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$_\mathrm{N}$=5660$^{+260}_{-380}$ K) and bright/hot (T$_\mathrm{D}$=8080$^{+470}_{-280}$ 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 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 radial velocity curves and the three-band light curves, we find a center-of-mass velocity of K$_2$=412.3$\pm$5.0 km s$^{-1}$ and an orbital inclination i=63.9$^\circ$$^{+2.4}_{-2.7}$. Our model is able to reproduce the observed fluxes and velocities without invoking irradiation by an extended source. We measure masses of M$_1$=2.27$^{+0.17}_{-0.15}$ M$_\odot$ and M$_2$=0.33$^{+0.03}_{-0.02}$ M$_\odot$ 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.