Studies on the Soft Intermediate State X-ray Flare of MAXI J1535-571 During Its 2017 Outburst

We analyzed an observation with the Nuclear Spectroscopic Telescope Array ofthe black-hole X-ray binary MAXI J1535-571 in the soft intermediate state, inwhich we detected a 2.5-ks long flare. Our spectral fitting results suggestthat MAXI J1535-571 possesses a high spin of 0.97 (-0.10/+0.02) and a lowinclination of approximately 24 deg. We observed a gradual increase in theinner disc radius, as determined from fits to the continuum spectrum. Thistrend is inconsistent with an increased flux ratio of the thermal component, aswell as the source evolving towards the soft state. This inconsistency may beattributed to a gradual decrease of the color correction factor. Additionally,with a flare velocity of approximately 0.5 c and a higher hardness ratio duringthe flare period, the quasi-simultaneous detection of a type-B QPO in theNeutron Star Interior Composition Explorer data, and quasi-simultaneous ejectalaunch through radio observations collectively provide strong evidencesupporting the possibility that the flare originated from a discrete jetejection.