2007ApJ...665L.147J

We report on our analysis of 300 ks of Chandra observations of the neutron star soft X-ray transient 1H 1905+000 in quiescence. We do not detect the source down to a 95% confidence unabsorbed flux upper limit of 2x 10^–16 ergs/cm2/s in the 0.5-10 keV energy range for an assumed Γ=2 power-law spectral model. A limit of 1.4x10^–16 ergs/cm2/s is derived if we assume that the spectrum of 1H 1905+000 in quiescence is described well with a blackbody of temperature of 0.2 keV. For the upper limit to the source distance of 10 kpc this yields a 0.5-10 keV luminosity limit of 2.4x10³⁰ ergs/s/1.7x10³⁰ ergs/s for the abovementioned power law or blackbody spectrum, respectively. This luminosity limit is lower than the luminosity of A0620-00, the weakest black hole soft X-ray transient in quiescence reported so far. Together with the uncertainties in relating the mass transfer and mass accretion rates we come to the conclusion that the claim that there is evidence for the presence of a black hole event horizon on the basis of a lower quiescent luminosity for black holes than for neutron stars is unproven. We also briefly discuss the implications of the low quiescent luminosity of 1H 1905+000 for the neutron star equation of state. Using deep Magellan images of the field of 1H 1905+000 obtained at excellent observing conditions, we do not detect the quiescent counterpart of 1H 1905+000 at the position of the outburst optical counterpart down to a magnitude limit of i'>25.3. This can be converted to a limit on the absolute magnitude of the counterpart of I>9.6, which implies that the counterpart can only be a brown or a white dwarf.