Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: jeanz@sron.nl
² Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ Physics Department, McGill University, 3600 Rue University, Montreal, QC, H3A 2T8, Canada
⁴ School of Physics & Astronomy, University of Minnesota, Twin Cities, Minneapolis, MN 55455, USA
⁵ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
⁶ Kapteyn Astronomical Institute, Groningen University, 9700 AV Groningen, The Netherlands

Received: 27 November 2008
Accepted: 26 January 2009

Abstract

We report the discovery of one-hour long tails on the few-minutes long X-ray bursts from the “clocked burster” GS 1826-24. We propose that the tails are due to enduring thermal radiation from the neutron star envelope. The enduring emission can be explained by cooling of deeper neutron star layers that were heated up through inward conduction of heat produced in the thermonuclear shell flash responsible for the burst. Similar, though somewhat shorter, tails are seen in bursts from EXO 0748-676 and 4U 1728-34. Only a small amount of cooling is detected in all these tails. This is either due to compton upscattering of the tail photons or, more likely, to a neutron star that is already fairly hot due to other stable nuclear processes.