X-Ray Flux Decrease of the Accretion-Powered Millisecond Pulsar SAX J1808.4-3658 in Quiescence Detected by ASCA

The accretion-powered millisecond X-ray pulsar, SAX J1808.4-3658, was observed in quiescence with ASCA in September 1999. We detected a dim X-ray source in the SIS data at the position consistent to SAX J1808.4-3658. The source count rate was (1.1+-0.4)x10E-3 cts/s (0.5-5 keV) for a single SIS, which corresponds to (3+-1)x10E-14 ergs/s/cmE2 if a power law energy spectrum of photon index 2 with low-energy absorption corresponding to a hydrogen column density of 1.3x10E21 cmE-2 is assumed. The statistical quality of the data was insufficient to constrain the energy spectrum or to detect the 401 Hz coherent pulsation. We compare the data with the BeppoSAX observation also made during the quiescent state, and find that the X-ray flux measured by ASCA is at least a factor of 4 smaller than that measured by BeppoSAX. We discuss the possible X-ray emission mechanisms that could explain the flux change, including the radio pulsar and the radio pulsar shock emission.