Discovery of a retrogradely rotating neutron star in the X-ray pulsar GX 301-2
A flashback from my PhD times... I've seen this wiggle long ago but did not understand what it means. Now Juhani Mönkkönen finally noticed it again, and after some time we've come up with an interesting interpretation.
We report on the analysis of the spin evolution of a slow X-ray pulsar GX 301-2 along the orbit using long-term monitoring by Fermi/Gamma-ray Burst Monitor. Based on the observationally confirmed accretion scenario and an analytical model for the accretion of angular momentum, we demonstrate that in this system, the neutron star spins retrogradely, that is, in a direction opposite to the orbital motion. This first-of-a-kind discovery of such a system proves the principal possibility of retrograde rotation in accreting systems with suitable accretion torque, and might have profound consequences for our understanding of the spin evolution of X-ray pulsars, estimates of their initial spin periods, and the ultimate result of their evolution.