  Only seven radio-quiet isolated neutron stars (INSs) emitting thermal X rays
are known, a sample that has yet to definitively address such fundamental
issues as the equation of state of degenerate neutron matter. We describe a
selection algorithm based on a cross-correlation of the ROSAT All-Sky Survey
(RASS) and the Sloan Digital Sky Survey (SDSS) that identifies X-ray error
circles devoid of plausible optical counterparts to the SDSS g~22 magnitudes
limit. We quantitatively characterize these error circles as optically blank;
they may host INSs or other similarly exotic X-ray sources such as radio-quiet
BL Lacs, obscured AGN, etc. Our search is an order of magnitude more selective
than previous searches for optically blank RASS error circles, and excludes the
99.9% of error circles that contain more common X-ray-emitting subclasses. We
find 11 candidates, nine of which are new. While our search is designed to find
the best INS candidates and not to produce a complete list of INSs in the RASS,
it is reassuring that our number of candidates is consistent with predictions
from INS population models. Further X-ray observations will obtain pinpoint
positions and determine whether these sources are entirely optically blank at
g~22, supporting the presence of likely isolated neutron stars and perhaps
enabling detailed follow-up studies of neutron star physics.
