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

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Abstract
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 the Rossi X-ray Timing Explorer (RXTE) over a period of 3 weeks. The spectrum is well described by a power law with a photon index of 1.86±0.01 that is exponentially cutoff 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 broadband (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 that 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 in their low states observed with RXTE. We compare theSAX J1808.4-3658 spectrum to three black hole candidates 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.