EDP Sciences
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Volume 396, Number 1, December II 2002
Page(s) 213 - 217
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20021405

A&A 396, 213-217 (2002)
DOI: 10.1051/0004-6361:20021405

Rapid variability of accretion in AM Herculis

D. de Martino1, G. Matt2, B. T. Gänsicke3, R. Silvotti1, J. M. Bonnet-Bidaud4 and M. Mouchet5, 6

1  Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
    e-mail: demartino@na.astro.it; silvotti@na.astro.it
2  Dipartimento di Fisica, Universita' degli Studi Roma Tre, via della Vasca Navale, Roma, Italy
    e-mail: matt@fis.uniroma3.it
3  Astronomy Group, University of Southampton, Hampshire, UK
    e-mail: btg@astro.soton.ac.uk
4  Service d'Astrophysique Saclay, Gif-Sur-Yvette, France
    e-mail: bobi@discovery.saclay.cea.fr
5  LUTH FRE2462 du CNRS, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
6  Université Denis Diderot, Place Jussieu, 75005 Paris, France
    e-mail: martine.mouchet@obspm.fr

(Received 25 July 2002 / Accepted 5 September 2002 )

We present the last pointed observation of AM Her carried out during the life of the BeppoSAX satellite. It was bright at the beginning of the observation, but dropped to the lowest X-ray level ever observed so far. The X-ray emission during the bright period is consistent with accretion occurring onto the main pole of the magnetized white dwarf. The rapid change from the active state to the low deep state indicates a drop by a factor of 17 in the accretion rate and hence that accretion switched-off. The short timescale (less than one hour) of this variation still remains a puzzle. Optical photometry acquired simultaneousy during the low state shows that the white dwarf remains heated, although a weak emission from the accretion stream could be still present. Cyclotron radiation, usually dominating the  V and  R bands, is negligible thus corroborating the possibility that AM Her was in an off-accretion state. The X-ray emission during the inactive state is consistent with coronal emission from the secondary late type star.

Key words: accretion, accretion disks -- stars: binaries: close -- stars: novae, cataclysmic variables -- stars: individual: AM Her -- X-rays: stars

Offprint request: D. de Martino, demartino@na.astro.it

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