The isolated neutron star candidate RBS1223 (1RXS J130848.6+212708)

TL;DR

This paper reports the identification of RBS1223 as a likely isolated neutron star based on X-ray and optical observations, with a very high X-ray to optical flux ratio and no optical counterpart at deep limits.

Contribution

The study confirms RBS1223 as an isolated neutron star candidate using precise X-ray positioning and deep optical imaging, adding to the small sample of known objects of this type.

Findings

RBS1223 has an X-ray to optical flux ratio >4.1.

Deep optical imaging shows no counterpart down to B ~ 26m.

RBS1223 is similar to two known isolated neutron stars.

Abstract

In the ROSAT Bright Survey (RBS) we have almost completely optically identified the brightest ~2000 high-galactic latitude sources from the ROSAT All-Sky Survey Bright Source Catalogue (1RXS). A small number of sources has empty X-ray error circles on optical images. ROSAT HRI follow-up observations of RBS1223 (=1RXS J130848.6+212708), a soft object with extreme X-ray to optical flux ratio, have confirmed a relatively bright X-ray source, whose position could be determined to an accuracy of 1.6 arcsec (90%) due to the presence of a nearby, X-ray detected bright star. Deep Keck R- and B-band images of the field were taken, but the refined X-ray error circle remains empty to a limiting magnitude B ~ 26m. With an X-ray to optical flux ratio of log (f_X/f_opt)>4.1 this object is almost certainly an isolated neutron star, similar to the two so far best-known examples RX J1856.4-3754 and RX J0720.4-3125. We discuss limits on the number of similar objects in the RBS catalogue.