We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M 15 in the far-ultraviolet ( FUV ) waveband .
Based on these observations , we identify the FUV counterpart of the recently discovered low-mass X-ray binary M 15 X-2 .
Our time-resolved FUV photometry shows a modulation with 0.062 \pm 0.004 mag semi-amplitude and we clearly detect a period of 22.5806 \pm 0.0002 min .
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 .
M 15 X-2 is FUV bright ( FUV \simeq 17 mag ) and is characterized by an extremely blue spectral energy distribution ( F _ { \lambda } \propto \lambda ^ { -2.0 } ) .
We also find evidence for an excess of flux between 1500 Å and 1600 Å and probably between 1600 Å and 2000 Å which might be due to C IV 1550 Å and He II 1640 Å emission lines .
We also show that M15 X-2 ’ s X-ray luminosity can be powered by accretion at the rate expected for gravitational-wave-driven mass transfer 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 \approx 34 ^ { \circ } .
We conclude that all observational characteristics of M15 X-2 are consistent with it being an ultracompact X-ray binary , only the third confirmed such object in a globular cluster .