The quasi-persistent neutron star soft X-ray transient 1M 1716−315 in quiescence

Abstract

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.

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