An XMM-Newton Study of the 401 Hz Accreting Pulsar SAX J1808.4-3658 in Quiescence

SAX J1808.4-3658 is a unique source, being the first low-mass X-ray binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first that assessed its quiescent luminosity and spectrum with a good signal-to-noise ratio. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658, likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X=5×10³¹ ergs s^-1, a relatively low value compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4-3658 was dominated by a hard (Γ~1.5) power law with only a minor contribution (<~10%) from a soft blackbody component. If the power law originates in the shock between the wind of a turned-on radio pulsar and matter outflowing from the companion, then a spin-down to an X-ray luminosity conversion efficiency of η~10^-3 is derived; this is in line with the value estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4-3658 likely hosts a massive neutron star (M>~1.7 M_solar).