The mass of the companion of PSR 1718-19

Abstract

The recently-discovered binary pulsar 1718-19 displays periodic obscuration. We show that free-free absorption in a spherical outflow from the companion star reproduces the observed light curves for a system inclination of order 35 deg. This implies a companion of mass approximately equal to 0.2 Solar Mass; if it is close to the main sequence this star is well inside its Roche lobe. The spindown torque in the wind could account for the large pulsar period derivative without the need to invoke a magnetic field which is unusually strong for a globular cluster pulsar. The stellar wind of a detached companion provides a natural source for the obscuring matter. This and the derived companion mass tend to support the capture formation scenario favored by Wijers & Paczynski (1993). However, it is also possible that the system descended from a low-mass X-ray binary which ceased mass transfer at an orbital period of about 3 hr, and has since been widened by adiabatic expansion of the companion through pulsar ablation.