Constraining the Temperature of the Neutron Star in SAX J1808.4-3658

We propose two 50ksec observations of the transient millisecond Xray pulsarSAX J1808.43658 (J1808) in quiescence. The previous (36 ksec XMM, MOS only)observation of J1808.s Xray spectrum did not show a clear blackbodylikecomponent from the neutron star (NS) surface. J1808.s known mass transfer rateand low photon cooling rate require rapid neutrino cooling. This is the mostrestrictive current constraint upon the structure of NS interiors, and thus onthe behavior of matter at high densities. We propose a substantial improvementof this constraint, capable of ruling out a pion condensate as the mechanism forrapid neutrino cooling. Our secondary objective is to measure or constrain anyvariability in the quiescent Xray luminosity and spectrum of J1808. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]