Lightcurve of 4U 212947 in a Low State

摘要

We report on the x-ray spectrum and lightcurve of the eclipsing Low-Mass X-ray Binary (LMXB) 4U 212947 as measured with the ROSAT PSPC during its current quiescent state. This object is the only accretion disk corona x-ray binary which is currently in a low state, therefore these observations can potentially provide new insights into the structure of LMXB's in quiescence. The quiescent x-ray luminosity of approx. 10(exp 33.5) erg/s and black-body temperature of kT approx. 0.16 keV (Bremss temperature kT approx. 0.25 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 full, square wave eclipse. We argue that 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 a black-body temperature of kT approx. 0.15 keV (Bremss temperature kT approx. 0.25 keV), again similar to other LMXB's in quiescence. The low temperatures observed in these two neutron star SXT are in agreement with the predictions of the standard alpha-disk models for accretion disks, which predict that the temperature of the disk should drop with the fourth root of the mass accretion rate, T approx. M(exp 1/4). This contrasts with the results from black hole transients, and points out a possible method to distinguish black hole and neutron star transients.