Free access article

A&A 451, L17-L21 (2006)
DOI: 10.1051/0004-6361:20065093

Letter

Evidence for precession of the isolated neutron star RX J0720.4-3125

F. Haberl¹, R. Turolla², C. P. De Vries³, S. Zane⁴, J. Vink⁵, M. Méndez³ and F. Verbunt<sup>5

1</sup>  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
    e-mail: fwh@mpe.mpg.de
²  Department of Physics, University of Padua, via Marzolo 8, 35131 Padova, Italy
³  SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴  Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
⁵  University Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands

(Received 26 February 2006 / Accepted 23 March 2006)

Abstract
The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 obtained over 4.5 years can be described by sinusoidal variations in the inferred blackbody temperature, the size of the emitting area and the depth of the absorption line with a period of years, which we suggest to be the precession period of the neutron star. Precession of a neutron star with two hot spots of different temperature and size, probably not located exactly in antipodal positions, may account for the variations in the X-ray spectra, changes in the pulsed fraction, shape of the light curve and the phase-lag between soft and hard energy bands observed from RX J0720.4-3125. An independent sinusoidal fit to published and new pulse timing residuals from a coherent analysis covering ~12 years yields a consistent period of  years supporting the precession model.

Key words: stars: individual: RX J0720.4-3125 -- stars: neutron -- stars: magnetic fields -- X-rays: stars

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© ESO 2006

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