ON THE MASS OF THE COMPACT OBJECT IN THE BLACK-HOLE BINARY A0620-00

Multicolor orbital light curves of the black hole candidate binary A06 20-00 are presented. The light curves exhibit ellipsoidal variations a nd a grazing eclipse of the mass donor companion star by the accretion disk. Synthetic light curves were generated using realistic mass dono r star fluxes and an isothermal blackbody disk. For mass ratios of q = M1/M2 = 5.0, 10.6, and 15.0 systematic searches were executed in para meter space for synthetic light curves that fit the observations. For each mass ratio, acceptable fits were found only for a small range of orbital inclinations. It is argued that the mass ratio is unlikely to exceed q = 10.6, and an upper limit of 0.8M. is placed on the mass of the companion star. These constraints imply 4.16 +/- 0.1M. less-than-o r-equal-to M1 less-than-or-equal-to 5.55 +/- 0.15 M.. The lower limit on M1 is more than 4 sigma above the mass of a maximally rotating neut ron star and constitutes further strong evidence in favor of a black h ole primary in this system.