arXiv subjects

Anuj Nandi

Publications and source records attributed to Anuj Nandi.

At least 37 records · Page 2Linked to original sources

arXiv2024

Discovery of evolving low-frequency QPOs in hard X-rays ($\sim 100$ keV) observed in black hole Swift J1727.8-1613 with $AstroSat$

We report the first detection of evolving Low-Frequency Quasi-periodic Oscillation (LFQPO) frequencies in hard X-rays upto $100$ keV with $AstroSat/LAXPC$ during `unusual' outburst phase of Swift J1727.8-1613 in hard-intermediate state (HIMS). The observed LFQPO in $20 - 100$ keV has a centroid $ν_{_{\rm QPO}}=1.43$ Hz, a coherence factor $Q= 7.14$ and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95\%$ with significance $σ= 5.46$. Type-C QPOs ($1.09-2.6$ Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays ($80 - 100$ keV), where ${\rm rms_{_{\rm QPO}}}$ decreases ($\sim 12-3\%$) with energy. Further, $ν_{_{\rm QPO}}$ is seen to correlate (anti-correlate) with low (high) energy flux in $2-20$ keV ($15-50$ keV). Wide-band ($0.7 - 40$ keV) energy spectrum of $NICER/XTI$ and $AstroSat/LAXPC$ is satisfactorily described by the `dominant' thermal Comptonization contribution ($\sim 88\%$) in presence of a `weak' signature of disk emissions ($kT_{\rm in} \sim 0.36$ keV) indicating the harder spectral distribution. Considering source mass $M_{\rm BH}=10M_\odot$ and distance $1.5 < {\rm d~(kpc)} < 5$, the unabsorbed bolometric luminosity is estimated as $\sim 0.03-0.92\%L_{\rm Edd}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.

astro-ph.HE↗

arXiv2023

First detection of Soft-lag in GRS 1915+105 at HFQPO using AstroSat observations

The Galactic black hole GRS 1915+105 exhibits generic High-Frequency Quasi-periodic Oscillations (HFQPOs) at $\sim$ 67 Hz only during the radio-quiet 'softer' variability classes. We present the time-lag properties associated with HFQPOs in the wide energy band (3$-$60 keV) using all AstroSat observations. For the first time, we detect soft-lag of 6$-$25 keV band w.r.t 3$-$6 keV band for all 'softer' variability classes ($δ$, $ω$, $κ$ and $γ$). Moreover, our findings reveal that soft-lag increases gradually with the energy of the photons. These features are entirely opposite to the previous report of hard-lag obtained with the RXTE observations. The energy-dependent time-lag study exhibits a maximum soft-lag of $\sim$ 3 ms and $\sim$ 2.5 ms for the $δ$ and $ω$ classes respectively, whereas the $κ$ and $γ$ classes both exhibit a maximum soft-lag of $\sim$ 2.1 ms. We find a coherent lag-energy correlation for all four variability classes, where the amplitude of soft-lag increases with energy and becomes maximum at $\sim$ 18 keV. We interpret this observed soft-lag as the reflection of hard photons in the 'cooler' accretion disc. A generic lag-rms correlation implies that the soft-lag increases with the rms amplitude of the HFQPO. The wideband (0.7$-$50 keV) spectral study suggests a high value of the optical depth ($τ$ $\sim$ 6.90$-$12.55) of the Comptonized medium and the magnitude of the soft-lag increases linearly with the increase in optical depth ($τ$). We explain the observed time-lag features at the HFQPOs in the context of a possible accretion disc scenario.

astro-ph.HE↗

arXiv2023

First detection of X-ray polarization in thermal state of LMC X-3: Spectro-polarimetric study with IXPE

We report a comprehensive spectro-polarimetric study of the black hole binary LMC X$-3$ using simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR} observations in 0.5$-$20 keV energy band. The broad-band energy spectrum (0.5$-$20 keV) with {\it NICER} and {\it NuSTAR} is well described by the disc emission of temperature $\sim$1.1 keV and a weak Comptonizing tail beyond $\sim$10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of $\sim$96\%. The lack of variability ($rms$$\sim$0.5\%) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04$\pm$0.40\% (>7$σ$) at a polarization angle (PA) of $-$44.24$^{\circ}$$\pm$3.77$^{\circ}$ (>7$σ$) is found in 2$-$8 keV energy range of {\it IXPE}. In addition, PD is seen to increase with energy up to $\sim$4.35$\pm$0.98\% (>3$σ$) in 4$-$8 keV band. Further, we attempt to constrain the source spin ($a_{*}$) using broad-band spectral modelling that indicates a weakly rotating `hole' in LMC X$-3$ with $a_{*}=0.273_{-0.012}^{+0.011}-0.295_{-0.021}^{+0.008}$ ($90\%$ confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X$-3$ is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings.

astro-ph.HE↗

arXiv2023

Spectro-polarimetry of 4U 1957+115 with IXPE: Effects of spin and returning radiation on polarised emission of black hole in thermal state

We present a comprehensive spectro-polarimetric study of persistent Black hole X-ray binary 4U $1957\!+\!115$ with {\it IXPE} and {\it NICER} observations. The source is observed in disk dominated thermal state with disk temperature, kT$_{in}\approx1.4$ keV. The emission during thermal state from the source is found to be moderately polarized and \textit{IXPE} measures a degree of polarization (PD) $= 1.95\pm0.37\%$ ($>4.79σ$) along with a polarization angle (PA) $= -42.78^{\circ}\pm5.41^{\circ}$ in the energy range of $2-8$ keV. PD is found to be an increasing function of energy, whereas PA indicates switching within the energy range which could be due to high inclination and the returning radiation within the system. Simultaneous energy spectra ($0.6-10$ keV) from {\it NICER} are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.988\pm0.001\,(1σ)$, which is corroborated by {\it NICER} observations. Finally, we discuss the implications of our findings.

astro-ph.HE↗

arXiv2023

Unveiling the accretion scenario of BH-ULXs using XMM-Newton observations

We present a comprehensive spectro-temporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival {\it XMM-Newton} observations. These sources, namely NGC 1313 X-1, NGC 5408 X-1, NGC 6946 X-1, M82 X-1 and IC 342 X-1, reveal short-term variability with fractional variance of $1.42-27.28\%$ and exhibit QPOs with frequency $ν_{\rm QPO} \sim 8-667$ mHz. Long-term evolution of ULXs energy spectra ($0.3 - 10$ keV; excluding M82 X$-$1) are described satisfactorily with a model combination that comprises a thermal Comptonization component (\texttt{nthComp}, yielding $Γ_{\rm nth} \sim 1.48 - 2.65$, $kT_{\rm e} \sim 1.62 - 3.76$ keV, $τ\sim 8 - 20$, y-par $\sim 1.16 - 6.24$) along with a standard disc component (\texttt{diskbb}, $kT_{\rm in} \sim 0.16 - 0.54$ keV). We find that these ULXs generally demonstrate anti-correlation between disc luminosity and inner disc temperature as $L_{\rm disc} \propto T_{\rm in}^α$, where $α= - 3.58 \pm 0.04$ for NGC 1313 X-1 and IC 342 X-1, $α= - 8.93 \pm 0.11$ for NGC 6946 X-1, and $α= - 10.31 \pm 0.10$ for NGC 5408 X-1. We also obtain a linear correlation between bolometric luminosity $L_{\rm bol}$ and $Γ_{\rm nth}$ that indicates spectral softening of the sources when $L_{\rm bol}$ increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between $\sim 5 - 20 \%$, while the Comptonized flux contribution ($50 - 90\%$) dominates over disc flux. Utilizing $ν_{\rm QPO}$ and $L_{\rm bol}$, we constrain ULXs mass by varying their spin ($a_{\rm k}$) and accretion rate ($\dot m$). We find that NGC 6946 X-1 and NGC 5408 X-1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC 342 X-1 and NGC 1313 X-1 can accrete in sub/super-Eddington limit irrespective to their spin values.

astro-ph.HE↗

arXiv2023

Multi-mission view of low-luminosity 'obscured' phase of GRS 1915+105

GRS 1915+105 is observed in an 'obscured' phase since May 2019, exhibiting steady and low X-ray luminosities, while being intervened by sporadic re-brightenings. In this work, we perform a comprehensive and wide-band analysis of the spectral and timing properties of the source during the period $2019-2021$ using AstroSat (SXT: $0.5-8$ keV; LAXPC: $3-60$ keV), NICER ($0.5-12$ keV), and NuSTAR ($3-60$ keV) observations. Spectral analysis reveals the presence of a highly variable obscurer (N$_{H_{1}}\sim~10^{22} - 10^{24}$ atoms cm$^{-2}$) throughout the observation period. Source is detected in the Low/Hard state for most of the time, with the spectra being described by a Comptonised component ($Γ\sim 1.16 - 1.79$, kT$_{e}\sim 2-31$ keV). The source spectra steepen ($Γ\sim2.5$) indicating softening of the spectrum during the rise of the re-brightenings. Various emission and absorption lines corresponding to the neutral Fe-K$α$, Fe-XXV K$α$, Fe-XXVI K$α$, and the Ni-XXVIII K$α$ were detected with equivalent widths varying between 70 eV $-$ 3.5 keV. The column density of the absorbing plasma varied between $10^{16} - 10^{18}$ atoms cm$^{-2}$ at a distance $\leq2\times$10$^{10}$ cm. Interestingly, the source is also seen exhibiting various variability classes ($ρ, λ, δ, χ$) at relatively low luminosities ($\sim$0.01L$_{Edd}$) during the re-brightening phases. Different variability classes show signature of QPOs ($ν_{QPO}$: 20--180 mHz, rms$_{QPO}$: 7.5% - 16%). The source showed a maximum bolometric luminosity {(L$_{bol}$)} of $\sim$0.01L$_{Edd}$ (Re-brightening phases) and a minimum L$_{bol}$ of 0.004L$_{Edd}$ (Quiet phase) during the period. We discuss the possible disc dynamics around the black hole during this low-luminosity `obscured' phase.

astro-ph.HE↗

arXiv2023

IXPE and NICER view of Black hole X-ray binary 4U 1630-47: First significant detection of polarized emission in thermal state

We present a detailed spectro-polarimetric study of Black hole X-ray binary 4U $1630-47$ during its $2022$ outburst with IXPE and NICER observations. The source is observed in disk dominated thermal state (kT$_{in}\approx1.4$ keV) with clear detection of absorption features at $6.69\pm0.01$ keV and $6.97\pm0.01$ keV from both NICER as well as IXPE spectra, likely indicating a coupling of disk-wind. A significant degree of polarization(PD) $= 8.33\pm0.17\%$ and polarization angle(PA) $=17.78^{\circ}\pm0.60^{\circ}$ in the energy range of $2-8$ keV are measured with IXPE. PD is found to be an increasing function of energy whereas PA remains roughly same within the energy range. Simultaneous energy spectra from NICER in the range of $0.5-12$ keV are modelled to study the spectral properties. Furthermore, the spin parameter of the black hole is estimated with spectro-polarimetric data as a$_{\ast}=0.927\pm0.001\,(1σ)$ which is corroborated by NICER observations. Finally, we discuss the implications of our findings.

astro-ph.HE↗

arXiv2023

AOTF based spectro-polarimeter for observing Earth as an Exoplanet

Earth is the only known habitable planet and it serves as a testbed to benchmark the observations of temperate and more Earth-like exoplanets. It is required to observe the disc-integrated signatures of Earth for a large range of phase angles, resembling the observations of an exoplanet. In this work, an AOTF (Acousto-Optic Tunable Filter) based experiment is designed to observe the spectro-polarimetric signatures of Earth. The results of spectroscopic and polarimetric laboratory calibration are presented here along with a brief overview of a possible instrument configuration. Based on the results of the spectro-polarimetric calibration, simulations are carried out to optimize the instrument design for the expected signal levels for various observing conditions. The usefulness of an AOTF based spectro-polarimeter is established from this study and it is found that, in the present configuration, the instrument can achieve a polarimetric accuracy of $<0.3$\% for linear polarization for an integration time of 100 ms or larger. The design configuration of the instrument and the planning of conducting such observations from Lunar orbit are discussed.

astro-ph.IM↗

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, $Γ\sim5.9$, $L_{bol}=1 %$ of Eddington luminosity L$_{Edd}$) and hard state ($Γ=1.64-2.22$, $kT_{e}$=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$-$2942 instead is found only in hard state ($Γ$=1.67$-$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 $logξ$=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 ($Γ\sim 2-2.6$) 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 ($θ\sim 32^\circ - 40^\circ$). The accretion disk is highly ionized (log $ξ$ > 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

AstroSat view of `Clocked` burster GS 1826-238:broad-band spectral nature of persistent and burst emission during soft state

In this paper, we have carried out a detailed study of the `Clocked' burster GS $1826-238$ using $\sim$ 90 ks broad-band (0.7 - 60.0 keV) data obtained with {\it AstroSat} observatory. The source was observed during a soft spectral state and traced a `banana' type track in the colour-colour diagram (CCD). We find that a combination of thermal component (multi-colour disc/bbodyrad) and Comptonized component is statistically good description for all the sections of the track in the CCD. The corona becomes optically thick ($τ$ increases from $\sim$ 5 to 21) and cooler ($kT_e$ decreases from $\sim$ 4.8 to 2.2 keV) as the source moves up in the `banana' branch. Probably cooling is caused by increase in the supply of soft-seed photons from the disc/boundary-layer. Reflection signature is observed at upper `banana' branch of the source. Two type-I X-ray bursts are detected during the {\it AstroSat} observations. During the bursts, hard X-rays increased unlike previous observations where a reduction in hard X-rays is observed during the bursts. Decrease in the electron temperature and increase in the optical depth are observed during the bursts. The PSD (Power Spectral Density) of all the sections of the CCD can be represented by a pure power-law component. The strength of this component increases from $\sim$ 1\% to 4.5\% as the source moves up in the `banana' track. Search for burst oscillations gave a null result. We discuss the implications of our results in the context of previous findings.

astro-ph.HE↗

arXiv2022

On the origin of core radio emissions from black hole sources in the realm of relativistic shocked accretion flow

We study the relativistic, inviscid, advective accretion flow around the black holes and investigate a key feature of the accretion flow, namely the shock waves. We observe that the shock-induced accretion solutions are prevalent and such solutions are commonly obtained for a wide range of the flow parameters, such as energy (${\cal E}$) and angular momentum ($λ$), around the black holes of spin value $0\le a_{\rm k} < 1$. When the shock is dissipative in nature, a part of the accretion energy is released through the upper and lower surfaces of the disc at the location of the shock transition. We find that the maximum accretion energies that can be extracted at the dissipative shock ($Δ{\cal E}^{\rm max}$) are $\sim 1\%$ and $\sim 4.4\%$ for Schwarzschild black holes ($a_{\rm k}\rightarrow 0$) and Kerr black holes ($a_{\rm k}\rightarrow 1$), respectively. Using $Δ{\cal E}^{\rm max}$, we compute the loss of kinetic power (equivalently shock luminosity, $L_{\rm shock}$) that is enabled to comply with the energy budget for generating jets/outflows from the jet base ($i.e.$, post-shock flow). We compare $L_{\rm shock}$ with the observed core radio luminosity ($L_R$) of black hole sources for a wide mass range spanning $10$ orders of magnitude with sub-Eddington accretion rate and perceive that the present formalism seems to be potentially viable to account $L_R$ of $16$ Galactic black hole X-ray binaries (BH-XRBs) and $2176$ active galactic nuclei (AGNs). We further aim to address the core radio luminosity of intermediate-mass black hole (IMBH) sources and indicate that the present model formalism perhaps adequate to explain core radio emission of IMBH sources in the sub-Eddington accretion limit.

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

Wide-band view of High Frequency QPOs of GRS 1915+105 in 'softer' variability classes observed with AstroSat

We present a comprehensive temporal and spectral analysis of the 'softer' variability classes ($i.e.$, $θ$, $β$, $δ$, $ρ$, $κ$, $ω$ and $γ$) of the source GRS 1915+105 observed by AstroSat during $2016-2021$ campaign. Wide-band ($3-60$ keV) timing studies reveal the detection of High Frequency Quasi-periodic Oscillations (HFQPOs) with frequency of $68.14-72.32$ Hz, significance of $2.75-11 σ$, and rms amplitude of $1.48-2.66\%$ in $δ$, $κ$, $ω$ and $γ$ variability classes. Energy dependent power spectra impart that HFQPOs are detected only in $6-25$ keV energy band and rms amplitude is found to increase ($1-8\%$) with energy. The dynamical power spectra of $κ$ and $ω$ classes demonstrate that HFQPOs seem to be correlated with high count rates. We observe that wide-band ($0.7-50$ keV) energy spectra can be described by the thermal Comptonization component (nthComp) with photon index ($Γ_{\rm nth}$) of $1.83-2.89$ along with an additional steep ($Γ_{\rm PL}\sim3$) powerlaw component. The electron temperature ($kT_e$) of $1.82 -3.66$ keV and optical depth ($τ$) of $2-14$ indicate the presence of a cool and optically thick corona. In addition, nthComp components ($1.97 \lesssim Γ_{\rm nth} \lesssim 2.44$, $1.06 \times 10^{-8} \lesssim F_{\rm nth} ~({\rm erg} {\rm ~cm}^{-2} {\rm ~s}^{-1}) \lesssim 4.46\times 10^{-8}$) are found to dominate in presence of HFQPOs. Overall, these findings infer that HFQPOs are possibly resulted due to the modulation of the 'Comptonizing corona'. Further, we find that the bolometric luminosity ($0.3-100$ keV) of the source lies within the sub-Eddington ($3-34\%$ $L_{\rm Edd}$) regime. Finally, we discuss and compare the obtained results in the context of existing models on HFQPOs.

astro-ph.HE↗

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 ($δ,χ,ρ,κ,ω$ and $γ$). We also report possible transitions from one class to another ($χ\rightarrowρ,ρ\rightarrowκ$ via an 'unknown' class and $ω\rightarrowγ\rightarrowω+γ$) 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 ($Γ$) from $1.83$ to $3.8$, disc temperature ($kT_{in}$) from $1.33$ to $2.67$ keV, and Quasi-periodic Oscillation (QPO) frequency ($ν$) from $4$ to $5.64$ Hz during the rise, while the parameters decrease to $Γ$ ~$1.18$, $kT_{in}$ ~$1.18$ keV, and $ν$ ~$1.38$ 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 ($Γ$) 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, $Γ$ 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↗

arXiv2021

Multi-mission view of extragalactic black hole X-ray binaries LMC X-1 and LMC X-3: evolution of broadband spectral features

Extragalactic black hole X-ray binaries LMC X-1 and LMC X-3 are the persistent sources, usually found in soft spectral state. We present the results from spectral and timing study of these sources using MAXI, NICER, NuSTAR and AstroSat observations carried out during 2014-2020. Study of long-term MAXI lightcurve shows that the fractional variability of flux in 2-10keV is moderate ($\sim$20%) in LMCX-1 and high ($\sim$50%) in LMCX-3. The energy spectra of both sources are characterized by a disc blackbody and a Comptonization component with LMCX-1 having an additional Fe-line emission feature. NICER (0.3-10keV), NuSTAR (3.0-40keV) and AstroSat (0.5-20keV) spectral analysis collectively show that LMC X-1 remained in the soft state (disc flux contribution $f_{disc}>80$%, photon index$Γ\sim2.06-4.08$) throughout 2014-2020. Mass accretion rate, $\dot{M}$ of LMC X-1 calculated from bolometric luminosity (0.1-50keV) is found to be within $0.07-0.24\dot{M}_{Edd}$ (Eddington mass accretion rate). Although LMC X-3 remained in the soft state ($f_{disc}>95\%,Γ\sim2.3$) during most of the time, it exhibits transition into intermediate ($f_{disc}=47-73\%,Γ\sim2.02-2.36$) and hard state ($f_{disc}\sim26\%,Γ\sim1.6$). $\dot{M}$ of LMC X-3 through different spectral states varies within $0.01-0.42\dot{M}_{Edd}$. Temporal study show that the Power Density Spectra (PDS) in 0.3-10keV follow a red-noise with rms of 2% for LMC X-1 and in case of LMC X-3, it is 0.08-2.35% during the soft state, but relatively high in the intermediate(3.05-4.91%) and hard state($\sim$17.06%). From continuum-fitting method we constrain spin of LMC X-1 and LMC X-3 to be within 0.85-0.94 and 0.16-0.33 respectively and from Fe-line fitting method, spin of LMC X-1 is found to be 0.93-0.94. Finally, we discuss the implication of our findings in the context of accretion disc dynamics around the vicinity of the BHs.

astro-ph.HE↗

arXiv2021

Relativistic viscous accretion flow model for ULX sources: A case study for IC 342 X-1

In this letter, we develop a model formalism to study the structure of a relativistic, viscous, optically thin, advective accretion flow around a rotating black hole in presence of radiative coolings. We use this model to examine the physical parameters of the Ultra-luminous X-ray sources (ULXs), namely mass ($M_{\rm BH}$), spin ($a_{\rm k}$) and accretion rate (${\dot m}$), respectively. While doing this, we adopt a recently developed effective potential to mimic the spacetime geometry around the rotating black holes. We solve the governing equations to obtain the shock induced global accretion solutions in terms of ${\dot m}$ and viscosity parameter ($α$). Using shock properties, we compute the Quasi-periodic Oscillation (QPO) frequency ($ν_{\rm QPO}$) of the post-shock matter (equivalently post-shock corona, hereafter PSC) pragmatically, when the shock front exhibits Quasi-periodic variations. We also calculate the luminosity of the entire disc for these shock solutions. Employing our results, we find that the present formalism is potentially promising to account the observed $ν_{\rm QPO}$ and bolometric luminosity ($L_{\rm bol}$) of a well studied ULX source IC 342 X-1. Our findings further imply that the central source of IC 342 X-1 seems to be rapidly rotating and accretes matter at super-Eddington accretion rate provided IC 342 X-1 harbors a massive stellar mass black hole ($M_{\rm BH} < 100 M_\odot$) as indicated by the previous studies.

astro-ph.HE↗