NuSTAR discovery of a luminosity dependent cyclotron line energy in Vela X-1

We present an analysis of NuSTAR observations of Vela X-1, making full use of the observatory's high sensitivity and high spectral resolution. We find clear evidence for two cyclotron lines, the fundamental line at 25keV and the harmonic line at 55keV. By extracting spectra with exposure times as short as 1ks, we investigate the behavior of the cyclotron lines as the flux range extends over one order of magnitude. We discover that the energy of the harmonic line is correlated with the 3-79keV X-ray flux. As Vela X-1 has a luminosity of only ~3e36 erg/s, this correlation can only be explained if the accretion column is very narrow, with a radius ~0.4km. In that case, Coulomb interactions can be strong enough to decelerate the in-falling material above the stellar surface and explain the correlation between line energy and flux. Additionally we found that the strength of the fundamental and harmonic lines are anti-correlated. We explain this by photon-spawning, where strong harmonic lines result in many spawned photons near the fundamental line energy, filling the line up to a level where it becomes undetectable. This is the first time that the evolution of the cyclotron parameters with flux can be studied in a persistent high-mass X-ray binary on short time-scales. These data are used to put constraints on the accretion geometry. We put our findings into context with comparisons to transient binaries and give an outlook to future observations.