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). 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 11.371 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 × 10 19 cm −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 22.7 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 × 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, 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 ± 0.3 mag from VLT imaging.
Highlights
The two soft X-ray sources RX J0806.4−4123 and RX J0420.0−5022 belong to the group of X-ray dim isolated neutron stars (XDINs) discovered in the ROSAT all-sky survey data
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 discovered by Haberl et al (1998) in a dedicated search for XDINs applying spectral hardness ratio selection criteria to the ROSAT survey source catalogue restricted to the galactic plane
Summary
The two soft X-ray sources RX J0806.4−4123 and RX J0420.0−5022 belong to the group of X-ray dim isolated neutron stars (XDINs) discovered in the ROSAT all-sky survey data (for recent reviews see Treves et al 2000; Motch 2001; Haberl 2004). Follow-up ROSAT PSPC and HRI observations and ESO-NTT images, which revealed no optical counterpart brighter than B = 25.25, ruled out possible kinds of known X-ray emitters other than an isolated neutron star (Haberl et al 1999). We derive accurate X-ray positions utilizing the imaging capabilities of Chandra and derive X-ray spectral parameters from the high statistical quality spectra collected by the EPIC-pn instrument on board XMM-Newton. We discuss the measured properties in comparison with the other known XDINs
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