http://hdl.handle.net/2289/101 2026-04-10T10:40:25Z http://hdl.handle.net/2289/8700 Title: Synchrotron Radiation from NGC 470 HLX1—a Hidden Hyperluminous Accreting Neutron Star? Authors: Ghosh, Tanuman; Sethi, Shiv; Dewangan, Gulab Chand; Bachetti, Matteo; Rana, Vikram; Misra, Ranjeev Abstract: The origin of the large-amplitude, quasi-periodic X-ray flux variations in several classes of the Galactic microquasar GRS 1915+105 remains unresolved. We address this issue through flux-resolved, broadband (0.8–20 keV) spectral modeling and simultaneous covariance spectral analysis during two κ and two ω class observations using AstroSat/SXT and LAXPC. The lightcurves show strong, quasi-periodic oscillations involving rapid transitions between bright bursts and deep dips on timescales of a few tens of seconds. Flux-resolved spectroscopy indicates that high-flux intervals in both classes are dominated by a hot, optically thick accretion disk with steep Comptonized emission, whereas low-flux intervals correspond to a cooler or partially recessed disk and a harder coronal continuum. These transitions involve a systematic 1–2 keV drop in disk temperature and a pronounced hardening of the Comptonized component, with flux reductions of up to a factor of five. Using covariance spectra across 0.015–5 Hz, we show that the rapid coherent variability arises almost entirely from the disk, which exhibits strong energy-dependent variations, while the Comptonized component contributes minimally. The combined results suggest that radiation-pressure-driven structural changes in the disk, with a slower coronal response, produce the observed oscillations, consistent with cyclic disk evacuation and refilling in the κ and ω classes. Description: Open Access 2026-03-16T00:00:00Z http://hdl.handle.net/2289/8699 Title: Observing octupolar charge orders and their transition dynamics in Coulomb clusters Authors: Ayyadevara, Akhil; Prakash, Anand; Dutta, Shovan; Paramekanti, Arun; Rangwala, S A Abstract: Interacting many-particle systems can self-organize into a rich variety of crystalline structures. While symmetry provides a powerful framework for predicting whether transitions between crystal states are continuous or discontinuous, collective lattice dynamics offer complementary insights into the microscopic mechanisms that drive these transitions. Trapped laser-cooled ions present a pristine and highly controllable system for studying this interplay of symmetry and dynamics. Here, we use real-time fluorescence imaging while deforming the trap potential to observe a variety of structural transitions in three-dimensional (3D), unit-cell-like ion clusters. We identify a set of unexplored transitions signaled by parity-odd octupole order parameters and probe their distinct dynamical signatures. Our experiments reveal the softening of a collective Higgs-like mode, indicating spontaneous symmetry breaking, hysteresis resulting from a catastrophe where a metastable state vanishes abruptly, and stochastic switching between metastable states of distinct symmetries. We also uncover a remarkable coincidence of symmetry-breaking and discontinuous transitions, analogous to a thermodynamic triple point. Our results establish 3D trapped-ion clusters as a versatile platform for engineering complex potential energy landscapes, opening avenues for studying collective phenomena such as geometric frustration with multipolar orders and the stochastic kinetics of rare events in exotic mesoscopic settings. Description: Open Access. Also available at arXiv.org (one of the alternative locations) 2026-03-18T00:00:00Z http://hdl.handle.net/2289/8697 Title: A Bayesian estimator for peculiar velocity correction in cosmological inference from supernovae data Authors: Upadhyay, Ujjwal; Saini, Tarun Deep; Sethi, Shiv K Abstract: The peculiar motion of the host galaxies can shift the observed redshifts from their true values, introducing bias in estimating cosmological parameters from supernova data. The coherent component of peculiar motion is typically corrected using velocity-field reconstruction, while the random component is handled statistically by inflating the magnitude uncertainty through standard error propagation. However, velocity-field reconstruction requires assuming an underlying cosmology, which can bias the final inference, whereas the statistical treatment of the random component relies on a locally linear magnitude–redshift relation and a Gaussian velocity distribution. We present a Bayesian estimator for simultaneously correcting for peculiar motion while fitting a cosmological model to supernova data, relaxing the assumption of linearity of the model and Gaussianity of the peculiar motion. Our approach is based on considering the problem of fitting the magnitude–redshift relation as a non-linear model with errors in both dependent and independent variables. To this end, we develop a general method for fitting non-linear errors-in-variables models and apply it to the magnitude–redshift relation, validating it with simulated data sets at the precision of current and upcoming surveys and testing it on the pantheon sample. Our method provides an alternative approach for accounting for the peculiar velocity effects, which is a complementary method for the coherent component, as it does not require independent velocity measurements, and generalizes the treatment of the random component. Moreover, our general method is applicable to various other problems in cosmology and astronomy. Description: Open Access. Also available at arXiv.org (one of the alternative locations) 2026-04-01T00:00:00Z http://hdl.handle.net/2289/8696 Title: Spectro-timing Origin of Large Amplitude X-Ray Variability in GRS 1915+105 Using AstroSat/LAXPC and SXT Authors: Suman, Shree; Khatua, Shuvajit; Jadoliya, Vishal; Gupta, Prathamesh Narayan; Pahari, Mayukh Abstract: The origin of the large-amplitude, quasi-periodic X-ray flux variations in several classes of the Galactic microquasar GRS 1915+105 remains unresolved. We address this issue through flux-resolved, broadband (0.8–20 keV) spectral modeling and simultaneous covariance spectral analysis during two κ and two ω class observations using AstroSat/SXT and LAXPC. The lightcurves show strong, quasi-periodic oscillations involving rapid transitions between bright bursts and deep dips on timescales of a few tens of seconds. Flux-resolved spectroscopy indicates that high-flux intervals in both classes are dominated by a hot, optically thick accretion disk with steep Comptonized emission, whereas low-flux intervals correspond to a cooler or partially recessed disk and a harder coronal continuum. These transitions involve a systematic 1–2 keV drop in disk temperature and a pronounced hardening of the Comptonized component, with flux reductions of up to a factor of five. Using covariance spectra across 0.015–5 Hz, we show that the rapid coherent variability arises almost entirely from the disk, which exhibits strong energy-dependent variations, while the Comptonized component contributes minimally. The combined results suggest that radiation-pressure-driven structural changes in the disk, with a slower coronal response, produce the observed oscillations, consistent with cyclic disk evacuation and refilling in the κ and ω classes. Description: Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations) 2026-03-12T00:00:00Z