Burst–Disk Interaction in 4U 1636–536 as Observed by NICER

Abstract

We present the detection of 51 thermonuclear X-ray bursts observed from 4U 1636–536 by the Neutron Star Interior Composition Explorer (NICER) over the course of a 3 yr monitoring campaign. We perform time-resolved spectroscopy for 40 of these bursts and show the existence of a strong soft excess in all the burst spectra. The excess emission can be characterized by the use of a scaling factor (the f a method) to the persistent emission of the source, which is attributed to the increased mass accretion rate onto the neutron star due to Poynting–Robertson drag. The soft excess emission can also be characterized by the use of a model taking into account the reflection of the burst emission off the accretion disk. We also present time-resolved spectral analysis of five X-ray bursts simultaneously observed by NICER and AstroSat, which confirm the main results with even greater precision. Finally, we present evidence for Compton cooling using seven X-ray bursts observed contemporaneously with NuSTAR, by means of a correlated decrease in the hard X-ray lightcurve of 4U 1636–536 as the bursts start.