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Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5–6932 during its 2024 outburst

Sun, 01 Feb 2026 00:00:00 GMT

Title: Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5–6932 during its 2024 outburst Authors: Sharma, Rahul; Beri, Aru; Paul, Biswajit; Sanna, Andrea; Maitra, Chandreyee; Yang, Haonan Abstract: Context. After nearly a decade of quiescence, the transient Be/X-ray binary pulsar RX J0520.5–6932 underwent an outburst in 2024. We performed X-ray monitoring of the source with NICER and AstroSat near the peak of the event. Aims. Our primary objective was to investigate the energy and luminosity dependence of the pulsed emission, characterize the spin evolution, and study the broadband X-ray spectral properties of RX J0520.5–6932 during the outburst. Methods. We extracted light curves and spectra from NICER and AstroSat observations carried out during the outburst. Pulsations were detected using epoch-folding techniques, enabling a detailed study of pulse-profile evolution as a function of energy and intensity. Broadband spectral modeling was performed using simultaneous data from SXT, LAXPC, and NICER. The spectra from individual NICER observations were used to study spectral variability. Results. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting ∼400–700 s with enhancements in intensity by about a factor of two. The pulse profile exhibits a strong dependence on both energy and intensity, evolving from a simple single-peaked structure at low energies to complex multi-peaked shapes at intermediate energies and reverting to simpler morphologies at higher energies. Pulse profiles during the flares differ significantly from those in the persistent state, indicating changes in the pulsed beam pattern with a change in the intensity on a short timescale. Broadband spectral analysis revealed a soft excess and an emission feature at ∼1 keV likely arising from reprocessed emission in the accretion disk and fluorescence from Ne K and Fe L ions. Continuous NICER monitoring over nearly one orbital cycle enabled us to track spin evolution with accretion-driven spin-up and spectral variability in the soft X-ray band. Additionally, we observed a declining spin-up rate during the outburst, likely due to a gradual reduction in mass accretion rate. Conclusions. Our results provide a comprehensive view of the complex accretion dynamics in RX J0520.5–6932 during its 2024 outburst. The strong variability in pulse shape and spin behavior highlights rapid changes in the accretion geometry and torque as a function of accretion rate. Description: Open Access. Also available at arXiv.org (one of the alternative locations)

Next-generation Digital Receiver Platforms for Cosmology/Radio Astronomy experiments

Fri, 20 Sep 2024 00:00:00 GMT

Title: Next-generation Digital Receiver Platforms for Cosmology/Radio Astronomy experiments Authors: Srivani, K S; Girish, B S; Rao, Mayuri S; Singh, Saurabh; Kamini, P A; Madhavi, S; Keerthipriya, S; Dash, Adarsh Kumar; Vishakha, S P; Thiagaraj, Prabu; Somashekar, R; Agrawal, Yash; Sethi, Shiv K; Shankar, Udaya N; Seetha, S Abstract: Over the last decade, Field-Programmable Gate Array (FPGA) technology has made significant strides, notably with Advanced Micro Devices Inc. (AMD) pioneering Multi-processor Systems-on-Chip (MPSoC). The addition of RF-class analog technology has further advanced MPSoCs, resulting in Radio Frequency System-on-a-Chip (RFSoC). RFSoCs combine data converters with programmable logic, dedicated processors, and high-speed transceivers, making them ideal for large-N applications in radio astronomy/cosmology experiments. The Raman Research Institute has embarked upon building next-generation digital correlation spectrometers using RFSoC evaluation boards like AMD's ZCU111 and ZCU216, advancing towards a compact, custom-designed third generation RFSoC-based platform named "Cosmology and radio astronomy experiments with RFSoC Integrated Signal Processing" (CRISP). These technological advancements are pivotal for high precision cosmology experiments, detecting subtle signals from recombination and reionization epochs. The paper describes the digital correlation receivers based on evaluation platforms, signal processing firmware, and the architecture of CRISP leveraging Size, Weight, and Power (SWaP) attributes. Description: Restricted Access.

Beyond radial profiles: using log-normal distributions to model the multiphase circumgalactic medium

Sat, 27 Apr 2024 00:00:00 GMT

Title: Beyond radial profiles: using log-normal distributions to model the multiphase circumgalactic medium Authors: Dutta, Alankar; Bisht, Mukesh Singh; Sharma, Prateek; Ghosh, Ritali; Roy, Manami; Nath, Biman B Abstract: Recent observations and simulations reveal that the circumgalactic medium (CGM) surrounding galaxies is multiphase, with the gas temperatures spanning a wide range at most radii, ∼104 K to the virial temperature (∼106 K for Milky Way). Traditional CGM models using simple density profiles are inadequate at reproducing observations that indicate a broad temperature range. Alternatively, a model based on probability distribution functions (PDFs) with parameters motivated by simulations can better match multiwavelength observations. In this work, we use log-normal distributions, commonly seen in the simulations of the multiphase interstellar and circumgalactic media, to model the multiphase CGM. We generalize the isothermal background model by Faerman et al. to include more general CGM profiles. We extend the existing probabilistic models from 1D-PDFs in temperature to 2D-PDFs in density–temperature phase space and constrain its parameters using a Milky Way-like Illustris TNG50-1 halo. We generate various synthetic observables such as column densities of different ions, UV/X-ray spectra, and dispersion and emission measures. X-ray and radio (Fast Radio Burst) observations mainly constrain the hot gas properties. However, interpreting cold/warm phase diagnostics is not straightforward since these phases are patchy, with inherent variability in intercepting these clouds along arbitrary lines of sight. We provide a tabulated comparison of model predictions with observations and plan to expand this into a comprehensive compilation of models and data. Our modelling provides a simple analytical framework that is useful for describing important aspects of the multiphase CGM. Description: Open Access. Also available at arXiv.org (one of the alternative locations)

Revisiting cosmic distance duality with megamasers and DESI DR2 observations: Model-independent constraints on early-late calibration

Thu, 15 Jan 2026 00:00:00 GMT

Title: Revisiting cosmic distance duality with megamasers and DESI DR2 observations: Model-independent constraints on early-late calibration Authors: Kanodia, Brijesh; Upadhyay, Ujjwal; Tiwari, Yashi Abstract: The cosmic distance duality relation (CDDR) connects the angular diameter distance (𝑑𝐴) and the luminosity distance (𝑑𝐿) at a given redshift. This fundamental relation holds in any metric theory of gravity, provided that photon number is conserved and light propagates along null geodesics. A deviation from this relation could indicate new physics beyond the standard cosmological model. In this work, we test the validity of the CDDR at very low redshifts (𝑧 <0.04) by combining 𝑑𝐴 from the Megamaser Cosmology Project with 𝑑𝐿 from the Pantheon + sample of type Ia supernovae (SNIa). We further incorporate high-redshift baryon acoustic oscillations- (BAO) based 𝑑𝐴 measurements from DESI DR2 in combination with SNIa data, highlighting the critical role of the 𝑟𝑑−𝑀𝑏 (early-late) calibration in testing the CDDR using these two probes. Assuming CDDR holds, we perform a Bayesian analysis to derive model-independent constraints on the calibration parameters. Using only BAO and SNIa data, we observe a strong degeneracy between 𝑟𝑑 and 𝑀𝑏. However, the inclusion of calibration-free megamaser measurements breaks this degeneracy, enabling independent constraints without relying on a specific cosmological model or distance-ladder techniques. Additionally, we present a forecast incorporating the expected precision from future megamaser and SNIa observations, demonstrating their potential to significantly tighten constraints on early-late calibration parameters, under the assumption of validity of CDDR. Description: Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)