The identity and position of X-ray sources in globular clusters: Radio emission from NGC6712

NINE globular clusters are known to contain luminous (L_x> 10³⁶ erg s^-1) X-ray sources, but because the X-ray position error boxes, a few arcseconds in size, contain many stars, optical iden-tification of the sources has not been possible, with the exception of that in NGC7078 (ref. 19). This has prevented detailed investiga-tion of the emitting objects. If the X-ray sources are similar to low-mass X-ray binaries (LMXBs) outside globular clusters, low-level radio emission would be expected^1,2, and the detection of such radio emission would both strengthen the case that luminous globular cluster X-ray sources are LMXBs and aid optical iden-tification by means of an accurate radio position. We report here the detection of radio emission from the position of the luminous X-ray source A1850-08 (ref. 3) in the globular cluster NGC6712. The centroid of the slightly elongated radio source coincides with a previously suggested 20th magnitude stellar optical counterpart⁴. If the radio emission is synchrotron radiation, the minimum energy requirement and magnetic field strength are similar to what is found in the radio counterparts to other high-luminosity X-ray sources such as Sco X-l and Cyg X-3.