NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1

TL;DR

This study uses NICER observations of Aql X-1 to analyze how a Type I X-ray burst influences the accretion disk and corona, revealing soft X-ray excesses caused by reprocessing and flux enhancement, marking a first for short sub-Eddington bursts.

Contribution

First NICER observation of a short sub-Eddington X-ray burst showing disk reprocessing and flux enhancement effects in Aql X-1.

Findings

Detection of soft X-ray excess below 1 keV during the burst

Evidence of reprocessing by the photoionized accretion disk

Possible flux enhancement due to Poynting-Robertson drag or coronal effects

Abstract

Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type I X-ray bursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impact on the disk during one of the first X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hard spectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits an excess of soft X-ray photons below 1 keV, where NICER's sensitivity peaks. We interpret the excess as a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burst persistent flux, possibly due to Poynting Robertson drag or coronal reprocessing. This is the first such detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER will be able to study how X-ray bursts affect the disk and corona for a range of accreting neutron star systems and disk states.