Peering into the Dark Side: Magnesium Lines Establish a Massive Neutron Star in PSR J2215+5135

New millisecond pulsars (MSPs) in compact binaries provide a good opportunity to search for the most massive neutron stars. Their main-sequence companion stars are often strongly irradiated by the pulsar, displacing the effective center of light from their barycenter and making mass measurements uncertain. We present a series of optical spectroscopic and photometric observations of PSR. J2215+5135, a redback binary MSP in a 4.14. hr orbit, and measure a drastic temperature contrast between the dark/cold (T-N = 5660(-380)(+260) K) and bright/hot (T-D = 8080(-280)(+470) K) sides of the companion star. We find that the radial velocities depend systematically on the atmospheric absorption lines used to measure them. Namely, the semi- amplitude of the radial velocity curve (RVC) of J2215 measured with magnesium triplet lines is systematically higher than that measured with hydrogen Balmer lines, by 10%. We interpret this as a consequence of strong irradiation, whereby metallic lines dominate the dark side of the companion (which moves faster) and Balmer lines trace its bright (slower) side. Further, using a physical model of an irradiated star to fit simultaneously the two-species RVCs and the three-band light curves, we find a center-of- mass velocity of K-2 = 412.3 +/- 5.0 km s(-1) and an orbital inclination i = 63 degrees.9(-2.7)(+2.4). Our model is able to reproduce the observed fluxes and velocities without invoking irradiation by an extended source. We measure masses of M-1 = 2.27(0.15)(+0.17) M circle dot and M-2 = 0.33(0.02)(+0.03). M circle dot for the neutron star and the companion star, respectively. If confirmed, such a massive pulsar would rule out some of the proposed equations of state for the neutron star interior.