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

Abstract

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. 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 x 10^{33} erg/s (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 x 10^{33} erg/s. 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 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 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. We discuss this possibility and alternative quiescent models to explain our results.