The millisecond X-ray pulsar/burster SAX J1808.4-3658: the outburst light curve and the power law spectrum

The X-ray light curve and broad band spectral properties of the millisecond X-ray pulsar/burster SAX J1808.4-3658 are reported. In the course of RXTE observations in April-May 1998 the 3-150 keV luminosity of the source decreased by a factor of similar to 100 from the peak value of similar to 9 x 10(36) erg/s (for a 4 kpc distance). However, the spectrum was remarkably stable and maintained a roughly power law shape with a photon index of similar to 2 without a strong high energy cut-off below 100 keV, similar to that sometimes observed in the low spectral state of X-ray bursters. An approximation of the averaged spectrum with an exponentially cut-off power law with a superimposed reflected component yields the 90% lower limit on the e-folding energy E-f greater than or similar to 270 keV. We speculate that Comptonization on the bulk motion in the radiation dominated shock might be a possible mechanism of spectral formation. The decaying part of the X-ray light curve was cut off abruptly at luminosity similar to few x 10(35) erg/s. Such behavior might be due to centrifugal inhibition of accretion (transition to the "propeller" regime) as well as to disk instability. In either case an upper limit on the neutron star magnetic field strength is B less than or similar to few x 10(7) Gauss.