Burst-Disk Interaction in 4U 1636-536 as observed by NICER

TL;DR

This study analyzes 51 thermonuclear X-ray bursts from 4U 1636-536 observed by NICER, revealing a consistent soft excess, reflection effects, and evidence of Compton cooling, enhancing understanding of burst-disk interactions.

Contribution

It provides detailed time-resolved spectroscopy of multiple bursts, demonstrating the soft excess and reflection phenomena, and confirms Compton cooling effects using multi-instrument observations.

Findings

Detection of a strong soft excess in all burst spectra

Confirmation of reflection effects from the accretion disk

Evidence of Compton cooling during bursts

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 three year monitoring campaign. We performed time resolved spectroscopy for 40 of these bursts and showed 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 (f_a method) to the persistent emission of the source, which is attributed to the increased mass accretion rate on to 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 of the accretion disk. We also present time resolved spectral analysis of 5 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 7 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.