A Chandra Observation of the Globular Cluster Terzan 1: The Neutron Star X-Ray Transient X1732-304 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 X1732-304, which was active during the 1980s and most of the 1990s. But a BeppoSAX observation performed in 1999 only showed a very weak source, indicating that the source had become quiescent. During our Chandra observation, we detect one source with a 0.5-10 keV luminosity of approximately (1-2 d) x 10(33) ergs s(-1) (for an assumed distance of 5.2 kpc). However, its position is not consistent with that of X1732-304. We do not conclusively detect X1732-304 with a 0.5-10 keV luminosity upper limit of (0.5-1) x 10(33) ergs 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 X1732-304 was still 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 X1732-304, 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 similar to200 yr. This would be the second system after KS 1731-260 for which quiescent episodes longer than several hundreds of years have been inferred. We discuss this possibility and alternative quiescent models to explain our results.