Crustal heating and quiescent emission from transiently accreting neutron stars

Nuclear reactions occurring at densities approximate to 10(12) g cm(-3) in the crust of a transiently accreting neutron star efficiently maintain the core at a temperature approximate to(5-10) x 10(7) K. When accretion halts, the envelope relaxes to a thermal equilibrium set by the flux from the hot core, as if the neutron star were newly born. For the time-averaged accretion rates (less than or similar to 10(-10) M. yr(-1)) typical of low-mass X-ray transients, standard neutrino cooling is unimportant and the core thermally reradiates the deposited heat. The resulting luminosity is similar to 5 x 10(32)-5 x 10(33) ergs s(-1) and agrees with many observations of transient neutron stars in quiescence. Confirmation of this mechanism would strongly constrain rapid neutrino cooling mechanisms for neutron stars (e.g., a pion condensate). Thermal emission had previously been dismissed as a predominant source of quiescent emission since blackbody spectral fits implied an emitting area much smaller than a neutron star's surface. However, as with thermal emission from radio pulsars, fits with realistic emergent spectra will imply a substantially larger emitting area. Other emission mechanisms, such as accretion or a pulsar shock, can also operate in quiescence and generate intensity and spectral variations over short timescales. Indeed, quiescent accretion may produce gravitationally redshifted metal photoionization edges in the quiescent spectra (detectable with AXAF and XMM). We discuss past observations of Aq1 X-1 and note that the low-luminosity (less than 10(34) ergs s(-1)) X-ray sources in globular clusters and the Be star/X-ray transients are excellent candidates for future study.