A SPECTROSCOPIC STUDY OF THE EXTREME BLACK WIDOW PSR J1311-3430

We report on a series of spectroscopic observations of PSR J1311-3430, an extreme black-widow gamma-ray pulsar with a helium-star companion. In a previous study we estimated the neutron star mass as M-NS = 2.68 +/- 0.14 M-circle dot (statistical error), based on limited spectroscopy and a basic (direct heating) light-curve model; however, much larger model-dependent systematics dominate the mass uncertainty. Our new spectroscopy reveals a range of complex source behavior. The variable He I companion wind emission lines can dominate broadband photometry, especially in red filters or near minimum brightness, and the wind flux should complete companion evaporation in a spin-down time. The heated companion face also undergoes dramatic flares, reaching similar to 40,000 K over similar to 20% of the star; this is likely powered by a magnetic field generated in the companion. The companion center-of-light radial velocity is now well measured with K-CoL = 615.4 +/- 5.1 km s(-1). We detect non-sinusoidal velocity components due to the heated face flux distribution. Using our spectra to excise flares and wind lines, we generate substantially improved light curves for companion continuum fitting. We show that the inferred inclination and neutron star mass, however, remain sensitive to the poorly constrained heating pattern. The neutron star's mass, M-NS, is likely less than the direct heating value and could range as low as 1.8 M-circle dot for extreme equatorial heating concentration. While we cannot yet pin down M-NS, our data imply that an intrabinary shock reprocesses the pulsar emission and heats the companion. Improved spectra and, especially, models that include such shock heating are needed for precise parameter measurement.