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Astron. Astrophys. 351, L53-L57 (1999)

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Letter to the Editor

RX J0420.0-5022: an isolated neutron star candidate with evidence for 22.7 s X-ray pulsations *

F. Haberl 1, W. Pietsch 1 and C. Motch 2

1 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
2 Observatoire de Strasbourg, 11, rue de l'Universite, 67000 Strasbourg, France

Received 2 September 1999 / Accepted 19 October 1999


We report the discovery of a new isolated neutron star candidate, RX J0420.0-5022, showing evidence ([FORMULA]) for 22.7 s X-ray pulsations in ROSAT data. NTT observations of the field around the soft X-ray source do not reveal any likely optical counterpart brighter than B = 25.25 implying an X-ray to optical flux ratio of [FORMULA] and ruling out other possible kinds of X-ray emitters. The X-ray spectrum of RX J0420.0-5022 can be described with blackbody emission with temperature kT of [FORMULA]57 eV and four ROSAT detections are consistent with no flux variations on time scales of years. The X-ray pulsations, if confirmed, make RX J0420.0-5022 the second long-period isolated neutron star candidate after RX J0720.4-3125. As for this latter source similar conclusions about the magnetic field strength of the neutron star can be drawn depending on evolutionary scenarios.

Key words: stars: individual: RX J0420.0 – 5022 – stars: neutron – X-rays: stars

* Partly based on NTT observations performed at the European Southern Observatory, La Silla, Chile

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

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999