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.
XMM-Newton observation of SAX J1808.4-3658 in quiescence were the first which assessed its quiescent luminosity and spectrum with good signal to noise. 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 if with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X=5x10³¹ erg 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 (Gamma ~ 1.5) power law with only a minor contribution (< 10%) from a soft black body 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 X-ray luminosity conversion efficiency of ~ 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_o). This is the first evidence for massive neutron stars in Low Mass X-ray Binary systems.
 
 
 
 

      
Left panel: XMM-Newton image of SAX J1808.4-3658. Small circles refer to XMM-Newton sources; large circles to previous positions of SAX J1808.4-3658 obtained with BeppoSAX and ASCA. Right panel: XMM-Newton MOS1 (filled circles) and MOS2 (open circles) spectrum of SAX J1808.4-3658 with only a power law component.
 
 

For more information about this discovery, refer to http://babbage.sissa.it/abs/astro-ph/0206376