EDP Sciences
Free access
Issue
A&A
Volume 375, Number 3, September 2001
Page(s) 922 - 926
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20010820


A&A 375, 922-926 (2001)
DOI: 10.1051/0004-6361:20010820

A BeppoSAX observation of Her X-1 during the first main-on after an anomalous low-state: Evidence for rapid spin-down

T. Oosterbroek1, A. N. Parmar1, M. Orlandini2, A. Segreto3, A. Santangelo3 and S. Del Sordo3

1  Astrophysics Division, Space Science Department of ESA, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
2  Istituto TESRE, CNR, via Gobetti 101, 40129 Bologna, Italy
3  IFCAI, CNR, via La Malfa 153, 90146 Palermo, Italy

(Received 17 April 2001 / Accepted 5 June 2001 )

Abstract
Results of a BeppoSAX observation of Her X-1 in 2000 October during the first main-on state after the longest recorded anomalous low-state are presented. The 0.1-30 keV spectrum, light curve and pulse profile are all consistent with those measured during previous main on-states, indicating that Her X-1 has resumed its regular 35 day cycle with similar on-state properties as before. However, from a comparison of the measured pulse period with that obtained close to the start of the anomalous low-state, it is evident that Her X-1 continued to spin-down strongly during the anomalous low-state such that the pulse period has returned to a similar value as ~15 years ago. Additionally, the occurrence time of the main-on states after the end of the anomalous low-state indicate that a change in the length, or phasing, of the 35-day cycle occurred during the anomalous low-state.


Key words: accretion, accretion disks -- X-rays: binaries -- stars individual: Her X-1

Offprint request: T. Oosterbroek, toosterb@astro.estec.esa.nl

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