Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1

TL;DR

This study analyzes Vela X-1's spectral variability and accretion geometry using NuSTAR observations, revealing correlations between spectral parameters and complex absorption features linked to stellar wind structures.

Contribution

The paper provides new insights into the variability of cyclotron lines and absorption features in Vela X-1, enhancing understanding of accretion processes in high-mass X-ray binaries.

Findings

Confirmed anti-correlations between photon index and luminosity.

Observed a drop in cyclotron line energy after a flare.

Detected significant absorption variability linked to wind structures.

Abstract

Because of its complex clumpy wind, prominent cyclotron resonant scattering features, intrinsic variability and convenient physical parameters (close distance, high inclination, small orbital separation) which facilitate the observation and analysis of the system, Vela X-1 is one of the key systems to understand accretion processes in high-mass X-ray binaries on all scales. We revisit Vela X-1 with two new observations taken with NuSTAR at orbital phases ~0.68-0.78 and ~0.36-0.52 which show a plethora of variability and allow us to study the accretion geometry and stellar wind properties of the system. We follow the evolution of spectral parameters down to the pulse period time-scale using a partially covered powerlaw continuum with a Fermi-Dirac cut-off to model the continuum and local absorption. We could confirm anti-correlations between the photon index and the luminosity and, for low fluxes, between the folding energy and the luminosity, implying a change of properties in the Comptonising plasma. We could not confirm a previously seen correlation between the cyclotron line energy and the luminosity of the source in the overall observation, but we observed a drop in the cyclotron line energy following a strong flare. We see strong variability in absorption between the two observations and within one observation (for the ~0.36-0.52 orbital phases) that can be explained by the presence of a large-scale structure, such as accretion- and photoionisation wakes in the system and our variable line of sight through this structure.