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

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

Corresponding author: T. Oosterbroek, toosterb@astro.estec.esa.nl

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.