We report on follow-up observations of candidate X-ray-bright, radio-quiet isolated neutron stars (INSs) identified from correlations of the ROSAT All-Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS) Data Release 4 in Agüeros et?al. We obtained Chandra X-ray Observatory exposures for 13 candidates in order to pinpoint the source of X-ray emission in optically blank RASS error circles. These observations eliminated 12 targets as good INS candidates. We discuss subsequent observations of the remaining candidate with XMM-Newton, the Gemini North Observatory, and the Apache Point Observatory. We identify this object as a likely extragalactic source with an unusually high log (f X/f opt) ~ 2.4. We also use an updated version of the population synthesis models of Popov et?al. to estimate the number of RASS-detected INSs in the SDSS Data Release 7 footprint. We find that these models predict ~3-4 INSs in the 11,000?deg2 imaged by SDSS, which is consistent with the number of known INSs that fall within the survey footprint. In addition, our analysis of the four new INS candidates identified in the SDSS footprint implies that they are unlikely to be confirmed as INSs; together, these results suggest that new INSs are not likely to be found from further correlations of the RASS and SDSS.
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机译：我们报告了从ROSAT全天调查（RASS）和斯隆数字天空调查（SDSS）数据发布4的相关性中识别出的候选X射线亮，放射性安静的孤立中子星（INS）的后续观察结果。 Agüeros等人。我们获得了13位候选人的钱德拉X射线天文台曝光量，以便在光学空白的RASS误差圈内查明X射线的发射源。这些观察结果消除了12个目标作为INS的良好候选对象。我们将与XMM-Newton，Gemini North天文台和Apache Point天文台讨论其余候选人的后续观察。我们认为该物体是可能的河外源，其对数高（f X / f opt）〜2.4。我们还使用Popov等人的人口综合模型的更新版本。估计SDSS Data Release 7占用空间中RASS检测到的INS的数量。我们发现，这些模型在SDSS成像的11,000？deg2中预测了约3-4个INS，这与调查范围内的已知INS的数量一致。此外，我们对SDSS足迹中确定的四个新INS候选者的分析表明，它们不太可能被确认为INS;总之，这些结果表明，不可能从RASS和SDSS的进一步关联中发现新的INS。
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