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We present a comprehensive spectrotemporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival XMM–Newton observations. These sources, namely NGC1313 X−1, NGC5408 X-1, NGC6946 X−1, M82 X−1, and IC342 X−1, reveal short-term variability with fractional variance of 1.42 |$-27.28~{{\ \rm per\ cent}}\, $| and exhibit Quasi-periodic Oscillations (QPOs) with frequency νQPO ∼ 8−667 mHz. Long-term evolution of ULXs energy spectra (0.3–10 keV; excluding M82 X−1) are described satisfactorily with a model combination that comprises a thermal Comptonization component (nthComp , yielding Γnth ∼ 1.48–2.65, kT e ∼ 1.62–3.76 keV, τ ∼ 8–20, y-par∼1.16–6.24) along with a standard disc component (diskbb , kT in ∼ 0.16–0.54 keV). We find that these ULXs generally demonstrate anticorrelation between disc luminosity and inner disc temperature as |$L_{\rm disc} \propto T_{\rm in}^\alpha$|⁠ , where α = −3.58 ± 0.04 for NGC1313 X−1 and IC342 X−1, α = −8.93 ± 0.11 for NGC6946 X−1, and α = −10.31 ± 0.10 for NGC5408 X−1. We also obtain a linear correlation between bolometric luminosity L bol and Γnth that indicates spectral softening of the sources when L bol increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between |$\sim 5-20~{{\ \rm per\ cent}}\, $|⁠ , while the Comptonized flux contribution (50 |$-90~{{\ \rm per\ cent}}\, $|⁠) dominates over disc flux. Utilizing νQPO and L bol, we constrain ULXs mass by varying their spin (a k) and accretion rate (⁠|$\dot{m}$|⁠). We find that NGC6946 X−1 and NGC5408 X−1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC342 X−1 and NGC1313 X−1 can accrete in sub/super-Eddington limit irrespective to their spin values. [ABSTRACT FROM AUTHOR]

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 significant detection of energy-dependent polarization in the X-ray signal from the source on 2022 September 29 in the 2–4, 4–8, and 2–8 keV energy bands with a polarization degree of 3.9 ± 0.3, 5.5 ± 0.6, and 4.5 ± 0.4 per cent, respectively. The polarization angle in the overall 2–8 keV band was ∼143° ± 2°. 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 had an upper limit of ∼11.5 per cent, and that of the Comptonized emission was constrained at 7.7 ± 2.5 per cent (at the 3σ 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 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. [ABSTRACT FROM AUTHOR]

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 disc dominated thermal state (kT in ≈ 1.4 keV) with clear detection of absorption features at 6.69 ± 0.01 keV and 6.97 ± 0.01 keV from both NICER as well as IXPE spectra, likely indicating a coupling of disc-wind. A significant degree of polarization (PD) |$= 8.33\pm 0.17~{{\ \rm per\ cent}}$| and polarization angle (PA) =17.78° ± 0.60° 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 |$_{\ast }=0.920\pm 0.001\, (1\sigma)$| which is corroborated by NICER observations. Finally, we discuss the implications of our findings. [ABSTRACT FROM AUTHOR]

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 |$1.7\pm 0.4~{{\ \rm per\ cent}}$| over the 2–8 keV energy band, with a polarization angle of 63° ± 7°. The polarization is found to be energy-dependent, with a 3σ polarization degree consistent with null polarization in 2–4 keV, and 3.2 per cent 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 3σ upper limit of |$\sim 11~{{\ \rm per\ cent}}$| on the polarization degree of the thermal component, and constrain that of the Comptonized component to |$\sim 3~{{\ \rm per\ cent}}$|⁠. We comment on the possible corona geometry of the system based on our results. [ABSTRACT FROM AUTHOR]

We present the results on broad-band 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 modelling of both sources after eliminating the contamination in their LAXPC spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission (∼6.4 keV) models suggest that GRS 1758−258 occupies both dim-soft state (kT bb = 0.37 ± 0.01 keV, Γ ∼ 5.9, |$L_{bol}=1{{\ \rm per\ cent}}$| of Eddington luminosity L Edd) and hard state (Γ = 1.64–2.22, kT e = 4–45 keV, L bol = 1–5 per cent L Edd) that requires a multicolour disc blackbody model (kT in = 0.54 ± 0.01 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 per cent L Edd). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broad-band spectra. Our results from AstroSat and NuSTAR consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter log ξ = 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 modelling 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 ± 2°. In case of 1E 1740.7−2942, i is constrained to be 55 ± 1°. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies. [ABSTRACT FROM AUTHOR]

In this paper, we have carried out a detailed study of the 'Clocked' burster GS 1826 − 238 using ∼ 90 ks broad-band (0.7–60.0 keV) data obtained with 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 ∼ 5 to 21) and cooler (kTe decreases from ∼ 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 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 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 ∼ 1 per cent to 4.5 per cent 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. [ABSTRACT FROM AUTHOR]

In this paper, 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 BH), spin (a k), and accretion rate (⁠|${\dot{m}}$|⁠), respectively. While doing this, we adopt a recently developed effective potential to mimic the space–time 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 (νQPO) 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 νQPO and bolometric luminosity 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 harbours a massive stellar mass black hole (⁠|$M_{\rm BH} \lt 100 \, \mathrm{M}_\odot$|⁠) as indicated by the previous studies. [ABSTRACT FROM AUTHOR]

We present an in-depth analysis and results of eleven XMM–Newton data sets, spanning 2000–2016, of the anomalous X-ray Pulsar CXOU J010043.1−721134 that 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 ± 0.02) × 10−11 s s−1, which translate to a dipolar magnetic field strength of 3.8 × 1014 G and characteristic age of ∼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 (Γ) between 2.0 and 2.7, and the electron temperature (kT e) between 0.5 and 0.9 keV, for a seed blackbody photon distribution of 0.2 keV. Finally, we conclude by discussing our results briefly. [ABSTRACT FROM AUTHOR]

IGR J17091−3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as 'heartbeat', similar to the one observed in GRS 1915 + 105. In this paper, we report the results of ∼125 ks AstroSat observations of this source during the 2016 outburst. For the first time, a double-peaked QPO (DPQ) is detected in a few time segments of this source with a difference of δ f   ∼ 12 mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low-frequency (25–48 mHz) 'heartbeat' single-peak QPO (SPQ) was observed at different intervals of time along with harmonics (50–95 mHz). Broad-band spectra in the range 0.7–23 keV, obtained with Soft X-ray Telescope and Large Area X-ray Proportional Counter , could be fitted well with combination of a thermal Comptonization and a multicolour disc component model. During AstroSat observation, the source was in the soft-intermediate state (SIMS) as observed with Swift /XRT. We present a comparative study of the 'heartbeat' state variability in IGR J17091−3624 with GRS 1915 + 105. Significant difference in the timing properties is observed although spectral parameters (Γ ∼ 2.1–2.4 and T max ∼ 0.6–0.8 keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase-resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915 + 105 to that in IGR J17091−3624 as ∼25: 1. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities. [ABSTRACT FROM AUTHOR]