Abstracts of Interest

Selected by: Adila Abdul Halim


Abstract: 2504.19958
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Title:Machine Learning Identification of Gravimentally Microlensed Gamma-Ray Bursts

Authors:Mohammad H. Zhoolideh Haghighi, Zeinab Kalantari, Sohrab Rahvar, Alaa Ibrahim
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Abstract:Gravitational microlensing of gamma-ray bursts (GRBs) provides a unique opportunity to probe compact dark matter and small-scale structures in the universe. However, identifying such microlensed GRBs within large datasets is a significant challenge. In this study, we develop a machine learning approach to distinguish Lensed GRBs from their Non-lensed counterparts using simulated light curves. A comprehensive dataset was generated, comprising labeled light curves for both categories. Features were extracted using the Cesium package, capturing critical temporal properties of the light curves. Multiple machine learning models were trained on the extracted features, with Random Forest achieving the best performance, delivering an accuracy of 94\% and an F1 score of 95\% (94\%) for Non-Lensed (Lensed) class. This approach successfully demonstrates the potential of machine learning for identifying gravitational lensing in GRBs, paving the way for future observational applications.



Abstract: 2504.19964
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Title:XMM-Newton Observations of the High Temperature Plasma in the Large Magellanic Cloud Supernova Remnant N132D

Authors:Adam R. Foster, Paul P.Plucinsky, Terrance J. Gaetz, Xi Long, Diab Jerius
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Abstract:We present an analysis of the archival XMM-Newton observations of the Large Magellanic Cloud (LMC) supernova remnant N132D totaling more than 500ks. We focus on the high temperature plasma ($kt\sim 4.5$keV) that is responsible for the high energy continuum and exciting the Fe K emission. An image analysis shows that the Fe K emission is mainly concentrated in the southern part of the remnant interior to the region defined by the forward shock. This Fe K distribution would be consistent with an asymmetric distribution of the Fe ejecta and/or an asymmetric interaction between the reverse shock and the Fe ejecta. We compare the EPIC-pn and EPIC-MOS spectra in the 3.0 -- 12.0keV bandpass with a model based on RGS data plus a higher temperature component, in collisional ionization equilibrium (CIE), or non-equilibrium (NEI) (both ionizing and recombining). We find that the data are equally well-fitted by the CIE and ionizing models. Assuming the CIE and ionizing spectral models, the Fe in this high temperature component is significantly enhanced with respect to typical LMC abundances. We can place only an upper limit on the neutral Fe K line. We conclude that the Fe~K emission is due to ejecta heated by the reverse shock given the spatial distribution, relatively high temperature, and enhanced abundance. We estimate the progenitor mass based on the Ca/Fe and Ni/Fe mass ratios to be $13\le M_P \le 15 M_\odot$.

Comments: 19 Pages, 8 Figures, accepted by ApJ


Abstract: 2504.19562
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Title:CGM cloud sizes from refractive FRB scattering

Authors:Lluis Mas-Ribas, Matthew McQuinn, J. Xavier Prochaska
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Abstract:We explore constraints on the size of cool gas clouds in the circumgalactic medium (CGM) obtainable from the presence, or lack thereof, of refractive scattering in fast radio bursts (FRBs). Our refractive analysis sets the most conservative bounds on parsec-scale CGM clumpiness as it does not make assumptions about the turbulent density cascade. We find that the bulk of low-redshift cool CGM gas, constrained to have densities of $n_{\rm e} \lesssim 10^{-2}\,{\rm cm^{-3}}$, likely cannot produce two refractive images and, hence, scattering. It is only for extremely small cloud sizes $\lesssim 0.1$ pc (about a hundred times smaller than the so-called shattering scale) that such densities could result in detectable scattering. Dense $n_{\rm e} \gtrsim 0.1\,{\rm cm^{-3}}$ gas with shattering-scale cloud sizes is more likely to inhabit the inner several kiloparsecs of the low-redshift CGM: such clouds would result in multiple refractive images and large scattering times $\gtrsim 1 - 10$ ms, but a small fraction FRB sightlines are likely to be affected. We argue that such large scattering times from an intervening CGM would be a signature of sub-parsec clouds, even if diffractive scattering from turbulence contributes to the overall scattering. At redshift $z\sim 3$, we estimate $\sim 0.1\%$ of FRBs to intersect massive proto-clusters, which may be the most likely place to see scattering owing to their ubiquitous $n_{\rm e} \approx 1\,{\rm cm^{-3}}$ cold gas. While much of our discussion assumes a single cloud size, we show similar results hold for a CGM cloud-size distribution motivated by hydrodynamic simulations.

Comments: Submitted to ApJ


Abstract: 2504.20132
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Title:Expanding Ejecta Method: I. Mapping Supernova Morphology with Intensity Interferometry

Authors:I-Kai Chen, David Dunsky, Ken Van Tilburg, Junwu Huang, Robert V. Wagoner
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Abstract:We explore the potential of optical intensity interferometry to extract angularly resolved information from supernova explosions, introducing the "expanding ejecta method" (EEM) as a robust alternative to the classical expanding photosphere method (EPM). Foreseeing future improvements to intensity interferometers of large light collection area ($25\pi\,\rm{m}^2$ per telescope) equipped with spectral multiplexing ($10^4$ spectral resolution) and fast photodetectors ($10\,\mathrm{ps}$ timing resolution, $50\%$ overall efficiency), we demonstrate that high signal-to-noise measurements of the visibility modulus are achievable for Type IIP (Type Ia) supernovae out to $3~(12)\,\mathrm{Mpc}$. By focusing on generic line emission and absorption in ballistic ejecta, the EEM can relax assumptions about spherical symmetry, blackbody radiation, and extinction. The EEM enables angular diameter distances to be determined with $\sim2\%$ precision for supernovae of apparent magnitude $m = 12$ from a 60-hour observation by an intensity interferometer with those same instrumental specifications. We argue that the EEM is significantly more robust to modeling uncertainties and systematic effects than (variants of) the EPM. In a companion paper, we show how the EEM can be used to provide geometric anchors for cosmic distance ladder calibration, or to construct a wholly independent Hubble diagram based on angular diameter distances.

Comments: 29+4 pages, 13+3 figures


Abstract: 2504.20163
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Title:Implications of the KM3NeT Ultrahigh-energy Event on Neutrino Self-interactions

Authors:Yuxuan He, Jia Liu, Xiao-Ping Wang, Yi-Ming Zhong
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Abstract:Neutrino self-interactions ({\nu}SI) mediated by light bosonic particles can produce characteristic spectral dips in astrophysical neutrino fluxes, thereby modifying the expected energy spectrum. The high-energy astrophysical neutrino spectrum has been extensively used to probe {\nu}SI models through these distinctive features. The recent detection of the ultrahigh-energy event KM3-230213A offers a new opportunity to explore {\nu}SI phenomenology at extreme energies. In this work, we investigate two implications of this observation under the assumption that the event originates from a diffuse power-law spectrum. First, we find that {\nu}SI induces spectral distortions that can mildly alleviate tensions between the KM3-230213A detection and the previous non-observation of PeV-scale neutrinos in IceCube data. Second, we derive new constraints on the {\nu}SI coupling strength for mediator masses around 100 MeV. Our analysis shows that neutrino telescopes can surpass existing collider constraints in this mass range. In the near future, IceCube-Gen2 is expected to substantially enhance the sensitivity to {\nu}SI over a broader range of parameter space.

Comments: 9 pages, 2 figures, 2 tables


Abstract: 2504.20263
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Title:Gamma-ray burst taxonomy: looking for the third class on the spectral peak energy-duration plane in the rest frame

Authors:Anastasia Tsvetkova, Lorenzo Amati, Mattia Bulla, Luciano Burderi, Dmitry Frederiks, Filippo Frontera, Cristiano Guidorzi, Alessandro Riggio, Tiziana di Salvo, Andrea Sanna, Fyodor Sviridov
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Abstract:Two classes of Gamma-ray bursts (GRBs), corresponding to the short/hard and the long/soft events, with a putative intermediate class, are typically considered in the observer frame. However, when considering GRB characteristics in the cosmological rest frame, the boundary between the classes becomes blurred. The goal of this research is to check for the evidences of a third "intermediate" class of GRBs and investigate how the transformation from the observer to the rest frame affects the hardness-duration-based classification. We applied fits with skewed and non-skewed (symmetric) Gaussian and Student distributions to the sample of 409 GRBs with reliably measured redshifts to cluster the bursts on the hardness (Ep) - duration (T90) plane. We found that, based on AIC/BIC criteria, the statistically preferred number of clusters on the GRB rest-frame hardnesses-duration plane does not exceed two. We also assess the robustness of the clustering technique. We did not find any solid evidence of an intermediate GRB class on the rest-frame hardness-duration plane.

Comments: 22 pages, 4 figures, 15 tables, A&A accepted


Abstract: 2504.20601
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Title:How to turn a Supernova into a PeVatron

Authors:Robert Brose, Iurii Sushch, Jonathan Mackey
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Abstract:Context. It is important to determine which Galactic cosmic-ray sources can accelerate particles to the knee of the cosmic ray spectrum at a few PeV, and in particular whether supernova remnants may contribute. Current models for particle acceleration in very young remnants assume the circumstellar material consists of smooth, freely expanding winds. There is strong evidence that some supernovae expand into much denser circumstellar material including dense shells ejected by eruptions shortly before explosion.
Aims. We investigate the effects of dense circumstellar shells on particle acceleration in supernova shocks during the first few years post-explosion, to quantify whether such interaction supernovae may act as PeVatrons.
Methods. We used the pion code to model the circumstellar medium around Luminous Blue Variables after having a brief episode with a mass-loss rate of up to dM/dt = 2Msol/yr. Consequently, we performed spherically symmetric 1-D simulations using our time-dependent acceleration-code RATPaC in which we simultaneously solve the transport equations for cosmic-rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit.
Results. We find that the interaction with the circumstellar shells can significantly boost the maximum energy by enhancing particle escape during the onset of the shock-shell interaction followed by the reacceleration of the shock propagating into a medium with a pre-amplified field. Early interactions boost the maximum energy to a greater degree and interactions within the first 5 months after explosion can increase Emax to more then 1 PeV.

Comments: 9 pages, 4 figures, accepted in A&A


Abstract: 2504.20627
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Title:Modeling of the time-resolved spectral energy distribution of blazar OJ 287 from 2008 to 2023: a comprehensive multi-epoch study

Authors:G. Harutyunyan, N. Sahakyan, D. Bégué
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Abstract:We present a comprehensive analysis of the time-resolved spectral energy distributions (SEDs) of the blazar OJ 287 over a 15-year period (2008-2023), using multi-wavelength data. In the $\gamma$-ray band, multiple flaring episodes were observed, with the strongest flare reaching a peak flux of $(5.60\pm1.11)\times10^{-7}\:{\rm photons\:cm^{-2}\:s^{-1}}$ on MJD 55869.03 (04 November 2011). In the optical/UV band, the source was in an active state between MJD 57360 (04 December 2015) and 57960 (26 July 2017), during which the highest flux of $(1.07\pm0.02)\times10^{-10}\:{\rm erg\:cm^{-2}\:s^{-1}}$ was observed on MJD 57681.23 (20 October 2016). In the X-ray band, both the flux and spectral index exhibit variability. To investigate the origin of the broadband emission from OJ 287, we systematically modeled 739 quasi-simultaneous SEDs using a leptonic model that self-consistently accounts for particle injection and cooling. This analysis is possible thanks to the recent development of a surrogate neural-network-based model, trained on kinetic simulations. This innovative, time-resolved, neural network-based approach overcomes the limitations of traditional single-epoch SED modeling, enabling to explore the temporal evolution of key model parameters, such as the magnetic field strength, Doppler factor, and electron injection distribution, across different states of the source. We identified distinct emission states characterized by unique combinations of magnetic field $ B $, electron index $ p $, and Doppler boost $ \delta $, associated to different underlying mechanisms such as varying acceleration processes (e.g., shocks, turbulence) and magnetic confinement. The analysis provides insights into the jet physics processes, including particle acceleration mechanisms and dynamic changes in the jet structure.

Comments: Accepted for publication in MNRAS


Abstract: 2504.21221
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Title:The H.E.S.S. extragalactic sky survey with the first decade of observations

Authors:H.E.S.S. Collaboration, F. Aharonian, F. Ait Benkhali, J. Aschersleben, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, M. de Bony de Lavergne, J. Borowska, M. Bouyahiaoui, F. Bradascio, R. Brose, A. Brown, F. Brun, B. Bruno, T. Bulik, C. Burger-Scheidlin, T. Bylund, S. Casanova, J. Celic, M. Cerruti, T. Chand, S. Chandra, A. Chen, J. Chibueze, O. Chibueze, G. Cotter, P. Cristofari, J. Damascene Mbarubucyeye, I.D. Davids, J. Devin, J. Djuvsland, A. Dmytriiev, K. Egberts, S. Einecke, S. Fegan, G. Fontaine, S. Funk, S. Gabici, J.F. Glicenstein, J. Glombitza, P. Goswami, G. Grolleron, L. Haerer, B. Heß, J.A. Hinton, W. Hofmann, T. L. Holch, M. Holler, D. Horns, Zhiqiu Huang, M. Jamrozy, F. Jankowsky, A. Jardin-Blicq, E. Kasai, K. Katarzyński, R. Khatoon, B. Khélifi, Nu. Komin, K. Kosack, D. Kostunin, A. Kundu, R.G. Lang, S. Le Stum, A. Lemière, M. Lemoine-Goumard, J.-P. Lenain, F. Leuschner, A. Luashvili, J. Mackey, D. Malyshev, V. Marandon, G. Martí-Devesa, R. Marx, A. Mehta, A. Mitchell, R. Moderski, L. Mohrmann, A. Montanari, M. de Naurois, J. Niemiec, P. O'Brien, L. Olivera-Nieto, E. de Ona Wilhelmi, M. Ostrowski, S. Panny, M. Panter, U. Pensec, G. Pühlhofer, M. Punch, A. Quirrenbach, S. Ravikularaman, M. Regeard et al. (43 additional authors not shown)
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Abstract:The results of the first extragalactic gamma-ray survey by the High Energy Stereoscopic System (H.E.S.S.) are presented. The survey comprises 2720 hours of very high-energy gamma-ray observations of the extragalactic sky, recorded with H.E.S.S. from 2004 up to the end of 2012. These data have been re-analysed using a common consistent set of up-to-date data calibration and analysis tools. From this analysis, a list of 23 detected objects, predominantly blazars, was obtained. This catalogue was assessed in terms of the source class populations that it contains. The level of source parameter bias for the blazar sources, probed by this observational dataset, was evaluated using Monte-Carlo simulations. Spectral results obtained with the H.E.S.S. data were compared with the \textit{Fermi}-LAT catalogues to present the full gamma-ray picture of the detected objects. Lastly, this unique dataset was used to assess the contribution of BL Lacertae objects and flat-spectrum radio quasars to the extragalactic gamma-ray background light at several hundreds of giga-electronvolts. These results are accompanied by the release of the high-level data to the astrophysical community.

Comments: 23 pages, 13 figures. Published in A&A


Abstract: 2505.00175
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Title:Extended TeV Halos May Commonly Exist around Middle-Aged Pulsars

Authors:A. Albert, R. Alfaro, C. Alvarez, J.C. Arteaga-Velázquez, D. Avila Rojas, H.A. Ayala Solares, R. Babu, E. Belmont-Moreno, A. Bernal, K.S. Caballero-Mora, T. Capistrán, A. Carramiñana, S. Casanova, U. Cotti, J. Cotzomi, S. Coutiño de León, E. De la Fuente, D. Depaoli, P. Desiati, N. Di Lalla, R. Diaz Hernandez, B.L. Dingus, M.A. DuVernois, J.C. Díaz-Vélez, K. Engel, C. Espinoza, K.L. Fan, N. Fraija, J.A. García-González, F. Garfias, H. Goksu, M.M. González, J.A. Goodman, S. Groetsch, J.P. Harding, S. Hernández-Cadena, I. Herzog, D. Huang, F. Hueyotl-Zahuantitla, A. Iriarte, S. Kaufmann, D. Kieda, J. Lee, H. León Vargas, J.T. Linnemann, A.L. Longinotti, G. Luis-Raya, K. Malone, O. Martinez, J. MartínezCastro, J.A. Matthews, P. Miranda-Romagnoli, J.A. Morales-Soto, E. Moreno, M. Mostafá, M. Najafi, A. Nayerhoda, L. Nellen, M.U. Nisa, N. Omodei, Y. Pérez Araujo, E.G. Pérez-Pérez, C.D. Rho, D. Rosa-González, E. Ruiz-Velasco, H. Salazar, D. Salazar-Gallegos, A. Sandoval, M. Schneider, J. Serna-Franco, Y. Son, R.W. Springer, O. Tibolla, K. Tollefson, I. Torres, R. Torres-Escobedo, R. Turner, F. Ureña-Mena, E. Varela, L. Villaseñor, X. Wang, E. Willox, H. Zhou, C. de León
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Abstract:Extended gamma-ray emission around isolated pulsars at TeV energies, also known as TeV halos, have been found around a handful of middle-aged pulsars. The halos are significantly more extended than their pulsar wind nebulae but much smaller than the particle diffusion length in the interstellar medium. The origin of TeV halos is unknown. Interpretations invoke either local effects related to the environment of a pulsar or generic particle transport behaviors. The latter scenario predicts that TeV halos would be a universal phenomena for all pulsars. We searched for extended gamma-ray emission around 36 isolated middle-aged pulsars identified by radio and gamma-ray facilities using 2321 days of data from the High-Altitude Water Cherenkov (HAWC) Observatory. Through a stacking analysis comparing TeV flux models against a background-only hypothesis, we identified TeV halo-like emission at a significance level of $5.10\,\sigma$. Our results imply that extended TeV gamma-ray halos may commonly exist around middle-aged pulsars. This reveals a previously unknown feature about pulsars and opens a new window to identify the pulsar population that is invisible to radio, x-ray, and GeV gamma-ray observations due to magnetospheric configurations.

Comments: 12 pages, 6 figures, published as Editor's suggestion in Phys. Rev. Lett., April 30, 2025


Abstract: 2505.00184
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Title:Absence of TeV halos around millisecond pulsars

Authors:A. U. Abeysekara, R. Alfaro, C. Alvarez, J. C. Arteaga-Velázquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. Capistrán, A. Carramiñana, S. Casanova, U. Cotti, J. Cotzomi, S. Coutiño de León, E. De la Fuente, D. Depaoli, P. Desiati, N. Di Lalla, R. Diaz Hernandez, M. A. DuVernois, J. C. Díaz-Vélez, K. Engel, T. Ergin, K. L. Fan, K. Fang, N. Fraija, S. Fraija, J. A. García-González, F. Garfias, M. M. González, J. A. Goodman, J. P. Harding, S. Hernández-Cadena, D. Huang, F. Hueyotl-Zahuantitla, A. Iriarte, S. Kaufmann, D. Kieda, A. Lara, J. Lee, H. León Vargas, J. T. Linnemann, A. L. Longinotti, G. Luis-Raya, K. Malone, O. Martinez, J. Martínez-Castro, J. A. Matthews, P. Miranda-Romagnoli, J. A. Morales-Soto, E. Moreno, M. Mostafá, M. Najafi, L. Nellen, M. Newbold, M. U. Nisa, R. Noriega-Papaqui, N. Omodei, Y. Pérez Araujo, E. G. Pérez-Pérez, C. D. Rho, D. Rosa-González, E. Ruiz-Velasco, M. A. Roth, H. Salazar, A. Sandoval, J. Serna-Franco, Y. Son, R. W. Springer, O. Tibolla, K. Tollefson, I. Torres, R. Torres-Escobedo, R. Turner, F. Ureña-Mena, E. Varela, L. Villaseñor, X. Wang, Z. Wang, I. J. Watson, H. Wu, S. Yu, C. de León
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Abstract:TeV halos are extended very-high-energy (VHE; 0.1-100 TeV) gamma-ray emission around middle-aged pulsars. So far they have only been found around isolated pulsars, but it has been suggested that they may also be powered by millisecond pulsars (MSPs). We searched for VHE gamma-ray emission from MSPs reported by radio and GeV gamma-ray observatories in 2565 days of data from the High Altitude Water Cherenkov (HAWC) Observatory. We found no significant emission from individual pulsars. By combining the likelihood profiles of all MSPs accessible to HAWC, our analysis suggests that the excess emission around the MSP population is consistent with a background. Our result suggests that MSPs are not as efficient as isolated pulsars in producing TeV halos. This finding has strong implications on the physics interpretation of the Galactic Center GeV excess and high-latitude Galactic diffuse emission.

Comments: 11 pages, 8 figures. Published as Editor's Suggestion in Phys. Rev. D, 6 February, 2025


Abstract: 2504.20551
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Title:Accretion and ejection at work in the Narrow Line Seyfert 1 galaxy 1H 0323+342 -- A case of intermittent activity?

Authors:Valentina Rosa, Luigi Foschini, Stefano Ciroi
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Abstract:We present a comprehensive investigation into the properties of 1H 0323+342, a prominent jetted narrow-line Seyfert 1 galaxy. The primary objective is to understand the interplay between the relativistic jet, the hot corona, and the accretion disk around the supermassive black hole. This study spans the years 2006 to 2023, incorporating a rich dataset with 172 Swift observations, including the optical, UV, and X-ray bands, integrated with Fermi Large Area Telescope (LAT) observations. Spectral analysis was conducted on the X-ray observations using the XSPEC software, and the results were compared with optical, UV, and gamma-ray flux measurements and photon index values. Our key findings include the identification of three distinct zones in the X-ray photon index-flux plot, characterized by high flux and a hard photon index (zone 1), high flux and a soft photon index (zone 2), and low flux and a soft photon index (zone 3). Before 2017, 1H 0323+342 moved back and forth between zones 1 and 2; after that epoch, it transitioned to zones 2 and 3. Correspondingly, we observed a decreasing jet activity in the Fermi/LAT data and a reduction in the accretion rate in optical/UV data from Swift/UVOT. We interpret these observations in the framework of an intermittent jet scenario, driven by radiation-pressure instability in the accretion disk.

Comments: 7 pages, 6 figures, accepted for publication on Astronomy and Astrophysics. Data tables will be available online on Vizier


Abstract: 2504.20407
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Title:Discovery of a variable yellow supergiant progenitor for the Type IIb SN 2024abfo

Authors:Zexi Niu, Ning-Chen Sun, Justyn R. Maund, Zhen Guo, Wenxiong Li, Meng Sun, Jifeng Liu
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Abstract:We report the discovery of a progenitor candidate for the Type IIb SN 2024abfo using multi-epoch pre-explosion images from the Hubble Space Telescope and the Dark Energy Camera Legacy Survey. The progenitor exhibited a ~0.7 mag decline in F814W from 2001 to 2013, followed by significant brightness and color fluctuations in the g, r and z bands. This is the first time that a variable progenitor has been found for a SN IIb. We suggest that the variability is caused by intrinsic changes in the progenitor star instead of varying obscuration by circumstellar dust. Our results show that the progenitor of SN 2024abfo was likely a yellow supergiant star with an initial mass of 12--14, 14--15 and 15--18 Msun for different assumed values of circumstellar reddening of E(B-V)_CSM = 0, 0.1 and 0.2 mag. Our study underscores the critical role of multi-epoch imaging surveys in revealing the final stages of core-collapse supernovae progenitors.

Comments: 5 Figures. Submitted to ApJL. Comments are welcome


Abstract: 2504.21096
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Title:Search for the Optical Counterpart of Einstein Probe Discovered Fast X-ray Transients from Lulin Observatory

Authors:Amar Aryan, Ting-Wan Chen, Sheng Yang, James H. Gillanders, Albert K.H. Kong, S. J. Smartt, Heloise F. Stevance, Yi-Jung Yang, Aysha Aamer, Rahul Gupta, Lele Fan, Wei-Jie Hou, Hsiang-Yao Hsiao, Amit Kumar, Cheng-Han Lai, Meng-Han Lee, Yu-Hsing Lee, Hung-Chin Lin, Chi-Sheng Lin, Chow-Choong Ngeow, Matt Nicholl, Yen-Chen Pan, Shashi Bhushan Pandey, Aiswarya Sankar.K, Shubham Srivastav, Guanghui Sun, Ze-Ning Wang
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Abstract:The launch of the Einstein Probe (EP) mission has revolutionized the detection and follow-up observations of fast X-ray transients (FXTs) by providing prompt and timely access to their precise localizations. In the first year of its operation, the EP mission reports the discovery of 72 high signal-to-noise FXTs. Subjected to the visibility in the sky and weather conditions, we search for the optical counterparts of 42 EP-discovered FXTs from the Lulin observatory. We successfully detect the optical counterparts of 12 FXTs, and five of those are first discovered by us from the Lulin observatory. We find that the optical counterparts are generally faint (r>20 mag) and decline rapidly (>0.5 mag per day). We also find that 11 out of 42 FXTs had shown direct evidence of their association with Gamma-Ray Bursts (GRBs) through significant temporal and spatial overlapping. Furthermore, the luminosities and redshifts of FXTs with confirm optical counterparts in our observations are fully consistent with the faintest end of the GRB population. However, the non-detection of any associated optical counterpart with a significant fraction of FXTs suggests that EP FXTs are likely a subset of so-called `Dark' FXTs, similar to `Dark' GRBs. Additionally, the luminosities of two FXTs were also consistent with jetted tidal disruption events (TDEs). However, their luminosities differ significantly from those of typical supernova shock breakout or kilonova emissions. Thus, we conclude that a significant fraction of EP-discovered FXTs are associated with events having relativistic jets; either a GRB or a jetted TDE.

Comments: The manuscript has 14 Figures, 7 Tables and a total of 49 pages (including Appendix). Submitted to ApJS


Abstract: 2504.21238
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Title:A new candidate of a cluster of galaxies behind the Galactic plane, AX J145732-5901

Authors:Shigeo Yamauchi, Masaru Ueno
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Abstract:AX J145732-5901 is an unidentified X-ray source discovered in the ASCA Galactic plane survey. Its extended nature and heavily absorbed X-ray spectrum suggest that AX J145732-5901 is a cluster of galaxies behind the Galactic plane. However, due to limited photon statistics, the spectral shape was not well examined. Using the results of the Galactic ridge X-ray emission and Cosmic X-ray background studies based on the Suzaku observations, we reanalyzed the ASCA data of AX J145732-5901. We confirmed that the source is more extended than the point spread function and the angular size is 14'x10'. The spectrum was heavily absorbed by interstellar matter equivalent to an N_{H} of ~10^{23} cm^{-2} and the emission line feature was confirmed. The spectrum was represented by a thin thermal plasma model with a temperature of 2.6 keV and a redshift of 0.12. Assuming the redshift value, the X-ray luminosity is calculated to be 2.6x10^{44} erg s^{-1} in the 1-10 keV energy band. The observational results indicate that AX J145732-5901 is a cluster of galaxies behind the Galactic plane.

Comments: Accepted for publication in PASJ


Abstract: 2504.21615
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Title:Rapid Spectral Evolution of SGR 1935+2154 During its 2022 Outburst

Authors:Chin-Ping Hu, Zorawar Wadiasingh, Wynn C. G. Ho, Matthew G. Baring, George A. Younes, Teruaki Enoto, Sebastien Guillot, Tolga Guver, Marlon L. Bause, Rachael Stewart, Alex Van Kooten, Chryssa Kouveliotou
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Abstract:During the 2022 outburst of SGR 1935+2154, a Fast-Radio-Burst-like event (FRB 20221014A) and X-ray activities occurred between two spin-up glitches, suggesting these glitches may connect to multiwavelength phenomenology. However, the mechanisms altering the magnetar's magnetosphere to enable radio emission remain unclear. This study presents high-cadence NICER and NuSTAR observations revealing spectral changes in burst and persistent emission. Hardness ratio and spectral analysis reveal significant changes during an "intermediate flare" 2.5 hours before FRB 20221014A. This 80-second flare, releasing $>(6.3\pm0.2)\times10^{40}$ erg, coincides with a rapid spectral softening in both burst and persistent emission and a notable decrease in burst occurrence rate. The intermediate flare is bright enough to be detected if placed at a few Mpc, and would appear as a fast X-ray transient. This implies that the connection between magnetar X-ray activity and FRBs can be observed in the local Universe. Post-flare burst spectra peak near 5 keV, resembling the characteristics of the FRB-associated X-ray burst of 2020. Such change persisted for a few hours, implying magnetospheric evolution on similar timescales. However, no radio emission was detected from post-flare bursts, suggesting that FRB emission requires conditions beyond peculiar short bursts. The burst waiting times exhibit a broken power-law distribution, likely resulting from contamination by enhanced persistent emission. Although the bursts appear randomly distributed in the spin phase, the hardness ratio profile as a function of spin phase follows that of the persistent emission, indicating that X-ray bursts originate at low altitudes.

Comments: 21 pages, 8 figures, submitted to ApJ


Abstract: 2504.21671
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Title:Broadband study of the SMC pulsar RX J0032.9-7348 during its X-ray brightening in 2024

Authors:Birendra Chhotaray, Gaurava K. Jaisawal, Sachindra Naik, Arghajit Jana
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Abstract:We present the results of the broadband timing and spectral analysis of the poorly understood SMC pulsar RX J0032.9-7348 (= SXP 7.02) using NuSTAR and NICER observations during its X-ray brightening in 2024. Our timing analysis revealed a pulsation period of approximately 7.02 s in the X-ray light curve. The pulse profile obtained in the broad energy range is double-peaked and asymmetric in nature and shows moderate variation with the energy. An absorbed power-law model describes the 0.5-8 keV NICER spectra well. The 3-50 keV NuSTAR spectrum is best described with an absorbed power-law modified with a high-energy cutoff model. We find no evidence of iron or cyclotron line features in the energy spectrum. During our observation period, the 0.5-50 keV luminosity varies in the range of $\sim 8\times10^{36} - 4\times10^{37}$ erg s$^{-1}$. We also discuss the dependence of spectral parameters on the rotational phase of the pulsar through phase-resolved spectroscopy.

Comments: Accepted for publication in MNRAS


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