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Institut für Astronomie und Astrophysik

Abteilung Astronomie

Sand 1, D-72076 Tübingen, Germany

Preprint Kf/98

On the Stability of the Cyclotron Line of Her X-1

D.E. Gruber(1), W.A. Heindl(1), R.E. Rothschild(1), R. Staubert(2), J. Wilms(2), and D.M. Scott(3)

(1)Center for Astrophysics and Space Sciences, UCSD, La Jolla, CA, USA
(2)Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3)Marshall Space Flight Center, Huntsville, AL, USA

1999, Highlights in X-ray Astronomy, ed. B. Aschenbach, MPE Report, in press

Abstract. Several recent observations with RXTE and BeppoSAX have shown a centroid for the Her X-1 cyclotron line of about 40 keV. Furthermore, the RXTE observations by themselves show that this centroid is constant to 1% over a period of many months. This centroid is rather higher than earlier measurements, which ranged from 34 to 38 keV, and may indicate a change over time. We review the observational evidence for change in view of the stability obtained from the RXTE measurements alone. We then discuss the implications of these results for several recent models of the radiating structure at the polar cap of the neutron star. The RXTE limit of 1% variability, in particular, implies that the line absorption occurs at a height above the polar cap which is constant to 50 meters.

Key words: -

Paper (89k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de)
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