STATISTICAL DETERMINATION OF THE MACHO MASS-SPECTRUM

The mass function of 51 massive compact halo objects (MACHOs) detected toward the Galactic bulge is statistically estimated from Einstein ri ng crossing times t(E). For a Gaussian mass function, the best-fit par ameters are [log m] = -1.12 and sigma(log m) = 0.57. If the mass funct ion follows a power-law distribution, the best-fit mass cutoff and pow er are m(cut) = 0.04 M(.) and p = -2.1. Both mass spectra indicate tha t a significant fraction of events are caused by MACHOs in the substel lar mass range. Both best determined mass functions are compared with that obtained from Hubble Space Telescope (HST) observations. The powe r law is marginally favored (2.3 sigma) over the Gaussian mass functio n and significantly favored over the HST mass function (5.5 sigma), in dicating that MACHOs have a different mass function from stars in the solar neighborhood. In addition, tile fact that all the models have ve ry poor fits to the longest four events, with t(E) greater than or equ al to 70 days, remains a puzzle.