Probing the stellar wind environment of Vela X-1 with MAXI

Abstract

Vela X-1 is among the best studied and most luminous accreting X-ray pulsars. The supergiant optical companion produces a strong radiatively-driven stellar wind, which is accreted onto the neutron star producing highly variable X-ray emission. A complex phenomenology, due to both gravitational and radiative effects, needs to be taken into account in order to reproduce orbital spectral variations. We have investigated the spectral and light curve properties of the X-ray emission from Vela X-1 along the binary orbit. These studies allow to constrain the stellar wind properties and its perturbations induced by the compact object. We took advantage of the All Sky Monitor MAXI/GSC data to analyze Vela X-1 spectra and light curves. By studying the orbital profiles in the $4-10$ and $10-20$ keV energy bands, we extracted a sample of orbital light curves ($\sim 15$% of the total) showing a dip around the inferior conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged and phase-resolved spectra of both the double-peaked and the standard sample. The dip in the double-peaked sample needs $N_H\sim 2\times 10^{24}$cm${}^{-2}$ to be explained by absorption solely, which is not observed in our analysis. We show how Thomson scattering from an extended and ionized accretion wake can contribute to the observed dip. Fitted by a cutoff power-law model, the two analyzed samples show orbital modulation of the photon index, hardening by $\sim 0.3$ around the inferior conjunction, compared to earlier and later phases, hinting a likely inadequacy of this model. On the contrary, including a partial covering component at certain orbital phase bins allows a constant photon index along the orbital phases, indicating a highly inhomogeneous environment. We discuss our results in the framework of possible scenarios.

arXiv comment10 pages, 9 figures, accepted for publication in A&A

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• Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1
• Dips and eclipses in the X-ray binary Swift J1858.6-0814 observed with NICER
• High-resolution X-ray spectroscopy of the stellar wind in Vela X-1 during a flare
• Observing the onset of the accretion wake in Vela X-1
• High resolution X-ray spectroscopy of V4641 Sgr during its 2020 outburst
• Looking through the photoionisation wake: Vela X-1 at ${\phi }_{\mathrm{orb}}\approx 0.75$ with Chandra/HETG
• XMM-Newton and NuSTAR simultaneous X-ray observations of IGR J11215-5952
• Variations of cyclotron resonant scattering features in Vela X-1 revealed with Insight-HXMT
• Variable structures in the stellar wind of the HMXB Vela X-1
• Cen x-3 as seen by MAXI during six years
• Stellar wind structures in the eclipsing binary system IGR J18027-2016
• Variability in high-mass X-ray binaries
• Pulse profile variations in the accreting X-ray pulsar Vela X-1