The neutron star soft X-ray transient 1H 1905+000 in quiescence

In this paper, we report on our analysis of a similar to 25 ks. Chandra X-ray observation of the neutron star soft X-ray transient (SXT) 1H 1905+000 in quiescence. Furthermore, we discuss our findings of the analysis of optical photometric observations which we obtained using the Magellan telescope and photometric and spectroscopic observations which we obtained using the Very Large Telescope (VLT) at Paranal. The X-ray counterpart of 1H 1905+000 was not detected in our Chandra data, with a 95 per cent confidence limit to the source count rate of 1.2 x 10(-4) counts s(-1). For different spectral models this yields an upper limit on the luminosity of 1.8 x 10(31) erg s(-1) (for an upper limit on the distance of 10 kpc). This luminosity limit makes 1H 1905+000 the faintest neutron star SXT in quiescence observed to date. The neutron star luminosity is so low that it is similar to the lowest luminosities derived for black hole (BH) SXTs in quiescence. This low luminosity for a neutron star SXT challenges the hypothesis presented in the literature that BH SXTs in quiescence have lower luminosities than neutron star SXTs as a result of the presence of a BH event horizon. Furthermore, the limit on the neutron star luminosity obtained less than 20 yr after the outburst has ceased, constrains the thermal conductivity of the neutron star crust. Finally, the neutron star core must be so cold that unless the time-averaged mass accretion rate is lower than 2 x 10(-12) M-circle dot yr(-1), core cooling has to proceed via enhanced neutrino emission processes. The time-averaged mass accretion rate can be derived from binary evolution models if the orbital period of the system is known. Our optical observations show that the optical counterpart discovered when the source was in outburst has faded. Near the outburst optical position we find two stars with a separation of 0.7 arcsec and I= 19.3 +/- 0.1 and 21.3 +/- 0.1. VLT optical spectroscopy revealed that the spectrum of the brighter of the two sources is a G5-7V star. However, the outburst astrometric position of the optical counterpart does not coincide with the position of the G5-7V star nor with that of the fainter star. We derive a limit on the absolute I-band magnitude of the quiescent counterpart of M-I > 7.8 assuming the source is at 10 kpc. This is in line with 1H 1905+000 being an ultracompact X-ray binary, as has been proposed based on the low-outburst V-band absolute magnitude.