PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star

Understand

We describe Keck-telescope spectrophotometry and imaging of the companion of the ``black widow" pulsar PSR~J0952$-$0607, the fastest known spinning neutron star (NS) in the disk of the Milky Way.

• The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated ``day" half of the orbit.
• The model fits indicate system inclination $i=59.8\pm 1.9^\circ$ and a pulsar mass $M_{NS} = 2.35\pm 0.17 M_\odot$, the largest well-measured mass found to date.
• Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit.