Fuerst, Felix3; Pottschmidt, Katja11; Wilms, Joern5; Tomsick, John A.6; Bachetti, Matteo7; Boggs, Steven E.6; Christensen, Finn Erland1; Craig, William W.6; Grefenstette, Brian W.12; Hailey, Charles J.13; Harrison, Fiona12; Madsen, Kristin K.12; Miller, Jon M.10; Stern, Daniel12; Walton, Dominic12; Zhang, William11
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 unknown4 NASA Goddard Space Flight Center5 Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics6 University of California at Berkeley7 Université de Toulouse8 California Institute of Technology9 Columbia University10 University of Michigan11 NASA Goddard Space Flight Center12 California Institute of Technology13 Columbia University
We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of Lx ~ 3x10^36 erg/s. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.