X-ray and optical studies of SAX J1808.4-3658 in quiescence

We have observed the accreting millisecond X-ray pulsar SAX J1808.4-3658 (1808) in quiescence during two 50 ksec XMM-Newton observations, and acquired near-simultaneous photometry with Gemini South. We find 1808's X-ray spectrum to be hard, describable with an absorbed power-law of photon index 1.7-1.9 and unabsorbed X-ray luminosity Lx = 5.2-7.9×10(31) ergs s-1. No thermal neutron star (NS) component is seen, with a limit on any possible NS component of LNS(0.01-10 keV)<6.2×10(30) ergs s-1. (However, an alternative thermal plasma continuum model for 1808 allows a NS component with up to LNS(0.01-10 keV) = 1.3-0.8+0.6×10(31) ergs/s.) This constraint, combined with 1808's accretion history, requires highly enhanced neutrino cooling in the core of 1808's NS.
The near-simultaneous Gemini observations find a large sinusoidal flux modulation on 1808's orbital period, consistent with predictions from an irradiated secondary star. We model the contributions of the disk and donor star, and find that the donor must be irradiated by an external flux of Lirr = 1.15-1.78×10(34) ergs/s, much larger than observed in the X-ray band. This irradiation may be in the form of relativistic particles from the NS turning on as a radio pulsar when not accreting, as suggested by Burderi et al. The amplitude and color dependence of the optical modulation constrain the system inclination and donor radius. These constraints, through the pulsar mass function, deliver constraints on the NS mass of MNS>2.2 Msolar, or for a distance uncertainty 10% larger, of MNS>1.8 Msolar. Such a heavy NS is consistent with the accelerated neutrino cooling found from the X-ray observations.