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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 ~100 from the peak value of ~9E36 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 ~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 ~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 ~few 1E35 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 < few 1E7 Gauss.