The X-Ray Spectrum of SAX J1808.4−3658

Abstract

We report on the X-ray spectrum of the 401 Hz X-ray pulsar and type I burst source SAXJ1808.4-3658 during its 1998 April/May hard outburst. The observationswere made with the Rossi X-ray Timing Explorer (RXTE) over a periodof 3 weeks. The spectrum is well described by a power law with aphoton index of 1.86+/-0.01 that is exponentially cutoff at highenergies. Excess soft emission above the power law is present as well asa weak Fe-K line. This is the first truly simultaneous broadband(2.5-250 keV) spectrum of a type I burst source in the hard state. Thespectrum is consistent with other hard-state burster spectra thatcover either only the soft (1-20 keV) or hard (>~20 keV) bands orcover both, but not simultaneously. The cutoff power-law spectrumresembles that of black hole candidates in their low states observedwith RXTE. We compare theSAX J1808.4-3658 spectrum to three blackhole candidates and find that the power law is somewhat softer. Thissuggests that the photon index may provide a way to distinguish betweenlow-state emission from Galactic black holes and type I bursters.