arXiv subjects

Anuj Nandi

Publications and source records attributed to Anuj Nandi.

At least 19 recordsLinked to original sources

arXiv2026

HEL1OS on Aditya-L1 Mission: Operations, Data Processing and Monitoring of Sun in Hard X-rays

HEL1OS (High Energy L1 Orbiting X-ray Spectrometer) is the hard X-ray spectrometer on-board Aditya-L1 solar mission, monitoring the Sun continuously in the hard X-ray band of 8 keV to 150 keV from the vantage observation platform around the Sun-Earth Lagragian Point 1 (L1). HEL1OS was commissioned in end of October, 2023 and the final instrument configuration was set by end of June, 2024 after all the performance verifications. The instrument always operates in the event mode, and the down-linked data is processed in an automated pipeline to generate Science-ready products comprising mainly of solar spectra in 8 keV to 150 keV energy band. The data products of observations since December, 2023 are hosted on the internet with open access. This includes data from the performance verification phase of the payload operations. In this paper, the details of the commissioning and performance verification phases, all the stages of the fully automated data pipeline, and the data products of HEL1OS are discussed. Also a few scientific results are discussed to illustrate the timing and spectral capabilities of the instrument in monitoring Sun in the X-ray energy bands.

astro-ph.IM↗

arXiv2026

Shaping SHAPE - A spectro-polarimeter onboard Chandrayaan-3 to observe Earth as an Exoplanet

Spectro-polarimetry of HAbitable Planet Earth (SHAPE) is an experimental instrument onboard the Propulsion Module (Orbiter) of the Chandrayaan-3 mission, designed to perform disc-integrated spectro-polarimetric observations of Earth from lunar and highly elliptical Earth orbits. SHAPE is a compact, lightweight spectro-polarimeter comprising three subsystems: the Electro-Optical Detector System (EODS)-Optics, EODS-Electronics, and Radio Frequency Source (RFS). An Acousto-Optic Tunable Filter (AOTF), driven by an in-house-developed 80$-$135 MHz RF source, provides spectral filtering in the near-infrared (NIR) wavelength range of 1.0$-$1.7 $\mu$m and produces two narrow-band beams with mutually perpendicular linear polarization states. The instrument optics, with a field of view of approximately 2.6{\deg}, focus the two beams onto InGaAs detectors. A spectral resolution of 2$-$4 nm is achieved using in-house-designed low-noise front-end electronics. The instrument also incorporates processing and power electronics for signal processing, detector biasing, and subsystem control. We present the overall instrument design, results from pre-launch ground-based testing, and in-orbit operational performance. The current configuration enables SHAPE to measure disc-integrated signatures of Earth over a range of phase angles, providing a test bed for characterizing Earth-like exoplanets and benchmarking future exoplanet observations.

astro-ph.IM↗

arXiv2026

Importance and Science Outcomes from the first XSPECT/XPoSat Workshop

This paper summarizes the science outcomes of the first Workshop on Data Analysis using observations from the XSPECT payload onboard the XPoSat, which brought together early-career researchers and experts to explore the instrument's scientific capabilities through lectures and hands-on analyses. Participants performed end-to-end data analysis, including calibration, spectral modeling, and timing studies, on seven sources comprising Neutron Star Low-Mass X-ray Binaries, pulsars, and Black Hole X-ray Binaries, demonstrating the instrument's scientific potential. The observations, obtained during the first year of XSPECT operations, together with in-house developed software, were provided to the participants, making them the first users outside the instrument team to analyze XSPECT data. For NS-LMXBs, Aql X-1 exhibited a classical Type-I X-ray burst, enabling constraints on the stellar radius through spectral fitting. Sco X-1, observed across its complete Z-track, revealed systematic spectral evolution driven by accretion-rate fluctuations and disk-corona coupling, while Cir X-1 displayed orbital phase-dependent transitions between hard and soft states, reflecting changes in accretion geometry. Among accretion-powered pulsars, GX 301-2 showed a double-peaked, energy-dependent pulse profile and strong iron fluorescence lines due to stellar wind reprocessing, whereas Vela X-1 exhibited orbital phase-dependent absorption and steady coronal temperatures. Among BH-XRBs, Cyg X-1 transitioned from a hard to soft-intermediate state with increasing disk contribution and spectral softening, while Cyg X-3 remained in the intermediate state with multiple emission lines originating from a clumpy stellar wind. The workshop outcomes highlight the scientific promise of XSPECT and the importance of collaborative training in maximizing the science from XSPECT and future Indian space astronomy missions.

astro-ph.HE↗

arXiv2026

Multimission Observations of GS 1354-64 during the 2025-2026 Outburst: First Results

The Galactic transient black hole GS 1354-64 recently showed a new outburst, which has been continuously monitored by {\it MAXI}, {\it NuSTAR} and {\it Insight-HXMT} missions. The ongoing $2025$--$2026$ outburst shows a slow rise with an unusually short period ($\sim 3$~days) of X-ray flare of peak flux $\sim 1.4$~Crab, followed by another relatively weak flare of intensity $\sim 0.8$ Crab. The source was observed to evolve through ``canonical'' spectral states in a hardness-intensity diagram (HID) during rising phase of the outburst, but the subsequent outburst profile during this study, the source did not follow the reverse trend of the HID. A rapid evolution of quasi-periodic oscillation (QPO) frequencies ($\sim 0.07-4$ Hz) is observed during hard and intermediate spectral states without any signature of QPOs in the soft state. The evolution of the observed low frequency QPOs shows a monotonically increasing (rising phase) signature as well as decreasing (decay phase) one, these are further studied with the propagating oscillatory shock model to understand the nature of the evolution of the shock wave responsible for the origin of the observed QPOs. The broadband energy spectra from {\it NuSTAR} ($3-70$ keV) and {\it Insight-HXMT} ($2-60$ keV) are well described by thermal (\textit{diskbb}) and reflection (\textit{relxill}) model components, indicating a strong signature of a relativistic reflection feature. Using ``canonical'' observations of a softer state, we found the source to be maximally rotating with $a_k \sim 0.998$ and inclination angle to be as $i \sim 63^\circ - 70^\circ$, which are consistent with earlier reports.

astro-ph.HE↗

arXiv2026

Time-Lag properties associated with LFQPO in X-ray variability classes of GRS 1915+105: Findings from AstroSat

We present a comprehensive analysis of Low Frequency Quasi-periodic Oscillation (LFQPO) associated time-lags in the persistently variable black hole binary GRS 1915+105 using 441 ks of \textit{AstroSat} observations from March 2016 to March 2019. LFQPO frequency ($1.38-7.38$ Hz) are detected across the $\theta$, $\beta$, $\rho$, and $\chi$ classes, with the $\chi$ class further subdivided into $\chi_1$, $\chi_2$, $\chi_3$, and $\chi_4$ based on spectro-temporal characteristics. Class transitions occur on timescales of a few hours, appearing either as a simultaneous increase in X-ray count rate and QPO frequency, or vice versa, indicating rapid changes in the accretion flow geometry. The $\text{rms}_{\rm QPO}$ increases with QPO frequency up to $\sim 3.4$ Hz and declines at higher frequencies, a trend similar to \textit{RXTE} observations, where peak occurred at $\sim 2$ Hz. Spectro-temporal correlations reveal that increasing $F_{\rm Comp}$ drives higher $\text{rms}_{\rm QPO}$ and decreases the soft-lag magnitude, while $\nu_{\rm QPO}$ and $\Gamma$ also decline, suggesting that the observed time lag may result from the combined effects of multiple physical mechanisms. The consistent increase of $\text{rms}_{\rm QPO}$ with $F_{\rm Comp}$ provides clear evidence that modulated Comptonized photons enhance the rms power ($\text{rms}_{\rm QPO}$). Moreover, the soft-lag ($1.59-13.49$ ms) observed across all QPO frequencies, without the sign reversal at $\sim$ 2 Hz observed in \textit{RXTE} observations, is interpreted within the framework of a dynamical accretion disk model around the black hole.

astro-ph.HE↗

arXiv2025

HEL1OS -- A Hard X-ray Spectrometer on Board Aditya-L1

HEL1OS (High Energy L1 Orbiting X-ray Spectrometer) is one of the remote sensing payloads on board Aditya-L1 mission designed to continuously monitor and measure the time-resolved spectra of solar flares between 8 keV and 150 keV. This broad energy range has been covered by using compound semiconductor detectors: cadmium telluride (CdTe: 8 - 70 keV) and cadmium zinc telluride (CZT: 20 - 150 keV) with geometric areas of 0.5 cm$^2$ and 32 cm$^2$, respectively. A stainless steel collimator provides a field-of-view of 6$^\circ$ $\times$ 6$^\circ$ optimized to limit the off-axis response while keeping the design within the instrument mass constraints. The in-house designed low-noise digital pulse processing-based front-end electronics has achieved a spectral resolution of $\approx$ 1 keV at 14 keV (CdTe) and $\approx$ 7 keV at 60 keV (CZT). The instrument is also equipped with processing and power electronics to process the signal, drive the electronics, bias the detectors with required low and high voltages for optimal performance of the overall system. In this article, we present design aspects of the instrument, results from the pre-launch ground-based tests, and the in-orbit operations, which have indicated optimal performance in line with that expected.

astro-ph.SR↗

arXiv2025

Possible detection of HFQPOs associated with 'unknown' variability class of GRS 1915+105

We present a comprehensive spectro-temporal analysis of GRS $1915+105$ observed with AstroSat during June, $2017$. A detailed study of the temporal properties reveals the appearance of an `unknown' variability class ($\tau$) during $\rho \rightarrow \kappa$ class transition of the source. This new `unknown' class ($\tau$) is characterized by the irregular repetition of low count `dips' along with the adjacent `flare' like features in between two successive steady count rate durations, resulting in uniform `$C$' shaped distribution in the color-color diagram. A detailed comparative study of the variability properties between the $\tau$ class and other known variability classes of GRS $1915+105$ indicates it as a distinct variability class of the source. Further, we find evidence of the presence of possible HFQPO features at $\sim 71$ Hz with quality factor $\sim 13$, rms amplitude $\sim 4.69\%$, and significance $3\sigma$, respectively. In addition, a harmonic-like feature at $\sim 152$ Hz is also seen with quality factor $\sim 21$, rms amplitude $\sim 5.75\%$ and significance $\sim 4.7\sigma$. The energy-dependent power spectral study reveals that the fundamental HFQPO and its harmonic are present in $3-15$ keV and $3-6$ keV energy ranges, respectively. Moreover, the wide-band ($0.7-50$ keV) spectral modelling comprising of thermal Comptonization component indicates the presence of a cool ($kT_{\rm e}\sim 1.7$ keV) and optically thick (optical depth $\sim 14$) Comptonizing `corona', which seems to be responsible in regulating the HFQPO features in GRS $1915$+$105$. Finally, we find the bolometric luminosity ($L_{\rm bol}$) to be about $42\% L_{\rm Edd}$ within $1-100$ keV, indicating the sub-Eddington accretion regime of the source.

astro-ph.HE↗

arXiv2025

Revisiting Disc-Jet Coupling in Black Hole X-ray Binaries: On the Nature of Disc Dynamics and Jet Velocity

We perform a comprehensive wide-band ($3-100$keV) spectro-temporal analysis of 13 outbursting BH-XRBs, using data (quasi)simultaneous with radio observations to unravel the complex disc-jet connection. RXTE observations are analyzed for XTEJ1859+226, GX339-4 (2002, 2006, and 2010 outbursts), 4U1543-47, H1743-322 (2003 and 2009 outbursts), XTEJ1550-564, XTEJ1752-223, XTEJ1650-500, SwiftJ1753.5-0127, XTEJ1748-288, and GROJ1655-40. For SwiftJ1727.8-1613 and MAXIJ1535-571, we utilize HXMT data, while both AstroSat and HXMT observations are analyzed for SwiftJ1658.2-4242. Type-C QPOs observed in harder states (LHS, HIMS; $F_{nth}\ge0.4$) exhibit positive lag for low-inclination sources ($i<50^{\circ}$), whereas it generally exhibits negative lag for high-inclination sources ($i>60^{\circ}$), except XTEJ1550-564, SwiftJ1727.8-1613, H1743-322 (2003 outburst) and GROJ1655-40. Notably, type-A QPOs exhibit negative lags ($\sim1-10$ms) regardless of source inclination, while type-B QPOs show positive lags in low-inclination sources, and both positive and negative lags ($\sim1-15$ms) in high-inclination sources, typically occurring in SIMS ($F_{nth}\lesssim0.45$). Systematic appearance of type-A QPOs preceding radio flares in several sources suggests that type-A QPOs indicate telltale signs of jet ejection, while type-B QPOs are closely linked with radio flares (i.e., transient jets). Present findings suggest the corona evolves from a radially extended to a vertically elongated structure during the type-C to type-B transition via type-A QPOs, with type-B QPOs linked to radially compact or vertically extended coronal geometries, resembling jet ejection. The strong radio-X-ray luminosity correlation seems to provide compelling evidence of accretion-powered jets. Finally, we find that jets in SIMS are moderately relativistic in nature with velocities $\gtrsim 0.3-0.8c$ in BH-XRBs under consideration.

astro-ph.HE↗

arXiv2025

Probing the accretion geometry of black hole X-ray binaries: A multi-mission spectro-polarimetric and timing study

We present a comprehensive spectro-polarimetric and timing analysis of twelve black hole X-ray binaries, namely Cyg X-1, 4U 1630-47, Cyg X-3, LMC X-1, 4U 1957+115, LMC X-3, Swift J1727.8-1613, GX 339-4, Swift J151857.0-572147, IGR J17091-3624, MAXI J1744-294 and GRS 1915+105, using quasi-simultaneous observations from {\it{IXPE}}, {\it{NICER}}, {\it{NuSTAR}}, and {\it{AstroSat}}. Timing analyses reveal type-B and type-C Quasi-periodic Oscillations across different spectral states, often associated with episodic radio ejections. Broadband ($0.7-60$ keV) spectral modeling, employing disc, Comptonization, and reflection components, reveals degeneracies in constraining disc-corona geometries. Polarimetric measurements in $2-8$ keV band detect significant polarization degrees (PDs) ranging from $3-20.6\%$ ($1.2-21.4\%$) in harder (softer) states, with moderate to strong energy dependence, except for LMC X$-1$, Swift J$151857.0-572147$, and MAXI J1744$-$294, where no significant polarization is detected. We report polarization detections of Cyg X$-3$ (PD $\sim 21.4\%$, SIMS), LMC X$-3$ (PD $\sim 2.4\%$, HSS) and IGR J$17091-3624$ (PD $\sim 9\%$, LHS) using recent {\it IXPE} observations. A positive correlation is found between PD and Comptonized photon fraction ($cov_{\rm frac}$), while an anti-correlation is observed with disc-to-Comptonized flux ratio ($F_{\rm ratio}$) across spectral states. Combined timing, spectral, and polarimetric results, together with constraints from radio jet observations, suggest a radially extended corona within a truncated disc for Cyg X$-1$, Swift J$1727.8-1613$, IGR J$17091-3624$, and GX $339-4$, whereas the disc-corona geometry remains poorly constrained for 4U $1957+115$, LMC X$-3$, and 4U $1630-47$. We discuss the implications of these findings for understanding accretion geometries and highlight prospects for future X-ray polarimetric studies.

astro-ph.HE↗

arXiv2025

Evidence of oscillating `compact' Comptonized corona in GRS 1915+105: Insights into HFQPOs with AstroSat

We present, for the first time, an in-depth dynamical analysis of the spectro-temporal properties of the soft variability classes ($\delta$, $\kappa$, $\omega$, and $\gamma$) of GRS 1915+105 during the detection of $\sim$70 Hz High-Frequency Quasi-periodic Oscillations (HFQPOs) using AstroSat data. The wide-band spectra ($0.7-50$ keV) are well described by thermal Comptonization along with an extended power-law component. Additionally, power spectra ($0.01-500$ Hz) indicate that Comptonized photons ($6-25$ keV) primarily contribute to the HFQPOs. Our findings reveal that high (low) count rates referred to as `non-dips' (`dips') in the light curves of the variability classes correspond to the detection (non-detection) of HFQPOs. Accumulated `non-dips' (`dips') spectra are modelled separately using thermal Comptonization (\texttt{nthComp}) as well as \texttt{kerrd} which indicates harder spectra and smaller inner disc radius during the detection of HFQPOs. We conduct dynamical analyses (every 32 s) to trace the presence of HFQPOs, and variations in thermal Comptonization parameters ($\Gamma_{\rm nth}$ and ${\rm N}_{\rm nth}$). Moreover, we observe a positive correlation of `non-dips' with QPO strength, ${\rm HR}1$, and ${\rm N}_{\rm nth}$, while $\Gamma_{\rm nth}$ shows an anti-correlation, suggesting that high-energy photons from the Comptonized corona are responsible for the HFQPOs. Furthermore, we estimate the size of the Comptonized corona using \texttt{kerrd} and \texttt{diskpn} to be $\sim 2.8 - 16$ $r_{\rm g}$. Thus, we infer that a `compact' oscillating corona likely modulates the high-energy radiation, exhibiting the $70$ Hz HFQPOs in GRS 1915$+$105.

astro-ph.HE↗

arXiv2025

Decoding the Origin of HFQPOs of GRS 1915+105 during 'Canonical' Soft States: An In-depth View using Multi-mission observations

We present a comprehensive analysis of the 'canonical' soft state ($\gamma$, $\delta$, and $\phi$ spectral variability classes) of the black hole binary GRS 1915+105, using RXTE, AstroSat, and NuSTAR data from 1996 to 2017 to investigate the origin of High Frequency Quasi-periodic Oscillations (HFQPOs). Our findings reveal that HFQPOs occur only in the $\gamma$ and $\delta$ classes, with frequencies of $65.07-71.38$ Hz and are absent in the $\phi$ class. We observe an evolution of time-lag from hard-lag (1.59$-$7.55 ms) in RXTE to a soft-lag (0.49$-$1.68 ms) in AstroSat observations. Wide-band (0.7$-$50 keV) spectral modelling suggests that HFQPOs are likely observed with a higher covering fraction ($f_{cov} \gtrsim 0.5$), i.e., the fraction of seed photons being Comptonized in the corona, enhanced Comptonized flux ($\sim$ 38%), and lower optical depth ($\tau \lesssim 8.5$ ) in contrast to observations where HFQPOs are absent. We observed similar constraints for observing HFQPOs during an inter-class ($\phi \rightarrow \delta$) transition as well as in a few intra-class ($\delta \rightarrow \delta$) variations. We also find that the time-lag decreases as $\tau$ increases, indicating that a higher $\tau$ reduces Compton up-scattering, thereby decreasing the hard-lag. Interestingly, in RXTE observations, the hard-lag ($\sim$ 7 ms) gradually decreases as optical depth and Comptonization ratio increases, eventually becoming a soft-lag ($\sim$1 ms) in AstroSat observations. These constraints on spectro-temporal parameters for the likelihood of observing HFQPOs support a 'compact' coronal oscillation mechanism for generating HFQPOs, which we attempt to explain within the framework of a possible accretion scenario.

astro-ph.HE↗

arXiv2025

Long-term evolution of BH-ULX candidates: An `unusual' $L_{\rm disc}$-$T_{\rm col}$ correlation associated with spectral states

We present the long-term spectral evolution of eight black hole ultra-luminous X-ray sources (BH-ULXs), namely NGC1313 X-1, NGC5408 X-1, NGC6946 X-1, IC342 X-1, NGC55 ULX1, NGC4395 ULX1, NGC5204 X-1 and NGC4190 ULX1 using {\it XMM-Newton} monitoring data spanning over a decade or more. An in-depth spectral modeling with thermal Comptonization ({\it nthComp}) and standard disc ({\it diskbb}) components reveals NGC5204 X-1, IC342 X-1, NGC4190 ULX1 and NGC1313 X-1 exhibiting harder spectral characteristics with dominant effect of Comptonization ($F_{nth}>F_{disc}$, $\Gamma_{nth}\lesssim2$). However, NGC6946 X-1 and NGC55 ULX1 remain in a disc-dominated state ($F_{disc}\sim2F_{nth}$, $\Gamma_{nth}\gtrsim2$), while NGC5408 X-1 shows intermediate spectral characteristics. The spectral analyses indicate an anti-correlation between disc luminosity ($L_{disc}$) and temperature ($T_{col}$) for all sources except NGC5204 X-1. These anti-correlations follow a relation $L_{disc}\propto T_{col}^{\alpha}$ with steeper exponents of $\alpha=-6.01\pm0.25$ (NGC55 ULX1), $-8.93\pm0.11$ (NGC6946 X-1), and $-10.31\pm0.10$ (NGC5408 X-1) for sources with softer or intermediate spectral characteristics. For harder sources, NGC1313 X-1 and IC342 X-1, the combined results provide $\alpha=-3.58\pm0.04$. However, for NGC5204 X-1, a positive correlation is found, yielding $\alpha=1.4\pm0.1$, suggesting that the emission mechanism is associated with the transition from the `standard disc' to the `slim disc' scenario. These findings suggest that the observed $L_{disc}-T_{col}$ correlations, along with the overall spectro-temporal properties of BH-ULXs, seems to be governed by disc-corona-wind driven accretion processes at various inclinations. Finally, we report a QPO-like feature ($\sim20$ mHz) with $rms\%\sim6.6$, Q-factor $\sim6.7$ and significant $2.8\sigma$ in NGC55 ULX1.

astro-ph.HE↗

arXiv2024

SHAPE -- A Spectro-Polarimeter Onboard Propulsion Module of Chandrayaan-3 Mission

SHAPE (Spectro-polarimetry of HAbitable Planet Earth) is an experiment onboard the Chandrayaan-3 Mission, designed to study the spectro-polarimetric signatures of the habitable planet Earth in the near-infrared (NIR) wavelength range (1.0 - 1.7 $\mu$m). The spectro-polarimeter is the only scientific payload (experimental in nature) on the Propulsion Module (PM) of the Chandrayaan-3 mission. The instrument is a compact and lightweight spectro-polarimeter with an Acousto-Optic Tunable Filter (AOTF) at its core. The AOTF operates in the frequency range of 80 MHz to 135 MHz with a power of 0.5 - 2.0 Watts. The two output beams (e-beam and o-beam) from the AOTF are focused onto two InGaAs detectors (pixelated, 1D linear array) with the help of focusing optics. The primary (aperture) optics, with a diameter of $\sim$2 mm, collects the NIR light for input to the AOTF, defining the field of view (FOV) of 2.6$^\circ$. The payload has a mass of 4.8 kg and operates at a power of 25 Watts. This manuscript highlights some of the ground-based results, including the post-launch initial performance of the payload while orbiting around the Moon to observe Earth.

astro-ph.IM↗

arXiv2024

Accretion disc dynamics in extragalactic black hole X-ray binaries: A comprehensive study of M33 X-7, NGC 300 X-1 and IC 10 X-1

Extragalactic Black Hole X-ray Binaries (BH-XRBs) are the most intriguing X-ray sources as some of them are `home' to the most massive stellar-mass BHs ever found. In this work, we conduct a comprehensive study of three massive, eclipsing extragalactic BH-XRBs i.e., M33X-7, NGC300X-1, and IC10X-1 and using entire X-ray observations available from \textit{XMM-Newton} and \textit{NuSTAR} till date. Preliminary analysis using \textit{diskbb} and \textit{powerlaw} models shows that the sources have steep spectra and sub-Eddington luminosities (L<0.69 L$_{Edd}$), with major flux contribution from non-thermal component, resembling the relatively uncharted Steep Powerlaw State (SPL). To understand the accretion disc properties in this state, we explore alternate modelling scenario that reveals the presence of a `hot' ($kT_{in}=1-2$ keV) slim-disc (\textit{diskpbb}) with radial temperature profile $T(r)\propto r^{-p}$ ($p=0.5-0.66$), along with a cooler ($kT_{in}=0.1-0.2$ keV) standard thermal disc (\textit{diskbb}). We carry out the continuum-fitting method using relativistic slim-disc model (\textit{slimbh}) and estimate the mass range of M33 X-7, NGC300X-1 and IC10X-1 is to be 9$-$15 M$_{\odot}$, 9$-$28 M$_{\odot}$ and 10$-$30 M$_{\odot}$, respectively. Further, eclipse periods are determined by modelling the lightcurve, using which we estimate the size of the eclipsing bodies. Modelling of the eclipse spectra revealed the complete obscuration of soft spectral component during eclipse, implying the emission of hard component from an extended accretion region. Based on our findings, we provide an inference on geometry of accretion disc in these wind-fed systems and compare their properties with the other two extragalactic BH-XRBs.

astro-ph.HE↗

arXiv2024

Long term wide-band spectro-temporal studies of outbursting black hole candidate sources GX 339-4 and H 1743-322: AstroSat and NuSTAR results

We present a comprehensive spectro-temporal analyses of recurrent outbursting black hole sources GX 339$-$4 and H 1743$-$322 using available AstroSat and NuSTAR archival observations during 2016-2024. The nature of the outburst profiles of both sources are examined using long-term MAXI/GSC and Swift/BAT lightcurves, and failed as well as successful outbursts are classified. Wide-band (0.5-60 keV) spectral modelling with disc (diskbb) and Comptonized (Nthcomp) components indicates that GX 339-4 transits from hard ($kT_{bb}=0.12-0.77$ keV, $\Gamma_{nth}=1.54-1.74$ and $L_{bol}=0.91-11.56$\% $L_{Edd}$) to soft state ($kT_{in}~[\approx{kT}_{bb}]=0.82-0.88$ keV, $\Gamma_{nth}=1.46-3.26$, $L_{bol}=19.59-30.06\%L_{Edd}$) via intermediate state ($kT_{in}~[\approx{kT}_{bb}]=0.56-0.88$ keV, $\Gamma_{nth}=1.76-2.66$, $L_{bol}=2.90-16.09\%L_{Edd}$), whereas H 1743-322 transits from quiescent to hard state ($\Gamma_{nth}=1.57-1.71$, $L_{bol}=2.08-3.48\%L_{Edd}$). We observe type-B and type-C Quasi-periodic Oscillations (QPOs) in GX 339-4 with increasing frequencies ($0.10-5.37$ Hz) along with harmonics. For H 1743-322, prominent type-C QPOs are observed in frequency range 0.22-1.01 Hz along with distinct harmonics. Energy-dependent power spectral studies reveal that fundamental QPO and harmonics disappear beyond 20 keV in GX 339-4, whereas fundamental QPO in H 1743-322 persists up to 40 keV. We also observe that type-C $QPO_{rms}\%$ decreases with energy for both sources although such variations appear marginal for type-B QPOs. Additionally, we report non-monotonic behavior of photon index with plasma temperature and detection of annihilation line. Finally, we discuss the relevance of the observational findings in the context of accretion dynamics around black hole binaries.

astro-ph.HE↗

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 $\nu_{_{\rm QPO}}=1.43$ Hz, a coherence factor $Q= 7.14$ and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95\%$ with significance $\sigma = 5.46$. 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, $\nu_{_{\rm QPO}}$ 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↗

arXiv2024

Spectro-polarimetric study to constrain accretion-ejection properties of MCG-5-23-16 using IXPE and NuSTAR observations

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.

astro-ph.HE↗

arXiv2024

First detection of X-ray polarization in Galactic ULX pulsar Swift J0243.6$+$6124 with {\it IXPE}

We report the results of first ever spectro-polarimetric analyses of the Galactic ultra-luminous X-ray pulsar Swift J0243.6$+6124$ during the 2023 outburst using quasi-simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR} observations. A pulsation of period $\sim 9.79$ s is detected in {\it IXPE} and {\it NuSTAR} observations with pulse fractions (PFs) $\sim 18\%$ ($2-8$ keV) and $\sim 28\%$ ($3-78$ keV), respectively. Energy dependent study of the pulse profiles with {\it NuSTAR} indicates an increase in PF from $\sim 27\%$ ($3-10$ keV) to $\sim 50\%$ ($40-78$ keV). Further, epoch-dependent polarimetric measurements during the decay phase of the outburst confirm the detection of significant polarization, with the polarization degree (PD) and polarization angle (PA) ranging between $\sim 2 -3.1\%$ and $\sim8.6^{\circ}-10.8^{\circ}$, respectively, in the $2-8$ keV energy range. We also observe that the PD increases up to $\sim 4.8\%$ at higher energies ($\gtrsim 5$ keV) with dominating \texttt{bbodyrad} flux contribution ($1.5 \lesssim F_{\rm BB}/F_{\rm PL} \lesssim 3.4$) in the {\it IXPE} spectra. The phase-resolved polarimetric study yields PD as $\sim 1.7-3.1\%$ suggesting a marginal correlation with the pulse profiles. Moreover, the broad-band ($0.6-70$ keV) energy spectrum of combined {\it NICER} and {\it NuSTAR} observations is well described by the combination of \texttt{bbodyrad} and \texttt{cutoffpl} components with seed photon temperature ($kT_{\rm bb}$) $\sim 0.86 \pm 0.03$ keV and photon index ($\Gamma$) $\sim 0.98 \pm 0.01$. With the above findings, we infer that the observed `low' PD in Swift J0243.6$+6124$ is attributed possibly due to `vacuum resonance' effect between the overheated and relatively cooler regions of the neutron star boundary layer.

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