A long, broadband X-ray study of Vela X-1 and its environment

We present preliminary results from a comprehensive observation campaign of the high-mass X-ray binary (HMXB) Vela X-1, using INTEGRAL, NuSTAR, Swift, and NICER. The campaign was organized as a follow-up to the first long-duration flight of X-Calibur, a hard X-ray polarization balloon mission.Vela X-1 is an archetypical HMXB, hosting a strongly magnetized neutron star with a spin period of around 283s. The magnetic field strongly influences the expected polarization signature, making it an ideal target for the hard X-ray polarization measurements done by X-Calibur.As a wind accreting system it shows strong flux and absorption variability, requiring simultaneous observations across the X-ray band to characterize the system completely and put the X-Calibur results into context.INTEGRAL observed the Vela X-1 system over a complete orbit of the binary system, from 2019-01-05 - 2019-01-12 (excluding eclipses), while NuSTAR observed in during the latter part of that orbit, between 2019-10-10 - 2019-01-11, together with NICER and Swift. Swift also provided daily monitoring between 2018-12-22 and 2019-01-05.As expected, the source shows strong variability, with flux variations of over a factor 10 between large flares and off-states and we measured an average luminosity of 2e36 erg/s (between 5-50keV). The INTEGRAL data show that the variability increases towards the later phases of the orbit, with the most extreme flux variations seen just before eclipse ingress.In NuSTAR the Cyclotron Resonant Scattering Feature (CRSF) around 55keV can be clearly measured. Using time-resolved spectroscopy we find significant changes in the CRSF energy, which seem to correlate a strong flare and a hardening of the continuum occurring around the middle of the observation. These data provide new insights into the location of the line-forming region as function of luminosity and time.From the soft X-ray coverage with NICER and Swift/XRT we find highly variable absorption, likely caused by wind clumps moving through our line-of-sight.We discuss these preliminary results in the context of recently developed theoretical models of the accretion column, the CRSF production, as well as the wind structure and ionization state of the system.