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The remarkable astrometric capabilities of the Chandra Observatory offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RX J0420.0-5022, RX J0806.4-4123and RX J1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as a reference the background of extragalactic or remote Galactic X-ray sources. We derive 2σupper limits of 123 mas yr-1and 86 mas yr-1on the proper motion of RX J0420.0-5022and RX J0806.4-4123, respectively. RX J1308.6+2127exhibits a very significant displacement (~9σ) yielding μ= 220 ±25 mas yr-1, the second fastest measured among all ROSAT-discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ~740 (d/700 pc) km s-1might be the largest of all ROSAT INSs and its corresponding spatial velocity lies among the fastest recorded for neutron stars. RX J1308.6+2127is thus a middle-aged (age ~1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. Overall, the distribution in transverse velocity of the ROSAT INSs is not statistically different from that of normal radio pulsars.

We obtained deep optical imaging of the thermally emitting X-ray bright and radio-quiet isolated neutron star RX J1605.3+3249with the Subaru telescope in 1999 and 2003. Together with archival HST images acquired in 2001 these data reveal a proper motion of $\mu = 144.5\pm13.2$mas/yr. This implies a relatively high spatial velocity and indicates that the star is unlikely to be re-heated by accretion of matter from the interstellar medium. Assuming that RX J1605.3+3249is a young (10$^{5}{-}10^{6}$yr) cooling neutron star, its apparent trajectory is consistent with a birth in the nearby Sco OB2 OB association at a location close to that derived for RX J1856.5-3754and perhaps also to that of RX J0720.4-3125. This suggests that the X-ray bright part of ROSAT-discovered isolated neutron stars is dominated by the production of the Sco OB2 complex which is the closest OB association and a part of the Gould belt. The Band Rmagnitudes of the faint optical counterpart did not vary from 1999 to 2003 at $B = 27.22\pm0.10$. Its $B-R$colour index of $+0.32\pm0.17$is significantly redder than that of other isolated neutron stars and the optical flux lies a factor 11.5 above the extrapolation of the X-ray blackbody-like spectrum. The red optical colour reveals the presence of an additional emitting component in the optical regime over the main neutron star thermal emission. We also discovered a small elongated Hαnebula approximately centered on the neutron star and aligned with the direction of motion. The width of the nebula is unresolved and smaller than ~0.4´´ for a length of about 1´´. The shape of the Balmer emitting nebula around RX J1605.3+3249is very different from those seen close to other neutron stars and should be confirmed by follow-up observations. We shortly discuss the possible mechanisms which could give rise to such a geometry.

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 $kT = 96$eV and NH= 4$\,\times \,10^{19}$cm-2without 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 $kT = 45$eV, the lowest value found from the small group of XDINs and NH= 1.0$\,\times \,10^{20}$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 ,  and . For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for  with $B = 26.6\pm0.3$mag from VLT imaging.

The XMM-Newton spectra of the isolated neutron star  show deviations from a Planckian energy distribution below 400 eV, similar to the spectra of RBS1223, another long-period X-ray pulsar, as reported recently by [CITE]. For a Gaussian-shaped absorption line we derive an energy of 271 eV and an equivalent width of -40 eV from the phase-averaged spectra of . We investigate the spectral variations seen in hardness ratios as function of pulse phase and find that they are best described by changes in the depth of the absorption line. The line equivalent width changes between -31 eV around intensity maximum of the pulse and -58 eV at the declining part of the pulse. Small variations (<20 eV) of the line energy with pulse phase may still be caused by statistical fluctuations. On the other hand, the black-body temperature varies significantly by 2.5 eV (statistical 90% errors typically 0.7 eV) reaching the highest value at pulse maximum. One possible interpretation for the absorption line is cyclotron resonance scattering of protons in a magnetic field with $B \simeq 5\,\times\, 10^{13}$G. This field strength is compatible with estimates inferred from recent spin down measurements of the pulsar.

We present results of the analysis of data collected in 57-ks XMM-Newtonand 505-ks Chandraobservations of the nearby ($\simeq $120 pc) isolated neutron star RX J1856.5–3754. We confirm most of the statements made by Burwitz et al. (2001) who discussed the original 55-ks Chandradata. Detailed spectral analysis of the combined X-ray and optical data rules out the currently available nonmagnetic light and heavy element neutron star atmosphere (LTE) models with hydrogen, helium, iron and solar compositions. We find that strongly magnetized atmosphere models also are unable to represent the data. The X-ray and optical data show no spectral features and are best fitted with a two-component blackbody model with $kT_{\rm bb,X}^\infty \simeq 63.5$eV and $R_{\rm bb,X}^\infty\simeq\,4.4\,(d/120\,\rm pc)$km for the hot X-ray emitting region, and $kT_{\rm bb,opt}^\infty < 33$eV and $R_{\rm bb,opt}^\infty > $17 (d/120 pc) km for the rest of the neutron star surface responsible for the optical flux. The large number of counts collected with XMM-Newtonallows us to reduce the upper limit on periodic variation in the X-ray range down to 1.3% (at a 2σconfidence level) in the $10^{-3}{-}50$Hz frequency range. In an attempt to explain this small variability, we discuss an one-component model with $kT_{\rm bb}^{\infty}\simeq 63$eV and $R_{\rm bb}^{\infty}\simeq 12.3\,(d/120\,{\rm pc})$km. This model requires a low radiative efficiency in the X-ray domain, which may be expected if the neutron star has a condensed matter surface.

The isolated neutron star  was observed with XMM-Newton in November 2000. The data from the three EPIC instruments allowed us (i) to derive an improved X-ray position to an accuracy of 2–3´´, (ii) to accumulate the first medium-resolution soft X-ray spectra of high statistical quality and (iii) to find a candidate for the neutron star rotation period. Although this period of 11.3714 s is formally detected at a 3.5σlevel in the EPIC-pn data, the similar pulse profiles deduced from all three EPIC instruments increase the confidence that the period is real. The pulsed fraction of ~6% would then be the weakest X-ray flux modulation detected from dim isolated neutron stars. We fitted the X-ray spectra with blackbody and neutron star atmosphere models and discuss the results with respect to the brightness limit placed by optical images. The reduced size of the error circle on the X-ray position should allow deeper searches for an optical counterpart.

We present the ChandraLETGS X-ray spectrum of the nearby (${\simeq} 60$pc) neutron star RXJ1856.5-3754. Detailed spectral analysis of the combined X-ray and optical data rules out the nonmagnetic neutron star atmosphere models with hydrogen, helium, iron and solar compositions. We also conclude that strongly magnetized atmosphere models are unable to represent the data. The data can be explained with a two-component blackbody model. The harder component with temperature of $kT_{\rm bb}^\infty \simeq 63$eV and a radius $R_{\rm bb}^\infty\simeq 2.2$km of the emitting region well fits the X-ray data and can be interpreted as radiation from a hot region on the star's surface.

We present the XMM-Newtonfirst light image, taken in January 2000 with the EPIC pn camera during the instrument's commissioning phase, when XMM-Newtonwas pointing towards the Large Magellanic Cloud (LMC). The field is rich in different kinds of X-ray sources: point sources, supernova remnants (SNRs) and diffuse X-ray emission from LMCinterstellar gas. The observations are of unprecedented sensitivity, reaching a few 10$\sp{32}$erg/s for point sources in the LMC. We describe how these data sets were analysed and discuss some of the spectroscopic results. For the SNR N157Bthe power law spectrum is clearly steeper than previously determined from ROSAT and ASCA data. The existence of a significant thermal component is evident and suggests that N157Bis not a Crab-like but a composite SNR. Most puzzling is the spectrum of the LMChot interstellar medium, which indicates a significant overabundance of Ne and Mg of a few times solar.

We present the high resolution spectrum of the isolated neutron star RX J0720.4-3125, obtained with the Reflection Grating Spectrometer on XMM-Newton, complemented with the broad band spectrum observed with the EPIC PN camera. The spectrum appears smooth, with no evidence for strong photospheric absorption or emission features. We briefly discuss the implications of our failure to detect structure in the spectrum.

An approximate method is presented for computing the ionization equilibrium and opacities in the photospheres of cooling neutron stars. The method is based on occupation probability formalism which describes the lowering of the ionization potential due to plasma microfield effect. The approach is simple and reproduces rather accurately the results of a more selfconsistent and sophisticated Hummer-Mihalas theory for a low-temperature and high-density H-He plasma. It is shown that the opacities and spectra of radiation outgoing from the photospheres of cooling neutron stars are insensitive to the type of the microfield distribution, and to the method of including the ionization potential lowering, as long as the neutron star effective temperatures Teff ≳ 105 K.

Context.Since the launch of the Fermi satellite, the number of known gamma-ray pulsars has increased tenfold. Most gamma-ray detected pulsars are young and energetic, and many are associated with TeV sources. PSR J1357−6429 is a high spin-down power pulsar (Ė= 3.1 × 1036erg s-1), discovered during the Parkes multibeam survey of the Galactic plane, with significant timing noise typical of very young pulsars. In the very-high-energy domain (E> 100 GeV), H.E.S.S. has reported the detection of the extended source HESS J1356−645 (intrinsic Gaussian width of 12′) whose centroid lies 7′ from PSR J1357−6429.