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

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 Catalogue (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 (delta) > -39..., among which are two previously known INSs, 17 sources that we show are not INSs, and 13 the classification of which are as yet undetermined. These results require a limit of less than 67 INSs (90% confidence, full sky, assuming isotropy) in the RASS BSC. This limit modestly constrains a naive and optimistic model for cooling neutron stars in the Galaxy.