The discovery of quiescent X-ray emission from SAX J1808.4-3658, the transient 2.5 millisecond pulsar

We report on a 20 ks BeppoSAX observation of the transient 2.5 ms X-ray pul sar SAX J1808.4-3658 in its quiescent state. A source at a level of similar to 3 x 10(-3) counts s(-1) was detected with the BeppoSAX Medium Energy Co ncentrator Spectrometer at a position consistent with that of SAX J1808.4-3 658. The inferred 0.5-10 keV luminosity was similar to(2-3) x 10(32) ergs s (-1) (for a distance of 4 kpc), a value similar to that measured from other quiescent low-mass X-ray transients hosting an old neutron star. This resu lt is discussed in the light of the propeller and radio pulsar shock emissi on models for the quiescent emission of these systems.