arXiv subjects

Sudip Bhattacharyya

Publications and source records attributed to Sudip Bhattacharyya.

At least 55 records · Page 3Linked to original sources

arXiv2024

Probing outbursts of the transient neutron star low mass X-ray binary Aql X-1 with NICER: a study of spectral evolution

X-ray observations of neutron star (NS) low mass X-ray binaries (LMXBs) are useful to probe physical processes close to the NS and to constrain source parameters. Aql X-1 is a transient NS LMXB which frequently undergoes outbursts provides an excellent opportunity to study source properties and accretion mechanism in strong gravity regime over a wide range of accretion rates. In this work, we systematically investigate the spectral evolution of Aql X-1 using NICER observations during the source outbursts in 2019 and 2020. The NICER observations cover the complete transition of the source from its canonical hard state to soft state and back. The spectra extracted from most observations can be explained by a partially Comptonised accretion disc. We find that the system can be described by an accretion disk with an inner temperature of $\approx0.7$ keV and a Comptonising medium of thermal electrons at $\approx2$ keV, while the photon index is strongly degenerate with the covering fraction of the medium. We also find evidence of Fe K$α$ fluorescence emission in the spectra indicating reprocessing of the Comptonised photons. We observe an absorption column density higher than the Galactic column density for most of the observations indicating a significant local absorption. But for some of the observations in 2020 outburst, the local absorption is negligible.

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arXiv2024

AstroSat Observations of the Dipping Low Mass X-ray Binary XB 1254-690

XB 1254-690 is a neutron star low-mass X-ray binary with an orbital period of 3.88 hrs, and it exhibits energy-dependent intensity dips, thermonuclear bursts, and flares. We present the results of an analysis of a long observation of this source using the AstroSat satellite. The X-ray light curve gradually changed from a high-intensity flaring state to a low-intensity one with a few dips. The hardness intensity diagram showed that the source is in a high-intensity banana state with a gradually changing flux. Based on this, we divide the observation into four flux levels for a flux-resolved spectral study. The X-ray spectra can be explained by a model consisting of absorption, thermal emission from the disc and non-thermal emission from the corona. From our studies, we detect a correlation between the temperature of the thermal component and the flux and we examine the implications of our results for the accretion disc geometry of this source.

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arXiv2024

An IXPE-Led X-ray Spectro-Polarimetric Campaign on the Soft State of Cygnus X-1: X-ray Polarimetric Evidence for Strong Gravitational Lensing

We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May-June. Companion multiwavelength data during the campaign are likewise shown. The 2-8 keV X-rays exhibit a net polarization degree PD=1.99%+/-0.13% (68% confidence). The polarization signal is found to increase with energy across IXPE's 2-8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA=-25.7+/-1.8 deg. East of North (68% confidence). This is consistent with being aligned to Cyg X-1's AU-scale compact radio jet and its pc-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree, but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trendline in the polarization degree between soft and hard states. Our favored polarimetric model indicates Cyg X-1's spin is likely high (a* above ~0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE's bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole.

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arXiv2024

Near- and sub-solar-mass naked singularities and black holes from transmutation of white dwarfs

Recent gravitational wave events have suggested the existence of near-solar-mass black holes which cannot be formed via stellar evolution. This has opened up a tantalizing possibility of future detections of both black holes and naked singularities in this mass range. Existence of naked singularities is a topical and fundamental physics issue, but their formation mechanism is not yet clear. Here, we show that some white dwarfs can realistically transmute into black holes and naked singularities with a wide range of near- and sub-solar-mass values by capturing asymmetric or non-self-annihilating primordial dark matter (PDM) particles. We argue that, while a type Ia supernova due to the accumulation of dark matter at the core of a white dwarf could also be a possibility, the transmutation of a white dwarf into a black hole or a naked singularity is a viable consequence of the capture of non-self-annihilating PDM particles. These white dwarf transmutations can have a significant role in probing the physics of dark matter and compact objects, and could be tested using the rates and locations of mergers over the cosmological time scale.

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arXiv2024

AstroSat and NICER timing view of the Z-type Neutron Star X-ray binary GX 340+0

The timing properties of the Z-type low-mass X-ray binaries provide insights into the emission components involved in producing the unique Z-shaped track in the hardness-intensity diagrams of these sources. In this work, we investigate the AstroSat and NICER observations of the GX 340+0 covering the complete 'Z'-track from the horizontal branch (HB) to the extended flaring branch (EFB). For the first time, we present the Z-track as seen in soft X-rays using the AstroSat/SXT and NICER (the soft colour is defined as a ratio of 3-6 keV to 0.5-3 keV). The shape of the track is distinctly different in soft X-rays, strongly suggesting the presence of additional components active in soft X-rays. The detailed timing analysis revealed significant quasi-periodic oscillation throughout the HB and the normal branch (NB) using LAXPC and the first NICER detection of 33.1 +/- 1.1 Hz horizontal branch oscillation (HBO) in 3-6 keV. The oscillations at the HB/NB vertex are observed to have higher frequencies (41-52 Hz) than the HB oscillations (16-31 Hz) and NB oscillations (6.2-8 Hz) but significantly lower rms (~1.6%). The HB oscillation is also limited to the energy range of 3-20 keV, indicating an association of HBO origin with the non-thermal component. It is also supported by earlier studies that found the strongest X-ray polarisation during HB.

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arXiv2023

AstroSat view of the neutron star low-mass X-ray binary GX 340+0

Understanding the spectral evolution along the `Z'-shaped track in the hardness-intensity diagram of Z-sources, which are a class of luminous neutron star low-mass X-ray binaries, is crucial to probe accretion processes close to the neutron star. Here, we study the horizontal branch (HB) and the normal branch (NB) of the Z source GX 340+0 using $AstroSat$ data. We find that the HB and the NB appear as two different types of X-ray intensity dips, which can appear in any sequence and with various depths. Our $0.8-25$ ~keV spectra of dips and the hard apex can be modeled by the emissions from an accretion disk, a Comptonizing corona covering the inner disk, and the neutron star surface. We find, as the source moves onto the HB the corona is replenished and energized by the disk and a reduced amount of disk matter reaches the neutron star surface. We also conclude that quasi-periodic oscillations during HB/NB are strongly associated with the corona, and explain the evolution of strength and hard-lag of this timing feature using the estimated coronal optical depth evolution.

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arXiv2023

Flux-resolved Spectropolarimetric Evolution of the X-Ray Pulsar Hercules X-1 Using IXPE

We conduct a spectro-polarimetric study of the accreting X-ray pulsar Hercules X-1 using observations with the Imaging X-ray Polarimetry Explorer (IXPE). IXPE monitored the source in three different Epochs, sampling two Main-on and one Short-on state of the well-known super-orbital period of the source. We find that the 2-7 keV polarization fraction increases significantly from ~ $7-9$ % in the Main-on state to ~ $15-19$ % in the Short-on state, while the polarization angle remains more or less constant or changes slightly, ~ $47-59$ degrees, in all three Epochs. The polarization degree and polarization angle are consistent with being energy-independent for all three Epochs. We propose that in the Short-on state, when the neutron star is partially blocked by the disk warp, the increase in the polarization fraction can be explained as a result of the preferential obstruction of one of the magnetic poles of the neutron star.

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arXiv2023

Broadband spectro-temporal investigation of neutron star low-mass X-ray binary GX 349+2

We report a broadband investigation of the Z-type neutron star (NS) low mass X-ray binary (LMXB) GX 349+2 using AstroSat and NICER. AstroSat observed the source exhibiting large scale variability in its normal branch (NB) /flaring branch (FB) vertex and flaring branch (FB) and a moderate evolution during NICER observations. The power spectra exhibit very low-frequency noise (VLFN) and low-frequency noise (LFN)/flaring branch noise (FBN), described by a power law and an evolving Lorentzian. We investigate the energy dependence of variability components and their correlation with the spectral state to probe their origin. The joint spectra of GX 349+2 are modeled by two thermal and one non-thermal component. The source moves along the Z track, with the increasing accretion rate, further heating of the NS boundary layer, and increasing temperature/radius of the brightened hotspot at the disc-boundary layer interface/NS surface. A power law well represents the hard non-thermal coronal emission. As predicted by the gravitational redshift, we find a correlation between the line energy detected in NICER spectra and the inner disc radius with the Spearman rank correlation coefficient of 1. Using this correlation, we demonstrate the potential of a method to constrain the accreting compact object properties, including evolving continuum and line spectroscopy. We report the first detection of hard lag providing evidence of the VLFN originating from the accretion disc in NS LMXBs, representing fluctuation of propagation through the disc.

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arXiv2023

Disk-corona modeling for spectral index and luminosity correlation of tidal disruption events

We present a relativistic disk-corona model for a steady state advective accretion disk to explain the UV to X-ray spectral index $α_{\text{OX}}$ evolution of \textbf{four} tidal disruption event (TDE) sources XMMSL2J1446, XMMSL1J1404, XMMSL1J0740, \textbf{and AT2018fyk}. The viscous stress in our model depends on gas ($P_g$) and total ($P_t$) pressures as $τ_{rϕ} \propto P_g^μ P_t^{1-μ}$, where $μ$ is a constant. We compare various steady and time-dependent sub-Eddington TDE accretion models along with our disk-corona model to the observed $α_{\text{OX}}$ of TDE sources and find that the disk-corona model agrees with the observations better than the other models. We find that $μ$ is much smaller than unity for TDE sources XMMSL2J1446, XMMSL1J1404, and XMMSL1J0740. We also compare the relativistic model with a non-relativistic disk-corona model. The relativistic accretion dynamics reduce the spectral index relative to the non-relativistic accretion by increasing the energy transport to the corona. We estimate the mass accretion rate for all the sources and find that the observed luminosity follows a nearly linear relation with the mass accretion rate. The ratio of X-ray luminosity from the disk to the corona increases with the mass accretion rate. The observed $α_{\text{OX}}$ shows positive and negative correlations with luminosity. The disk-corona model explains the negative correlation seen in the TDE sources XMMSL1J0740, XMMSL2J1446, and XMMSL1J1404. However, TDE AT2018fyk shows a positive correlation at higher luminosity and shows a better fit when a simple spherical adiabatic outflow model is added to the relativistic disk-corona model. Even though the disk luminosity dominates at a higher mass accretion rate, we show that the accretion models without a corona are unable to explain the observations, and the presence of a corona is essential.

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arXiv2023

Automated classification of Chandra X-ray point sources using machine learning methods

A large number of unidentified sources found by astronomical surveys and other observations necessitate the use of an automated classification technique based on machine learning methods. The aim of this paper is to find a suitable automated classifier to identify the point X-ray sources in the Chandra Source Catalogue (CSC) 2.0 in the categories of active galactic nuclei (AGN), X-ray emitting stars, young stellar objects (YSOs), high-mass X-ray binaries (HMXBs), low-mass X-ray binaries (LMXBs), ultra luminous X-ray sources (ULXs), cataclysmic variables (CVs), and pulsars. The catalogue consists of approx 3,17,000 sources, out of which we select 2,77,069 point sources based on the quality flags available in CSC 2.0. In order to identify unknown sources of CSC 2.0, we use multi-wavelength features, such as magnitudes in optical/UV bands from Gaia-EDR3, SDSS and GALEX, and magnitudes in IR bands from 2MASS, WISE and MIPS-Spitzer, in addition to X-ray features (flux and variability) from CSC 2.0. We find the Light Gradient Boosted Machine, an advanced decision tree-based machine learning classification algorithm, suitable for our purpose and achieve $93\%$ precision, $93\%$ recall score and 0.91 Mathew's Correlation coefficient score. With the trained classifier, we identified 54,770 (14,066) sources with more than $3σ$ (4$σ$) confidence, out of which there are 32,600 (8,574) AGNs, 16,148 (5,166) stars, 5,184 (208) YSOs, 439 (46) HMXBs, 197 (71) LMXBs, 50 (0) ULXs, 89 (1) CVs, and 63 (0) pulsars. This method can also be useful for identifying sources of other catalogues reliably.

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arXiv2022

Probing the tidal disruption event iPTF16axa with CLOUDY and disc-wind models

We present both a disc-wind model on the optical/UV emission continuum and CLOUDY modelling on the spectral lines of the tidal disruption event (TDE) iPTF16axa to understand the disc-wind emission and the properties of the atmosphere that impacts the line luminosity of the TDE. Assuming the optical/UV emission from the wind due to the disc super-Eddington phase, we use the steady structured disc-wind model with a spherical wind with constant velocity to fit the observations on multiple days. The extracted parameters are stellar-mass $M_{\star} = 6.20 \pm 1.19 M_{\odot}$, disc radiative efficiency $\log_{\rm 10}(η) = -1.22 \pm 1.327$, wind inner radius $r_l = (2.013 \pm 0.551) \times 10^{14}~{\rm cm}$ and velocity $v_w = 18999.4 \pm 1785.1 ~{\rm km~s^{-1}}$. The photosphere temperature for wind emission is $ \sim 2 \times 10^4~{\rm K}$ and the disc single blackbody temperature is $\sim 0.995 \times 10^5~{\rm K}$. We also perform CLOUDY modelling to explain the observed He and H line luminosities that estimate a wind inner radius $r_l = 7.07 \times 10^{14}~{\rm cm}$ and velocity $v_w = 1.3 \times 10^4~{\rm km~s^{-1}}$. The independent analyses of iPTF16axa using CLOUDY and disc-wind models show comparable results that agree with observations. The CLOUDY modelling finds that both the super solar abundance of He and a smaller He II line optical depth is responsible for the enhancement of He II line luminosity over the H$α$ line luminosity. The super-solar abundance of He II agrees with a relatively large stellar mass and suggests that the disrupted star might have been a red giant.

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arXiv2022

Advective accretion disc-corona model with fallback for tidal disruption events

Tidal disruption events (TDEs) show a correlation between the UV to X-ray spectral index and the Eddington ratio, with non-thermal X-ray emission at the low Eddington ratio. We consider the corona surrounding the accretion disc as a non-thermal X-ray source. We construct a time-dependent and non-relativistic advective accretion disc-corona model for TDEs. The infalling debris is assumed to form a seed disc in time $t_c$, that evolves due to the mass gain from the infalling debris at the constant outer radius with a mass fallback rate $\dot{M}_{\rm fb}$ and the mass loss through accretion onto the black hole. The viscous stress in our model depends on gas ($P_g$) and total ($P_t$) pressures as $τ_{rϕ} \propto P_g^{1-μ} P_t^μ$, where $μ$ is a constant. We find that the mass accretion rate $\dot{M}_a$ evolves from Eddington to sub-Eddington accretion with a late-time evolution close to $t^{-5/3}$, where $t$ is the time. We find that the bolometric disc luminosity follows a late-time evolution close to $t^{-5/3}$. The ratio of total X-ray luminosity from corona to bolometric disc luminosity increases with $μ$ and increases at late times for $μ\neq 1$. We obtain the X-ray blackbody temperature of the disc that agrees with the temperature from X-ray observations ($\sim~10^5~{\rm K}$). We find the radiative efficiency of the disc increases with time and decreases for a disc when the corona is included. We have neglected the outflow, and our model is more applicable for near-to-sub-Eddington accretion and when $\dot{M}_{\rm fb}$ is sub-Eddington.

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arXiv2022

Primordial Black Holes having Gravitomagnetic Monopole

A primordial black hole (PBH) is thought to be made of the regular matter or ordinary mass ($M$) only, and hence could have already been decayed due to the Hawking radiation if its initial ordinary mass were $\lesssim 5 \times 10^{11}$ kg. Here, we study the role of gravitomagnetic monopole for the evaporation of PBHs, and propose that the lower energy PBHs (equivalent to ordinary mass $M << 5\times 10^{11}$ kg) could still exist in our present Universe, if it has gravitomagnetic monopole. If a PBH was initially made of both regular matter and gravitomagnetic monopole, the regular matter could decay away due to the Hawking radiation. The remnant gravitomagnetic monopole might not entirely decay, which could still be found as a PBH in the form of the pseudo `mass-energy'. If a PBH with $M \gtrsim 5 \times 10^{11}$ kg is detected, one may not be able to conclude if it has gravitomagnetic monopole. But, a plausible detection of a relatively low energy (equivalent to $2.176 \times 10^{-8}$ kg $< M \lesssim 5\times10^{11}$ kg) PBH in future may imply the existence of a gravitomagnetic monopole PBH, which may or may not contain the ordinary mass.

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arXiv2022

Probing spectral and temporal evolution of the neutron star low-mass X-ray binary 4U 1724-30 with AstroSat

We report the broadband spectro-temporal study of the poorly studied accreting neutron star (NS) low mass X-ray binary (LMXB) 4U 1724-30 using data from Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC) instruments on board AstroSat. The dim persistent LMXB source was observed with AstroSat over 4 epochs in 2017, all of which corresponded to a low-luminosity non-thermal emission dominated (hard/island) emission state with modest spectral evolution. All the X-ray broadband spectra can be modelled by a combination of thermal emission from the NS boundary layer (BL) or NS surface and a non-thermal emission component possibly originating from the inverse Comptonization of the disc seed photons. We investigate the presence of frequency and energy-dependent variabilities to probe the origin of the disc/coronal fluctuations. We also report the detection of a Type-I X-ray burst displaying a photospheric radius expansion (PRE). During the burst, a hard X-ray shortage in the 30-80 keV energy band and the enhancement of the persistent emission reveal the burst feedback on the overall accretion process. Using the touch-down burst flux $\sim$ $4.25 \times 10^{-8}$ erg s$^{-1}$ cm$^{-2}$, the distance of the source is estimated as $\sim$ 8.4 kpc.

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arXiv2021

NuSTAR monitoring of MAXI J1348-630: evidence of high density disc reflection

We present the broadband spectral analysis of all the six hard, intermediate and soft state NuSTAR observations of the recently discovered transient black hole X-ray binary MAXI J1348-630 during its first outburst in 2019. We first model the data with a combination of a multi-colour disc and a relativistic blurred reflection, and, whenever needed, a distant reflection. We find that this simple model scheme is inadequate in explaining the spectra, resulting in a very high iron abundance. We, therefore, explore the possibility of reflection from a high-density disc. We use two different sets of models to describe the high-density disc reflection: relxill-based reflection models, and reflionx-based ones. The reflionx-based high-density disc reflection models bring down the iron abundance to around the solar value, while the density is found to be $10^{20.3-21.4} \rm cm^{-3}$. We also find evidence of a high-velocity outflow in the form of $\sim$7.3 keV absorption lines. The consistency between the best-fit parameters for different epochs and the statistical significance of the corresponding model indicates the existence of high-density disc reflection in MAXI J1348-630.

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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.

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arXiv2021

Spectroscopic modelling of two high-mass X-ray binaries, Cyg X--3 and 4U 1538--522

We report a detailed modelling of soft X-ray emission lines from two stellar-wind fed Galactic high mass X-ray binary (HMXB) systems, Cyg X-3 and 4U 1538-522, and estimate physical parameters, e.g., hydrogen density, radiation field, chemical abundances, wind velocity, etc. The spectral synthesis code CLOUDY is utilized for this modelling. We model highly ionised X-ray spectral lines such as Fe XXV (6.700 keV), Fe XXVI (6.966 keV), and reproduce the observed line flux values. We find that for Cyg X--3 and 4U 1538-522, the inner radius of the ionised gas is at a distance of 10$^{12.25}$ cm and 10$^{10.43}$ cm respectively from the primary star, which is the main source of ionisation. The densities of the ionised gas for Cyg X--3 and 4U 1538--522 are found to be $\sim$ 10$^{11.35}$ cm$^{-3}$ and 10$^{11.99}$ cm$^{-3}$, respectively. The corresponding wind velocities are 2000 km s$^{-1}$ and 1500 km s$^{-1}$. The respective predicted hydrogen column densities for Cyg X--3 and 4U 1538--522 are $10^{23.2}$ cm$^{-2}$ and 10$^{22.25}$ cm$^{-2}$. In addition, we find that a magnetic field affects the strength of the spectral lines through cyclotron cooling. Hence, we perform separate model comparisons including a magnetic field for both the sources. Most of the parameters, except the hydrogen column density, have similar values with and without a magnetic field. We estimate that the most probable strength of the magnetic field for Cyg X--3 and 4U 1538--522, where the Fe XXV and Fe XXVI lines originate, is $\sim$ 10$^{2.5}$G.

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arXiv2021

X-ray Observations of 1ES 1959+650 in its high activity state in 2016-2017 with AstroSat and Swift

We present a comprehensive multi-frequency study of the HBL 1ES 1959+650 using data from various facilities during the period 2016-2017, including X-ray data from {\it AstroSat} and {\it Swift} during the historically high X-ray flux state of the source observed until February 2021. The unprecedented quality of X-ray data from high cadence monitoring with the {\it AstroSat} during 2016-2017 enables us to establish a detailed description of X-ray flares in 1ES 1959+650. The synchrotron peak shifts significantly between different flux states, in a manner consistent with a geometric (changing Doppler factor) interpretation. A time-dependent leptonic diffusive-shock-acceleration and radiation transfer model is used to reproduce the spectral energy distributions (SEDs) and X-ray light curves, to provide insight into the particle acceleration during the major activity periods observed in 2016 and 2017. The extensive data of {\it Swift}-XRT from December 2015 to February 2021 (Exp. = 411.3 ks) reveals a positive correlation between flux and peak position.

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