arXiv subjects

Samir Mandal

Publications and source records attributed to Samir Mandal.

At least 19 recordsLinked to original sources

arXiv2026

The Reawakening of 4U 1755-338 after 25 Years of Quiescence: Spectro-temporal Analysis Using Multi-instrument X-ray Data

The black hole X-ray binary 4U 1755$-$338 underwent an outburst in 2020 after 25 years of quiescence. The comprehensive spectral analysis revealed that the system has a low interstellar neutral hydrogen column density of $0.34\pm0.01 \times$10$^{22}$ cm$^{-2}$. The outburst began with a low mass-accretion rate and was characterized as a low-luminosity outburst. The radius of the inner accretion disc remained constant throughout the outburst. Additionally, a growing neutral medium with constant density was detected in the local environment of 4U 1755$-$338.The hardness-intensity diagram (HID) did not follow the standard q-shaped pattern, indicating a non-canonical outburst. Instead, the HID showed a correlated evolution of hardness and source flux, suggesting a thermal disc origin of the flux. A wideband spectral analysis was performed using simultaneous NICER-NuSTAR data in two frameworks, based on kerrbb and bhspec. The results of bhspec (kerrbb) based modeling indicate that 4U 1755$-$338 is a high-inclination system, $67.44_{-3.03}^{+9.75}$ ($75.25_{-4.68}^{+5.59}$) degrees, and harbors a moderately spinning black hole with a spin parameter of $0.78_{-0.14}^{+0.02}$ ($0.50_{-0.43}^{+0.19}$) and a mass of $3.37_{-1.04}^{+0.45} (3.28_{-1.1}^{+1.7})M_{\odot}$ respectively. The inferred key parameters: black hole mass, spin, and system inclination are consistent across both modeling approaches. No reflection features were detected in the spectra of 4U 1755$-$338. The high spectral index, the blackbody nature ($L\propto T^4$) of the hardness ratio, the absence of reflection signatures, and the weak variability in the power density spectra indicate that the source remained in the high/soft state throughout the outburst.

astro-ph.HE↗

arXiv2026

Explainable machine learning classification of \textit{Chandra} X-ray sources: SHAP analysis of multi-wavelength features

Extensive astronomical surveys, like those conducted with the {\em Chandra} X-ray Observatory, detect hundreds of thousands of unidentified cosmic sources. Machine learning (ML) methods offer an efficient, probabilistic approach to classify them, which can be useful for making discoveries and conducting deeper studies. In earlier work, we applied the LightGBM (ML model) to classify 277,069 {\em Chandra} point sources into eight categories: active galactic nuclei (AGN), X-ray emitting stars, young stellar objects (YSO), high-mass X-ray binaries, low-mass X-ray binaries, ultraluminous X-ray sources, cataclysmic variables, and pulsars. In this work, we present the classification table of 54,770 robustly classified sources (over $3\sigma$ confidence), including 14,066 sources at $>4\sigma$ significance. To ensure classification reliability and gain a deeper insight, we investigate the multiwavelength feature relationships learned by the LightGBM model, focusing on AGNs, Stars, and YSOs. We employ Explainable Artificial Intelligence (XAI) techniques, specifically, SHapley Additive exPlanations (SHAP), to quantify the contribution of individual features and their interactions to the predicted classification probabilities. Among other things, we find infrared-optical and X-ray decision boundaries for separating AGN/Stars, and infrared-X-ray boundaries for YSOs. These results are crucial for estimating object classes even with limited multiwavelength data. This study represents one of the earliest applications of XAI to large-scale astronomical datasets, demonstrating ML models' potential for uncovering physically meaningful patterns in data in addition to classification. Finally, our publicly available, extensive, and interactive catalogue will be helpful to explore the contributions of features and their combinations in greater detail in the future.

astro-ph.IM↗

arXiv2026

Particle Acceleration and Depolarization in the Protostellar jet knots HH 80 and HH 81

Linearly polarized emission is a powerful tracer of magnetic field geometry and particle acceleration in protostellar jets. We present a polarimetric study of the HH objects HH 80 and HH 81 from where non-thermal emission has been confirmed through spectral index measurements at low frequencies. We carried out observations of HH 80 and HH 81 with the Karl G. Jansky Very Large Array in 4-6 GHz. Unlike the inner jet knots, no linear polarization is detected towards the knots HH 80 and HH 81. We place a $3\sigma$ upper limit of $30~\mu$Jy on the polarization intensity, corresponding to fractional polarization limits of $\Pi_{\max}\approx0.02$ and $0.01$ for HH 80 and HH 81, respectively. To interpret this non-detection, we assess the conditions for synchrotron polarization and the impact of depolarization mechanisms. The shock cooling parameter $\chi_\mathrm{s}$ is lower in these outermost HH objects than in the inner knots, indicating that the reverse shocks in HH 80-81 are less efficient at accelerating relativistic electrons compared with the inner knots. Moreover, Faraday depolarization appears severe: the dispersion in the estimated rotation measure $\sigma_{\rm RM}\sim400~\mathrm{rad~m^{-2}}$ is comparable to or larger than observed RM values themselves. This is consistent with strong fluctuations and turbulence. Together with beam depolarization, these effects can suppress the observable fractional polarization flux densities below the detectable thresholds. We conclude that reduced acceleration efficiency (when compared to inner knots) and strong depolarization account for the absence of polarized emission towards HH 80 and HH 81.

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arXiv2025

First detection of Circular Polarization in radio continuum towards a Massive Protostar

Polarization measurements provide strong constraints on magnetic fields in star-forming systems. While magnetic field estimates of a few kiloGauss (kG) have been obtained near the surface of low-mass protostars, there are no analogous measurements in the immediate vicinity of the surface of massive protostars. We report the measurement of radio continuum circular polarization (CP) towards a massive protostar IRAS 18162-2048 for the first time wielding Karl G. Jansky Very Large Array (VLA) observations. The fractional CP varies between $3-5\%$ across the observed frequency range of $4-6$ GHz. We consider multiple hypotheses for the production of CP and propose (i) gyrosynchrotron emission and (ii) Faraday conversion due to turbulence in the magnetic medium - both driven by mildly relativistic electrons as plausible mechanisms. We estimate, for the first time, a magnetic field $B\gtrsim20-35$ G close to the massive protostar. The Lorentz factor of the low energy electrons is estimated to be in the range $\gamma_{min}\sim5-7$ for gyrosynchrotron emission and $80-100$ for Faraday conversion from our observations. The magnetic field estimate can provide important constraints to the formation models of massive stars.

astro-ph.GA↗

arXiv2024

Probing properties of nearly two-hundred new active galactic nuclei

We present a comprehensive analysis of the X-ray spectral properties of 198 newly identified active galactic nuclei (AGNs), leveraging archival data from the {\it Chandra} X-ray Observatory. All these AGNs exhibit a powerlaw spectral signature spanning a broad energy range of $0.5-7.0$ keV, characterized by the photon index ($\Gamma$) values ranging from $0.3^{+0.16}_{-0.14}$ to $2.54^{+0.14}_{-0.13}$. Particularly, 76 of these AGNs display discernible levels of intrinsic absorption, after considering the Galactic absorption. The column densities associated with this local absorption ($n_{\rm H}^{\rm local}$) are within a range of $\sim 10^{19} - 10^{22}\ {\rm cm^{-2}}$. We study the cosmological evolution of AGNs using the variation of $n_{\rm H}^{\rm local}$ and $\Gamma$ with their estimated redshift. The intrinsic spectral signature did not reveal any significant cosmological evolution; however, a deficit of hard sources at high redshift is possibly intrinsic. Our sample covers several decades of broadband intrinsic luminosity ($L_{\rm B}^{\rm intr}$) ranging from $4.59^{+0.41}_{-0.41} \times 10^{42}$ to $2.4^{+0.12}_{-0.12} \times 10^{46}\, {\rm erg~s}^{-1}$ with peak at 1.84 redshift. We also investigate the hardness-luminosity diagram (HLD) to further probe the AGNs. We conduct a sanity check by applying our findings to known AGNs, and the results are consistent with our observations.

astro-ph.HE↗

arXiv2024

AT2020ohl: its nature and probable implications

ASASSN-20hx, a.k.a AT2020ohl, is an ambiguous nuclear transient (ANT), which was discovered in the nearby galaxy NGC6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We have investigated the evolution of AT2020ohl using a multi-wavelength dataset to explain the geometry of the system and the energy radiated by it between X-ray and radio wavelengths. Our X-ray, UV/optical, and radio observations of the object jointly clarify the association of AT2020ohl with the nuclear activity of NGC6297. We detected radio counterpart of AT2020ohl 111 days and 313 days after the discovery in Jansky Very Large Array X-band with flux densities 47$\pm$14 $\mu$Jy and 34$\pm$3 $\mu$Jy, respectively. Using multi-wavelength data analysis, we nullify the possibility of associating any stellar disruption process with this event. We found some evidence showing that the host galaxy is a merger remnant, so the possibility of a binary SMBH system can not be ruled out. The central SMBH has a mass of $\sim1.2\times10^7$ M$_\odot$. We propose the accretion disk activity as the origin of AT2020ohl $-$ it is either due to disk accretion event onto the central SMBH or due to the sudden accretion activity in a preexisting accretion disk of the system during the interaction of two SMBHs which became gravitationally bound during a merger process. However, we also admit that with the existing dataset, it is impossible to say definitively, among these two probabilities, which one is the origin of this nuclear transient.

astro-ph.HE↗

arXiv2023

Modeling of thermal and non-thermal radio emission from HH80-81 jet

Protostellar jets are one of the primary signposts of star formation. A handful of protostellar objects exhibit radio emission from ionized jets, of which a few display negative spectral indices, indicating the presence of synchrotron emission. In this study, we characterize the radio spectra of HH80-81 jet with the help of a numerical model that we have developed earlier, which takes into account both thermal free-free and non-thermal synchrotron emission mechanisms. For modeling the HH80-81 jet, we consider jet emission towards the central region close to the driving source along with two Herbig-Haro objects, HH80 and HH81. We have obtained the best-fit parameters for each of these sources by fitting the model to radio observational data corresponding to two frequency windows taken across two epochs. Considering an electron number density in the range $10^3 - 10^5$ cm$^{-3}$, we obtained the thickness of the jet edges and fraction of relativistic electrons that contribute to non-thermal emission in the range $0.01^{\circ} - 0.1^{\circ}$ and $10^{-7} - 10^{-4}$, respectively. For the best-fit parameter sets, the model spectral indices lie in the range of -0.15 to +0.11 within the observed frequency windows.

astro-ph.GA↗

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 $3{\sigma}$ (4${\sigma}$) 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.

astro-ph.HE↗

arXiv2023

Broadband X-ray properties of black holes GRS 1758-258 and 1E 1740.7-2942: AstroSat and NuSTAR results

We present the results on broadband X-ray properties of persistent black hole binaries GRS 1758$-$258 and 1E 1740.7$-$2942 using AstroSat, NuSTAR and Swift-XRT observations carried out during 2016$-$2022. We perform spectral modeling of both sources after eliminating the contamination in their \textit{LAXPC} spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission ($\sim$ 6.4 keV) models suggest that GRS 1758$-$258 occupies both dim-soft state ($kT_{bb}=0.37\pm0.01$ keV, $\Gamma\sim5.9$, $L_{bol}=1 %$ of Eddington luminosity L$_{Edd}$) and hard state ($\Gamma=1.64-2.22$, $kT_{e}$=4$-$45 keV, $L_{bol}$=1$-$5 % L$_{Edd}$) that requires a multi-colour disc blackbody model ($kT_{in}=0.54\pm0.01$ keV) occasionally. 1E 1740.7$-$2942 instead is found only in hard state ($\Gamma$=1.67$-$2.32, $kT_{e}$=5$-$16 keV, $L_{bol}$=1$-$2 % L$_{Edd}$). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broadband spectra. Our results from \textit{AstroSat} and \textit{NuSTAR} consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter $log\xi$=2.7$-$3.8 and 2.7$-$4.7 erg cm s$^{-1}$ in GRS 1758$-$258 and 1E 1740.7$-$2942 respectively) in both sources. Reflection modeling revealed GRS 1758$-$258 to have a high metal abundance ($A_{fe}=3.9^{+0.4}_{-0.3}$ times solar metal abundance) and inclination angle ($i$) of $61\pm2^{\circ}$. In case of 1E 1740.7$-$2942, $i$ is constrained to be $55\pm1^{\circ}$. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies.

astro-ph.HE↗

arXiv2023

Wideband Study of the Brightest Black Hole X-ray Binary 4U 1543-47 in the 2021 Outburst: Signature of Disk-Wind Regulated Accretion

A comprehensive wideband spectral analysis of the brightest black hole X-ray binary 4U $1543-47$ during its 2021 outburst is carried out for the first time using NICER, NuSTAR, and AstroSat observations by phenomenological and reflection modelling. The source attains a super-Eddington peak luminosity and remains in the soft state, with a small fraction ($< 3\%$) of the inverse-Comptonized photons. The spectral modelling reveals a steep photon index ($\Gamma \sim 2-2.6$) and relatively high inner disk temperature ($T_{in}\sim 0.9-1.27$ keV). The line-of-sight column density varies between ($0.45-0.54$)$\times10^{22}$ cm$^{-2}$. Reflection modelling using the RELXILL model suggests that 4U $1543-47$ is a low-inclination system ($\theta \sim 32^\circ - 40^\circ$). The accretion disk is highly ionized (log $\xi$ > 3) and has super solar abundance (3.6$-$10 $A_{Fe,\odot}$) over the entire period of study. We detected a prominent dynamic absorption feature between $\sim 8-11$ keV in the spectra throughout the outburst. This detection is the first of its kind for X-ray binaries. We infer that the absorption of the primary X-ray photons by the highly ionized, fast-moving disk-winds can produce the observed absorption feature. The phenomenological spectral modelling also shows the presence of a neutral absorption feature $\sim 7.1 - 7.4$ keV, and both ionized and neutral absorption components follow each other with a delay of a typical viscous timescale of $10-15$ days.

astro-ph.HE↗

arXiv2022

Accretion Scenario of MAXI J1820+070 during 2018 Outbursts with Multi-mission Observations

We present a comprehensive spectral and temporal study of the black hole X-ray transient MAXI J1820+070 during its outbursts in 2018 using Swift/XRT, NICER, NuSTAR and AstroSat observations. The Swift/XRT and NICER spectral study shows a plateau in the light curve with spectral softening (hardness changes from $\sim$ $2.5$ to $2$) followed by a gradual decline without spectral softening during the first outburst. Also, spectral modelling suggests that the first outburst is in the low/hard state throughout with a truncated disk whereas the thermal disk emission dominates during the second outburst. During the entire outburst, strong reflection signature (reflection fraction varies between $\sim$ $0.38 - 3.8$) is observed in the simultaneous wideband (NICER-NuSTAR, XRT-NuSTAR, AstroSat) data due to the presence of a dynamically evolving corona. The NICER timing analysis shows Quasi-periodic Oscillation (QPO) signatures and the characteristic frequency increases (decreases) in the plateau (decline) phase with time during the first outburst. We understand that the reduction of the electron cooling timescale in the corona due to spectral softening and the resonance oscillation with the local dynamical timescale may explain the above behavior of the source during the outburst. Also, we propose a possible scenario of outburst triggering and the associated accretion geometry of the source.

astro-ph.HE↗

arXiv2022

Radio spectra of protostellar jets: Thermal and non-thermal emission

Protostellar jets and outflows are pointers of star-formation and serve as important sources of momentum and energy transfer to the interstellar medium. Radio emission from ionized jets have been detected towards a number of protostellar objects. In few cases, negative spectral indices and polarized emission have also been observed suggesting the presence of synchrotron emission from relativistic electrons. In this work, we develop a numerical model that incorporates both thermal free-free and non-thermal synchrotron emission mechanisms in the jet geometry. The flux densities include contribution from an inner thermal jet, and a combination of emission from thermal and non-thermal distributions along the edges and extremities, where the jet interacts with the interstellar medium. We also include the effect of varying ionization fraction laterally across the jet. An investigation of radio emission and spectra along the jet shows the dependence of the emission process and optical depth along the line of sight. We explore the effect of various parameters on the turnover frequencies and the radio spectral indices (between 10 MHz and 300 GHz) associated with them.

astro-ph.GA↗

arXiv2021

Unraveling the foretime of GRS 1915+105 using AstroSat observations: Wide-band spectral and temporal characteristics

We present a comprehensive study of GRS 1915+105 in wide energy band ($0.5-60$ keV) using AstroSat observations during the period of $2016-2019$. The MAXI X-ray lightcurve of the source shows rise and decay profiles similar to canonical outbursting black holes. However, the source does not follow the exemplary 'q'-diagram in the Hardness-Intensity Diagram (HID). Model independent analysis of lightcurves suggests that GRS 1915+105 displays various types of variability classes ($\delta,\chi,\rho,\kappa,\omega$ and $\gamma$). We also report possible transitions from one class to another ($\chi\rightarrow\rho,\rho\rightarrow\kappa$ via an 'unknown' class and $\omega\rightarrow\gamma\rightarrow\omega+\gamma$) within a few hours duration. Broadband energy spectra are well modeled with multi-coloured disc blackbody and Comptonised components. We explore the 'spectro-temporal' features of the source in the different variability classes, transitions between classes, and evolution during $2016-2019$. Detailed analysis indicates a gradual increase in the photon index ($\Gamma$) from $1.83$ to $3.8$, disc temperature ($kT_{in}$) from $1.33$ to $2.67$ keV, and Quasi-periodic Oscillation (QPO) frequency ($\nu$) from $4$ to $5.64$ Hz during the rise, while the parameters decrease to $\Gamma$ ~$1.18$, $kT_{in}$ ~$1.18$ keV, and $\nu$ ~$1.38$ Hz respectively in the decline phase. The source shows maximum bolometric luminosity (L$_{bol}$) during the peak at ~$36$% of Eddington luminosity (L$_{EDD}$), and a minimum of ~$2.4$% L$_{EDD}$ during the decay phase. Further evolution of the source towards an obscured low-luminosity (L$_{bol}$ of ~ 1% L$_{EDD}$) phase, with a decrease in the intrinsic bolometric luminosity of the source due to obscuration, has also been indicated from our analysis. The implication of our results are discussed in the context of accretion disc dynamics around the black hole.

astro-ph.HE↗

arXiv2021

Revealing the nature of the transient source MAXI J0637-430 through spectro-temporal analysis

We study the spectral and temporal properties of MAXI J0637-430 during its 2019-2020 outburst using \textit{NICER}, \textit{AstroSat} and \textit{Swift-XRT} data. The source was in a disc dominant state within a day of its detection and traces out a `c' shaped profile in the HID, similar to the `mini'-outbursts of the recurrent BHB 4U 1630-472. Energy spectrum is obtained in the $0.5-10$ keV band with \textit{NICER} and \textit{Swift-XRT}, and $0.5-25$ keV with \textit{AstroSat}. The spectra can be modelled using a multicolour disc emission (\textit{diskbb}) convolved with a thermal Comptonisation component (\textit{thcomp}). The disc temperature decreases from 0.6 keV to 0.1 keV during the decay with a corresponding decrease in photon index ($\Gamma$) from 4.6 to 1.8. The fraction of Compton scattered photons ($f_{cov}$) remains $<$ 0.3 during the decay upto mid-January 2020 and gradually increases to 1 as the source reaches hard state. Power Density Spectra (PDS) generated in the 0.01-100 Hz range display no Quasi-periodic Oscillations (QPOs) although band-limited noise (BLN) is seen towards the end of January 2020. During \textit{AstroSat} observations, $\Gamma$ lies in the range $2.3-2.6$ and rms increases from 11 to 20\%, suggesting that the source was in an intermediate state till 21 November 2019. Spectral fitting with the relativistic disc model (\textit{kerrbb}), in conjunction with the soft-hard transition luminosity, favour a black hole with mass $3-19$ $M_{\odot}$ with retrograde spin at a distance $<15$ kpc. Finally, we discuss the possible implications of our findings.

astro-ph.HE↗

arXiv2020

Broadband 'spectro-temporal' features of extragalactic black hole binaries LMC X-1 and LMC X-3: An AstroSat perspective

We present the first results of extragalactic black hole X-ray binaries LMC X-1 and LMC X-3 using all the archival and legacy observations by AstroSat during the period of $2016-2020$. Broadband energy spectra ($0.5-20$ keV) of both sources obtained from the SXT and LAXPC on-board AstroSat are characterized by strong thermal disc blackbody component ($kT_{in}\sim1$keV, $f_{disc}>79\%$) along with a steep power-law ($\Gamma\sim2.4-3.2$). Bolometric luminosity of LMC X-1 varies from $7-10\%$ of Eddington luminosity ($L_{Edd}$) and for LMC X-3 is in the range $7-13\%$ of $L_{Edd}$. We study the long-term variation of the light curve using MAXI data and find the fractional variance to be $\sim25\%$ for LMC X-1 and $\sim53\%$ for LMC X-3. We examine the temporal properties of both sources and obtain fractional rms variability of PDS in the frequency range $0.002-10$ Hz to be $\sim9\%-17\%$ for LMC X-1, and $\sim7\%-11\%$ for LMC X-3. The `spectro-temporal' properties indicate both sources are in thermally dominated soft state. By modelling the spectra with relativistic accretion disc model, we determine the mass of LMC X-1 and LMC X-3 in the range $7.64-10.00$ $M_{\odot}$ and $5.35-6.22$ $M_{\odot}$ respectively. We also constrain the spin of LMC X-1 to be in the range $0.82-0.92$ and that of LMC X-3 in $0.22-0.41$ with 90\% confidence. We discuss the implications of our results in the context of accretion dynamics around the black hole binaries and compare it with the previous findings of both sources.

astro-ph.HE↗

arXiv2020

AstroSat view of GRS 1915+105 during the Soft State: Detection of HFQPOs and estimation of Mass and Spin

We report the results of AstroSat observations of GRS 1915$+$105 obtained using 100 ks guaranteed-time (GT) during the soft state. The Color-Color Diagram (CCD) indicates a variability class of $\delta$ with the detection of High Frequency QPO (HFQPO) in the power density spectra (PDS). The HFQPO is seen to vary in the frequency range of $67.96 - 70.62$ Hz with percentage rms $\sim 0.83 - 1.90$ % and significance varying from $1.63 - 7.75$. The energy dependent power spectra show that the HFQPO features are dominant only in $6 - 25$ keV energy band. The broadband energy spectra ($0.7 - 50$ keV) of SXT (Soft X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) modelled with nthComp and powerlaw imply that the source has an extended corona in addition to a compact 'Comptonizing corona' that produces high energy emission and exhibits HFQPOs. The broadband spectral modelling indicates that the source spectra are well described by thermal Comptonization with electron temperature (kT$_{\rm e}$) of $2.07 - 2.43$ keV and photon-index ($\Gamma_{\rm nth}$) between $1.73-2.45$ with an additional powerlaw component of photon-index ($\Gamma_{\rm PL}$) between $2.94 - 3.28$. The norm of nthComp component is high ($\sim 8$) during the presence of strong HFQPO and low ($\sim 3$) during the absence of HFQPO. Further, we model the energy spectra with the kerrbb model to estimate the accretion rate, mass and spin of the source. Our findings indicate that the source accretes at super-Eddington rate of $1.17-1.31~ \dot{M}_{\rm Edd}$. Moreover, we find the mass and spin of the source as $12.44 - 13.09~M_{\odot}$ and $0.990-0.997$ with $90\%$ confidence suggesting that GRS 1915$+$105 is a maximally rotating stellar mass X-ray binary black hole source.

astro-ph.HE↗

arXiv2020

AstroSat and MAXI view of the Black Hole binary 4U 1630-472 during 2016 and 2018 Outbursts

We present an in-depth spectral and timing analysis of the Black Hole binary 4U 1630-472 during 2016 and 2018 outbursts as observed by \textit{AstroSat} and \textit{MAXI}. The extensive coverage of the outbursts with \textit{MAXI} is used to obtain the Hardness Intensity Diagram (HID). The source follows a `c'-shaped profile in agreement with earlier findings. Based on the HIDs of previous outbursts, we attempt to track the evolution of the source during a `super'-outburst and `mini'-outbursts. We model the broadband energy spectra ($0.7-20.0$ keV) of \textit{AstroSat} observations of both outbursts using phenomenological and physical models. No Keplerian disc signature is observed at the beginning of 2016 outburst. However, the disc appears within a few hours after which it remains prominent with temperature ($T_{in}$) $\sim$ 1.3 keV and increase in photon index ($\Gamma$) from 1.8 to 2.0, whereas the source was at a disc dominant state throughout the \textit{AstroSat} campaign of 2018 outburst. Based on the HIDs and spectral properties, we classify the outbursts into three different states - the `canonical' hard and soft states along with an intermediate state. Evolution of rms along different states is seen although no Quasi-periodic Oscillations (QPOs) are detected. We fit the observed spectra using a dynamical accretion model and estimate the accretion parameters. Mass of the black hole is estimated using inner disc radius, bolometric luminosity and two component flow model to be $3-9$ $M_{\odot}$. Finally, we discuss the possible implications of our findings.

astro-ph.HE↗

arXiv2019

Properties of two-temperature magnetized advective accretion flow around rotating black hole

We study the two-temperature magnetized advective accretion flow around the Kerr black holes. During accretion, ions are heated up due to viscous dissipation, and when Coulomb coupling becomes effective, they transfer a part of their energy to the electrons. On the contrary, electrons lose energy due to various radiative cooling processes, namely bremsstrahlung, synchrotron, and Comtonization processes, respectively. To account for the magnetic contribution inside the disc, we consider the toroidal magnetic fields which are assumed to be dominant over other components. Moreover, we adopt the relativistic equation of state to describe the thermal characteristics of the flow. With this, we calculate the global transonic accretion solutions around the rotating black holes. We find that accretion solution containing multiple critical points may harbor shock wave provided the standing shock conditions are satisfied. Further, we investigate the shock properties, such as shock location ($x_s$) and compression ratio ($R$) that delineate the post-shock corona (hereafter PSC) and find that the dynamics of PSC is controlled by the flow parameters, such as accretion rate (${\dot m}$) and magnetic fields ($\beta$, defined as the ratio of gas pressure to the magnetic pressure), etc. Finally, we calculate the emission spectra of the accretion flows containing PSC and indicate that both ${\dot m}$ and $\beta$ play the pivotal roles in explaining the spectral state transitions commonly observed for black hole X-ray binaries.

astro-ph.HE↗