2022MNRAS.512.5686L

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 x 10³⁸ erg s^–1 is found to be 0.25 phase offset from that below 1.5 x 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 L_t ∼ 1.3 x 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 L_t. The spin-up rate ({dot}ν) is linearly correlated with luminosity (L) below L_t, consistent with the prediction of a radiation pressure dominated disc. The {dot}ν-L relation flattens above L_t, indicating a less efficient transfer of angular momentum and a change of accretion disc geometry above L_t. 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.