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. © 2022. The Author(s). Published by the American Astronomical Society.