The power of the dark side: hunting spiders to find the most massive neutron stars

The maximum neutron star mass has profound impacts across a variety of fields. Gravitational wave astronomy, nuclear physics, and stellar astrophysics all depend on this critical value that is still poorly constrained. Compact binary millisecond pulsars, with rapidly-spinning neutron stars spun up by a sustained accretion phase, can harbour the most massive neutron stars, and their numbers are quickly growing.
Using the largest ground-based telescopes and phase-resolved spectroscopy, we study these cannabilistic systems, also known as spiders, in which the pulsar ablates its companion with intense relativistic winds. This can produce strong irradiation effects that shift the companion's centre of light away from its centre of mass. The dark side of the companion is the key to obtaining a robust orbital solution.
By using precise cross-correlation techniques on both the hydrogen lines that dominate the irradiated face, and the metallic line species of the dark side, we can accurately determine the true motion of the companion.
Combining this with optimal subtraction techniques, we can follow the temperature variation of the companion throughout its orbit to obtain the orbital inclination. By taking advantage of the two sides of the companion and leveraging the different temperature-dependent line species, we can obtain the most precise neutron star mass measurements, and continue the hunt for the maximum neutron star mass.