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Gulab Dewangan

Publications and source records attributed to Gulab Dewangan.

At least 19 recordsLinked to original sources
arXiv2026

XRISM Resolves the Circum-nuclear Environment of NGC 4945

Compton-thick Active Galactic Nuclei (AGN) represent one of the most elusive phases of massive black hole growth, yet are expected to contribute substantially to the Cosmic X-ray Background and the integrated growth of massive black holes. NGC 4945 is the closest Compton-thick AGN and amongst the brightest AGN in the hard X-ray sky, making it an important benchmark for more distant Compton-thick AGN. We present the first high-resolution X-ray spectral analysis of NGC 4945 using XRISM/Resolve. The entire 4-15 keV Resolve spectrum, including a strong Fe K$α$ doublet and weak Compton Shoulder, is well described by a de-coupled dual-obscurer model. The model features a low-covering-factor Compton-thick primary obscurer intersecting the line-of-sight that permits the rapidly variable, direct transmitted coronal continuum to dominate above 10 keV. A Compton-thin secondary reprocessor with a high covering factor dominates the reprocessed emission between $\sim$4-10 keV. Assuming that virial motion accounts for line broadening, the secondary reprocessor can exist at $\sim$0.12 pc, and could help explain the weak high-ionisation optical and infrared emission lines observed in NGC 4945. If such obscuration geometries are common among more distant and/or fainter Compton-thick AGN, our results suggest that simpler coupled X-ray spectral modelling could substantially over-estimate Compton-thick covering factors and under-estimate intrinsic X-ray luminosities.

astro-ph.GA↗
arXiv2026

Model dependence of XRISM black-hole spin constraints in Cyg X-1

We study the persistent black hole X-ray binary Cyg X-1, recently observed by XRISM Resolve and simultaneously by NICER and NuSTAR in its hard spectral state. We confirm the result of Draghis et al. that fits of the Resolve data alone with the simplest available relativistic reflection model, relxill, yield a black hole spin parameter close to the maximum, $a_* = 0.99$. However, fitting with an improved, Comptonization-based model, relxillCp, yields a low $a_*=0.0^{+0.17}$. A similarly low range is obtained with another Comptonization-based model, reflkerrD. Then, fits to the combined data require two Comptonization models but are consistent with any spin value. We conclude that the spin value of Cyg X-1 is strongly model-dependent. However, low spin values are consistent with the constraints from gravitational waves. All of the models constrain the inner disk radius to be <10 gravitational radii, which is consistent with a recent finding of the weakness of thermal reverberation in Cyg X-1. The suggested source geometry is that of an outflowing disk corona, which was also proposed to explain the X-ray polarization observed from this source.

astro-ph.HE↗
arXiv2026

The strong Fe K line and spin of the black-hole X-ray binary MAXI J1631-479

We study the transient black hole binary MAXI J1631--479 in its soft spectral state observed simultaneously by the NICER and NuSTAR instruments. Its puzzling feature is the presence of a strong and broad Fe K line, while the continuum consists of a strong disk blackbody and a very weak power-law tail. The irradiation of the disk by a power-law spectrum fitting the tail is much too weak to account for the strong line. Two solutions were proposed in the past. One invoked an intrinsic Fe K disk emission, and the other invoked disk irradiation by the returning blackbody emission. We instead find that the strong line is naturally explained by the irradiation of the disk by the spectrum from Comptonization of the disk blackbody by coronal relativistic electrons. The shape of the irradiating spectrum at $\lesssim$10 keV reflects that of the disk blackbody; it is strongly curved and has a higher flux than that of a fit with a power-law irradiation. That flux accounts for the line. While this result is independent of the physical model used for the disk intrinsic emission, the value of the fitted spin strongly depends on it. When using a Kerr disk model for a thin disk with a color correction, the fitted spin corresponds to a retrograde disk, unlikely for a Roche-lobe overflow binary. Then, a model accounting for both the disk finite thickness and radiative transfer yields a spin of $a_*\approx0.8$--0.9, which underlines the strong model-dependence of X-ray spin measurements.

astro-ph.HE↗
arXiv2026

Understanding coronal geometry in NGC 4593 using Fourier frequency-resolved covariance and time-lag spectral analysis

Understanding disc-corona geometry through X-ray reverberation variability studies in Seyfert galaxies is crucial, yet our knowledge mostly relies on flux-averaged mean spectral analysis. In this study, we investigate the origin of the large X-ray variability of the Seyfert 1 galaxy NGC 4593 using two \xmm{} observations, which are at least 65 ksec long and have a 0.3-10 keV X-ray flux difference by a factor of $\sim$2.5. We extracted mean spectra, Fourier-frequency resolved covariance, and time-lag spectra and performed modelling of all spectra in a self-consistent manner. From the best-fit covariance spectra, we have shown that energy-dependent covariance during low flux shows dominances of direct powerlaw continuum over reflection continuum at all Fourier frequencies (2.1 $-$ 390 $\times$ 10$^{-5}$ Hz). However, during high flux, the variabilities are dominated by the reflection components most of the time. Our results are further supported by the Fourier frequency-dependent time-lag (between soft: 0.3-1 keV and hard: 1-5 keV bands) spectral modeling during high and low fluxes. A significant change is observed in the X-ray reverberation delay timescale from 483 $\pm$ 135 sec (during high flux) to $<$96 sec (during low flux), indicating a change in coronal size at least by a factor of $\sim$2 (from $<$3.3 R$_g$ to $>$7.2 R$_g$) during low to high flux transitions.

astro-ph.HE↗
arXiv2025

Is the Spin of the Black Hole in GX 339-4 Negative?

We have studied the accreting black hole binary GX 339--4 using two highly accurate broad-band X-ray data sets in very soft spectral states from simultaneous NICER and NuSTAR observations. Joint fitting of both data sets with relativistic models of the disk, its Comptonization and reflection allows us to relatively accurately determine the black-hole mass and spin, and the distance and inclination. However, we find the measured values strongly depend on the used disk model. With widely used Kerr disk models treating departures from local blackbody spectra using color corrections, we find relatively low black-hole masses and strongly negative spins (i.e., retrograde accretion). Then, models employing radiative transfer calculations of the disk atmosphere predict moderately positive spins and high masses. When adding a warm corona above the disk (as proposed before for both AGNs and accreting binaries), we find the spin is weakly constrained, but consistent with zero. In all cases, the fitted inclination is low, $\approx 30$--$34^\circ$. For the spin axis aligned with the binary axis, the mass function for this source implies large values of the mass, consistent only with those obtained with either disk-atmosphere models or the presence of a warm corona. We also test different disk models for an assumed set of mass, distance and inclination. We find that different models yield values of the spin parameter differing up to $\sim$0.3. Our results confirm previously found strong model dependencies of the measured black-hole spin, now by comparing different disk models and for a low-mass X-ray binary.

astro-ph.HE↗
arXiv2024

Millinovae: A New Class of Transient Supersoft X-ray Sources without a Classical Nova Eruption

Some accreting binary systems containing a white dwarf (such as classical novae or persistent supersoft sources) are seen to emit low-energy X-rays with temperatures of ~10^6 K and luminosities exceeding 10^35 erg/s. These X-rays are thought to originate from nuclear burning on the white dwarf surface, either caused by a thermonuclear runaway (classical novae) or a high mass-accretion rate that sustains steady nuclear burning (persistent sources). The discovery of transient supersoft X-rays from ASASSN-16oh challenged these ideas, as no clear signatures of mass ejection indicative of a classical nova eruption were detected, and the origin of these X-rays remains controversial. It was unclear whether this star was one of a kind or representative of a larger, as yet undiscovered, group. Here, we present the discovery of 29 stars located in the direction of the Magellanic Clouds exhibiting long-duration, symmetrical optical outbursts similar to that seen in ASASSN-16oh. We observed one of these objects during an optical outburst and found it to be emitting transient supersoft X-rays, while no signatures of mass ejection (indicative of a classical nova eruption) were detected. We therefore propose that these objects form a homogeneous group of transient supersoft X-ray sources, which we dub ``millinovae'' because their optical luminosities are approximately a thousand times fainter than those of ordinary classical novae.

astro-ph.SR↗
arXiv2024

Investigating Polarization characteristics of GRB200503A and GRB201009A

We present results of a comprehensive analysis of the polarization characteristics of GRB 200503A and GRB 201009A observed with the Cadmium Zinc Telluride Imager (CZTI) on board AstroSat. Despite these GRBs being reasonably bright, they were missed by several spacecraft and had thus far not been localized well, hindering polarization analysis. We present positions of these bursts obtained from the Inter-Planetary Network (IPN) and the newly developed CZTI localization pipeline. We then undertook polarization analyses using the standard CZTI pipeline. We cannot constrain the polarization properties for GRB 200503A, but find that GRB 201009A has a high degree of polarization.

astro-ph.HE↗
arXiv2024

What Is the Black Hole Spin in Cyg X-1?

We perform a detailed study of the black hole spin of Cyg X-1, using accurate broad-band X-ray data obtained in the soft spectral state by simultaneous NICER and NuSTAR observations, supplemented at high energies by INTEGRAL data. We use the relativistic disk model kerrbb together with different models of the Comptonization high energy tail and the relativistically-broadened reflection features. Unlike most previous studies, we tie the spin parameters of the disk and relativistic broadening models, thus combining the continuum and reflection methods of spin determination. We also consider a likely increase of the disk color correction due to a partial support of the disk by large scale magnetic fields. We find that such models yield the spin parameter of $a_*= 0.87^{+0.04}_{-0.03}$ if the disk inclination is allowed to be free, with $i= 39^{+1}_{-1}$ degree. Assuming $i=27.5$ degree, as determined by optical studies of the binary, worsens the fit, but leads to similar values of the spin, $a_*= 0.90^{+0.01}_{-0.01}$. In addition, we consider the presence of a warm Comptonization layer on top of the disk, motivated by successful modeling of soft X-ray excesses in other sources with such a model. This dramatically lowers the spin, to $a_*\lesssim 0.1$, consistent with the spin measurements from black-hole mergers. On the other hand, if the natal spin of Cyg X-1 was low but now $a_*\approx 0.9$, a period of effective super-critical accretion had to take place in the past. Such accretion could be facilitated by photon advection, as proposed for ultraluminous X-ray sources.

astro-ph.HE↗
arXiv2024

Localisation of Gamma Ray Bursts using AstroSat Mass Model

The Cadmium Zinc Telluride Imager (CZTI) aboard AstroSat has good sensitivity to Gamma Ray Bursts (GRBs), with close to 600 detections including about 50 discoveries undetected by other missions. However, CZTI was not designed to be a GRB monitor and lacks localisation capabilities. We introduce a new method of localising GRBs using "shadows" cast on the CZTI detector plane due to absorption and scattering by satellite components and instruments. Comparing the observed distribution of counts on the detector plane with simulated distributions with the AstroSat Mass Model, we can localise GRBs in the sky. Our localisation uncertainty is defined by a two-component model, with a narrow Gaussian component that has close to 50% probability of containing the source, and the remaining spread over a broader Gaussian component with an 11.3 times higher $σ$. The width ($σ$) of the Gaussian components scales inversely with source counts. We test this model by applying the method to GRBs with known positions and find good agreement between the model and observations. This new ability expands the utility of CZTI in the study of GRBs and other rapid high-energy transients.

astro-ph.HE↗
arXiv2024

Science with the Daksha High Energy Transients Mission

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1~keV to $>1$~MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes.

astro-ph.HE↗
arXiv2024

Daksha: On Alert for High Energy Transients

We present Daksha, a proposed high energy transients mission for the study of electromagnetic counterparts of gravitational wave sources, and gamma ray bursts. Daksha will comprise of two satellites in low earth equatorial orbits, on opposite sides of earth. Each satellite will carry three types of detectors to cover the entire sky in an energy range from 1 keV to >1 MeV. Any transients detected on-board will be announced publicly within minutes of discovery. All photon data will be downloaded in ground station passes to obtain source positions, spectra, and light curves. In addition, Daksha will address a wide range of science cases including monitoring X-ray pulsars, studies of magnetars, solar flares, searches for fast radio burst counterparts, routine monitoring of bright persistent high energy sources, terrestrial gamma-ray flashes, and probing primordial black hole abundances through lensing. In this paper, we discuss the technical capabilities of Daksha, while the detailed science case is discussed in a separate paper.

astro-ph.IM↗
arXiv2024

Black hole spin measurements in LMC X-1 and Cyg X-1 are highly model-dependent

The black-hole spin parameter, $a_*$, was measured to be close to its maximum value of 1 in many accreting X-ray binaries. In particular, $a_*\gtrsim 0.9$ was found in a number of studies of LMC X-1. These measurements were claimed to take into account both statistical and systematic uncertainties. We perform new measurements using a recent simultaneous observation by NICER and NuSTAR, providing a data set of high quality. We use the disk continuum method together with improved models for coronal Comptonization. With the standard relativistic disk model and optically thin Comptonization, we obtain values of $a_*$ similar to those obtained before. We then consider modifications to the standard model. Using a color correction of 2, we find $a_*\approx 0.64$--0.84. We then consider disks with dissipation in surface layers. To account for that, we assume the standard disk is covered by a warm and optically thick Comptonizing layer. Our model with the lowest $χ^2$ yields then $a_*\approx 0.40^{+0.41}_{-0.32}$. In order to test the presence of such effects in other sources, we also study an X-ray observation of Cyg X-1 by Suzaku in the soft state. We confirm the previous findings of $a_*>0.99$ using the standard model, but then we find a weakly constrained $a_*\approx 0.82^{+0.16}_{-0.74}$ when including an optically thick Comptonizing layer. We conclude that determinations of the spin using the continuum method can be highly sensitive to the assumptions about the disk structure.

astro-ph.HE↗
arXiv2023

High hard X-ray polarization in Cygnus X-1 confined to the intermediate hard state: evidence for a variable jet component

Cygnus X-1, the well-known accreting black hole system, exhibits several observational features hinting at an intricate interplay between the accretion disk, its atmosphere known as the corona and the putative relativistic jet. It has been extensively studied using all available observational methods, including using the newly available technique of sensitive X-ray polarimetry. X-ray polarization characteristics are distinct for coronal and jet emissions. The low X-ray polarization measured below $\sim$100 keV is understood as arising from the corona. In contrast, the high polarization measurements reported above $\sim$400 keV required a separate jet-dominated spectral component, which spectroscopy does not demonstrate conclusively. Here we report precise polarization measurements in the 100-380 keV region made during three different sub-classes of spectral states of the source using the CZTI instrument onboard {\em AstroSat}. A high polarization (23$\pm$4 \%) is found mainly in the Intermediate Hard State of the source, and the energy-resolved measurements smoothly connect the coronal and the jet regimes. When high polarization is observed, the simultaneous spectral data hints at a separate power law component above 100 keV. We examine the possible sources of this energy-dependent high polarization in Cygnus X-1.

astro-ph.HE↗
arXiv2023

Multi-epoch hard X-ray view of Compton-thick AGN Circinus Galaxy

The circumnuclear material around Active Galactic Nuclei (AGN) is one of the essential components of the obscuration-based unification model. However, our understanding of the circumnuclear material in terms of its geometrical shape, structure and its dependence on accretion rate is still debated. In this paper, we present the multi-epoch broadband X-ray spectral modelling of a nearby Compton-thick AGN in Circinus galaxy. We utilise all the available hard X-ray ($> 10$ keV) observations taken from different telescopes, $i.e.,$ $BeppoSAX$, $Suzaku$, $NuSTAR$ and $AstroSat$, at ten different epochs across $22$ years from $1998$ to $2020$. The $3.0-79$ keV broadband X-ray spectral modelling using physically-motivated models, namely MYTORUS, BORUS02 and UXCLUMPY, infers the presence of a torus with a low covering factor of $0.28$, an inclination angle of $77^{\circ}$ $-$ $81^{\circ}$ and Compton-thick line-of-sight column densities ($N_{\rm H,LOS} = 4.13~-~9.26~\times~10^{24}$ cm$^{-2}$) in all the epochs. The joint multi-epoch spectral modelling suggests that the overall structure of the torus is likely to remain unchanged. However, we find tentative evidence for the variable line-of-sight column density on timescales ranging from one day to one week to a few years, suggesting a clumpy circumnuclear material located at sub-parsec to tens of parsec scales.

astro-ph.GA↗
arXiv2022

The Birth of a Relativistic Jet Following the Disruption of a Star by a Cosmological Black Hole

A black hole can launch a powerful relativistic jet after it tidally disrupts a star. If this jet fortuitously aligns with our line of sight, the overall brightness is Doppler boosted by several orders of magnitude. Consequently, such on-axis relativistic tidal disruption events (TDEs) have the potential to unveil cosmological (redshift $z>$1) quiescent black holes and are ideal test beds to understand the radiative mechanisms operating in super-Eddington jets. Here, we present multi-wavelength (X-ray, UV, optical, and radio) observations of the optically discovered transient \target at $z=1.193$. Its unusual X-ray properties, including a peak observed luminosity of $\gtrsim$10$^{48}$ erg s$^{-1}$, systematic variability on timescales as short as 1000 seconds, and overall duration lasting more than 30 days in the rest-frame are traits associated with relativistic TDEs. The X-ray to radio spectral energy distributions spanning 5-50 days after discovery can be explained as synchrotron emission from a relativistic jet (radio), synchrotron self-Compton (X-rays), and thermal emission similar to that seen in low-redshift TDEs (UV/optical). Our modeling implies a beamed, highly relativistic jet akin to blazars but requires extreme matter-domination, i.e, high ratio of electron-to-magnetic field energy densities in the jet, and challenges our theoretical understanding of jets.

astro-ph.HE↗
arXiv2017

Estimation of error on the cross-correlation, phase and time lag between evenly sampled light curves

Temporal analysis of radiation from Astrophysical sources like Active Galactic Nuclei, X-ray Binaries and Gamma-ray bursts provide information on the geometry and sizes of the emitting regions. Establishing that two light-curves in different energy bands are correlated and measuring the phase and time-lag between them is an important and frequently used temporal diagnostic. In the present work, expressions to estimate the errors on the cross-correlation, phase and time-lag between two light-curves are presented and the same have been verified using simulations. Earlier estimates depended upon numerically expensive simulations or on dividing the light-curves in large number of segments to find the variance. The estimates presented here allow for analysis of light-curves with relatively small (~1000) number of points, as well as to obtain information on the longest time-scales available. The error estimation is verified using simulations of light-curves derived from both white and 1/f stochastic processes with measurement errors. As a demonstration, we apply this technique to the XMM-Newton light-curves of the Active Galactic Nucleus, Akn 564.

astro-ph.HE↗
arXiv2016

On the reality of broad iron L lines from the narrow line Seyfert 1 galaxies 1H0707-495 and IRAS 13224-3809

We performed time resolved spectroscopy of 1H0707-495 and IRAS 13224-3809 using long XMM-Newton observations. These are strongly variable narrow line Seyfert 1 galaxies and show broad features around 1 keV that has been interpreted as relativistically broad Fe L$α$ lines. Such features are not clearly observed in other AGN despite sometimes having high iron abundance required by the best fitted blurred reflection models. Given the importance of these lines, we explore the possibility if rapid variability of spectral parameters may introduce broad bumps/dips artificially in the time averaged spectrum, which may then be mistaken as broadened lines. We tested this hypothesis by performing time resolved spectroscopy using long (> 100 ks) XMM-Newton observations and by dividing it into segments with typical exposure of few ks. We extracted spectra from each such segment and modelled using a two component phenomenological model consisting of a power law to represent hard component and a black body to represent the soft emission. As expected both the sources showed variations in the spectral parameters. Using these variation trends, we simulated model spectra for each segment and then co-added to get a combined simulated spectrum. In the simulated spectra, we found no broad features below 1 keV and in particular no deviation near 0.9 keV as seen in the real average spectra. This implies that the broad Fe L? line that is seen in the spectra of these sources is not an artifact of the variation of spectral components and hence providing evidence that the line is indeed genuine.

astro-ph.HE↗
arXiv2013

The kilo-second variability of X-ray sources in nearby galaxies

Chandra observations of 17 nearby galaxies were analysed and 166 bright sources with X-ray counts > 100, were chosen for temporal analysis. Fractional root mean square variability amplitudes were estimated for lightcurves binned at ~ 4 ksec and of length ~ 40 ksec. While there are nine ultra-luminous X-ray sources (ULXs) with unabsorbed luminosity (in 0.3-8.0 keV band) L > 10^39 erg/s in the sample for which the fractional r.m.s variability is constrained to be < 10%, only two of them show variability. One of the variable ULXs exhibits a secular transition and has a ultra-soft spectrum with temperature ~ 0.3 keV while the other is a rapidly varying source in NGC 0628, which has been previously compared to the Galactic micro-quasar GRS1915+105. These results seem to indicate that ULXs are typically not highly variable in ksec time-scales, except for some ultra-soft ones. Among the relatively low luminosity sources (L ~ 10^38 erg/s) we find five of them to be variable. Apart from an earlier known source in NGC 1569, we identify a source in NGC 2403, which exhibits persistent high amplitude fluctuations.The variability of the sources in general, do not seem to be correlated with hardness,which indicates that they may not be due to variations in any absorbing material, but instead could reflect inner accretion disk instabilities.

astro-ph.HE↗