Speaker
Description
Vela X-1 is an archetypal high-mass X-ray binary pulsar in which strong X-ray irradiation from the neutron star significantly affects the companion’s stellar wind. A smooth, isotropic wind cannot explain the observed variability in the system, especially the orbital variation in the absorption column density along the sight line, suggesting the presence of additional structures such as photoionisation or accretion wakes. Furthermore, the source intensity exhibits strong variability from orbit to orbit. India’s first dedicated X-ray polarimetry mission, XPoSat, was launched on 1 January 2024 aboard PSLV-C58. The mission carries two payloads: the primary Polarimeter Instrument in X-rays (POLIX), operating in the 8.0-30.0 keV band to measure X-ray polarization, and the secondary X-ray Spectroscopy and Timing (XSPECT) instrument, covering the 0.8-15.0 keV energy range. XSPECT has so far observed Vela X-1 during two epochs, comprising five consecutive out-of-eclipse observations and one full orbital cycle. Using these data, we perform a detailed timing and spectroscopic study of Vela X-1 on orbital-period timescales. We find that the variability observed over the orbital period is driven by a comparable contribution from clumpy stellar winds, intrinsic source variability, and accretion/photoionization wake structures. Furthermore, our results confirm that the wake structure has variability at shorter time scales.