An euve observation of the globular cluster M15: The first detection of a low-mass x-ray binary?

We have detected EUV emission from the globular cluster M15 using the Deep Survey Photometer aboard the Extreme Ultraviolet Explorer. The emission is variable at the 97% confidence level. The minimum EUV luminosity implied by our detection is ~∼ × 1036 ergs s^-1 for a distance and reddening appropriate to M15. We have examined a number of possible origins for this emission including post-asymptotic giant branch stars, a population of hitherto unknown, optically faint, stellar merger products or "supersoft sources," or the well-known M15 low-mass X-ray binary (LMXB) AC 211. A significant EUV flux from AC 211 is supported by the relatively strong He II λ4686 emission observed from this system. If the observed EUV flux indeed originates from AC 211, this is the first detection of an LMXB at EUV energies. Furthermore, such a luminosity is comparable to the 2-10 keV X-ray luminosity of AC 211 and may dominate the energetics of the system if absorption local to the binary is taken into account. Further observations of this and other low column LMXBs (e.g., that in NGC 1851) are required to establish the ubiquity of globular cluster/LMXB EUV emission.