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

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). RXJ0806.4-4123 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 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N-H = 4 x 10(19) cm(-2) without significant changes between the two observations. Four XMM-Newton observations of RXJ0420.0-5022 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 kT = 45 eV, the lowest value found from the small group of XDINs and N-H = 1.0 x 10(20) 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 RBS1223, RXJ1605.3+3249 and RXJ0720.4-3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RXJ0420.0-5022 with B = 26.6 +/- 0.3 mag from VLT imaging.