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Register a new userAn Optical Counterpart Candidate for the Isolated Neutron Star RBS1774
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Multiwavelength studies of the seven identified X-ray dim isolated neutron stars (XDINSs) offer a unique opportunity to investigate their surface thermal and magnetic structure and the matter-radiation interaction in presence of strong gravitational and magnetic fields. As a part of an ongoing campaign aimed at a complete identification and spectral characterization of XDINSs in the optical band, we performed deep imaging with the ESO Very Large Telescope (VLT) of the field of the XDINS RBS1774 (1RXS J214303.7 +065419). The recently upgraded FORS1 instrument mounted on the VLT provided the very first detection of a candidate optical counterpart in the B band. The identification is based on a very good positional coincidence with the X-ray source (chance probability ~2E-3). The source has B=27.4 +/- 0.2 (1 sigma confidence level), and the optical flux exceeds the extrapolation of the X-ray blackbody at optical wavelengths by a factor ~35 (+/- 20 at 3sigma confidence level). This is barely compatible with thermal emission from the neutron star surface, unless the source distance is d~200-300 pc, and the star is an almost aligned rotator or its spin axis is nearly aligned with the line of sight. At the same time, such a large optical excess appears difficult to reconcile with rotation-powered magnetospheric emission, unless the source has an extremely large optical emission efficiency. The implications and possible similarities with the optical spectra of other isolated NSs are discussed.
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Over the last decade, X-ray observations unveiled the existence of several classes of isolated neutron stars (INSs) which are radio-quiet or exhibit radio emission with properties much at variance with those of ordinary radio pulsars. The identification of new sources is crucial in order to understand the relations among the different classes and to compare observational constraints with theoretical expectations. A recent analysis of the 2XMMp catalogue provided less than 30 new thermally emitting INS candidates. Among these, the source 2XMM J104608.7-594306 appears particularly interesting because of the softness of its X-ray spectrum and of the present upper limits in the optical, which imply a logarithmic X-ray-to-optical flux ratio greater than 3.1, corrected for absorption. We present the X-ray and optical properties of 2XMM J104608.7-594306 and discuss its nature in the light of two possible scenarios invoked to explain the X-ray thermal emission from INSs: the release of residual heat in a cooling neutron star, as in the seven radio-quiet ROSAT-discovered INSs, and accretion from the interstellar medium. We find that the present observational picture of 2XMM J104608.7-594306 is consistent with a distant cooling INS with properties in agreement with the most up-to-date expectations of population synthesis models: it is fainter, hotter and more absorbed than the seven ROSAT sources and possibly located in the Carina Nebula, a region likely to harbour unidentified cooling neutron stars. The accretion scenario, although not entirely ruled out by observations, would require a very slow (~10 km/s) INS accreting at the Bondi-Hoyle rate.
Deep optical B band images of the ROSAT HRI error region of RX J0720.4-3125 reveal the presence of two faint stellar-like objects with B = 26.1 +/- 0.25 and B = 26.5 +/- 0.30. Exposures obtained through U, V and I filters are not sensitive enough to detect the two candidates and provide upper limits of U = 24.9, V = 23.2 and I = 21.9. These new observations virtually establish that RX J0720.4-3125 is a slowly rotating, probably completely isolated neutron star. The absence of an optical counterpart brighter than B = 26.1 seems incompatible with a neutron star atmosphere having a chemical composition dominated by Hydrogen or Helium. UBI photometry of field stars shows astonishingly little interstellar reddening in the direction of the X-ray source. Together with the small column density detected by the ROSAT PSPC, this suggests a mean particle density in the range of n = 0.1 - 0.4 cm-3. Such average densities would imply very low velocities relative to interstellar medium (Vrel < 10 km/s) if the source were powered by accretion. These stringent constraints may be relaxed if the neutron star is presently crossing a small size structure of higher density or if the effective temperature of the heated atmosphere is overestimated by the blackbody approximation. Alternatively, RX J0720.4-3125 could be a young and highly magnetized cooling neutron star.
In the ROSAT Bright Survey (RBS) we have almost completely optically identified the brightest ~2000 high-galactic latitude sources from the ROSAT All-Sky Survey Bright Source Catalogue (1RXS). A small number of sources has empty X-ray error circles on optical images. ROSAT HRI follow-up observations of RBS1223 (=1RXS J130848.6+212708), a soft object with extreme X-ray to optical flux ratio, have confirmed a relatively bright X-ray source, whose position could be determined to an accuracy of 1.6 arcsec (90%) due to the presence of a nearby, X-ray detected bright star. Deep Keck R- and B-band images of the field were taken, but the refined X-ray error circle remains empty to a limiting magnitude B ~ 26m. With an X-ray to optical flux ratio of log (f_X/f_opt)>4.1 this object is almost certainly an isolated neutron star, similar to the two so far best-known examples RX J1856.4-3754 and RX J0720.4-3125. We discuss limits on the number of similar objects in the RBS catalogue.
We report on the identification of a new possible Isolated Neutron Star candidate in archival ROSAT observations. The source 1RXS J214303.7+065419, listed in the ROSAT Bright Survey as RBS 1774, is very soft, exhibits a thermal spectrum well fitted by a blackbody at ${T}sim 90$ eV and has a low column density, ${N_H}sim 5times 10^{20}$ ${cm}^{-2}$. Catalogue searches revealed no known sources in other energy bands close to the X-ray position of RBS 1774. Follow-up optical observations with NTT showed no peculiar object within the X-ray error circle. The absence of any plausible optical counterpart down to ${m_R}sim 23$ results in an X-ray to optical flux ratio in excess of 1000.
The region of the Small Magellanic Cloud (SMC) with which this paper is concerned contains the highest concentration of IRAS/Spitzer sources, H I emission, and molecular clouds in this neighboring galaxy. However very few studies have been devoted to it, despite these signs of star formation. We present the first detailed study of the compact H II region N33 in the SMC by placing it in a wider context of massive star formation. Moreover, we show that N33 is a particularly interesting candidate for isolated massive star formation. This analysis is based mainly on optical ESO NTT observations, both imaging and spectroscopy, coupled with other archive data, notably Spitzer images (IRAC 3.6, 4.5, 5.8, and 8.0 mic) and 2MASS observations. We derive a number of physical characteristics of the compact H II region N33 for the first time. This gas and dust formation of 7.4 (2.2 pc) in diameter is powered by a massive star of spectral type O6.5-O7 V. The compact H II region belongs to a rare class of H II regions in the Magellanic Clouds, called high-excitation blobs (HEBs). We show that this H II region is not related to any star cluster. Specifically, we do not find any traces of clustering around N33 on scales larger than 10 (~ 3 pc). On smaller scales, there is a marginal stellar concentration, the low density of which, below the 3 sigma level, does not classify it as a real cluster. We also verify that N33 is not a member of any large stellar association. Under these circumstances, N33 is also therefore attractive because it represents a remarkable case of isolated massive-star formation in the SMC. Various aspects of the relevance of N33 to the topic of massive-star formation in isolation are discussed.
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