A bright thermonuclear X-ray burst simultaneously observed with Chandra and RXTE

TL;DR

This study reports the observation of the brightest thermonuclear X-ray burst from SAX J1808.4-3658 using Chandra and RXTE, revealing spectral excesses and suggesting scattering in the accretion disk corona.

Contribution

First simultaneous high-resolution broad-band observations of a thermonuclear X-ray burst from a millisecond pulsar, highlighting spectral features and disk interactions.

Findings

Detected the brightest X-ray burst from the source to date.

Observed spectral excesses at high and low energies during the burst.

Suggested scattering in the accretion disk corona as a possible explanation.

Abstract

The prototypical accretion-powered millisecond pulsar SAX J1808.4-3658 was observed simultaneously with Chandra-LETGS and RXTE-PCA near the peak of a transient outburst in November 2011. A single thermonuclear (type-I) burst was detected, the brightest yet observed by Chandra from any source, and the second-brightest observed by RXTE. We found no evidence for discrete spectral features during the burst; absorption edges have been predicted to be present in such bursts, but may require a greater degree of photospheric expansion than the rather moderate expansion seen in this event (a factor of a few). These observations provide a unique data set to study an X-ray burst over a broad bandpass and at high spectral resolution (lambda/delta-lambda=200-400). We find a significant excess of photons at high and low energies compared to the standard black body spectrum. This excess is well described by a 20-fold increase of the persistent flux during the burst. We speculate that this results from burst photons being scattered in the accretion disk corona. These and other recent observations of X-ray bursts point out the need for detailed theoretical modeling of the radiative and hydrodynamical interaction between thermonuclear X-ray bursts and accretion disks.