Mass measurements of black holes in X-ray transients: Is there a mass gap?

We explore possible systematic errors in the mass measurements of stellar mass black holes (BHs). We find that significant errors can arise from the assumption of zero or constant emission from the accretion flow, which is commonly used when determining orbital inclination by modeling ellipsoidal variations. For A0620-00, the system with the best available data, we show that typical data sets and analysis procedures can lead to systematic underestimates of the inclination by 10° or more. A careful examination of the available data for the 15 other X-ray transients with low-mass donors suggests that this effect may significantly reduce the BH mass estimates in several other cases, most notably that of GRO J0422+32. Assuming that GRO J0422+32 behaves similarly to A0620-00, the reduction in the mass of GRO J0422+32 fills the mass gap between the low end of the distribution and the maximum theoretical neutron star mass, as has been identified in previous studies. Otherwise, we find that the mass distribution retains other previously identified characteristics, namely a peak around 8 M_⊙, a paucity of sources with masses below 5 M_⊙, and a sharp drop-off above 10 M_⊙.