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A Multiwavelength Study of a Long-Duration VHE Flare from BL Lacertae with VERITAS
Authors:
Atreya Acharyya,
Avery Archer,
Priyadarshini Bangale,
Joshua Bartkoske,
Wystan Benbow,
James Buckley,
Yu Chen,
Jodi Christiansen,
Alisha Chromey,
Anne Duerr,
Manel Errando,
Miguel Escobar Godoy,
Juan Escudero Pedrosa,
Abe Falcone,
Sydney Feldman,
Qi Feng,
Simon Filbert,
Lucy Fortson,
Amy Furniss,
William Hanlon,
Olivier Hervet,
Claire Hinrichs,
Jamie Holder,
Zach Hughes,
Thomas Humensky
, et al. (50 additional authors not shown)
Abstract:
We report the first observations of a long-duration very-high-energy (VHE; $E > 100$ GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On October 15, 2022, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observation…
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We report the first observations of a long-duration very-high-energy (VHE; $E > 100$ GeV) flare from BL Lacertae (VER J2202+422), taken with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). On October 15, 2022, the Fermi-Large Area Telescope (LAT) detected elevated GeV activity originating from this blazar. This triggered a multiwavelength campaign, which includes observations from VERITAS, Swift, NuSTAR, and select optical and radio observatories. VERITAS observed the source for a total of $\sim 9.8$ hours between September 1, 2022 and December 1, 2022. An analysis of these data yields a $\sim 28 σ$ detection of the source. While previously observed VHE flares from BL Lacertae have lasted on time-scales of minutes to days, VERITAS continued to detect flaring activity from the source for over a month ($\sim 40$ days) after the original flaring activity was detected with Fermi-LAT. Broadband spectral modeling shows that a synchrotron self-Compton (SSC) model with an external inverse-Compton (EC) component is preferred over a one-zone SSC model.
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Submitted 16 July, 2026;
originally announced July 2026.
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The exceptional 2017 gamma-ray flare of the radio galaxy NGC 1275: VERITAS and Multiwavelength Observations
Authors:
A. Acharyya,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. Escobar Godoy,
A. Falcone,
S. Feldman,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
Z. Hughes,
M. Iskakova,
W. Jin,
M. N. Johnson
, et al. (44 additional authors not shown)
Abstract:
The radio galaxy NGC 1275 is the Brightest Cluster Galaxy in the Perseus cluster. It is well-studied across all wavebands, including Very High Energy (VHE; E>100GeV gamma-rays, and with radio observations over the last 20 years tracking an unusual radio component, "C3". NGC 1275 was observed in an exceptional VHE flaring state between 2016 December 31 and 2017 January 3. The flare peak reached ~1.…
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The radio galaxy NGC 1275 is the Brightest Cluster Galaxy in the Perseus cluster. It is well-studied across all wavebands, including Very High Energy (VHE; E>100GeV gamma-rays, and with radio observations over the last 20 years tracking an unusual radio component, "C3". NGC 1275 was observed in an exceptional VHE flaring state between 2016 December 31 and 2017 January 3. The flare peak reached ~1.5 Crab units as measured by the MAGIC observatory. We report on the observations of NGC~1275 conducted by VERITAS and multi-wavelength data collected during this flaring state, and for context, data taken between 2009 and 2017 inclusive. VERITAS detected the declining state of the flare on 2017 January 2 (MJD 57755) and 3 (MJD 57756) at an average flux state of 0.5 Crab units. VERITAS spectra show an overall long-term trend of harder-when-brighter. During the flare, the gamma-ray spectrum obtained from the combined Fermi-LAT, MAGIC, and VERITAS observations, changes from a power law with an exponential cut-off on January 1 to a log-parabola on January 2. To study the evolution of the flare in more detail, multi-band spectral energy distributions (SEDs) were constructed for the nights of 2017 January 1 and 2 corresponding to the shift from the peak to the decline of the flare. A blob-in-jet modeling of the SEDs results in support for a two-component model with a jet angle of 10 degrees to the line of sight and the gamma-ray emission zone located in the vicinity of the C3 radio component.
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Submitted 1 June, 2026; v1 submitted 19 May, 2026;
originally announced May 2026.
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Chasing Gamma-Ray Signals from Binary Neutron Star Coalescences with the Cherenkov Telescope Array: Prospects and Observing Strategies
Authors:
S. Abe,
J. Abhir,
A. Abhishek,
F. Acero,
A. Acharyya,
R. Adam,
A. Aguasca-Cabot,
I. Agudo,
I. Albanese,
J. Alfaro,
C. Alispach,
R. Alves Batista,
E. Amato,
G. Ambrosi,
D. Ambrosino,
F. Ambrosino,
L. Angel,
C. Aramo,
A. Arbet-Engels,
C. Arcaro,
C. Arena,
T. T. H. Arnesen,
K. Asano,
H. Ashkar,
C. Bakshi
, et al. (435 additional authors not shown)
Abstract:
The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB~170817A, heralded the birth of multi-messenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), ideal…
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The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB~170817A, heralded the birth of multi-messenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV--TeV GW follow-up strategies for CTAO using a multi-step simulation pipeline and to estimate the expected rate of joint GW-GRB detections during observing run O5.
Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated.
We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV--TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star-black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.
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Submitted 9 April, 2026;
originally announced April 2026.
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Investigation of the Microquasar SS 433 with VERITAS
Authors:
The VERITAS Collaboration,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
N. R. Bond,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. E. Godoy,
J. E. Pedrosa,
S. Feldman,
Q. Feng,
S. Filbert,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
T. B. Humensky,
M. Iskakova,
W. Jin,
M. N. Johnson
, et al. (35 additional authors not shown)
Abstract:
Microquasars such as SS 433 are considered potential contributors to cosmic rays up to the knee of the cosmic ray energy spectrum ($\sim4\,\mathrm{PeV}$), where a transition in the dominant acceleration processes is expected. The SS 433 system, located within the W50 supernova remnant, is a Galactic microquasar with relativistic jets interacting with the surrounding medium over parsec scales, prov…
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Microquasars such as SS 433 are considered potential contributors to cosmic rays up to the knee of the cosmic ray energy spectrum ($\sim4\,\mathrm{PeV}$), where a transition in the dominant acceleration processes is expected. The SS 433 system, located within the W50 supernova remnant, is a Galactic microquasar with relativistic jets interacting with the surrounding medium over parsec scales, providing an example for studying jet-driven particle acceleration. A deep morphological and spectral study of SS 433 is performed using more than 150 hours of observations with VERITAS, sensitive to $γ$-ray energies $>100\,\mathrm{GeV}$. With an angular resolution better than $0.1^°$, extended TeV $γ$-ray emission is resolved from both the eastern and western jet lobes, located tens of parsecs from the central binary. The emission appears elongated along the jet axis and coincides with regions where the jets interact with the surrounding supernova remnant. No TeV emission is detected from the central binary, nor is significant emission observed between the central binary and the jet lobes. Phase-resolved analyses show no evidence for variability with orbital or precessional phase, supporting a steady emission scenario. The observed morphology and spectra are consistent with scenarios where particles are accelerated in the lobes of the jets, possibly through shocks or alternative processes such as magnetic reconnection. The extended TeV emission from the jet lobes of SS 433 favors a leptonic origin in the VERITAS energy range, suggesting any hadronic acceleration is subdominant. The results offer valuable constraints on how microquasar jets may contribute to the Galactic cosmic-ray population toward the knee.
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Submitted 23 March, 2026;
originally announced March 2026.
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VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278
Authors:
The VERITAS Collaboration,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
J. H. Buckley,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. Escobar Godoy,
S. Feldman,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
Z. Hughes,
T. B. Humensky,
M. Iskakova,
W. Jin
, et al. (44 additional authors not shown)
Abstract:
Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Sw…
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Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Swift-XRT observations have been used to explore the double-peaked broad-band emission. Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multi-wavelength data at either peak. Here we report serendipitous observations of NGC 4278 with VERITAS, made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within $2^\circ$ of NGC 4278. VERITAS did not detect any gamma-ray emission and a flux upper limit was calculated. The flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with Fermi-LAT, indicate a peak in the SED between 100 GeV and 2 TeV. We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the AGN, followed by p-$γ$ interactions and optically thin $γ$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.
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Submitted 18 March, 2026;
originally announced March 2026.
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VERITAS contributions to the 39th International Cosmic Ray Conference
Authors:
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Escobar Godoy,
J. Escudero Pedrosa,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
T. B. Humensky,
M. Iskakova,
W. Jin,
M. N. Johnson,
E. Joshi,
M. Kertzman,
M. Kherlakian
, et al. (33 additional authors not shown)
Abstract:
Compilation of papers presented by the VERITAS Collaboration at the 39th International Cosmic Ray Conference (ICRC), held July 14 through July 24, 2025 in Geneva, Switzerland.
Compilation of papers presented by the VERITAS Collaboration at the 39th International Cosmic Ray Conference (ICRC), held July 14 through July 24, 2025 in Geneva, Switzerland.
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Submitted 6 January, 2026; v1 submitted 22 December, 2025;
originally announced December 2025.
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Design and Performance of the Upgraded Prototype Schwarzschild-Couder Telescope Camera Module
Authors:
Giovanni Ambrosi,
Carla Aramo,
Mattia Barbanera,
Chiara Bartolini,
Wystan Benbow,
Bruna Bertucci,
Elisabetta Bissaldi,
Massimiliano Bitossi,
Massimo Capasso,
Mirco Caprai,
Davide Cerasole,
Zachary Curtis-Ginsberg,
Gaia De Palma,
Leonardo Di Venere,
Miguel Escobar Godoy,
Qi Feng,
Emanuele Fiandrini,
Lucy Fortson,
Stefan Funk,
Amy Furniss,
Alasdair Gent,
Stefano Germani,
Nicola Giglietto,
Francesco Giordano,
William Hanlon
, et al. (31 additional authors not shown)
Abstract:
The Cherenkov Telescope Array Observatory (CTAO) is a ground-based observatory that will improve upon the sensitivities of the current generation of very-high-energy gamma-ray instruments. The Schwarzschild-Couder Telescope (SCT) is a dual-mirror candidate design for a CTAO Medium-Sized Telescope (MST). The prototype Schwarzschild-Couder Telescope (pSCT) was inaugurated in 2019 at Fred Lawrence Wh…
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The Cherenkov Telescope Array Observatory (CTAO) is a ground-based observatory that will improve upon the sensitivities of the current generation of very-high-energy gamma-ray instruments. The Schwarzschild-Couder Telescope (SCT) is a dual-mirror candidate design for a CTAO Medium-Sized Telescope (MST). The prototype Schwarzschild-Couder Telescope (pSCT) was inaugurated in 2019 at Fred Lawrence Whipple Observatory (FLWO) in Arizona and observed significant gamma-ray emission from the Crab Nebula with a partially populated camera. The pSCT camera is currently being upgraded to fully instrument the focal plane with 11,328 silicon photomultiplier (SiPM) pixels split between 177 camera modules. Additionally, the modules will feature upgraded electronics designed to reduce electronics crosstalk and noise. A module calibration procedure has been developed using a preproduction test module. Following this calibration procedure, performance testing shows that the upgrade module has low noise, minimal electronics crosstalk, and excellent charge resolution. After calibration and optimization, the 177 production modules will be installed in the pSCT camera for commissioning. This will be followed by observations of known VHE gamma-ray sources for camera performance validation.
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Submitted 13 December, 2025;
originally announced December 2025.
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Probing a cosmogenic origin of astrophysical neutrinos and cosmic rays using gamma-ray observations of TXS 0506+056
Authors:
A. Acharyya,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
J. H. Buckley,
Y. Chen,
J. L. Christiansen,
A. Duerr,
M. Errando,
M. Escobar Godoy,
A. Falcone,
S. Feldman,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
Z. Hughes,
M. Iskakova,
W. Jin,
P. Kaaret
, et al. (36 additional authors not shown)
Abstract:
In September 2017, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the $3σ$ level, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pion…
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In September 2017, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the $3σ$ level, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pions and hence neutrinos, while interactions with the cosmic microwave background predominantly drive inverse Compton scattering, contributing to electromagnetic cascades in intergalactic space. The resulting secondary gamma-ray emission can be detected with high-energy gamma-ray telescopes. Here, we report on a new search for such cosmogenic cascade emission from the blazar TXS 0506+056, using a combined data set from the Fermi-Large Area Telescope and VERITAS. We compare the gamma-ray spectrum and neutrino observations with the predictions of cosmic-ray induced cascades in intergalactic space. The observed gamma-ray spectrum is modeled as a combination of the primary spectrum and the cascade spectrum. We apply a Monte Carlo simulation with a $Δχ^2$-based likelihood analysis to jointly determine the best-fit parameters of a proton emission spectrum describing the data and derive constraints on the proton escape luminosity. Assuming a log-parabola primary photon spectrum, we find consistency with a proton injection spectral index of $α_{p} \simeq 2.0$ and a cutoff energy of $E_{p,\text{max}} \simeq 1.3 \times 10^{16}$ eV, and constrain the isotropic proton escape luminosity to $1 \times 10^{44}$ erg s$^{-1}$ $\lesssim L_{p, esc} \lesssim 3 \times 10^{45}$ erg s$^{-1}$ at the 90 % confidence level.
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Submitted 8 November, 2025;
originally announced November 2025.
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Constraints on Axion-Like Particles from VERITAS Observations of a Flaring Radio Galaxy in the Perseus Cluster
Authors:
C. B. Adams,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. Escobar Godoy,
J. Escudero Pedrosa,
S. Feldman,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
Z. Hughes,
T. B. Humensky,
M. Iskakova,
W. Jin
, et al. (40 additional authors not shown)
Abstract:
Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing…
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Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing would be expected to leave an imprint on the spectra of TeV gamma-ray sources.
Data: In 2017, the VERITAS gamma-ray observatory recorded the second day of a dramatic flare of the radio galaxy NGC 1275, embedded at the center of the Perseus galaxy cluster. This serendipitous locale provides a spatially-extended magnetic field of strength O(10$μ$G) through which escaping photons traverse, making it an excellent target to study ALPs.
Methods: We analyze the VERITAS data of NGC 1275's 2017 flare with the gammapy analysis package. Extensive fitting and modeling are performed to ultimately conduct a likelihood analysis used to search for any evidence of a preference for ALPs and to explore the confidence with which constraints can be set. We adopt the CLs method for this study for its conservative approach to setting limits in regimes where the search has limited sensitivity.
Results: No evidence for the existence of ALPs is found, and no combination of mass and coupling strength can be excluded at or above 95% confidence level. We provide a map showing the strength of our exclusions in the mass and coupling parameter space. The strongest exclusions are found in the mass range $2 \times 10^{-7}$eV $\lesssim m_a \lesssim 4 \times 10^{-7}$eV and at the coupling strength of $g_{aγ} \gtrsim 3 \times 10^{-11}$ GeV$^{-1}$ up to 80% confidence level, which are consistent with previous studies.
Conclusions: We find the CLs method to be a trustworthy approach, and advocate for its...
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Submitted 21 October, 2025;
originally announced October 2025.
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Combined dark matter search towards dwarf spheroidal galaxies with Fermi-LAT, HAWC, H.E.S.S., MAGIC, and VERITAS
Authors:
Fermi-LAT Collaboration,
:,
S. Abdollahi,
L. Baldini,
R. Bellazzini,
B. Berenji,
E. Bissaldi,
R. Bonino,
P. Bruel,
S. Buson,
E. Charles,
A. W. Chen,
S. Ciprini,
M. Crnogorcevic,
A. Cuoco,
F. D'Ammando,
A. de Angelis,
M. Di Mauro,
N. Di Lalla,
L. Di Venere,
A. Domínguez,
S. J. Fegan,
A. Fiori,
P. Fusco,
V. Gammaldi
, et al. (582 additional authors not shown)
Abstract:
Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive cu…
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Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive currently operating gamma-ray telescopes, namely: the satellite-borne Fermi-LAT telescope; the ground-based imaging atmospheric Cherenkov telescope arrays H.E.S.S., MAGIC, and VERITAS; and the HAWC water Cherenkov detector. Individual datasets were analyzed using a common statistical approach. Results were subsequently combined via a global joint likelihood analysis. We obtain constraints on the velocity-weighted cross section $\langle σ\mathit{v} \rangle$ for DM self-annihilation as a function of the DM particle mass. This five-instrument combination allows the derivation of up to 2-3 times more constraining upper limits on $\langle σ\mathit{v} \rangle$ than the individual results over a wide mass range spanning from 5 GeV to 100 TeV. Depending on the DM content modeling, the 95% confidence level observed limits reach $1.5\times$10$^{-24}$ cm$^3$s$^{-1}$ and $3.2\times$10$^{-25}$ cm$^3$s$^{-1}$, respectively, in the $τ^+τ^-$ annihilation channel for a DM mass of 2 TeV.
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Submitted 27 August, 2025;
originally announced August 2025.
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Prospects for dark matter observations in dwarf spheroidal galaxies with the Cherenkov Telescope Array Observatory
Authors:
K. Abe,
S. Abe,
J. Abhir,
A. Abhishek,
F. Acero,
A. Acharyya,
R. Adam,
A. Aguasca-Cabot,
I. Agudo,
A. Aguirre-Santaella,
J. Alfaro,
R. Alfaro,
C. Alispach,
R. Alves Batista,
J. -P. Amans,
E. Amato,
G. Ambrosi,
D. Ambrosino,
F. Ambrosino,
L. Angel,
L. A. Antonelli,
C. Aramo,
C. Arcaro,
K. Asano,
Y. Ascasibar
, et al. (469 additional authors not shown)
Abstract:
The dwarf spheroidal galaxies (dSphs) orbiting the Milky Way are widely regarded as systems supported by velocity dispersion against self-gravity, and as prime targets for the search for indirect dark matter (DM) signatures in the GeV-to-TeV $γ$-ray range owing to their lack of astrophysical $γ$-ray background. We present forecasts of the sensitivity of the forthcoming Cherenkov Telescope Array Ob…
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The dwarf spheroidal galaxies (dSphs) orbiting the Milky Way are widely regarded as systems supported by velocity dispersion against self-gravity, and as prime targets for the search for indirect dark matter (DM) signatures in the GeV-to-TeV $γ$-ray range owing to their lack of astrophysical $γ$-ray background. We present forecasts of the sensitivity of the forthcoming Cherenkov Telescope Array Observatory (CTAO) to annihilating or decaying DM signals in these targets. An original selection of candidates is performed from the current catalogue of known objects, including both classical and ultra-faint dSphs. For each, the expected DM content is derived using the most comprehensive photometric and spectroscopic data available, within a consistent framework of analysis. This approach enables the derivation of novel astrophysical factor profiles for indirect DM searches, which are compared with results from the literature. From an initial sample of 64 dSphs, eight promising targets are identified -- Draco I, Coma Berenices, Ursa Major II, Ursa Minor and Willman 1 in the North, Reticulum II, Sculptor and Sagittarius II in the South -- for which different DM density models yield consistent expectations, leading to robust predictions. CTAO is expected to provide the strongest limits above $\sim$10 TeV, reaching velocity-averaged annihilation cross sections of $\sim$5$\times$10$^{-25}$ cm$^3$ s$^{-1}$ and decay lifetimes up to $\sim$10$^{26}$ s for combined limits. The dominant uncertainties arise from the imprecise determination of the DM content, particularly for ultra-faint dSphs. Observation strategies are proposed that optimise either deep exposures of the best candidates or diversified target selections.
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Submitted 13 October, 2025; v1 submitted 26 August, 2025;
originally announced August 2025.
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HAWC, VERITAS, Fermi-LAT and XMM-Newton follow-up observations of the unidentified ultra-high-energy gamma-ray source LHAASO J2108+5157
Authors:
The VERITAS collaboration,
C. B. Adams,
P. Bangale,
W. Benbow,
J. H. Buckley,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
M. Escobar Godoy,
S. Feldman,
Q. Feng,
J. Foote,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
Z. Hughes,
T. B. Humensky,
W. Jin,
P. Kaaret,
M. Kertzman,
M. Kherlakian,
D. Kieda
, et al. (121 additional authors not shown)
Abstract:
We report observations of the ultra-high-energy gamma-ray source LHAASO J2108$+$5157, utilizing VERITAS, HAWC, Fermi-LAT, and XMM-Newton. VERITAS has collected $\sim$ 40 hours of data that we used to set ULs to the emission above 200 GeV. The HAWC data, collected over $\sim 2400$ days, reveal emission between 3 and 146 TeV, with a significance of $7.5~σ$, favoring an extended source model. The bes…
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We report observations of the ultra-high-energy gamma-ray source LHAASO J2108$+$5157, utilizing VERITAS, HAWC, Fermi-LAT, and XMM-Newton. VERITAS has collected $\sim$ 40 hours of data that we used to set ULs to the emission above 200 GeV. The HAWC data, collected over $\sim 2400$ days, reveal emission between 3 and 146 TeV, with a significance of $7.5~σ$, favoring an extended source model. The best-fit spectrum measured by HAWC is characterized by a simple power-law with a spectral index of $2.45\pm0.11_{stat}$. Fermi-LAT analysis finds a point source with a very soft spectrum in the LHAASO J2108+5157 region, consistent with the 4FGL-DR3 catalog results. The XMM-Newton analysis yields a null detection of the source in the 2 - 7 keV band. The broadband spectrum can be interpreted as a pulsar and a pulsar wind nebula system, where the GeV gamma-ray emission originates from an unidentified pulsar, and the X-ray and TeV emission is attributed to synchrotron radiation and inverse Compton scattering of electrons accelerated within a pulsar wind nebula. In this leptonic scenario, our X-ray upper limit provides a stringent constraint on the magnetic field, which is $\lesssim 1.5\ μ$G.
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Submitted 25 August, 2025; v1 submitted 3 August, 2025;
originally announced August 2025.
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Multi-wavelength Study of HESS J0632+057: New Insights into Pulsar-Disk Interaction
Authors:
Jaegeun Park,
Hongjun An,
Chanho Kim,
Natalie Matchett,
Kaya Mori,
Brian van Soelen,
VERITAS Collaboration,
:,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
J. H. Buckley,
Y. Chen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. Escobar Godoy,
A. Falcone,
S. Feldman,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon
, et al. (38 additional authors not shown)
Abstract:
We present an analysis of new multi-wavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023--2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source's X-ray spectrum, including increased X-ray absorption at the orbital phase interval of…
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We present an analysis of new multi-wavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023--2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source's X-ray spectrum, including increased X-ray absorption at the orbital phase interval of $φ\approx0.3\textrm{--}0.4$. The source's X-ray flux within this phase interval seems to have exhibited a significant change on an orbital timescale. Additionally, occasional short-term variations in the X-ray band on a timescale of less than 3 days have been observed. The measured duration of the increased absorbing column density and the flux variability timescales can provide clues about the interaction between the putative pulsar and the Be companion's disk if, as previously suggested, the pulsar crosses the disk at this phase interval. Moreover, the new contemporaneous X-ray and TeV observations around the pulsar-crossing phases revealed independent variability in the X-ray and TeV fluxes, contrary to a previous observation of concurrent flux increases. While these observations alone cannot provide definitive conclusions, we discuss our results in the context of pulsar-disk interaction and intrabinary shock emission scenarios.
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Submitted 31 July, 2025;
originally announced July 2025.
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Measurement of the photosphere oblateness of $γ$ Cassiopeiae via Stellar Intensity Interferometry with the VERITAS Observatory
Authors:
A. Archer,
J. P. Aufdenberg,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
J. H. Buckley,
Y. Chen,
N. B. Y. Chin,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Escobar Godoy,
S. Feldman,
Q. Feng,
S. Filbert,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
Z. Hughes,
T. B. Humensky,
W. Jin,
M. N. Johnson
, et al. (36 additional authors not shown)
Abstract:
We use the stellar intensity interferometry system implemented with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at Fred Lawrence Whipple Observatory (FLWO) as a light collector to obtain measurements of the rapid rotator star $γ$ Cassiopeiae, at a wavelength of 416 nm. Using data from baselines sampling different position angles, we extract the size, oblateness, and proje…
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We use the stellar intensity interferometry system implemented with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at Fred Lawrence Whipple Observatory (FLWO) as a light collector to obtain measurements of the rapid rotator star $γ$ Cassiopeiae, at a wavelength of 416 nm. Using data from baselines sampling different position angles, we extract the size, oblateness, and projected orientation of the photosphere. Fitting the data with a uniform ellipse model yields a minor-axis angular diameter of $0.43\pm0.02$ mas, a major-to-minor-radius ratio of $1.28\pm0.04$, and a position angle of $116^\circ\pm5^\circ$ for the axis of rotation. A rapidly-rotating stellar atmosphere model that includes limb and gravity darkening describes the data well with a fitted angular diameter of $0.604^{+0.041}_{-0.034}$ mas corresponding to an equatorial radius of 10.9$^{+0.8}_{-0.6}~R_\odot$, a rotational velocity with a $1~σ$ lower limit at $97.7\%$ that of breakup velocity, and a position angle of $114.7^{+6.4}_{-5.7}$ degrees. These parameters are consistent with H$α$ line spectroscopy and infrared-wavelength Michelson interferometric measurements of the star's decretion disk. This is the first measurement of an oblate photosphere using intensity interferometry.
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Submitted 17 June, 2025;
originally announced June 2025.
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Multiwavelength observation of a candidate pulsar halo LHAASO J0621+3755 and the first X-ray detection of PSR J0622+3749
Authors:
C. B. Adams,
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
J. H. Buckley,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. Escobar Godoy,
A. Falcone,
S. Feldman,
Q. Feng,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
T. B. Humensky,
W. Jin,
M. N. Johnson,
P. Kaaret
, et al. (49 additional authors not shown)
Abstract:
Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755…
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Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755 with VERITAS and XMM-Newton in the TeV and X-ray bands, respectively. For this work, we developed a novel background estimation technique for imaging atmospheric Cherenkov telescope observations of such extended sources. No halo emission was detected with VERITAS (0.3--10 TeV) or XMM-Newton (2--7 keV) within 1 degree and 10 arcmin around PSR J0622+3749, respectively. Combined with the LHAASO-KM2A and Fermi-LAT data, VERITAS flux upper limits establish a spectral break at ~1--10 TeV, a unique feature compared with Geminga, the most studied pulsar halo. We model the gamma-ray spectrum and LHAASO-KM2A surface brightness as inverse Compton emission and find suppressed diffusion around the pulsar, similar to Geminga. A smaller diffusion suppression zone and harder electron injection spectrum than Geminga are necessary to reproduce the spectral cutoff. A magnetic field <= 1 uG is required by our XMM-Newton observation and synchrotron spectral modeling, consistent with Geminga. Our findings support slower diffusion and lower magnetic field around pulsar halos than the Galactic averages, hinting at magnetohydrodynamic turbulence around pulsars. Additionally, we report the detection of an X-ray point source spatially coincident with PSR J0622+3749, whose periodicity is consistent with the gamma-ray spin period of 333.2 ms. The soft spectrum of this source suggests a thermal origin.
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Submitted 2 April, 2025;
originally announced April 2025.
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Constraints on the X-ray and Very High Energy $γ$-ray Flux from Supernova Remnant W44
Authors:
A. Archer,
P. Bangale,
J. T. Bartkoske,
W. Benbow,
J. H. Buckley,
Y. Chen,
J. L. Christiansen,
A. J. Chromey,
A. Duerr,
M. Errando,
M. Escobar Godoy,
S. Feldman,
Q. Feng,
J. Foote,
L. Fortson,
A. Furniss,
W. Hanlon,
O. Hervet,
C. E. Hinrichs,
J. Holder,
T. B. Humensky,
W. Jin,
M. N. Johnson,
P. Kaaret,
M. Kertzman
, et al. (33 additional authors not shown)
Abstract:
Observations of GeV gamma-ray emission from the well-studied mixed-morphology supernova remnant (SNR) W44 by Fermi-LAT and AGILE imply that it is a site of significant cosmic ray acceleration. The spectral energy distribution (SED) derived from the GeV data suggest that the gamma-ray emission likely originates from the decay of neutral pions generated by cosmic-ray interactions. It is essential to…
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Observations of GeV gamma-ray emission from the well-studied mixed-morphology supernova remnant (SNR) W44 by Fermi-LAT and AGILE imply that it is a site of significant cosmic ray acceleration. The spectral energy distribution (SED) derived from the GeV data suggest that the gamma-ray emission likely originates from the decay of neutral pions generated by cosmic-ray interactions. It is essential to measure the SED of W44 in the X-ray and very high energy (VHE) gamma-ray bands to verify the hadronic origin of the emission and to gauge the potential contributions from leptonic emission. We report an upper-limit of the nonthermal X-ray flux from W44 of 5 $\times$ 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5 - 8.0 keV band based on $\sim$ 300 ks of XMM-Newton observations. The X-ray upper limit is consistent with previously estimated hadronic models, but in tension with the leptonic models. We estimate the VHE flux upper limit of $\sim$ 1.2 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$ in the 0.5 - 5.0 TeV range from W44 using data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS). Our non-detection of W44 at VHE wavlengths is in agreemnent with observations from other imaging atmospheric Cherenkov telescopes (IACTs) and is perhaps consistent with the evolutionary stage of the SNR.
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Submitted 12 March, 2025;
originally announced March 2025.
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VERITAS and multiwavelength observations of the Blazar B3 2247+381 in response to an IceCube neutrino alert
Authors:
Atreya Acharyya,
Colin B. Adams,
Priyadarshini Bangale,
J. T. Bartkoske,
Wystan Benbow,
James H. Buckley,
Yu Chen,
Jodi Christiansen,
Alisha Chromey,
Anne Duerr,
Manel Errando,
Miguel E. Godoy,
Abe Falcone,
Qi Feng,
Juniper Foote,
Lucy Fortson,
Amy Furniss,
William Hanlon,
David Hanna,
Olivier Hervet,
Claire E. Hinrichs,
Jamie Holder,
Thomas B. Humensky,
Weidong Jin,
Madalyn N. Johnson
, et al. (473 additional authors not shown)
Abstract:
While the sources of the diffuse astrophysical neutrino flux detected by the IceCube Neutrino Observatory are still largely unknown, one of the promising methods used towards understanding this is investigating the potential temporal and spatial correlations between neutrino alerts and the electromagnetic radiation from blazars. We report on the multiwavelength target-of-opportunity observations o…
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While the sources of the diffuse astrophysical neutrino flux detected by the IceCube Neutrino Observatory are still largely unknown, one of the promising methods used towards understanding this is investigating the potential temporal and spatial correlations between neutrino alerts and the electromagnetic radiation from blazars. We report on the multiwavelength target-of-opportunity observations of the blazar B3 2247+381, taken in response to an IceCube multiplet alert for a cluster of muon neutrino events compatible with the source location between May 20, 2022 and November 10, 2022. B3 2247+381 was not detected with VERITAS during this time period. The source was found to be in a low-flux state in the optical, ultraviolet and gamma-ray bands for the time interval corresponding to the neutrino event, but was detected in the hard X-ray band with NuSTAR during this period. We find the multiwavelength spectral energy distribution is well described using a simple one-zone leptonic synchrotron self-Compton radiation model. Moreover, assuming the neutrinos originate from hadronic processes within the jet, the neutrino flux would be accompanied by a photon flux from the cascade emission, and the integrated photon flux required in such a case would significantly exceed the total multiwavelength fluxes and the VERITAS upper limits presented here. The lack of flaring activity observed with VERITAS, combined with the low multiwavelength flux levels, and given the significance of the neutrino excess is at 3$σ$ level (uncorrected for trials), makes B3 2247+381 an unlikely source of the IceCube multiplet. We conclude that the neutrino excess is likely a background fluctuation.
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Submitted 6 February, 2025;
originally announced February 2025.
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An in-depth study of Gamma rays from the Starburst Galaxy M 82 with VERITAS
Authors:
Atreya Acharyya,
Colin B. Adams,
Priyadarshini Bangale,
Joshua T. Bartkoske,
Wystan Benbow,
Yu Chen,
Jodi L. Christiansen,
Alisha J. Chromey,
Anne Duerr,
Manel Errando,
Miguel E. Godoy,
Abe Falcone,
Sydney Feldman,
Qi Feng,
Juniper Foote,
Lucy Fortson,
Amy Furniss,
William Hanlon,
David Hanna,
Olivier Hervet,
Claire E. Hinrichs,
Jamie Holder,
Thomas B. Humensky,
Weidong Jin,
Madalyn N. Johnson
, et al. (38 additional authors not shown)
Abstract:
Assuming Galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce very-high-energy (VHE; E$>$100 GeV) gamma-ray emission via the interaction of their copious quantities of cosmic rays with the large reservoirs of dense gas within the galaxies. Such VHE emission was detected by VERITAS from the starburst galaxy M 82 in 2008-09. An extensive, mul…
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Assuming Galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce very-high-energy (VHE; E$>$100 GeV) gamma-ray emission via the interaction of their copious quantities of cosmic rays with the large reservoirs of dense gas within the galaxies. Such VHE emission was detected by VERITAS from the starburst galaxy M 82 in 2008-09. An extensive, multi-year campaign followed these initial observations, yielding a total of 254 h of good quality VERITAS data on M 82. Leveraging modern analysis techniques and the larger exposure, these VERITAS data show a more statistically significant VHE signal ($\sim$6.5 standard deviations ($σ$)). The corresponding photon spectrum is well fit by a power law ($Γ= 2.3 \pm 0.3_{stat} \pm0.2_{sys}$) and the observed integral flux is F($>$450 GeV) = $(3.2 \pm0.6_{stat} \pm 0.6_{sys}) \times 10^{-13}~\mathrm{cm^{-2}~s}^{-1}$, or $\sim$0.4\% of the Crab Nebula flux above the same energy threshold. The improved VERITAS measurements, when combined with various multi-wavelength data, enable modeling of the underlying emission and transport processes. A purely leptonic scenario is found to be a poor representation of the gamma-ray spectral energy distribution (SED). A lepto-hadronic scenario with cosmic rays following a power-law spectrum in momentum (index $s\simeq 2.25$), and with significant bremsstrahlung below $1$~GeV, provides a good match to the observed SED. The synchrotron emission from the secondary electrons indicates that efficient non-radiative losses of cosmic-ray electrons may be related to advective escape from the starburst core.
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Submitted 17 January, 2025;
originally announced January 2025.
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Dark Matter Line Searches with the Cherenkov Telescope Array
Authors:
S. Abe,
J. Abhir,
A. Abhishek,
F. Acero,
A. Acharyya,
R. Adam,
A. Aguasca-Cabot,
I. Agudo,
A. Aguirre-Santaella,
J. Alfaro,
R. Alfaro,
N. Alvarez-Crespo,
R. Alves Batista,
J. -P. Amans,
E. Amato,
G. Ambrosi,
L. Angel,
C. Aramo,
C. Arcaro,
T. T. H. Arnesen,
L. Arrabito,
K. Asano,
Y. Ascasibar,
J. Aschersleben,
H. Ashkar
, et al. (540 additional authors not shown)
Abstract:
Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of sele…
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Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g.~box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.
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Submitted 23 July, 2024; v1 submitted 7 March, 2024;
originally announced March 2024.
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Chasing Gravitational Waves with the Cherenkov Telescope Array
Authors:
Jarred Gershon Green,
Alessandro Carosi,
Lara Nava,
Barbara Patricelli,
Fabian Schüssler,
Monica Seglar-Arroyo,
Cta Consortium,
:,
Kazuki Abe,
Shotaro Abe,
Atreya Acharyya,
Remi Adam,
Arnau Aguasca-Cabot,
Ivan Agudo,
Jorge Alfaro,
Nuria Alvarez-Crespo,
Rafael Alves Batista,
Jean-Philippe Amans,
Elena Amato,
Filippo Ambrosino,
Ekrem Oguzhan Angüner,
Lucio Angelo Antonelli,
Carla Aramo,
Cornelia Arcaro,
Luisa Arrabito
, et al. (545 additional authors not shown)
Abstract:
The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very…
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The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA.
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Submitted 5 February, 2024; v1 submitted 11 October, 2023;
originally announced October 2023.
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Prospects for $γ$-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope Array
Authors:
The Cherenkov Telescope Array Consortium,
:,
K. Abe,
S. Abe,
F. Acero,
A. Acharyya,
R. Adam,
A. Aguasca-Cabot,
I. Agudo,
A. Aguirre-Santaella,
J. Alfaro,
R. Alfaro,
N. Alvarez-Crespo,
R. Alves Batista,
J. -P. Amans,
E. Amato,
E. O. Angüner,
L. A. Antonelli,
C. Aramo,
M. Araya,
C. Arcaro,
L. Arrabito,
K. Asano,
Y. Ascasíbar,
J. Aschersleben
, et al. (542 additional authors not shown)
Abstract:
Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster med…
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Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster medium. We estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse gamma-ray emission from the Perseus galaxy cluster. We perform a detailed spatial and spectral modelling of the expected signal for the DM and the CRp components. For each, we compute the expected CTA sensitivity. The observing strategy of Perseus is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio within the radius $R_{500}$ down to about $X_{500}<3\times 10^{-3}$, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index $α_{\rm CRp}=2.3$. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure $α_{\rm CRp}$ down to about $Δα_{\rm CRp}\simeq 0.1$ and the CRp spatial distribution with 10% precision. Regarding DM, CTA should improve the current ground-based gamma-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to $\sim 5$, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with $τ_χ>10^{27}$s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.
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Submitted 7 September, 2023;
originally announced September 2023.
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Testing Lepton Flavor Universality and CKM Unitarity with Rare Pion Decays in the PIONEER experiment
Authors:
PIONEER Collaboration,
W. Altmannshofer,
H. Binney,
E. Blucher,
D. Bryman,
L. Caminada,
S. Chen,
V. Cirigliano,
S. Corrodi,
A. Crivellin,
S. Cuen-Rochin,
A. Di Canto,
L. Doria,
A. Gaponenko,
A. Garcia,
L. Gibbons,
C. Glaser,
M. Escobar Godoy,
D. Göldi,
S. Gori,
T. Gorringe,
D. Hertzog,
Z. Hodge,
M. Hoferichter,
S. Ito
, et al. (36 additional authors not shown)
Abstract:
The physics motivation and the conceptual design of the PIONEER experiment, a next-generation rare pion decay experiment testing lepton flavor universality and CKM unitarity, are described. Phase I of the PIONEER experiment, which was proposed and approved at Paul Scherrer Institut, aims at measuring the charged-pion branching ratio to electrons vs.\ muons, $R_{e/μ}$, 15 times more precisely than…
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The physics motivation and the conceptual design of the PIONEER experiment, a next-generation rare pion decay experiment testing lepton flavor universality and CKM unitarity, are described. Phase I of the PIONEER experiment, which was proposed and approved at Paul Scherrer Institut, aims at measuring the charged-pion branching ratio to electrons vs.\ muons, $R_{e/μ}$, 15 times more precisely than the current experimental result, reaching the precision of the Standard Model (SM) prediction at 1 part in $10^4$. Considering several inconsistencies between the SM predictions and data pointing towards the potential violation of lepton flavor universality, the PIONEER experiment will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles up to the PeV mass scale. The later phases of the PIONEER experiment aim at improving the experimental precision of the branching ratio of pion beta decay (BRPB), $π^+\to π^0 e^+ ν(γ)$, currently at $1.036(6)\times10^{-8}$, by a factor of three (Phase II) and an order of magnitude (Phase III). Such precise measurements of BRPB will allow for tests of CKM unitarity in light of the Cabibbo Angle Anomaly and the theoretically cleanest extraction of $|V_{ud}|$ at the 0.02\% level, comparable to the deduction from superallowed beta decays.
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Submitted 10 March, 2022;
originally announced March 2022.
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PIONEER: Studies of Rare Pion Decays
Authors:
PIONEER Collaboration,
W. Altmannshofer,
H. Binney,
E. Blucher,
D. Bryman,
L. Caminada,
S. Chen,
V. Cirigliano,
S. Corrodi,
A. Crivellin,
S. Cuen-Rochin,
A. DiCanto,
L. Doria,
A. Gaponenko,
A. Garcia,
L. Gibbons,
C. Glaser,
M. Escobar Godoy,
D. Göldi,
S. Gori,
T. Gorringe,
D. Hertzog,
Z. Hodge,
M. Hoferichter,
S. Ito
, et al. (36 additional authors not shown)
Abstract:
A next-generation rare pion decay experiment, PIONEER, is strongly motivated by several inconsistencies between Standard Model (SM) predictions and data pointing towards the potential violation of lepton flavor universality. It will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles even if their masses are at very high scales. Measurement of the c…
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A next-generation rare pion decay experiment, PIONEER, is strongly motivated by several inconsistencies between Standard Model (SM) predictions and data pointing towards the potential violation of lepton flavor universality. It will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles even if their masses are at very high scales. Measurement of the charged-pion branching ratio to electrons vs. muons $R_{e/μ}$ is extremely sensitive to new physics effects. At present, the SM prediction for $R_{e/μ}$ is known to 1 part in $10^4$, which is 15 times more precise than the current experimental result. An experiment reaching the theoretical accuracy will test lepton flavor universality at an unprecedented level, probing mass scales up to the PeV range. Measurement of pion beta decay, $π^+\to π^0 e^+ ν(γ)$, with 3 to 10-fold improvement in sensitivity, will determine $V_{ud}$ in a theoretically pristine manner and test CKM unitarity, which is very important in light of the recently emerged tensions. In addition, various exotic rare decays involving sterile neutrinos and axions will be searched for with unprecedented sensitivity. The experiment design benefits from experience with the recent PIENU and PEN experiments at TRIUMF and the Paul Scherrer Institut (PSI). Excellent energy and time resolutions, greatly increased calorimeter depth, high-speed detector and electronics response, large solid angle coverage, and complete event reconstruction are all critical aspects of the approach. The PIONEER experiment design includes a 3$π$ sr 25 radiation length calorimeter, a segmented low gain avalanche detector stopping target, a positron tracker, and other detectors. Using intense pion beams, and state-of-the-art instrumentation and computational resources, the experiments can be performed at the PSI ring cyclotron.
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Submitted 7 March, 2022; v1 submitted 3 March, 2022;
originally announced March 2022.