A Limit on the Number of Isolated Neutron Stars Detected in the ROSAT Bright Source Catalog

The challenge in searching for non-radio-pulsing isolated neutron stars (INSs) is in excluding association with objects in the very large error boxes (~13", 1 sigma radius) typical of sources from the largest X-ray all-sky survey, the ROSAT All-Sky-Survey/Bright Source Catalog (RASS/BSC). We search for candidate INSs using statistical analysis of optical (USNO-A2), infrared (IRAS), and radio (NVSS) sources near the ROSAT X-ray localization, and show that this selection would find 20% of the INSs in the RASS/BSC. This selection finds 32 candidates at declinations greater than -39 deg, among which are two previously known INSs, seventeen sources which we show are not INSs, and thirteen the classification of which are as yet undetermined. These results require a limit of <67 INSs (90% confidence, full sky, assuming isotropy) in the RASS/BSC. This limit modestly constrains a naive and optimistic model for cooling NSs in the galaxy.