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

We report on the X-ray spectrum of the 401 Hz X-ray pulsar and type I burstsource SAX J1808.4-3658, during its 1998 April/May hard outburst. Theobservations were made with RXTE over a period of three weeks. The spectrum iswell-described by a power law with photon index 1.86+/-0.01 that isexponentially cut off at high energies. Excess soft emission above the powerlaw is present as well as a weak Fe-K line. This is the first trulysimultaneous broad-band (2.5-250 keV) spectrum of a type I burst source in thehard state. The spectrum is consistent with other hard state burster spectrawhich cover either only the soft (1-20 keV) or hard (>20 keV) bands, or coverboth, but not simultaneously. The cut-off power law resembles black holecandidates (BHCs) in their low states, observed with RXTE. We compare the SAXJ1808.4-3658 spectrum to three BHCs and find that the power law is somewhatsofter. This suggests that the photon index may provide a way to distinguishbetween low state emission from Galactic black holes and type I bursters.Comment: Accepted for publication in the Astrophysical Journal Letters. 12 pages, 3 figure