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Astron. Astrophys. 326, 662-668 (1997)

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RX J0720.4-3125:
strong evidence for an isolated pulsating neutron star *

F. Haberl 1, C. Motch 2, D.A.H. Buckley 3, F.-J. Zickgraf 1 and W. Pietsch 1

1 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, D-85748 Garching, Germany
2 Observatoire de Strasbourg, 11, rue de l'Universite, F-67000 Strasbourg, France
3 South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa

Received 29 October 1996 / Accepted 14 May 1997


We report the discovery of RX J0720.4-3125, a soft X-ray source showing 8.391 s pulsations, from the ROSAT all-sky survey. The relative constant long-term X-ray intensity, the blackbody-like spectrum with little photo-electric absorption and the limit for f [FORMULA] /f [FORMULA] of more than 500 are consistent with an isolated neutron star accreting from the interstellar medium (all very similar to RX J1856.5-3754, so far the best candidate). We estimate the magnetic field strength of the neutron star to be less than 1010 G. RX J0720.4-3125 may have emerged from common envelope evolution of a high mass X-ray binary. The final neutron star is expected to be close the galactic plane, have a low space velocity and a low magnetic field, accreting interstellar matter very effectively. In this case RX J0720.4-3125 would not belong to the expected large group of old neutron stars evolved from single stars. The low derived magnetic field strength and the pulse period are however also compatible with an old (109 yr) neutron star in which case we see a neutron star from the very low end of the velocity distribution. The low number of isolated neutron star candidates from the ROSAT all-sky survey remains to be explained.

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

* Partly based on observations collected at the South African Astronomical Observatory and the European Southern Observatory at La Silla

Send offprint requests to: F. Haberl (fwh@mpe-garching.mpg.de)

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997