2004MNRAS.350..649V

We present integral field spectroscopy of X2127+119, the luminous X-ray binary in the globular cluster M15, obtained with INTEGRAL/WYFFOS on the William Herschel Telescope. We find that tomograms of HeIIλ4686 line profiles appear to be incompatible with the previously assumed view of X2127+119, in which the binary consists of a 1.4-M_☉neutron star and a 0.8-M_☉ subgiant companion near the main-sequence turn-off for M15. Our data imply a much smaller mass ratio M₂/M_Xof ∼0.1. We find that models of X2127+119 with black hole compact objects give a poor fit to our data, while a neutron-star compact object is consistent with the data, implying that the companion of X2127+119 may have a much lower mass (∼0.1 M_☉) than previously assumed. As a ∼0.1-M_☉main-sequence star would be unable to fill its Roche lobe in a binary with the orbital period of X2127+119 (17.1 h), the companion is likely to be the remnant of a significantly more massive star which has had most of its envelope stripped away.