The Decrease in X-Ray Flux 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 1999 September. We detected a dim X-ray source in the Solid-State Imaging Spectrometer (SIS) data at the position consistent with SAX J1808.4-3658. The source count rate was (1.1 ± 0.4) × 10^-3 counts s^-1 (0.5-5 keV) for a single SIS, which corresponds to (3 ± 1) × 10^-14 ergs s^-1 cm^-2 if a power-law energy spectrum of photon index 2 with low-energy absorption corresponding to a hydrogen column density of 1.3 × 10²¹ cm^-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.