Two Neutron Star SXT in Quiescence: 4U 2129+47 and EXO 0748-676

Abstract

We report on x-ray observations of two soft x-ray transients containing neutron stars, 4U 2129+47 and EXO 0748-676. Our emphasis is on the quiescent properties of these sources. The x-ray spectrum and lightcurve of the eclipsing Low-Mass X-ray Binary (LMXB) 4U 2129+47 was measured with the ROSAT PSPC during its current quiescent state. The quiescent x-ray luminosity of ~ 10**32.7 ergs/s and blackbody temperature of kT ~ 0.21 keV are similar to other quiescent LMXB. The quiescent x-ray light curve appears to show orbital modulation, but the statistics are insufficient to distinguish between a v-shaped partial eclipse (as seen in the high state) or a total, square wave eclipse. The similarity in the luminosity and temperature to other (non-eclipsing) quiescent LMXB implies that the vertical structure in the disk which blocked our direct view of the neutron star in the high state has collapsed, and the neutron star is seen directly. EXO 0748-676 was serendipitously observed with the Einstein IPC in quiescence before it was discovered as a bright transient with EXOSAT. Our re-analysis of this quiescent observation finds L_x ~ 10**34.0 ergs/s, and blackbody temperature of kT ~ 0.22 keV, again similar to other LMXBs in quiescence.