  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.
