A Chandra observation of the globular cluster Terzan 1 : the neutron - star X - ray transient X 1732 – 304 in quiescence

We present a short (∼3.6 ks) Chandra/HRC-I observation of the globular cluster Terzan 1. This cluster is known to contain the bright neutron star low-mass X-ray binary X 1732–304 which was active during the 1980’s and most of the 1990’s. During our observation, we detect one source with a 0.5–10 keV luminosity of approximately 1 − 2× 10 erg s−1 (for an assumed distance of 5.2 kpc). However, its position is not consistent with that of X 1732–304. We do not conclusively detect X 1732–304 with a 0.5–10 keV luminosity upper limit of 0.5 − 1 × 10 erg s−1. This limit is consistent with the luminosities observed for several neutron-star X-ray transients in our Galaxy when they are quiescent, strongly suggesting that X 1732–304 was quiescent during our Chandra observation. If the quiescent emission in neutron star X-ray transients is due to the thermal emission from the neutron star, then it is expected that the quiescent luminosity depends on the time-averaged accretion rate of the source. However, the upper limit on the quiescent luminosity of X 1732–304, combined with its very long accretion episode prior to the current quiescent episode, indicates that the quiescent episodes of the source have to be longer than ∼200 years. This would be the second system after KS 1731–260 for which quiescent episodes longer than several hundreds of years have been inferred. Remarkably, both X 1732–304 and KS 1731–260 have very long accretion episodes and long inferred quiescent episodes, contrary to what is seen in ordinary transients. This suggests that a correlation might be present between the duration of the active episodes and the quiescent ones if the quiescent emission is indeed due to the thermal emission from the neutron star. We discuss this possibility and alternative quiescent models to explain our results. Subject headings: accretion, accretion disks — stars: individual (X 1732–304)— X-rays: stars Center for Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA; [email protected] Harvard College Observatory, 60 Garden St., Cambridge, MA 02138 Chandra Fellow