arXiv subjects

Priya Bharali

Publications and source records attributed to Priya Bharali.

5 recordsLinked to original sources

arXiv2025

QPO signatures of disk restoration after type-I X-ray bursts from 4U~1636$-$536

Type--I thermonuclear bursts (TNBs) from neutron star low-mass X-ray binaries (NS LMXBs) originate on the neutron star's surface from the unstable burning of the accreted material. On the other hand, kHz quasi-periodic oscillations (QPOs) are thought to originate in the innermost regions of the in-spiralling accretion disk. Type-I TNBs are expected to impact the inner accretion flow, and consequently the kHz QPOs, due to the intense radiation pressure. In this work, we systematically study the evolution of the upper and the lower kHz QPOs immediately before and after a Type--I TNB on 4U 1636-536 using AstroSat observations in the 3--20,keV band. The analysis of the power-density-spectra show the presence of kHz QPOs within 200,seconds before the onset of the Type--I burst. However, we have not detected any prominent signature of the same within 100--200,sec after the burst. The kHz QPOs then re-emerges after $\approx$\,200\,sec. The fractional rms variation in the 3--20\,keV band drops by $\approx$\,5--6\,\%, supporting the non-existence of kHz QPOs in the 200\,sec post-Burst Zone. The time scale of 200\,sec coincides with the viscous time scale, highlighting a scenario where the inner disk is temporarily disrupted by the intense radiation from the Type--I TNB. The kHz QPO then re-establishes as the inner disk is restored.

astro-ph.HE↗

arXiv2023

The study of thermonuclear X-ray bursts in accreting millisecond pulsar MAXI J1816-195 with NuSTAR and NICER

The millisecond pulsar MAXI J1816-195 was recently discovered in an outburst by MAXI in 2022 May. We study different properties of the pulsar using data from NuSTAR and NICER observations. The unstable burning of accreted material on the surface of neutron stars induces thermonuclear (Type-I) bursts. Several such thermonuclear bursts have been detected by MAXI J1816-195 during its outburst. We investigate the evolution of the burst profile with flux and energy using NuSTAR and NICER observations. During the NuSTAR observation, a total of four bursts were detected from the source. The duration of each burst is around $\sim$ 30 s and the ratio of peak to persistent count rate is $\sim$ 26 as seen from the NuSTAR data. The burst profiles are modelled using a sharp linear rise and exponential decay function to determine the burst timing parameters. The burst profiles show a relatively long tail at lower energies. The broadband time-resolved spectra during the burst periods are successfully modelled with a combination of an absorbed blackbody along with a non-thermal component to account for the persistent emission. From our modelling results, we are able to estimate the maximum apparent emitting area of the blackbody of the neutron star to be $\sim$12.5 km during the peak of the outburst and the maximum distance to the object to be 8.7 kpc. Our findings for the mass accretion rate and the alpha factor indicate the stable burning of hydrogen via a hot CNO cycle during the bursts.

astro-ph.HE↗

arXiv2019

Broad-band spectral study of X-ray transient MAXI J1820+070 using Swift/XRT and NuSTAR

We report on a \textit{NuSTAR} and \textit{Swift}/XRT observation of the newly discovered X-ray transient MAXI J1820+070. \textit{Swift}/XRT and \textit{NuSTAR} have concurrently observed the newly detected source on 14 March 2018. We have simultaneously fitted the broad-band spectra obtained from \textit{Swift}/XRT and \textit{NuSTAR}. The observed joint spectra in the energy range 0.6--78.0 keV are well modeled with a weak disk black-body emission, dominant thermal Comptonization and relativistic reflection fraction. We have detected a fluorescent Iron-K$α$ line relativistically broadened, and a Compton hump at $\sim$ 30 keV. We constrain the inner disk radius as well as the disk inclination angle and their values are found to be 4.1$^{+0.8}_{-0.6}$ R$_{ISCO}$ (where R$_{ISCO}\equiv$ radius of the innermost stable circular orbit) or 5.1$^{+1.0}_{-0.7}$ r$_{g}$ (where r$_{g}\equiv$ gravitational radius) and 29.8$^{+3.0}_{-2.7}$ $^\circ$ respectively. The best fit broad-band spectra suggest that the source was in the hard state and evolving. The source emission is best described by weak thermal emission along with strong thermal Comptonization from a relatively cold, optically thick, geometrically thin and ionized accretion disk. X-ray spectral modeling helps us to understand the accretion and ejection properties in the vicinity of the compact object.

astro-ph.HE↗

arXiv2019

Re-awakening of GRS 1716--249 after 23 years, observed by {\it Swift}/XRT and {\it NuSTAR}

In this work, we present a spectral and temporal analysis of {\it Swift}/XRT and {\it NuSTAR} observations of GRS 1716--249 during its recent 2016--2017 outburst. This low mass X-ray binary underwent an extraordinary outburst after a long quiescence of 23 years, since its last major outburst in 1993. The source was observed over two different epochs during 2017 April, 07 and 10. The best fit joint spectral fitting in the energy range 0.5 $-$ 79.0 keV indicates that the spectrum is best described by relatively cold, weak disk blackbody emission, dominant thermal Comptonization emission, and a relativistically broadened fluorescent iron K$α$ emission line. We observed a clear indication of a Compton hump around 30 keV. We also detected an excess feature of $\sim1.3$ keV. Assuming a lamp-post geometry of the corona, we constrained the inner disk radius for both observations to 11.92$^{+8.62}_{-11.92}$ R$_{ISCO}$ (i.e., an upper limit) and 10.39$^{+9.51}_{-3.02}$ R$_{ISCO}$ (where R$_{ISCO}\equiv$ radius of the innermost stable circular orbit) for the first epoch (E1) and second epoch (E2), respectively. A significant ($\sim5σ$) type$-$C quasi-periodic oscillation (QPO) at $1.20\pm0.04$ Hz is detected for the first time for GRS 1716--249, which drifts to $1.55\pm0.04$ Hz ($\sim6σ$) at the end of the second observation. The derived spectral and temporal properties show a positive correlation between the QPO frequency and the photon index.

astro-ph.HE↗

arXiv2017

Broadening of the thermal component of the prompt GRB emission due to rapid temperature evolution

The observations of the prompt emission of gamma ray bursts (GRB) by GLAST Burst Monitor (GBM), on board Fermi Gamma-ray Space Telescope, suggest the presence of a significant thermal spectral component, whose origin is not well understood. Recently, it has been shown that for long duration GRBs, the spectral width as defined as the logarithm of the ratio of the energies at which the spectrum falls to half its peak value, lie in the range of 0.84-1.3 with a median value of 1.07. Thus, while most of the GRB spectra are found to be too narrow to be explained by synchrotron emission from an electron distribution, they are also significantly broader than a blackbody spectrum whose width should be 0.54. Here, we consider the possibility that an intrinsic thermal spectrum from a fire-ball like model, may be observed to be broadened if the system undergoes a rapid temperature evolution. We construct a toy-model to show that for bursts with durations in the range 5-70 seconds, the widths of their 1 second time-averaged spectra can be at the most $\lesssim 0.557$. Thus, while rapid temperature variation can broaden the detected spectral shape, the observed median value of $\sim 1.07$ requires that there must be significant sub-photospheric emission and/or an anisotropic explosion to explain the broadening for most GRB spectra.

astro-ph.HE↗