamma ray burst observed by the {\it Fermi} gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27} \%$; $5.3 σ$ detection), and (ii) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at $3.5 σ$ and $3.1 σ$ detections respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of $1/Γ$, where $Γ$ is the Lorentz factor of the outflow. △ Less

Submitted 28 August, 2019; originally announced August 2019.

Comments: 12 pages, 6 figures, accepted for publication in The Astrophysical Journal Letters

Magnetars as Astrophysical Laboratories of Extreme Quantum Electrodynamics: The Case for a Compton Telescope

Authors: Zorawar Wadiasingh, George Younes, Matthew G. Baring, Alice K. Harding, Peter L. Gonthier, Kun Hu, Alexander van der Horst, Silvia Zane, Chryssa Kouveliotou, Andrei M. Beloborodov, Chanda Prescod-Weinstein, Tanmoy Chattopadhyay, Sunil Chandra, Constantinos Kalapotharakos, Kyle Parfrey, Harsha Blumer, Demos Kazanas

Abstract: A next generation of Compton and pair telescopes that improve MeV-band detection sensitivity by more than a decade beyond current instrumental capabilities will open up new insights into a variety of astrophysical source classes. Among these are magnetars, the most highly magnetic of the neutron star zoo, which will serve as a prime science target for a new mission surveying the MeV window. This p… ▽ More A next generation of Compton and pair telescopes that improve MeV-band detection sensitivity by more than a decade beyond current instrumental capabilities will open up new insights into a variety of astrophysical source classes. Among these are magnetars, the most highly magnetic of the neutron star zoo, which will serve as a prime science target for a new mission surveying the MeV window. This paper outlines the core questions pertaining to magnetars that can be addressed by such a technology. These range from global magnetar geometry and population trends, to incisive probes of hard X-ray emission locales, to providing cosmic laboratories for spectral and polarimetric testing of exotic predictions of QED, principally the prediction of the splitting of photons and magnetic pair creation. Such fundamental physics cannot yet be discerned in terrestrial experiments. State of the art modeling of the persistent hard X-ray tail emission in magnetars is presented to outline the case for powerful diagnostics using Compton polarimeters. The case highlights an inter-disciplinary opportunity to seed discovery at the interface between astronomy and physics. △ Less

Submitted 13 March, 2019; originally announced March 2019.

Comments: 11 pages, 4 figures, Astro2020 Science White Paper submitted to the National Academies of Sciences