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Astron. Astrophys. 350, L5-L8 (1999)

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Letter to the Editor

X-ray observations during a Her X-1 anomalous low-state

A.N. Parmar 1, T. Oosterbroek 1, D. Dal Fiume 2, M. Orlandini 2, A. Santangelo 3, A. Segreto 3 and S. Del Sordo 3

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

Received 24 August 1999 / Accepted 2 September 1998


Results of a 1999 July 8-10 BeppoSAX observation during an anomalous low-state of Her X-1 are presented. The standard on-state power-law and blackbody continuum model is excluded at high confidence unless partial covering is included. This gives a power-law photon index of [FORMULA] and implies that [FORMULA] of the flux undergoes additional absorption of [FORMULA] atom cm-2. 11% of the observed 0.1-10 keV flux is from the [FORMULA] keV blackbody. 1.237747(2) s pulses with a semi-amplitude of [FORMULA]% are detected at [FORMULA]99.5% confidence and confirmed by RXTE measurements. This implies that Her X-1 underwent substantial spin-down close to the start of the anomalous low-state. The spectral and temporal changes are similar to those recently reported from 4U 1626-67. These may result from a strongly warped disk that produces a spin-down torque. The X-ray source is then mostly viewed through the inner regions of the accretion disk. A similar mechanism could be responsible for the Her X-1 anomalous low-states. Shadowing by such an unusually warped disk could produce observable effects in the optical and UV emission from the companion star.

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

Send offprint requests to: A.N. Parmar (aparmar@astro.estec. esa.nl)

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© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999