RX J185635-3754: X-Ray Astrometry

The ROSAT positions are subject to systematic uncertainties of up to 10 arcsec in rare cases. We calibrated the boresight correction by using another source in the HRI field which we have identified as a pre-main sequence star (GSC 7916 0050) [10]. Applying the offset between the default X-ray position and the optical position, and assuming an HRI pixel size of 0.4979 arcsec instead of the canonical 0.5" [10], we find the X-ray source position to be

18h 56m 35.29s, -37 54' 34.4"
on the HST GSC reference frame. The statistical uncertainty on the position is 0.5 arcsec; we quote a conservative total uncertainty of 5.0 arcsec.

references:
10: Neuhaeuser, R., Thomas, H.-C., Danner, R., Peschke, S., & Walter, F.M. 1996. A&A, 318, L43.

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