arXiv subjects

Koushal Vadodariya

Publications and source records attributed to Koushal Vadodariya.

5 recordsLinked to original sources

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

Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT

XSPECT instrument on-board XPoSat mission is a soft X-ray spectrometer sensitive in the energy band 0.8$-$15 keV. XSPECT has observed several bright neutron star low mass X-ray binaries since launch. Two well known sources, Aql X-1 and 4U 1608-52 which are soft X-ray transients, were observed by XPoSat during the decay phase of their recent outbursts in September 2024 and February 2025 respectively. During XSPECT observations, 4U 1608-52 exhibited a superburst which is a long duration thermonuclear burst, believed to be triggered by carbon burning. We carry out a detailed spectro-temporal analysis of the superburst, tracing its onset, rise, and decay over the next several hours. Using time-resolved spectroscopy, we probe the spectral evolution of the source and find that the persistent emission is suppressed during the superburst and the emission can be described by a gradually cooling blackbody component. The superburst was preceded by a precursor burst which is a normal type-I X-ray burst. We also observe a type-I burst $\sim 5$ days after the superburst, indicating resumption of burst activities which is typically quenched after a superburst. Aql X-1 also exhibited a type-I burst during XSPECT observations. The persistent emission of both the sources can be fitted using a combination of blackbody and disk blackbody emission or, alternatively, using a disk Comptonized by an optically thick plasma. Using the latter model, we find a clear flux dependence of the Comptonization parameters, with both the sources exhibiting harder spectra at higher accretion rates.

astro-ph.HE↗

arXiv2025

XSPECT on-board XPoSat: Calibration and First Results

XPoSat is India's first X-ray spectro-polarimetry mission, consisting of two co-aligned instruments, a polarimeter (POLIX) and a spectrometer (XSPECT), to study the X-ray emission from celestial sources. Since polarimetry is a photon-hungry technique, the mission is designed to observe sources for long integration times (~ few days to weeks). This provides an unique opportunity, enabling XSPECT to carry out long-term monitoring of sources, and study their spectro-temporal evolution. To ensure that the instrument is able to fulfill its scientific objectives, it was extensively calibrated on-ground. Post launch, these calibrations were validated using on-board observations. Additionally, some aspects of the instrument such as alignment and effective area were also derived and fine-tuned from in-flight data. In this paper, we describe the calibration of XSPECT instrument in detail, including some initial results derived from its data to establish its capabilities.

astro-ph.IM↗

arXiv2025

Spectral nature of Sco X-1 observed using the X-ray SPECtroscopy and Timing (XSPECT) payload on-board XPoSat

Scorpius X-1 is the brightest and first discovered X-ray source in the sky. Studying this source in the low-energy band has been challenging in the past due to its high brightness. However, with the X-ray SPECtroscopy and Timing (XSPECT) payload on-board Indias first X-ray Polarimetry Satellite (XPoSat), we have the capability to study the source despite its very high brightness, thanks to the fast (1 ms) readout of the instrument. We investigate the evolution of the spectral and timing properties of Sco X-1 across the horizontal, normal, and flaring branch, as observed with XSPECT. We examine changes in the spectral parameters as a function of position on the color-color diagram (CCD). Spectral studies indicate that the soft X-ray emission can be modeled using a multicolor disk component, with the inner disk temperature ranging from 0.6 to 0.8 keV. The hard component is described by a Comptonized continuum using either the nthComp or Comptb model with electron temperatures from 2.4 to 4.7 keV and optical depth between 5 and 14. Additionally, we observe the presence of an iron K-alpha line at 6.6 keV and an iron K-beta line at 7.6 keV. Both spectral models suggest a steep rise in Comptonization flux as well as disk flux in the flaring branch. An increase in neutron star blackbody temperature and inner disk temperature are also observed during flaring. The Z-track is driven by changes in the optical depth of the corona, the Comptonization flux and the disk flux and the inner disk temperature. No quasi-periodic oscillations are detected in any branch, suggesting their association with the high-energy spectrum.

astro-ph.HE↗

arXiv2025

X-Ray spectroscopy and timing (XSPECT) experiment on XPoSat -- instrument configuration and science prospects

X-ray Polarimeter Satellite (XPoSat) with POLarimeter Instrument in X-rays (POLIX), is India's first spacecraft dedicated to study medium energy X-ray polarisation from celestial objects. X-Ray Spectroscopy and Timing (XSPECT) instrument on XPoSat is configured to study long term spectral behaviour of select sources in Soft X-ray regime. The instrument uses Swept Charge Devices (SCD)s to provide large area and spectral performance with passive cooling arrangement. The instrument consists of set of collimators with two different FOVs, optical light blocking filters, and signal processing electronics. The instrument was designed, tested and calibrated on ground. The unique opportunity is provided by ISRO's XPoSat mission, where a source is observed for longer duration. The device used also enables spectroscopy study of brighter sources compared to the CCD based spectrometers. The first results demonstrate instrument capability for spectral studies in the 0.8 keV-15 keV energy band.

astro-ph.IM↗