arXiv subjects

Biki Ram

Publications and source records attributed to Biki Ram.

5 recordsLinked to original sources

arXiv2026

Investigating the disappearance of a type-C QPO in black hole X-ray binary Swift J151857-572147 with AstroSat

We present a timing and broadband spectral study of the AstroSat observation of the black hole low mass X-ray binary Swift J151857.0--572147, focusing on its evolution across two temporal segments, S1 and S2. During the first segment S1 lasting $\sim$ 50.6 ks, the source shows a clear type-C QPO at $\sim8.02$ Hz and a low-frequency bump (LFB) in the power density spectrum. By the later segment S2, the strength of the QPO feature decreases significantly and is no longer consistent with a coherent QPO, along with the simultaneous disappearance of the LFB. An energy-dependent analysis reveals that both the RMS amplitude and the time lag of the QPO in S1 increase monotonically with energy, suggesting a low inclination nature of the source. The QPO and LFB centroid frequencies show a strong positive correlation across orbital segments, with a Pearson coefficient of 0.98. Our variability analysis and hardness-intensity diagram reveal a spectral state transition of the source coincident with the turn-off of the QPO and the LFB, a conclusion supported by the evolution of broadband spectral parameters. Notably, the orbit-resolved spectral analysis reveals that the non-thermal parameters, photon index, and electron temperature of the corona exhibit significant deviation as the source traverses from S1 to S2, whereas the thermal parameters scatter without any systematic variation between the two segments. The simultaneous turn-off of the type-C QPO and the low-frequency broadband noise component, both originating from the hot inner flow, indicates a rapid reorganization of the inner accretion geometry. Our results strongly suggest a coronal origin for the QPO and demonstrate that the disappearance of both features reflects a substantial transformation of the Comptonizing region across the spectral transition.

astro-ph.HE↗

arXiv2026

Understanding corona and disk evolution in black hole X-ray binaries through a comprehensive study of their broadband variability and QPO characteristics

The shape of the power spectrum of the black hole low-mass X-ray binary evolves systematically over different spectral states during an outburst. Therefore, the power colours (ratio of the variability amplitude at different frequency ranges) and the hue parameter, quantifying the power spectral shape, can be utilised to identify the spectral states of the system. We present the comprehensive power colour analysis and subsequent identification of spectral states using the entire archival data (2016-2024) from AstroSat. We detected 29 QPOs (quasi-periodic oscillations), along with several associated harmonics and shoulders, and investigated their properties as a function of hue. We examined the evolution of the QPO RMS variability and time lag, along with hue and QPO frequency. We report a sign change in the average QPO time lag around the QPO frequency of ~2 Hz for high inclination sources, during the hard-to-hard intermediate state transition. At lower frequency, the hard lags showed an increasing trend reaching up to ~36 ms, but the soft lags above 2 Hz remained confined within ~10 ms, suggesting an evolution to a compact corona. Conversely, for low inclination sources, no such transition was found. Furthermore, for high inclination sources, the harmonic lag remains unaffected during state transition, in contrast to the QPO lag behaviour. Our results are consistent with a transition from an elongated jet-like corona to a compact corona and reveal vital clues about the dynamical evolution of the corona and disk.

astro-ph.HE↗

arXiv2024

Spectro-temporal investigation of the black hole X-ray transient 4U 1543-475 during the 2021 outburst

We report a detailed spectro-temporal analysis of the black hole (BH) low mass X-ray binary (LMXB) 4U 1543-475 during its 2021 outburst using the data from LAXPC (Large Area X-ray Proportional Counters) and SXT (Soft X-ray Telescope) instrument on board AstroSat. We investigate the energy and frequency dependency of the source variability to probe the origin of the disc/coronal fluctuations. Following the state transition (from soft to intermediate state), the emergence of a band-limited noise (BLN) component is observed along with the power law noise (PLN) when the disk is recovering from a sudden decrease in the inner disk radius. A possible correlation between the low-frequency variability amplitude (RMS) and the covering fraction of the non-thermal component is detected. During the final AstroSat observation, a flip-flop phenomenon is reported, where rapid variation in RMS occurs in concurrence with sudden flux transition. An indication of the evolution of inner disk temperature along with a significant change in thermal flux was observed during the flip-flop phase, arguing for a disk instability-driven origin for this phenomenon. Our results suggest that the long-term variability evolution is primarily affected by the coronal changes, whereas the disk behavior governs the short-term variability evolution.

astro-ph.HE↗

arXiv2023

Broadband spectro-temporal investigation of neutron star low-mass X-ray binary GX 349+2

We report a broadband investigation of the Z-type neutron star (NS) low mass X-ray binary (LMXB) GX 349+2 using AstroSat and NICER. AstroSat observed the source exhibiting large scale variability in its normal branch (NB) /flaring branch (FB) vertex and flaring branch (FB) and a moderate evolution during NICER observations. The power spectra exhibit very low-frequency noise (VLFN) and low-frequency noise (LFN)/flaring branch noise (FBN), described by a power law and an evolving Lorentzian. We investigate the energy dependence of variability components and their correlation with the spectral state to probe their origin. The joint spectra of GX 349+2 are modeled by two thermal and one non-thermal component. The source moves along the Z track, with the increasing accretion rate, further heating of the NS boundary layer, and increasing temperature/radius of the brightened hotspot at the disc-boundary layer interface/NS surface. A power law well represents the hard non-thermal coronal emission. As predicted by the gravitational redshift, we find a correlation between the line energy detected in NICER spectra and the inner disc radius with the Spearman rank correlation coefficient of 1. Using this correlation, we demonstrate the potential of a method to constrain the accreting compact object properties, including evolving continuum and line spectroscopy. We report the first detection of hard lag providing evidence of the VLFN originating from the accretion disc in NS LMXBs, representing fluctuation of propagation through the disc.

astro-ph.HE↗

arXiv2021

Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat

Broadband spectral studies of Type-I X-ray bursts can put strong constraints on the physics of burst spectra as well as their interaction with the environment. We present the results obtained from the broadband time-resolved spectroscopy of 15 thermonuclear bursts detected simultaneously from the neutron star atoll source 4U 1636-536 using LAXPC and SXT onboard AstroSat. During the observations with AstroSat, the Low mass X-ray binary (LMXB) 4U 1636-536 is observed to show a modest spectral evolution within the island state. The broadband burst spectra are observed to show an excess in addition to the thermal emission from the neutron star surface near the peak of the bursts. We investigate the interpretation of the excess observed near the peak of the burst as re-emission/reprocessing of the photons by the accretion disk/corona or scattering of the photons in the neutron star atmosphere or the enhanced persistent emission due to Poynting-Robertson drag. This is the first reported broadband simultaneous study of Type-I bursts using LAXPC and SXT onboard AstroSat. This kind of study may provide a better understanding of the burst-accretion interaction and how the bursts influence the overall accretion process contributed by the accretion disk as well as the corona.

astro-ph.HE↗