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We present the spectral and timing analyses of \textit{AstroSat} observations of the Black Hole X-ray Binary GX 339-4 when the source was close the peak of the outburst in 2024. We find that both the spectral and timing variability of the source is indicative of it in its steep power law (SPL) state during the observations. We used phenomenological and physical models to understand the physics and geometry of accretion during this spectral state of the source. Spectral fits indicate the presence of an accretion disc with a temperature of $kT\sim$0.82 keV and a hot corona with a spectral index of $\sim$2.2 along with a significant contribution from iron line emission from the accretion disc. Strong QPOs were detected at $\sim$4.6 Hz in the Power Density Spectra of the source along with a harmonics feature. Time and phase lag at the QPO frequency are studied and we find a hard lag at the QPO frequency and at the same time a soft lag at the harmonic frequency. We estimate the spin of the black hole and it was found that $a = 0.99 \pm 0.003$. The height of the coronal region is estimated to be about 2.5 $R_{g}$, which is found to be similar to that observed during the previous outbursts of the source. We attempt to discuss the possible physical scenario for the observed spectral and timing features exhibited by the source., Comment: 13 pages, 10 figures, 5 tables

We present the spectral and timing study of the bright NS-LMXB GX 5-1 using \textit{\textit{AstroSat}/LAXPC} and \textit{SXT} observations conducted in the year 2018. During the observation, the source traces out the complete HB and NB of the Z-track in the HID. Understanding the spectral and temporal evolution of the source along the 'Z' track can probe the accretion process in the vicinity of a neutron star. Spectral analysis was performed in the 0.7-20 keV energy range for different segments in the HID using a multi-temperature disc black body with an average temperature, $kT_{in} \sim$0.46 and a thermal Comptonization model. It is found that the optical depth of the corona drops from $\sim$6.68 in HB to $\sim$2.74 in NB. The Timing analysis using the LAXPC instrument indicates the presence of quasi-periodic oscillations in HB, NB, and the hard apex of the Z-track. The observed QPO frequencies are similar to the characteristic frequencies of horizontal branch and normal branch oscillations. The HBO frequency increase from $\sim$12-46 Hz towards the hard apex. The timing studies conducted in soft and hard band indicate the association of HBO and NBO origin with the non-thermal component. Further research could explore the implications of this relationship for understanding the dynamics of accretion onto neutron stars., Comment: 11 pages, 5 figures, 4 tables

We conducted a study on the X-ray polarization properties of MCG-5-23-16 by analyzing long-term monitoring data from {\it NuSTAR} jointly with {\it IXPE} observations made in May and November 2022. The re-analysis of {\it IXPE} data gives model-dependent polarization degree, PD (\%) = $1.08\pm0.66$ in the energy band $2-8$ keV. The model-independent analysis of PD poses an upper limit of $\leq3.8$ ($1\sigma$ level) for the same energy band. The observed upper limit of PD, along with broadband spectral analysis ($2-79$ keV) using an accretion-ejection based model, allowed us to derive the corona geometry (i.e. radius and height) and the accretion disk inclination ($\sim 33^\circ$). Additional {\it NuSTAR} observations were also analyzed to gain insights into the accretion flow properties of the source and to estimate the expected polarization during those epochs with PD $\sim 4.3\%$. The radius and height of the corona varies between $28.2\pm3.1 - 39.8\pm4.6$ r$_s$ and $14.3\pm1.7-21.4\pm1.9$ r$_s$ respectively, with a mass outflow rate from the corona measuring $0.14\pm0.03-0.2\pm0.03$ Eddington rate ($\dot m_{\rm Edd}$). The spectral analysis further provided an estimate for the mass of the central black hole $\sim 2\times 10^7$ M$_\odot$ and the velocity of the outflowing gas $\sim 0.16-0.19c$. A comparative broadband spectral study using reflection-based models estimates the disk inclination between $\sim 31^\circ\pm8^\circ-45^\circ\pm7^\circ$, and yields an expected PD of 3.4-6.0\%. The expected PD measured using accretion-ejection and reflection models is less compared to the expected PD measured for a given disk inclination of $45^\circ$. Our modeling of the disk-corona-outflows and polarization connection can be extended and validated with data from the recently launched \textit{XPoSat}, India's first X-ray Polarimeter Satellite, offering potential applications to other sources., Comment: 12 pages, 5 figures, 6 tables, in press

AstroSat observed transient neutron star low-mass X-ray binary XTE J1701-462 for a total duration of 135 ks during its 2022 outburst. The source traced a complete 'Z' shaped structure in the hardness intensity diagram (HID) during the observation. The source exhibited an extended horizontal branch and a short-dipping flaring branch in the HID. We find that most suitable spectral model comprises emission from a standard multi-color accretion disk and Comptonized radiation from a hot central corona. The observed disk component is cool having temperature in the range of 0.3-0.4 keV and truncated far ( ~100 - 520 km) from the compact object. The Compton corona has optical depth in the range of 3.3-5 and temperature in the range of 3.3-4.9 keV. The disk and corona flux as well as truncation radius varies significantly along the HID. We discuss the possible scenarios to explain the relationship between spectral evolution and motion of the source along the HID. The timing analysis revealed horizontal branch oscillations (HBOs) in the frequency range 34-60 Hz. The frequency and rms strength of HBO vary systematically as the source moves along the horizontal branch (HB). The observed correlation of HBO properties with the position on the HB is similar to that reported previously in this source using \textit{RXTE} data during the 2006 outburst of the source. The source also showed NBOs with frequency $\sim$ 6.7 Hz in the middle and lower normal branch. The energy dependent study of the HBO properties suggests that the HBO is stronger in the higher energy band. We also observed very-low frequency noise (VLFN) and band limited noise (BLN) components in the power-density spectra. The break frequency of BLN component was found to be tightly correlated with the HBO frequency. We discuss possible models to explain the origin and nature of the observed features in the PDS., Comment: Submitted to Publications of the Astronomical Society of Australia, 8 figures, 7 tables

We report the measurement of X-ray polarization in the high synchrotron peaked blazar 1ES 1959+650. Of the four epochsof observations from the {\it Imaging X-ray Polarimetry Explorer}, we detected polarization in the 2$-$8 keV band on two epochs. From model-independent analysis of the observations on 28 October 2022, in the 2$-$8 keV band, we found the degree of polarization of $\Pi_X$ = 9.0 $\pm$ 1.6\% and an electric vector position angle of $\psi_X$ = 53 $\pm$ 5 deg. Similarly, from the observations on 14 August 2023, we found $\Pi_X$ and $\psi_X$ values as 12.5 $\pm$ 0.7\% and 20 $\pm$ 2 deg, respectively. These values are also in agreement with the values obtained from spectro-polarimetric analysis of the I, Q, and U spectra. The measured X-ray polarization is larger than the reported values in the optical, that ranges between 2.5$-$9\% , when observed during 2008 to 2018. Broadband spectral energy distribution constructed for the two epochs are well described by the one zone leptonic emission model with the bulk Lorentz factor ($\Gamma$) of the jet larger on 14 August 2023 compared to 28 October 2022. Our results favour shock acceleration of the particles in the jet, with the difference in $\Pi_X$ between the two epochs being influenced by change in the $\Gamma$ of the jet., Comment: Accepted for publication in JOAA

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 $\nu_{\rm QPO} \sim 8-667$ 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 $\Gamma_{\rm nth} \sim 1.48 - 2.65$, $kT_{\rm e} \sim 1.62 - 3.76$ keV, $\tau \sim 8 - 20$, 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 $L_{\rm disc} \propto T_{\rm in}^\alpha$, where $\alpha = - 3.58 \pm 0.04$ for NGC 1313 X-1 and IC 342 X-1, $\alpha = - 8.93 \pm 0.11$ for NGC 6946 X-1, and $\alpha = - 10.31 \pm 0.10$ for NGC 5408 X-1. We also obtain a linear correlation between bolometric luminosity $L_{\rm bol}$ and $\Gamma_{\rm nth}$ 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 $\nu_{\rm QPO}$ 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., Comment: 21 pages, 11 figures, 6 tables, accepted for publication in MNRAS

X-ray polarimetry is a powerful tool to probe the geometry of the hot X-ray corona in active galactic nuclei (AGN). Here, we present our results on the characterisation of the X-ray polarization of the radio-quiet Seyfert-type AGN IC 4329A at a redshift of $z$ = 0.016. This is based on observations carried out by the {\it Imaging X-ray Polarimeter (IXPE)}. {\it IXPE} observed IC 4329A on January 5, 2023, for a total observing time of 458 ks. From the model-independent analysis, we found a polarization degree ($\Pi_{X}$) of 3.7$\pm$1.5$\%$ and a polarization position angle ($\Psi_{X}$) of 61$^{\circ}$$\pm$12$^{\circ}$ in the 2$-$8 keV energy range (at 1$\sigma$ confidence level). This is also in agreement with the values of $\Pi_{X}$ and $\Psi_{X}$ of 4.7$\pm$2.2$\%$ and 71$^{\circ}$ $\pm$14$^{\circ}$ respectively obtained from spectro-polarimetric analysis of the I, Q and U Stokes spectra in the 2$-$8 keV energy band (at the 90$\%$ confidence). The value of $\Pi_X$ in the 2-8 keV band obtained from the model-independent analysis is lower than the minimum detectable polarization (MDP) value of 4.5$\%$. However, $\Pi_X$ obtained from spectro-polarimetric analysis in the 2-8 keV band is larger than the MDP value. In the 3-5 keV band, we found $\Pi_X$ of 6.5 $\pm$ 1.8, which is larger than the MDP value of 5.5$\%$. The observed moderate value of $\Pi_{X}$ obtained from the analysis of the {\it IXPE} data in the 3$-$5 keV band argues against a spherical lamp$-$post geometry for the X-ray corona in IC 4329A; however, considering simulations, the observed polarization measurements tend to favour a conical shape geometry for the corona. This is the first time measurement of X-ray polarization in IC 4329A. Measurements of the X-ray polarization in many such radio-quiet AGN will help in constraining the geometry of the X-ray corona in AGN., Comment: 7 pages, 4 figures, Submitted to the Journal of Astronomy and Astrophysics

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$_{\ast}=0.927\pm0.001\,(1\sigma)$ which is corroborated by NICER observations. Finally, we discuss the implications of our findings., Comment: 6 pages, 4 figures, 4 tables, Accepted for publication in MNRAS Letters

In this paper, we performed a spectro-polarimetric analysis of the transient NS-LMXB XTE J1701-462 using IXPE, NICER, and NuSTAR data during its 2022 outburst. We report the significant detection of energy-dependent polarization in the X-ray signal from the source on 2022 September 29 in the $2-4$ keV, $4-8$ keV and $2-8$ keV energy bands with a polarization degree of $3.9 \pm 0.3$% (10.7$\sigma$), $5.5 \pm 0.6$% (9.1$\sigma$) and $4.5 \pm 0.4$% (12.6$\sigma$), respectively. The polarization angle in the overall $2-8$ keV band was found to be $\sim 143^{\circ}\pm 2^{\circ}$. The spectra were modelled using a combination of thermal emission from an accretion disc, Comptonized emission from a hot electron plasma (or corona) and a Gaussian line. From spectro-polarimetric analysis, the polarization degree due to the disc emission was found to have an upper limit of $\sim 11.5$%, and that of the Comptonized emission was constrained at $7.7 \pm 2.5$% (at the 3$\sigma$ level). The results suggest that the Comptonized component probably originates from a spreading-layer/boundary-layer above the neutron star surface. IXPE observation of the same source on 2022 October 8 does not show significant polarization which can be attributed to a weakening of the coronal Comptonized emission. The implication of these results are discussed., Comment: 6 pages, 3 figures, 4 tables; Accepted for publication in MNRAS

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., Comment: Accepted for publication in MNRAS

We have carried out the first spectro-polarimetric study of the bright NS-LMXB GX 9+9 using IXPE and AstroSat observations. We report a significant detection of polarization of $1.7\pm 0.4\%$ over the $2-8$ keV energy band, with a polarization angle of $63^{\circ}\pm 7^{\circ}$. The polarization is found to be energy-dependent, with a $3\sigma$ polarization degree consistent with null polarization in $2-4$ keV, and $3.2\%$ in $4-8$ keV. Typical of the spectra seen in NS-LMXBs, we find that a combination of soft thermal emission from the accretion disc and Comptonized component from the optically thick corona produces a good fit to the spectra. We also attempt to infer the individual polarization of these components, and obtain a $3\sigma$ upper limit of $\sim 11\%$ on the polarization degree of the thermal component, and constrain that of the Comptonized component to $\sim 3\%$. We comment on the possible corona geometry of the system based on our results., Comment: 5 pages, 6 figures, Accepted for publication in MNRAS Letters

We present the timing and spectral analysis of the Compton Thick Seyfert 2 active galactic nuclei NGC 1068 observed using {\it NuSTAR} and {\it XMM-Newton}. In this work for the first time we calculated the coronal temperature ($\rm{kT_{e}}$) of the source and checked for its variation between the epochs if any. The data analysed in this work comprised of (a) eight epochs of observations with {\it NuSTAR} carried out during the period December 2012 to November 2017, and, (b) six epochs of observations with {\it XMM-Newton} carried out during July 2000 to February 2015. From timing analysis of the {\it NuSTAR} observations, we found the source not to show any variations in the soft band. However, on examination of the flux at energies beyond 20 keV, during August 2014 and August 2017 the source was brighter by about 20\% and 30\% respectively compared to the mean flux of the three 2012 {\it NuSTAR} observations as in agreement with earlier results in literature. From an analysis of {\it XMM-Newton} data we found no variation in the hard band (2 $-$ 4 keV) between epochs as well as within epochs. In the soft band (0.2 $-$ 2 keV), while the source was found to be not variable within epochs, it was found to be brighter in epoch B relative to epoch A. By fitting physical models we determined $\rm{kT_{e}}$ to range between 8.46$^{+0.39}_{-0.66}$ keV and 9.13$^{+0.63}_{-0.98}$ keV. From our analysis, we conclude that we found no variation of $\rm{kT_{e}}$ in the source., Comment: 13 pages, 14 figures, accepted for publication in MNRAS

We performed a detailed timing study of the Atoll source 4U 1705-44 in order to understand the accretion disk geometry. Cross correlation function (CCF) studies were performed between soft (3-5 keV) and hard energy (15-30 keV) bands using the AstroSat LAXPC data. We detected hard as well as soft lags of the order of few ten to hundred seconds. A dynamical CCF study was performed in the same energy bands for one of the light curves and we found smaller lags of few tens of seconds ($<$ 50 s) suggesting that the variation is probably originating from the corona. We found a broad noise component around $\sim$ 13 Hz in the 3-10 keV band which is absent in 10-20 keV band. We interpret the observed lags as the readjustment timescales of the corona or a boundary layer around the neutron star and constrain the height of this structure to few tens of km. We independently estimated the coronal height to be around 15 km assuming that the 13 Hz feature in the PDS is originating from the oscillation of the viscous shell around the neutron star., Comment: Published in Astrophysics and Space Science

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 ($\alpha$). Using shock properties, we compute the Quasi-periodic Oscillation (QPO) frequency ($\nu_{\rm QPO}$) 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 $\nu_{\rm QPO}$ 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., Comment: 6 pages, 4 figures, Accepted for publication in MNRAS

For the first time, simultaneous broadband spectral and timing study of the atoll source 4U 1705-44 was performed using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data (0.8-70 keV). Based on the HID, the source was in the soft banana state during these observations. Spectral modeling was performed using the full reflection framework and an inner disk radii of 14 Rg was obtained. A hard powerlaw tail was noticed in the soft state and hot component fluxes and varying powerlaw indices point towards a varying corona/sub-keplerian flow. Based on the spectral fits the boundary layer radius and magnetospheric radius were constrained to be $\sim$ 14-18 km and $\sim$ 9-19 km respectively. Cross- Correlation Function studies were performed between the 0.8-3 keV soft SXT lightcurve and 10-20 keV hard LAXPC lightcurve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6-20 km (\b{eta}=0.1). Since the inner disk radius is not varying during the observations, we conclude that the detected lags are possibly caused by a varying structure of corona/boundary layer in the inner region of the accretion disk. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44., Comment: Accepted for publication in MNRAS

We present an in-depth analysis and results of eleven XMM-Newton datasets, spanning 2000 to 2016, of the anomalous X-ray Pulsar CXOU J010043.1$-$721134 which has been classified as a magnetar. We find a spin-period of 8.0275(1) s as of December 2016 and calculate the period derivative to be $(1.76\pm 0.02) \times 10^{-11}$ s s$^{-1}$, which translate to a dipolar magnetic field strength of $3.8\times 10^{14}$ G and characteristic age of $\sim 7200$ yr for the magnetar. It has a double-peaked pulse profile, with one broad and one narrow peak, in both soft ($0.3-1.3$ keV) and hard ($1.3-8$ keV) energy bands. The pulse fractions in the two energy bands are found to be consistent with constant values. These results are in agreement with previously published results for this source. Although two-component models produce acceptable fits to its energy spectra, single component models are much simpler and are able to explain the similarity of the pulse profiles in the low and high energy bands. We attempt fitting with four different single-component models and find that the best fit to the spectra is obtained by fitting a thermal Comptonization model with the photon index $(\Gamma)$ between $2.0-2.7$ and the electron temperature $(kT_e)$ between $0.5-0.9$ keV, for a seed blackbody photon distribution of 0.2 keV. Finally, we conclude by discussing our results briefly., Comment: 10 pages, 6 figures, Accepted for publication in MNRAS

We present a detailed and systematic investigation of correlated spectral and timing properties of the Z source GX 349+2, using a huge amount (about 40 ks) of good quality data from September 29, 1998 to October 13, 1998, obtained with the Proportional-Counter-Array on-board RXTE satellite. We, for the first time, give a detailed comparison between the normal-branch-properties and the flaring-branch-properties of this source, as it shows an extended normal branch (which is a rare phenomenon for it) for our case. In our work, the properties of the peaked noise are analyzed as functions of the position on the Z-track (in color-color-diagram), and they are discussed in connection with the peaked noise seen for another Z source Cyg X-2 (at low overall intensities). This will help to construct theoretical models for the peaked noise, as well as to understand the physics behind the Z-shaped tracks traced by these kind of sources in color-color-diagram. We also find a QPO (centroid frequency is about 3.8 Hz) at the upper part of the flaring branch. This is for the first time, a QPO is seen from this source and hence is very important in understanding the nature of its X-ray emitting components., Comment: 26 pages, 3 tables, 11 figures, submitted to A&A