HEAD Division Meeting 1999, April 1999
Session 27. X-Ray Binaries
Poster, Wednesday, April 14, 1999, 8:30am Wed. - 2:00pm Thurs., Gold Room

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[27.01] The Origin of The EUV Emission in Her X-1

D.A. Leahy (University of Calgary), H.L. Marshall (M.I.T.)

Her X-1 exhibits a strong orbital modulation of its EUV flux with a large decrease around time of eclipse of the neutron star, and a significant dip which appears at different orbital phases at different 35-day phases. We summarize the observations of Her X-1 in the EUVE by the Extreme Ultraviolet Explorer (EUVE), including data near the start and end of the Short High state. The observed EUV lightcurve has bright and faint phases. The bright phase can be explained as the low energy tail of the soft x-ray pulse. The faint phase emission has been modeled to understand its origin. We find: the x-ray heated surface of HZ Her is too cool to produce enough emission; the accretion disk does not explain the orbital modulation; however, reflection of x-rays off of HZ Her can produce the observed lightcurve with orbital eclipses. The dip can be explained by shadowing of the companion by the accretion disk. We discuss the constraints on the accretion disk geometry derived from the observed shadowing.

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