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Preprint 02g/02

A Her X-1 Turn-On: Using the pulse profile to probe the outer edge of an accretion disk

M. Kuster (1), J. Wilms (1), R. Staubert (1), P. Risse (1), W.A. Heindl (2), R. Rothschild (2), N.I. Shakura (3), K.A. Postnov (3,4)

(1) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
(2) Center for Astrophysics and Space Sciences, UCSD, La Jolla, CA 92093, USA
(3) Sternberg Astronomical Institute, Moscow University, 119899 Moscow, Russia
(4) Faculty of Physics, Moscow State University, 119899 Moscow, Russia

in: New Visions of the X-ray Universe in the XMM-Newton and Chandra Era, eds. F. Jansen & TBD, ESA SP-488, 2002

Abstract.

The X-ray binary pulsar Her X-1 shows a wide variety of long and short term variabilities in the X-ray light curve. The 35 d variability of the source is interpreted as the influence of a warped, inclined, and twisted accretion disk periodically covering the line of sight to the central neutron star. In 1997 September we observed the "turn-on" of a 35 d cycle with the Rossi X-ray Timing Explorer (RXTE). Spectral analysis reveals that during early phases of the turn-on the overall spectrum is composed of X-rays scattered into the line of sight plus heavily absorbed X-rays. This interpretation is consistent with the variation of the pulse profile observed at the same time. The overall shape of the pulse profile is not changing, but towards earlier phases of the turn-on the pulse signature is steadily "washed out". This behavior can be understood as an influence of scattering and absorption due to the presence of the accretion disk rim. Using a Monte Carlo code we simulate the influence of both processes on a time variable, beamed emission characteristic, similar to the pulse profile of Her X-1. By comparing the results of the simulation with the observed profiles we determine the amount of scattered radiation, absorbed radiation, and the size of the scattering region.

Key words: stars: individual (Hercules X-1) - X-rays: stars - stars: neutron - Accretion, accretion disks

Paper (74k gzip'ed Postscript including figures)

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