Sharp Chandra View of ROSAT All-Sky Survey Bright Sources: I. Improvement of Positional Accuracy

TL;DR

This study enhances the positional accuracy of ROSAT Bright Source Catalog X-ray sources by cross-matching with high-resolution Chandra data, improving optical identification prospects and refining source classifications.

Contribution

It introduces a method to improve ROSAT source positions using Chandra observations, aiding more accurate multi-wavelength identification and classification.

Findings

Positional accuracy of RASS sources is significantly improved.

Refined source identifications impact the classification of neutron star candidates.

Method enhances multi-wavelength studies of X-ray sources.

Abstract

The ROSAT All-Sky Survey (RASS) represents one of the most complete and sensitive soft X-ray all-sky surveys to date. However, the deficient positional accuracy of the RASS Bright Source Catalog (BSC) and subsequent lack of firm optical identifications affect the multi-wavelength studies of X-ray sources. The widely used positional errors $\sigma_{pos}$ based on the Tycho Stars Catalog (Tycho-1) have previously been applied for identifying objects in the optical band. The considerably sharper Chandra view covers a fraction of RASS sources, whose $\sigma_{pos}$ could be improved by utilizing the sub-arcsec positional accuracy of Chandra observations. We cross-match X-ray objects between the BSC and \emph{Chandra} sources extracted from the Advanced CCD Imaging Spectrometer (ACIS) archival observations. A combined counterparts list (BSCxACIS) with \emph{Chandra} spatial positions weighted by the X-ray flux of multi-counterparts is employed to evaluate and improve the former identifications of BSC with the other surveys. Based on these identification evaluations, we suggest that the point-likeness of BSC sources and INS (isolated neutron stars) candidates should be carefully reconsidered.