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Reducing the Dimensions of AGN Lightcurve Manifolds
Authors:
Shoubaneh Hemmati,
Jessica Krick,
Daniel Stern,
Vandana Desai,
Andreas Faisst,
Lucas Martin-Garcia,
Varoujan Gorjian,
Aryana Haghjoo,
Farnik Nikakhtar,
Troy Raen,
Sogol Sanjaripour,
Brigitta M Sipocz,
David Shupe
Abstract:
The Active Galactic Nuclei (AGN) glossary is vast and complex. Depending on selection method, observing wavelength, and brightness, AGNs are assigned distinct labels, yet the relationship between different selection methods and the diversity of time-domain behavior within and across classes remains difficult to characterize in a unified framework. Changing-look AGNs (CLAGNs), which transition betw…
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The Active Galactic Nuclei (AGN) glossary is vast and complex. Depending on selection method, observing wavelength, and brightness, AGNs are assigned distinct labels, yet the relationship between different selection methods and the diversity of time-domain behavior within and across classes remains difficult to characterize in a unified framework. Changing-look AGNs (CLAGNs), which transition between classifications over time, further complicate this picture. In this work, we learn a data-driven, low-dimensional representation of multi-wavelength photometric light curves of AGNs, in which the structure of the projected manifold correlates with AGN class and independent spectroscopic properties. Using the NASA Fornax Science Platform, we assemble light curves from ZTF, Pan-STARRS, Gaia, and WISE/NEOWISE for two samples: (1) a heterogeneous set of $\sim$2000 AGNs spanning $z \lesssim 1$, including SDSS quasars, variability-selected sources, and CLAGNs; and (2) a homogeneous sample of $\sim$65000 narrow-line AGNs at $z \approx 0.1$ with well-characterized optical emission-line measurements. Without using class labels during training, the learned manifolds organize variability-selected AGNs into coherent regions of the low-dimensional space, distinguish between turn-on and turn-off CLAGNs, and place tidal disruption events in distinct regions. Manifold coordinates correlate with key spectroscopic and host-galaxy properties, including stellar mass, [OIII] luminosity, and D$_n$(4000), demonstrating that heterogeneous multi-band variability can be combined in a purely data-driven manner to recover correlations with independent physical diagnostics, without requiring explicit physical modeling. These results show that manifold learning offers a practical, assumption-light approach for integrating time-domain surveys and prioritizing spectroscopic follow-up.
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Submitted 12 January, 2026;
originally announced January 2026.
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Magnetic Circular Dichroism at the Oxygen K edge in Microcrystals of Spinels Grown on Ru(0001)
Authors:
A. Mandziak,
V. Sosa,
P. Nita,
L. Martín-García,
J. E. Prieto,
M. Foerster,
M. A. Niño,
L. Aballe,
C. Granados-Miralles,
A. Quesada,
C. Tejera-Centeno,
S. Gallego,
J. de la Figuera
Abstract:
We have measured the circular magnetic dichroism in the x-ray absorption at the K-edge of oxygen in microcrystals of different spinel oxides. The microcrystals are islands of micrometric size and nanometric thickness, grown on Ru(0001) substrates using high-temperature oxygen-assisted molecular beam epitaxy. The domains observed in the oxygen K-edge dichroism have the same distribution and orienta…
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We have measured the circular magnetic dichroism in the x-ray absorption at the K-edge of oxygen in microcrystals of different spinel oxides. The microcrystals are islands of micrometric size and nanometric thickness, grown on Ru(0001) substrates using high-temperature oxygen-assisted molecular beam epitaxy. The domains observed in the oxygen K-edge dichroism have the same distribution and orientation as those observed in x-ray magnetic circular dichroism at the L$_{3}$ edge of the octahedral cations. Integrating the area from a single domain, x-ray magnetic circular dichroic spectra of oxygen were measured and, by the application of the K-edge sum rule, non vanishing orbital magnetic moments aligned with the octahedral cations were found. Density functional theory calculations, which did not show any orbital moment at the oxygen anions, indicate that the energy ranges where oxygen dichroism is observed correspond to those with significant hybridization with the cations d bands. They also show a correlation between the magnitude of the measured value of the oxygen orbital moment and the theoretical one for the cations, and demonstrate that this trend is preserved in the presence of Fe excess in the samples. Our experimental XMCD suggest, following the DFT calculations, that the origin of the oxygen magnetic moment lies in the hybridization of the oxygen unoccupied p-derived bands with the cation bands, mostly with the d-derived ones.
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Submitted 30 May, 2025;
originally announced May 2025.
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Magnetic domain wall pinning in cobalt ferrite microstructures
Authors:
Sandra Ruiz-Gómez,
Anna Mandziak,
Laura Martín-García,
Jose Emilio Prieto,
Pilar Prieto,
Carmen Munuera,
Michael Foerster,
Adrián Quesda,
Lucía Aballe,
Juan de la Figuera
Abstract:
A detailed correlative structural, magnetic and chemical analysis of non-stoichiometric cobalt ferrite micrometric crystals was performed by x-ray magnetic circular dichroism combined with photoemission microscopy, low energy electron microscopy, and atomic force microscopy. The vector magnetization at the nanoscale is obtained from magnetic images at different x-ray incidence angles and compared…
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A detailed correlative structural, magnetic and chemical analysis of non-stoichiometric cobalt ferrite micrometric crystals was performed by x-ray magnetic circular dichroism combined with photoemission microscopy, low energy electron microscopy, and atomic force microscopy. The vector magnetization at the nanoscale is obtained from magnetic images at different x-ray incidence angles and compared with micromagnetic simulations, revealing the presence of defects which pin the magnetic domain walls. A comparison of different types of defects and the domain walls location suggests that the main source of pinning in these microcrystals are linear structural defects induced in the spinel by the substrate steps underneath the islands.
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Submitted 12 January, 2022;
originally announced January 2022.