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 \pm 0.14 { M } _ { \odot } ( 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 broad-band 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 \sim 40,000 K over \sim 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 \pm 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 _ { \odot } for extreme equatorial heating concentration .
While we can not 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 .