X-ray observations of the accreting pulsar GX 301-2 during a spin-up episode

The accretion-powered high-mass X-ray binary GX 301-2 is composed of a pulsar accreting from a B1-type stellar companion, and is one of the brightest X-ray sources in the sky. Sources like GX 301-2 are important to our understanding of the physics underlying accretion and mass-transfer mechanisms onto compact objects. Especially useful are simultaneous measurements of the flux and the change in spin frequency, which can be tested against models of angular momentum transfer under different accretion scenarios. In the case of GX 301-2, NICER and Swift-XRT observations across two orbital periods between MJD 58469 and 58552 during a spin-up episode measured by Fermi GBM allow for testing for the formation of a transient accretion disk. Using this data, we examine the correlation between the spin frequency and flux based on accretion torque models and calculate how much of the observed luminosity results from the mass transfer. We determine that even though the contribution to the luminosity from the temporary accretion disk is negligible compared to the luminosity produced by wind accretion, the presence of an accretion disk is indicated by changes in the pulsed fraction of the emission.