No Confirmed New Isolated Neutron Stars In The SDSS Data Release 4

TL;DR

This study conducted follow-up observations on candidate isolated neutron stars from SDSS and RASS data, finding no new confirmed INSs and suggesting such discoveries are unlikely with current correlation methods.

Contribution

The paper provides a comprehensive follow-up analysis of candidate INSs, refining the candidate list and updating population models to estimate the expected number of INSs in SDSS data.

Findings

Most candidates were ruled out as INSs after follow-up observations.

Population models predict 3-4 INSs in the SDSS footprint, aligning with known numbers.

Additional candidates are unlikely to be confirmed as INSs.

Abstract

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 Ag\"ueros et al. (2006). We obtained Chandra X-ray Telescope 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 the XMM-Newton X-ray Observatory, the Gemini North Observatory, and the Apache Point Observatory. We identify this object as a likely extragalactic source with an unusually high log(fX/fopt) ~ 2.4. We also use an updated version of the population synthesis models of Popov et al. (2010) 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 square deg 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 by Turner et al. (2010) 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.