The quiescent state of the neutron-star X-ray transient GRS 1747−312 in the globular cluster Terzan 6

Abstract

We studied the transient neutron-star low-mass X-ray binary GRS 1747−312, located in the globular cluster Terzan 6, in its quiescent state after its outburst in August 2004, using an archival XMM–Newton observation. A source was detected in this cluster and its X-ray spectrum can be fitted with the combination of a soft, neutron-star atmosphere model and a hard, power-law model. Both contributed roughly equally to the observed 0.5–10 keV luminosity (∼4.8 × 1033 erg s−1). This type of X-ray spectrum is typically observed for quiescent neutron-star X-ray transients that are perhaps accreting in quiescence at very low rates. Therefore, if this X-ray source is the quiescent counterpart of GRS 1747−312, then this source is also accreting at low levels in-between outbursts. Since source confusion is a likely problem in globular clusters, it is quite possible that part, if not all, of the emission we observed is not related to GRS 1747−312, and is instead associated with another source or conglomeration of sources in the cluster. Currently, it is not possible to determine exactly which part of the emission truly originates from GRS 1747−312, and a Chandra observation (when no source is in outburst in Terzan 6) is needed to be conclusive. Assuming that the detected emission is due to GRS 1747−312, we discuss the observed results in the context of what is known about other quiescent systems. We also investigated the thermal evolution of the neutron star in GRS 1747−312, and inferred that GRS 1747−312 can be considered a typical quiescent system under our assumptions.