ASCA Observations of Transient X-Ray Sources in Quiescence

• Published in: Publications of the Astronomical Society of Japan 1998-12, Vol.50 (6), p.611-619
• Main Authors: Asai, Kazumi, Dotani, Tadayasu, Hoshi, Reiun, Tanaka, Yasuo, Robinson, Craig R., Terada, Kentaro
• Format: Article
• Language: English
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/50.6.611

Soft X-ray transients are most probably low-mass binary systems containing either a neutron star (NS-LMXB) or a black hole (BH-LMXB). We have systematically investigated thirteen soft X-ray transients (four NS-LMXBs and nine BH-LMXBs) in the quiescent state using the available ASCA data. The luminosities of the observed NS-LMXBs in quiescence are on the order of ∼ 1032−33 erg s−1, whereas those of the BH-LMXBs are found to be systematically lower, most of them giving upper limits in the range below 1032−33 erg s−1 . The quiescent luminosity of LMXBs may be explained in terms of an optically thin, advection-dominated accretion disk, for which most of the thermal and kinetic energies of accreting matter are advected into the compact object with little X-ray emission. However, this scenario predicts a much larger luminosity for NS-LMXBs than observed, if the advected energies are liberated at the neutron-star surface, which is absent in BH-LMXBs. Mass accretion onto the neutron-star surface may be suppressed through some mechanism, such as the so-called propeller effect. The soft component observed in the NS-LMXBs is most probably emission from the magnetic pole regions of the neutron star.