On the nature of two low- X-ray bursters: 1RXS J170854.4–321857 and 1RXS J171824.2–402934

¹ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
² Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ Department of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, UK

Received: 1 March 2005
Accepted: 10 May 2005

Abstract

We carried out Chandra observations of two low-luminosity low-mass X-ray binaries, 1RXS J170854.4–321857 and 1RXS J171824.2–402934, for which previously single X-ray bursts were detected with the Wide Field Cameras (WFCs) on board BeppoSAX. Both were detected in our Chandra observations in an actively accreting state three to eight years after the X-ray bursts, with 0.5–10 keV luminosities between and  erg s^-1. The apparently persistent nature is remarkable for 1RXS J171824.2–402934 given its low luminosity of 10^-3 L_Edd. The persistence of both sources also distinguishes them from 5 other low-L bursters, which have also been seen during bursts with the WFCs but were not detected during Chandra observations above a luminosity of 10³³ erg s^-1. Those are probably transient rather than persistent sources.