Measuring the Remnant Mass Function of the Galactic Bulge

Abstract

I show that by observing microlensing events both astrometrically and photometrically, the Space Interferometry Mission (SIM) can measure the mass function of stellar remnants in the Galactic bulge, including white dwarfs, neutron stars, and black holes. Neutron stars and black holes can be identified individually, while white dwarfs are detected statistically from the sharp peak in their mass function near M ~ 0.6 M☉. This peak is expected to be more than twice as high as the background of main-sequence microlenses. I estimate that of order 20% of the ~ 400 bulge microlensing events detected to date are due to remnants, but show that these are completely unrecognizable from their timescale distribution (the only observable that normal microlensing observations produce). To resolve the white dwarf peak, the SIM mass measurements must be accurate to ~ 5%, substantially better than is required to measure the mass function of the more smoothly distributed main sequence. Nevertheless, SIM could measure the masses of about 20 bulge remnants in 500 hours of observing time.