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Preprint C/00

The 1999 Hercules X1 Anomalous Low State

W. Coburn (1), W.A. Heindl (1), J. Wilms (2), D.E. Gruber (1), R. Staubert (2), R. E. Rothschild (1), K. A. Postnov (3), N. Shakura (4), P. Risse (2), I. Kreykenbohm (2), M. R. Pelling (1)

(1) Center for Astrophysics and Space Sciences, Code 0424, University of California at San Diego, La Jolla, CA, 920930424, USA
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) Faculty of Physics, Moscow State University, 119899 Moscow, Russia
(4) Sternberg Astronomical Institute, 119899 Moscow, Russia

To appear in: ApJ

Abstract. A failed main-on in the 35d cycle of Her X-1 was observed with the Rossi X-Ray Timing Explorer (RXTE) on 1999 April 26. Exceptions to the normal 35d cycle have been seen only twice before; in 1983 and again 1993. We present timing and spectral results of this latest Anomalous Low State (ALS) along with comparisons to the main-on and normal low states. Pulsations were observed in the 3-18 keV band with a fractional RMS variation of (0.037+-0.003). Spectral analysis indicates that the ALS spectrum has the same shape as the main-on but is modified by heavy absorption and scattering. We find that 70% of the observed emission has passed through a cold absorber (NH=5.0x1023cm-2). This partially absorbing spectral fit can be applied to the normal low state with similar results. We find that the ALS observations may be interpreted as a decrease in inclination of the accretion disk causing the central X-Ray source to be obscured over the entire 35d cycle.

Key words:

Paper (63k gzip'ed Postscript including figures), astro-ph/9911533

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