The X-Ray Binary X2127+119 in M15: Evidence for a Very-Low-Mass, Stripped-Giant Companion

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 He II λ4686 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⊙ sub-giant companion near the main-sequence turn-off for M15. Our data imply a much smaller mass ratio M2/MX of ∼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 X2127+119’s companion may have a much lower mass (∼0.1 M⊙) than previously assumed. As an ∼0.1-M⊙ main-sequence star would be unable to fill its Roche lobe in a binary with X2127+119’s orbital period (17.1 hours), the companion is likely to be the remnant of a significantly more massive star which has had most of its envelope stripped away.