XMM-NEWTON FINDS THAT SAX J1750.8–2900 MAY HARBOR THE HOTTEST, MOST LUMINOUS KNOWN NEUTRON STAR

We have performed the first sensitive X-ray observation of the low-mass X-ray binary (LMXB) SAX J1750.8–2900 in quiescence with XMM-Newton. The spectrum was fit to both a classical blackbody model, and a non-magnetized, pure hydrogen neutron star (NS) atmosphere model. A power-law component was added to these models, but we found that it was not required by the fits. The distance to SAX J1750.8–2900 is known to be D = 6.79 kpc from a previous analysis of photospheric radius expansion bursts. This distance implies a bolometric luminosity (as given by the NS atmosphere model) of (1.05 ± 0.12) × 1034 (D/6.79 kpc)2 erg s–1, which is the highest known luminosity for a NS LMXB in quiescence. One simple explanation for this surprising result could be that the crust and core of the NS were not in thermal equilibrium during the observation. We argue that this was likely not the case, and that the core temperature of the NS in SAX J1750.8–2900 is unusually high.