Contributed Talk - Splinter HotStars

Friday, 15 September 2023, 14:40   (H 2036)

Long-term monitoring study of Vela X-1 in the 2–10 keV energy band

L. Abalo, P. Kretschmar, F. Fürst, V. Grinberg, C. Diez, A. Manousakis, S. Martinez-Núñez, I. El Mellah, R. Amato, M. Guainazzi
cosine, Leiden University, ESA/ESAC, ESA/ESTEC, SAASST, CSIC, IRAP

Vela X-1 is important for the understanding of the accretion processes in high-mass X-ray binaries on account of its natural variability and inhomogeneous wind, being easily observable due to the close distance and high inclination of the system. The softer energy bands are much more susceptible to photoabsorption thanks to the rapidly declining cross-section of photoabsorption with increasing energy as sigma~E^-3. In this project, we conduct a systematic long-term X-ray monitoring study of the source in the 2 to 10 keV energy bands with the Monitor of All-sky X-ray Image mission, which offers a unique avenue to explore the significant fluctuations of absorption over the orbit. To do this, we use the hardness ratios as an indicator of variability in absorption and look at individual binary orbits in contrast to the average trends of the more than thirteen years of observations. Our output is consistent with previous works that explain this variability with a model of an inhomogeneous environment, where the overdensities arise from the line-driven instability triggered in the stellar wind near the photosphere. Furthermore, we look at individual binary orbits of the time intervals of spin-up and spin-down episodes aiming to find recurring evolutions. These results provide us insights into the accretion and wind physical properties of Vela X-1 in particular, but they also shed light on the properties of other similar sources.