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Issue A&A Volume 408, Number 1, September II 2003 Page(s) 323 - 330 Section Stellar atmospheres DOI 10.1051/0004-6361:20030802

A&A 408, 323-330 (2003)
DOI: 10.1051/0004-6361:20030802

The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125

C. Motch¹, V. E. Zavlin² and F. Haberl<sup>2

1</sup>  Observatoire Astronomique, UA 1280 CNRS, 11 rue de l'Université, 67000 Strasbourg, France
²  Max-Planck-Institut für extraterrestrische Physik, 85740 Garching bei München, Germany

(Received 12 March 2003 / Accepted 12 May 2003 )

Abstract
ESO 4 m class telescope and VLT deep imaging of the isolated neutron star RX J0720.4-3125 reveals a proper motion of mas/yr and a blue U-B color index. We show that a neutron star atmosphere model modified to account for a limited amount of hydrogen on the star's surface can well represent both the optical and X-ray data without invoking any additional thermal component. The large proper motion almost completely excludes the possibility that accretion from the interstellar medium is the powering mechanism of the X-ray emission. It also implies that the proposed spin down is entirely due to magnetic dipole losses. RX J0720.4-3125 is thus very likely a middle aged cooling neutron star. Its overall properties are quite similar to some of the long period radio pulsars recently discovered, giving further support to the idea that RX J0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.

Key words: stars: newton -- X-rays: individual: RX J0720.4-3125

Offprint request: C. Motch, motch@newb6.u-strasbg.fr

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© ESO 2003

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