A bright thermonuclear X-ray burst simultaneously observed withChandraand RXTE

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 (λ/Δλ = 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.

A NICER Thermonuclear Burst from the Millisecond X-Ray Pulsar SAX J1808.4–3658

“…We find that the burst spectrum shows a strong excess at the lowest energies that requires the inclusion of a second blackbody in the spectral model. A similar strong soft excess was previously observed in SAX J1808 by in 't Zand et al (2013). In contrast to that work, however, we also observe emission features at 1 keV and 6.7 keV.…”

Characterizing the Evolving X-Ray Spectral Features During a Superburst From 4u 1636-536

“…In the superburst tail, the cutoff power-law flux smoothly returns to the pre-superburst value. Worpel et al (2013) infer much larger increases for a selection of PRE bursts observed with the PCA, including from 4U 1636-536 (see also in 't Zand et al 2013). This may be because the superburst is less bright, as it does not reach the Eddington limit.…”

“…Alternatively, the enhanced cutoff power law could be the result of an increase in seed photons for the Comptonization process (in 't Zand et al 2013). These seed photons would be produced by the reprocessing in the accretion disk of the burst emission.…”

“…Reflection features have also been detected from neutron stars outside of bursts (e.g., Bhattacharyya & Strohmayer 2007;Cackett et al 2008;Miller et al 2013). Normal X-ray bursts last too short a time to collect enough photons to detect reflection features with current instrumentation (for constraints on the effect on the continuum for one of the brightest bursts, see in 't Zand et al 2013). 4U 1636-536 is a prolific burster that exhibits a wide range of bursting behavior, including both bursts with and without PRE (e.g., Hoffman et al 1977;Galloway et al 2008), burst oscillations (Strohmayer et al 1998), short recurrence time bursts (Galloway et al 2008;Keek et al 2010), mHz QPOs (Revnivtsev et al 2001;Altamirano et al 2008), double-and triple-peaked bursts Bhattacharyya & Strohmayer 2006;Zhang et al 2009), and four superbursts (Wijnands 2001;Strohmayer & Markwardt 2002;Kuulkers 2009).…”

“…A recent analysis of a large number of PRE bursts observed with the PCA, however, finds indications that the persistent spectrum is not constant, but has an increased normalization during the burst (Worpel et al 2013;in 't Zand et al 2013). One explanation is Poynting-Robertson drag (e.g., Walker 1992): the photons emitted by the neutron star surface transfer momentum to the disk, reducing its angular momentum and allowing for more material to fall in, which increases the flux generated by the accretion process.…”