The isolated neutron star X-ray pulsars RX J0420.0–5022 and RX J0806.4–4123: New X-ray and optical observations^*

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany e-mail: fwh@mpe.mpg.de
² Observatoire Astronomique, CNRS UMR 7550, 11 rue de l'Université, 67000 Strasbourg, France
³ Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany
⁴ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁶ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Dipartimento di Fisica, Universitá di Padova, via Marzolo 8, 35131 Padova, Italy

Received: 15 March 2004
Accepted: 18 May 2004

Abstract

We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs).  was observed with XMM-Newton in April 2003, 2.5 years after the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature eV and N_H = 4 cm^-2 without significant changes between the two observations. Four XMM-Newton observations of  between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields eV, the lowest value found from the small group of XDINs and N_H = 1.0 cm^-2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from ,  and . For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for  with mag from VLT imaging.