1998A&A...338L..83G

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 3-150keV luminosity of the source decreased by a factor of ∼100 from the peak value of ∼9x10³⁶erg/s (for a 4kpc 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 100keV, 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>270keV. 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 x10³⁵erg/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 x10⁷Gauss.