Super-Eddington accretion of the first Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124

Abstract

ABSTRACT We present a detailed timing study of the pulse profile of Swift J0243.6+6124 with Hard X-ray Modulation Telescope (HXMT) and Fermi/GBM (Gamma-ray Burst Monitor) data during its 2017 giant outburst. The double-peak profile at luminosity above 5 × 1038 erg s−1 is found to be 0.25 phase offset from that below 1.5 × 1038erg s−1, which strongly supports for a transition from a pencil beam to a fan beam, and thus for the formation of shock dominated accretion column. During the rising stage of the high double-peak regime, the faint peak got saturated in 10–100 keV band above a luminosity of Lt ∼ 1.3 × 1039erg s−1, which is coincident with sudden spectral changes of both the main and faint peaks. They imply a sudden change of emission pattern around Lt. The spin-up rate ($\dot{\nu }$) is linearly correlated with luminosity (L) below Lt, consistent with the prediction of a radiation pressure dominated disc. The $\dot{\nu }-L$ relation flattens above Lt, indicating a less efficient transfer of angular momentum and a change of accretion disc geometry above Lt. It is likely due to irradiation of the disc by the central accretion column and indicates significant radiation feedback well before the inner disc radius reaching the spherization radius.