arXiv subjects

Ajith Balu

Publications and source records attributed to Ajith Balu.

3 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↗

arXiv2025

Unveiling the intensity-dependent wake structure of Vela X-1 using MAXI/GSC

Context. Vela X-1 is among the earliest discovered high-mass X-ray binary (HMXB) pulsars. In such systems, the companion's stellar wind is strongly affected by ionisation from X-rays emitted by the compact object. A smooth, isotropic stellar wind model cannot account for the observed orbital variation in absorption column density. A stream-like photoionisation wake trailing the neutron star has been proposed to explain this variation. Aims. We investigated the variability of the circumbinary environment at different intensity levels of the Vela X-1 and used a model similar to the above-mentioned stream-like photoionisation wake to explain the asymmetric absorption column density present in the source. Methods. The 2.0-20.0 keV MAXI/GSC spectrum was well modelled with a Comptonised continuum absorbed by local and interstellar material. Nearly 13 years of MAXI/GSC data was used to constrain the variations in absorption column density in Vela X-1 from orbital-phase and intensity-and-orbital-phase resolved spectroscopy. Results. The long-term light curve of Vela X-1 shows orbit-to-orbit intensity level variations without any apparent super-orbital periodicity. The orbital-phase resolved spectroscopy in multiple intensity levels reveals asymmetric variation in absorption column density changes across the intensity levels. Conclusions. We confirm that the orbital variation in absorption column density in Vela X-1 cannot be explained by a smooth stellar wind alone using long-term MAXI/GSC data. An additional component, such as a photoionisation or accretion wake, is required. The wake structure is present across intensity levels, with geometry varying by intensity. The long-term MAXI/GSC data enabled us to vary wake parameters and derive best-fit stellar wind parameters for the time-averaged intensity, reproducing observed absorption column density across intensity levels. (Abbreviated.)

astro-ph.HE↗

arXiv2024

Studying the X-ray absorption characteristics of Centaurus X-3 using nearly 14 years of MAXI/GSC data

Centaurus X-3 is a persistent high-mass X-ray binary with the long-term light curve from the source exhibiting orbit-to-orbit intensity variations with no apparent superorbital periodicity. We used $\sim$13.5 years of MAXI/GSC data to study the long-term behaviour of X-ray absorption caused by the stellar wind from the companion star and any absorbing structures present in the binary. We used orbital-phase-resolved spectroscopy to study the variation in the photoelectric absorption along the line of sight of the source for both the intensity-averaged data and intensity-resolved data after dividing all the data binned with orbital period into three intensity levels. We find an asymmetric variation in the photoelectric absorption along the line of sight across an orbit of the source. The orbital-phase-resolved spectra show a clear increase in photoelectric absorption after $\phi_\text{orb}\sim$ 0.5, which deviates from a spherically symmetric stellar wind model. The flux of Cen X-3 shows significant variation between consecutive orbits. An intensity-resolved spectral analysis of the source was performed, followed by an intensity-resolved and orbital-phase-resolved spectral analysis, which showed that at the medium and high intensity levels, the orbital-phase-resolved photoelectric absorption is slightly asymmetric with respect to mid-phase ($\phi_\text{orb}=$ 0.5). The asymmetry is very pronounced at the lowest intensity level and cannot be explained by a spherically symmetric wind from the companion star. The differences in the orbital phase-dependence of absorption for different intensity levels suggest that the presence of an accretion wake, photoionization wake, or tidal stream is more prominent at a lower intensity level for Centaurus X-3 than at a higher intensity level.

astro-ph.HE↗