An Ultracompact X-Ray Binary in the Globular Cluster M15 (NGC 7078)

We have used the Advanced Camera for Surveys on board the Hubble SpaceTelescope to image the core of the globular cluster M15 in the far-ultraviolet(FUV) waveband. Based on these observations, we identify the FUV counterpart ofthe recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUVphotometry shows a modulation with 0.062+/-0.004 mag semi-amplitude and weclearly detect a period of 22.5806+/-0.0002 min. We have carried out extensiveMonte Carlo simulations which show that the signal is consistent with beingcoherent over the entire observational time range of more than 3000 cycles.This strongly suggests that it represents the orbital period of the binarysystem. M15 X-2 is FUV bright (approx. 17 mag) and is characterized by anextremely blue spectral energy distribution (F_lambda ~ lambda^{-2.0}). We alsofind evidence for an excess of flux between 1500 and 1600 \AA and probablybetween 1600 and 2000 \AA, which might be due to CIV 1550 and HeII 1640emission lines. We also show that M15 X-2's X-ray luminosity can be powered byaccretion at the rate expected for gravitational-wave-driven mass transfer atthis binary period. The observed FUV emission appears to be dominated by anirradiated accretion disk around the neutron star primary, and the variabilitycan be explained by irradiation of the low-mass white dwarf donor if theinclination of the system is approx. 34 degree. We conclude that allobservational characteristics of M15 X-2 are consistent with it being anultracompact X-ray binary, only the third confirmed such object in a globularcluster.Comment: 13 pages, including 4 figures, accepted for publication in ApJ Letter