arXiv subjects

Aru Beri

Publications and source records attributed to Aru Beri.

At least 19 recordsLinked to original sources

arXiv2026

The first comprehensive spectral and timing study of the ultra-compact X-ray binary 4U 1812-12 with NICER and NuSTAR

The source 4U 1812-12 is a persistent, weakly variable low-mass X-ray binary containing a neutron star. The source was observed by NICER between 2019 and 2021 and, more recently, by NuSTAR in 2025. During the NICER and NuSTAR observations, the source was detected in a hard spectral state with a bolometric luminosity of $\sim 1.90\times 10^{36}$ ergs s$^{-1}$. Its $3-70$ keV NuSTAR spectrum is characterized by a soft thermal emission from the disc, a hard Comptonized emission from the corona, and its reflection from the accretion disc. The NuSTAR energy spectrum exhibits the clear presence of disc reflection features, fitted using a self-consistent relativistic reflection model {\tt relxill}. Our reflection modeling indicates a moderately ionized accretion disc (log\:$\xi\sim2.72$) extending close to the neutron star surface ($R_{in}\lesssim 1.72\:R_{ISCO}$), and viewed through a small inclination angle ($i\sim 25$ degrees). Assuming that the magnetic field ($B$) truncates the disc, we found $B\lesssim 2.54\times 10^{8}$ G, comparable to the typical values observed for NS LMXBs. The $1.0-9.5$ keV NICER spectra are also characterized by a soft thermal component and a dominant hard Comptonized component. During NICER observations, the disc temperature exhibits a small variation within $\sim 0.69-0.84$ keV. In contrast, the power law photon index, $\Gamma$, exhibits a large variation of $\sim 0.8-1.5$, implying a substantial change in the Comptonized emission. Moreover, NICER timing analysis reveals broadband aperiodic variability with significant QPO-like features at $0.379\pm 0.008$ Hz and $0.724\pm 0.025$ Hz, having fractional rms amplitudes of $2.9\pm 0.6\%$ and $4.1\pm 0.5\%$, respectively.

astro-ph.HE↗

arXiv2026

Accreting Compact Object Binaries with the SKA

Accreting binary systems provide a time-resolved view of the accretion and ejection processes that are seen in all classes of compact objects, from stellar-mass to supermassive. They allow us to study the launching of relativistic jets on human timescales, probing how the jets are coupled to the underlying accretion flow, and providing unique insights that can be extended to supermassive black holes via the well-established mass scale invariance. Comparative studies of jets from different classes of compact objects allow us to determine the impact of mass, spin, magnetic fields, and stellar surfaces on the launching of jets. The jets from black hole X-ray binaries provide probes of the Galactic black hole population, and an important source of feedback to the surrounding interstellar medium. While existing radio facilities (including the SKA precursors) have made great progress in this field in recent years, the SKA will enhance such studies via its high point source and surface brightness sensitivity, high angular resolution, and broad frequency coverage. This will enable the extension of our existing black hole studies to the fainter accreting neutron star and white dwarf population, the detection of previously-undiscovered systems in a low-luminosity quiescent state, and the extension of our studies to nearby galaxies, revealing rare, high-accretion rate systems that probe a key phase of black hole growth.

astro-ph.HE↗

arXiv2026

X-rays breaking out of pre-explosion ejecta mark a supernova's first light

Massive stars die as core-collapse supernovae, whose optical light emerges days after the implosion. Theory predicts that the initial collapse-driven shock, upon breaking through the star and dense circumstellar medium, emits a brief thermal flash of soft X-rays and ultraviolet. Yet these elusive first signals have remained largely undetected, owing to limited wide-field soft X-ray monitoring. Here we report the discovery of a soft X-ray flash, EP260321a, followed days later by a broad-lined supernova from an envelope-stripped progenitor. Its X-ray spectrum, best modeled with blackbody, establishes it as the long-sought archetypal shock breakout. The burst's duration and energetics place the breakout at a radius of 300 solar radii, tracing a dense surrounding shell and revealing abrupt mass ejection within the final month before collapse.

astro-ph.HE↗

arXiv2026

Accretion geometry and spectral evolution in 1A 1118-61: a comparison of the 2009 and 2026 outbursts

We present a detailed spectro-temporal study of the Be X-ray binary pulsar $1A$ $1118-61$ during its brightest recorded outburst in 2026, using \textit{Swift} and \textit{NuSTAR} observations, and compare its properties with the 2009 outburst. Coherent pulsations at $\sim400$ s are detected throughout the outburst, with pulse profiles evolving strongly with energy and luminosity, indicating changes in emission geometry. A transient quasi-periodic oscillation (QPO) at $\sim$0.11 Hz is observed during the rising phase. The luminosity dependence of the QPO frequency during the current and previous outbursts suggests an origin associated with instabilities near the magnetospheric radius. The broadband spectra are well described by thermal Comptonization and show clear spectral hardening at higher luminosities. A cyclotron line is detected in the two \textit{NuSTAR} observations, with its energy remaining nearly constant despite a factor of $\sim25$ change in luminosity. Long-term monitoring reveals that the 2026 outburst is systematically harder and brighter, suggesting significant difference in the accretion structure and emission regions between the two outbursts.

astro-ph.HE↗

arXiv2025

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

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. Our primary objective is 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. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting ~400-700 s, with enhancements by a factor of ~2. 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 reveals a soft excess and an emission feature at ~1 keV, likely arising from reprocessed emission in the accretion disc 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, a declining spin-up rate is observed during the outburst, likely due to a gradual reduction in mass accretion rate. 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 behaviour highlights rapid changes in the accretion geometry and torque as a function of accretion rate. [Abridged]

astro-ph.HE↗

arXiv2025

Probing spectral variability in NGC 4490 ULX-8 over 24 years of XMM-Newton, Chandra and Swift-XRT observations

We present a spectral variability study of the ultraluminous X-ray source NGC 4490 ULX-8 based on 14 Chandra, 6 XMM-Newton and 19 Swift-XRT observations obtained between 2000 and 2024. The X-ray spectra are modelled using absorbed power-law and absorbed multicolour disc blackbody models. The best-fit photon indices span 0.9-2.7, while the inferred inner disc temperatures lie in the range 1.0-1.6 keV. We detect pronounced long-term variability in the unabsorbed X-ray luminosity on multi-year timescales, while variability within individual observations is comparatively modest. A Hardness-Intensity Diagram of the source shows no clear transition between hard and soft states; however, two recent observations taken on 2022 December 1 and 2024 May 4 show a sharp increase in brightness. The spectra across all observations are dominated by smooth, single-component curvature in the 0.3-10 keV band, consistent with the broadened-disc regime of ultraluminous X-ray sources. A correlation analysis reveals a weak positive X-ray luminosity-photon index trend that remains statistically supported after controlling for related degeneracies, indicating that it is not driven solely by fitting covariance. The luminosity-inner disc temperature relation is only weakly constrained, but remains compatible, within uncertainties, with both thin-disc and slim-disc scalings. Using disc parameters derived from higher-quality XMM-Newton spectra, we obtain model-dependent estimates of the characteristic inner disc radius and compact-object mass as functions of inclination and spin. The reported results are consistent with a stellar-mass black hole accretor operating at or near the Eddington limit.

astro-ph.HE↗

arXiv2025

Astrophysics with Compact Objects: An Indian Perspective, Present Status and Future Vision

Astrophysical compact objects, viz., white dwarfs, neutron stars, and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of the GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this article plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing on, and the opportunities in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

astro-ph.HE↗

arXiv2025

Sidebands to mHz QPOs in 4U 1626$-$67 in the second spin-down state

We report results from an $AstroSat$ Target-of-Opportunity (ToO) observation of 4U 1626$-$67, performed on 2023 May 18, soon after the discovery of torque reversal to spin-down in the source. The X-ray emission exhibited significant dependence on both energy and torque state. This work highlights the comparison of timing features of 4U 1626$-$67 with a previous $AstroSat$ observation from 2018, when the neutron star was in the spin-up state. The power density spectrum (PDS) of the 2023 observation comprised a sharp peak corresponding to $ν_{\rm NS}\sim$130 mHz X-ray pulsations along with a prominent quasi-periodic oscillation (QPO) feature at $ν_{\rm QPO}\sim$46 mHz with $\sim$20\% rms amplitude, which was positively correlated with energy. We also report the detection of sidebands to QPO occurring at a beat frequency ($ν_{\rm NS}-ν_{\rm QPO}$) of $\sim$83 mHz with $\sim$8\% rms amplitude, having $>3σ$ detection significance. Additionally, we utilized $Nuclear ~Spectroscopic ~Telescope ~ARray$ ($NuSTAR$) observations from the same torque state (2023 May-July) to analogize the presence and energy dependence of sidebands. The source retains timing properties in this spin-down torque state, similar to those seen in the previous spin-down phase. In sharp contrast, PDS from the 2018 observation was dominated by red noise, an absence of QPOs and a broadening in the wings of the pulse frequency peak, indicating a coupling between periodic and low-frequency aperiodic variability. Furthermore, we detected the known cyclotron resonance scattering feature (CRSF) at 37 keV in the Large Area X-ray Proportional Counter (LAXPC) spectrum. We explore various mechanisms that could possibly explain the presence of QPOs exclusively during the spin-down state.

astro-ph.HE↗

arXiv2024

X-Ray spectral and temporal properties of LMXB 4U 1608-52- observed with AstroSat and NICER

We report results from a detailed study of the neutron star X-ray binary, 4U 1608-52 using observations with {\it AstroSat} (LAXPC/SXT) and {\it NICER} during its 2016 and 2020 outbursts. The 0.7--20.0 keV spectra could be well described with the disk blackbody and thermal Comptonization model. The best-fitting inner disk temperature is $\sim$ 1 keV and radius { $\sim$ 22.17$^{+2.57}_{-2.38}$--27.19$^{+2.03}_{-1.85}$} km and no significant evolution was observed in the disk radius after performing flux and time-resolved spectroscopy. We used a multi-Lorentzian approach to fit the power density spectra and obtained broad-band noise variability. We estimated the energy-dependent fractional root mean square and time-lag of the broad-band noise, and these variations are quantitatively modelled as being due to the coherent variation of the disk emission and the coronal heating rate. Thus, the rapid temporal modeling is consistent with the longer term spectral evolution where the inner disk radius does not vary, and instead the variations can be attributed to accretion rate variations which changes the inner disk temperature and the coronal heating rate.

astro-ph.HE↗

arXiv2023

Millihertz X-ray variability during the 2019 outburst of black hole candidate Swift~J1357.2$-$0933

Swift J1357.2$-$0933 is a black-hole candidate X-ray transient, which underwent its third outburst in 2019, during which several multi-wavelength observations were carried out.~Here, we report results from the \emph{Neil Gehrels Swift} and \emph{NICER} observatories and radio data from \emph{AMI}.~For the first time,~millihertz quasi-periodic X-ray oscillations with frequencies varying between ${\sim}$~1--5~$\rm{mHz}$ were found in \emph{NICER} observations and a similar feature was also detected in one \emph{Swift}--\textsc{XRT} dataset.~Our spectral analysis indicate that the maximum value of the measured X-ray flux is much lower compared to the peak values observed during the 2011 and 2017 outbursts.~This value is ${\sim}$~100 times lower than found with \emph{MAXI} on MJD~58558 much ($\sim$~68 days) earlier in the outburst, suggesting that the \emph{Swift} and \emph{NICER} fluxes belong to the declining phase of the 2019 outburst.~An additional soft component was detected in the \textsc{XRT} observation with the highest flux level, but at a relatively low $L_{\rm X}$~$\sim$~$3{\times}10^{34}~(d/{\rm 6~kpc)}^2\rm{erg}~\rm{s}^{-1}$, and which we fitted with a disc component at a temperature of $\sim 0.17$~keV.~The optical/UV magnitudes obtained from \emph{Swift}--\textsc{UVOT} showed a correlation with X-ray observations, indicating X-ray reprocessing to be the plausible origin of the optical and UV emission.~However, the source was not significantly detected in the radio band.~There are currently a number of models that could explain this millihertz-frequency X-ray variability; not least of which involves an X-ray component to the curious dips that, so far, have only been observed in the optical.

astro-ph.HE↗

arXiv2023

AstroSat and NuSTAR observations of XTE J1739-285 during the 2019-2020 outburst

We report results from a study of XTE J1739-285, a transient neutron star low mass X-ray binary observed with AstroSat and NuSTAR during its 2019-2020 outburst. We detected accretion-powered X-ray pulsations at 386 Hz during very short intervals (0.5--1 s) of X-ray flares. These flares were observed during the 2019 observation of XTE J1739-285. During this observation, we also observed a correlation between intensity and hardness ratios, suggesting an increase in hardness with the increase in intensity. Moreover, a thermonuclear X-ray burst detected in our AstroSat observation during the 2020 outburst revealed the presence of coherent burst oscillations at 383 Hz during its decay phase. The frequency drift of 3 Hz during X-ray burst can be explained with r modes. Thus, making XTE J1739-285 belong to a subset of NS-LMXBs which exhibit both nuclear- and accretion-powered pulsations. The power density spectrum created using the AstroSat-LAXPC observations in 2020 showed the presence of a quasi-periodic oscillation at ~ 0.83 Hz. Our X-ray spectroscopy revealed significant changes in the spectra during the 2019 and 2020 outburst. We found a broad iron line emission feature in the X-ray spectrum during the 2020 observation, while this feature was relatively narrow and has a lower equivalent width in 2019,~when the source was accreting at higher rates than 2020.

astro-ph.HE↗

arXiv2023

Gender Equity in Physics in India: Interventions, Outcomes, and Roadmap

The gender imbalance in physics higher education and advanced professions is a global problem, and India is not an exception. Although the issue has been acknowledged widely, discrimination needs to be recognized as the driving force. The past three years have witnessed initiatives by different gender groups as well as the Government of India in addressing these lacunae. We report various activities, describe interventions, and present statistics indicating improvements achieved. The Gender in Physics Working Group has brought about significant gender reforms in the Indian Physics Association. The working group organized an open discussion on the issue of sexual harassment in physics professions for the first time in 2018. Subsequently, in 2019, GIPWG organized the first-ever national conference on gender issues, Pressing for Progress. The deliberations of the conference culminated in the Hyderabad Charter, a roadmap towards gender equity in India. The Working Group for Gender Equity constituted under the Astronomical Society of India, also played an impactful role. At the government level, notable new initiatives include Gender Advancement through Transforming Institutions and the proposed Science and Technology Innovation Policy for mainstreaming equity and inclusion.

physics.ed-ph↗

arXiv2022

The AstroSat observation of accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst

We report on the analysis of the AstroSat dataset of the accreting millisecond X-ray pulsar SAX J1808.4-3658, obtained during its 2019 outburst. We found coherent pulsations at $\sim 401$ Hz and an orbital solution consistent with previous studies. The 3-20 keV pulse profile can be well fitted with three harmonically related sinusoidal components with background-corrected fractional amplitude of $\sim 3.5 \%$, $\sim 1.2 \%$ and $\sim 0.37 \%$ for fundamental, second and third harmonic, respectively. Our energy-resolved pulse profile evolution study indicate a strong energy dependence. We also observed a soft lag in fundamental and hard lag during its harmonic. The broadband spectrum of SAX J1808.4-3658 can be well described with a combination of thermal emission component with $kT \sim 1$ keV, a thermal Comptonization ($Γ\sim 1.67$) from the hot corona and broad emission lines due to Fe.

astro-ph.HE↗

arXiv2022

Evidence of hard power-law spectral cutoff and disc reflection features from the X-ray transient XTE J1739-285

We report on the nearly simultaneous \nicer{} and \nustar{} observations of the known X-ray transient XTE~J1739-285. These observations provide the first sensitive hard X-ray spectrum of this neutron star X-ray transient. The source was observed on 2020 February 19 in the hard spectral state with a luminosity of $0.007$ of the Eddington limit. The broadband $1-70 \kev{}$ \nicer{} and \nustar{} observation clearly detects a cutoff of the hard spectral component around $34-40 \kev{}$ when the continuum is fitted by a soft thermal component and a hard power-law component. This feature has been detected for the first time in this source. Moreover, the spectrum shows evidence for disc reflection -- a relativistically broadened Fe K$α$ line around $5-8 \kev{}$ and a Compton hump in the $10-20 \kev{}$ energy band. The accretion disc reflection features have not been identified before from this source. Through accretion disc reflection modeling, we constrain the radius of the inner disc to be $R_{in}=3.1_{-0.5}^{+1.8}\;R_{ISCO}$ for the first time. In addition, we find a low inclination, $i\sim 33^{0}$. Assuming the magnetosphere is responsible for such truncation of the inner accretion disc above the stellar surface, we establish an upper limit of $6.2\times 10^8$ G on the magnetic field at the poles.

astro-ph.HE↗

arXiv2022

NuSTAR and AstroSat observations of thermonuclear X-ray bursts with short-recurrence times in 4U 1636$-$536

We report results from the spectro-timing analysis of the atoll source 4U 1636$-$536 observed with NuSTAR and AstroSat during its hard spectral state. In three observations of 207 ks total exposure, we identify 31 thermonuclear X-ray bursts including five doublets and a triplet. Recurrence time as short as 3.8 minutes is seen in one of the doublets. To the best of our knowledge, this is the shortest recurrence time known for this source. Our time-averaged spectroscopy during the bursts indicate the presence of an additional powerlaw or a blackbody component in a few cases, perhaps due to varying temperatures during bursts or plausible deviation from ideal blackbody behavior, however, it is difficult to probe this using the time-resolved spectroscopy owing to limited statistics. Time-resolved bursts are well fit using an absorbed blackbody model with temperatures varying between 1.7 and 2.2 keV. Burst oscillations around 581 Hz are detected with 3$σ$ confidence during the decay phase in two of the X-ray bursts. One of the burst oscillations is seen at 582 Hz, a frequency observed during the 2001 superburst in this source.

astro-ph.HE↗

arXiv2021

Thermonuclear X-ray bursts from LMXB 4U 1636$-$536 observed with AstroSat

We report results obtained from the study of 12 thermonuclear X-ray bursts in 6 AstroSat observations of a neutron star X-ray binary and well-known X-ray burster, 4U 1636$-$536. Burst oscillations at $\sim$581 Hz are observed with 4$-$5$σ$ confidence in three of these X-ray bursts. The rising phase burst oscillations show a decreasing trend of the fractional rms amplitude at 3$σ$ confidence,by far the strongest evidence of thermonuclear flame spreading observed with AstroSat. During the initial 0.25 second of the rise a very high value (34.0$\pm$6.7%) is observed. The concave shape of the fractional amplitude profile provides a strong evidence of latitude-dependent flame speeds, possibly due to the effects of the Coriolis force. We observe decay phase oscillations with amplitudes comparable to that observed during the rising phase, plausibly due to the combined effect of both surface modes as well as the cooling wake. The Doppler shifts due to the rapid rotation of the neutron star might cause hard pulses to precede the soft pulses, resulting in a soft lag. The distance to the source estimated using the PRE bursts is consistent with the known value of $\sim$6 kpc.

astro-ph.HE↗

arXiv2020

Evolution of timing and spectral characteristics of 4U 1901+03 during its 2019 outburst using the Swift and NuSTAR observatories

We report the results from a detailed timing and spectral study of transient X-ray pulsars, 4U 1901+03 during its 2019 outburst. We performed broadband spectroscopy in the 1-70 keV energy band using four observations made with Swift and NuSTAR at different intensity levels. Our timing results reveal the presence of highly variable pulse profiles dependent on both luminosity and energy. Our spectroscopy results showed the presence of a cyclotron resonance scattering feature (CRSF) at ~30 keV. This feature at 30 keV is highly luminosity and pulse-phase dependent. Phase-averaged spectra during the last two observations, made close to the declining phase of the outburst showed the presence of this feature at around 30 keV. The existence of CRSF at 30 keV during these observations is well supported by an abrupt change in the shape of pulse profiles found close to this energy. We also found that 30 keV feature was significantly detected in the pulse-phase resolved spectra of observations made at relatively high luminosities. Moreover, all spectral fit parameters showed a strong pulse phase dependence. In line with the previous findings, an absorption feature at around 10 keV is significantly observed in the phase-averaged X-ray spectra of all observations and also showed a strong pulse phase dependence.

astro-ph.HE↗

arXiv2020

AstroSat Observations of the first Galactic ULX Pulsar Swift J0243.6+6124

SwiftJ0243.6+6124, the first Galactic ultra-luminous X-ray pulsar, was observed during its 2017-2018 outburst with \emph{AstroSat} at both sub- and super-Eddington levels of accretionwith X-ray luminosities of $L_{X}{\sim}7{\times}10^{37}$ and $6{\times}10^{38}$$ergs^{-1}$, respectively.Our broadband timing and spectral observations show that X-ray pulsations at ${\sim}9.85\rm{s}$ have been detected up to 150keV when the source was accreting at the super-Eddington level.The pulse profiles are a strong function of both energy and source luminosity,showing a double-peaked profile with pulse fraction increasing from $\sim$$10{\%}$ at $1.65\rm{keV}$ to 40--80$\%$ at $70\rm{keV}$.The continuum X-ray spectra are well-modeled with a high energy cut-off power law($Γ$${\sim}$0.6-0.7) and one or two blackbody components with temperatures of $\sim$0.35$\rm{keV}$ and $1.2\rm{keV}$, depending on the accretion level.No iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9keV is observed at the super-Eddington level, along with a blackbody radius($121-142\rm{km}$) that indicates the presence of optically thick outflows.

astro-ph.HE↗