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 burst source SAX J1808.4-3658 during its 1998 April/May hard outburst. The observations were made with RXTE over a period of three weeks. The spectrum is well-described by a power law with photon index 1.86 ± 0.01 that is exponentially cut off at high energies. Excess soft emission above the power law is present as well as a weak Fe-K line. This is the first truly simultaneous broad-band (2.5–250 keV) spectrum of a type I burst source in the hard state. The spectrum is consistent with other hard state burster spectra which cover either only the soft (1–20 keV) or hard (> ∼ 20 keV) bands, or cover both, but not simultaneously. The cutoff power law spectrum resembles that of black hole candidates (BHCs) in their low states, observed with RXTE. We compare the SAX J1808.4-3658 spectrum to three BHCs and find that the power law is somewhat softer. This suggests that the photon index may provide a way to distinguish between low state emission from Galactic black holes and type I bursters.