2004A&A...424..635H

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). RX J0806.4-4123 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 11.371s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT=96eV and N_H=4x10¹⁹cm^–2 without significant changes between the two observations. Four XMM-Newton observations of RX J0420.0-5022 between December 2002 and July 2003 did not confirm the 22.7s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT=45eV, the lowest value found from the small group of XDINs and N_H=1.0x10²⁰cm^–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 RBS1223, RX J1605.3+3249 and RX J0720.4-3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0-5022 with B=26.6±0.3mag from VLT imaging.