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

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: jeanz@sron.nl
² Monash Centre for Astrophysics, School of Mathematical Sciences & School of Physics, VIC 3800, Monash University, Australia
³ Dept. Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴ Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ Columbia Astrophysics Laboratory, 550 West 120th Street, New York, NY 10027, USA
⁸ Los Alamos National Laboratory, B244, Los Alamos, NM 87545, USA
⁹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

Received: 8 January 2013
Accepted: 19 March 2013

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.