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

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.

arXiv comment18 pages, 13 figures, A&A accepted

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• A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375
• Observing the onset of the accretion wake in Vela X-1
• Vacuum polarization alters the spectra of accreting X-ray pulsars
• The $\text{False Widow}$ Link Between Neutron Star X-ray Binaries and Spider Pulsars
• An in-depth analysis of the variable cyclotron lines in GX 301$-$2
• Variable structures in the stellar wind of the HMXB Vela X-1
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• SAS in ESA Datalabs: A New Platform for XMM-Newton Analysis
• Low-frequency spectra of neutron star + OB supergiant binaries: Does wind density drive persistent and flaring modes of accretion?
• Flux variability of the "10 keV feature" of 4U 0115+63
• Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA NuSTAR and X-shooter
• Timing and spectral studies of the Be/X-ray binary EXO 2030+375 using Insight-HXMT observations
• Pulse profile variations in the accreting X-ray pulsar Vela X-1