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Session 26 - X-ray Emission Flow, Neutron Stars & White Dwarfs.
Oral session, Monday, January 15
Salon del Rey South, Hilton

[26.02] Hard X-ray Outbursts from the X-ray Burster 4U 1608--522 Observed by BATSE/CGRO

S. N. Zhang (USRA), B. A. Harmon, G. J. Fishman (NASA/MSFC), W. S. Paciesas (UAH), D. Barret, J. E. Grindlay, P. Bloser (Harvard/CfA), M. Tavani, P. Kaaret, E. Ford (Columbia)

Several hard X-ray outbursts have been observed by the Large Area Detectors (LADs) of BATSE/CGRO since April 1991 from the X-ray burster 4U 1608--522, with durations ranging from 30 days to 160 days and peak fluxes from 70--150 mCrab (20--100 keV). The outburst from June to December 1991 was the brightest and longest one. A near simultaneous Ginga observation in the middle of this outburst detected the source in a soft X-ray (< 10 keV) low state with its hard X-ray spectrum and flux in agreement with the BATSE data. Combined with the Ginga results, the low energy spectrum before the break energy is well determined to be a power law with a photon index of about 1.8. The high energy spectrum above the break energy can be fit equally well by either the Sunyaev-Titarchuk Comptonization model or a broken power law model. For the first time, we have determined that a rather intense hard X-ray outburst from a neutron star system is associated with a soft X-ray low state and the power law photon spectrum breaks at about 65 keV. We show that most of the previously considered black hole `signatures' can be found from 4U 1608--522. New criteria for identifying galactic black hole candidates will be discussed.

Program listing for Monday