The quasi-persistent neutron star soft X-ray transient 1M 1716−315 in quiescence
We report on our analysis of a 20 ks Chandra X-ray observation of the quasi-persistent neutron star soft X-ray transient (SXT) 1M 1716-315 in quiescence. Only one source was detected in the HEAO-1 error region. Its luminosity is 1.6 x 10^(32)-1.3 x 10^(33) erg s^(-1). In this, the range is dominated by the uncertainty in the source distance. The source spectrum is well described by an absorbed soft spectrum, e.g. a neutron star atmosphere or blackbody model. No optical or near-infrared counterpart is present at the location of the X-ray source, down to a magnitude limit of I empty set 23.5 and K-s empty set 19.5. The positional evidence, the soft X-ray spectrum together with the optical and near-infrared non-detections provide strong evidence that this source is the quiescent neutron star SXT. The source is 10-100 times too bright in X-rays in order to be explained by stellar coronal X-ray emission. Together with the interstellar extinction measured in outburst and estimates for the source distance, the reported optical and near-infrared limit give an upper limit on the absolute magnitude of the counterpart of I > 8.6 and K-s > 5.1. This implies that the system is either an ultra-compact X-ray binary having P-orb < 1 h or the companion star is an M-dwarf. We reconstructed the long-term X-ray light curve of the source. 1M 1716-315 has been active for more than 12 yr before returning to quiescence, the reported Chandra observation started 16.9 +/- 4.1 yr after the outburst ended.
© 2007 The Authors. Journal Compilation ©2007 RAS. Accepted 2007 March 1. Received 2007 February 28; in original form 2006 August 1. PGJ and SW acknowledge support from NASA grant GO4-5033X. PGJ and CGB acknowledge support from the Netherlands Organization for Scientific Research. We also acknowledge Frank Verbunt for useful discussions and the referee for useful comments that helped improve the manuscript.
Published - JONmnras07.pdf