An ultracompact X-ray binary in the globular cluster M15 (NGC 7078)

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet ( FUV) wave band. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062 +/- 0.004 mag semiamplitude, and we clearly detect a period of 22.5806 +/- 0.0002 minutes. We have carried out extensive Monte Carlo simulations, which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV-bright ( FUV similar or equal to 17 mag) and is characterized by an extremely blue spectral energy distribution ( F-lambda proportional to lambda(-2.0)). We also find evidence for an excess of flux between 1500 and 1600 angstrom and probably between 1600 and 2000 that might be due to C IV 1550 angstrom and He II 1640 angstrom emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for mass transfer driven by gravitational waves at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is approximate to 34 degrees. We conclude that all observational characteristics of M15 X-2 are consistent with its being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.