Title: Probing the Stellar Wind in Vela X−1 with MAXI/GSC

Authors: Abhisek Tamang 1,2, Kinjal Roy 1, Hemanth Manikantan 1, Ajith Balu 1 and Biswajit Paul 1

1 Raman Research Institute, Bangalore, Karnataka - 560080, India

2 Indian Institute of Science, Bangalore, Karnataka - 560012, India

Vela X−1 is one of the first few discovered High Mass X-ray Binary (HMXB) Pulsars with a B-type supergiant companion. In compact HMXB pulsars like Vela X−1, the companion’s stellar wind is significantly affected by ionization due to X-rays from the compact object. An isotopic stellar wind model alone cannot explain the orbital variations of absorption column density in Vela X−1. A simple accretion wake toy model was previously implemented to explain the observed orbital column density variations. We modelled the long-term spectrum of the source obtained with MAXI/GSC with the Comptonized emission of cold photons on hot electrons behind a thick column of photo-absorbing material. We use ∼14 years of MAXI/GSC data to constrain the variations of absorption column density in Vela X−1 using orbital-phase resolved spectral analysis. In this work, we use the above-mentioned model for the photo-ionized accretion wake around the NS to explain the asymmetric absorption column density present in the source. The long-term light curve of the source also showed orbit-to-orbit intensity level variations without any apparent super-orbital periodicity. Further analysis involving orbital-phase-resolved spectroscopy in multiple intensity levels revealed asymmetric variation in absorption column density changes across multiple intensity levels. The high-quality MAXI/GSC data allowed us to vary the different wake parameters to obtain the best-fit model parameters that closely resemble the observed orbital variations of absorption column density for different intensity levels of the source.