A Limit on the Number of Isolated Neutron Stars Detected in the ROSAT All-Sky Survey Bright Source Catalog

TL;DR

This study sets new upper limits on the number of isolated neutron stars in the ROSAT catalog using X-ray observations, identifies new candidates, and discusses future survey prospects to improve detection.

Contribution

The paper provides the most stringent limits to date on the number of isolated neutron stars in the ROSAT catalog and identifies new candidates for follow-up.

Findings

Number of INSs <=48 at 90% confidence

All-sky limit of <=31 for soft, non-variable sources

Future surveys could detect up to 1500 INSs

Abstract

Using new and archival observations made with the Swift satellite and other facilities, we examine 147 X-ray sources selected from the ROSAT All-Sky-Survey Bright Source Catalog (RASS/BSC) to produce a new limit on the number of isolated neutron stars (INSs) in the RASS/BSC, the most constraining such limit to-date. Independent of X-ray spectrum and variability, the number of INSs is <=48 (90% confidence). Restricting attention to soft (having an effective temperature of < 200 eV), non-variable X-ray sources -- as in a previous study -- yields an all-sky limit of <=31 INSs. In the course of our analysis, we identify five new high-quality INS candidates for targeted follow-up observations. A future all-sky X-ray survey with eROSITA, or another mission with similar capabilities, can be expected to increase the detected population of X-ray-discovered INSs from the 8 to 50 in the BSC, to (for a disk population) 240 to 1500, which will enable a more detailed study of neutron star population models.