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 × 1032–1.3 × 1033 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 23.5 and Ks 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 Ks > 5.1. This implies that the system is either an ultra-compact X-ray binary having Porb < 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.