ON THE PREDICTED AND OBSERVED COLOR BOUNDARIES OF THE RR LYRAE INSTABILITY STRIP AS A FUNCTION OF METALLICITY

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 mag 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 Lac objects and obscured active galactic nuclei. 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, 9 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 INSs and perhaps enabling detailed follow-up studies of neutron star physics.