2002A&A...392..909C

We present the results of a detailed non-LTE analysis of the ultraviolet and optical spectrum of the O6.5Iaf⁺ star HD 153919 - the mass donor in the high-mass X-ray binary 4U 1700-37. We find that the star has a luminosity log(L_*/L_☉)=5.82±0.07, T_eff=35000±1000K, radius R_*=21.9^+1.3_–0.5R_☉, mass-loss rate {dot}(M)=9.5x10^–6M_☉/yr, and a significant overabundance of nitrogen (and possibly carbon) relative to solar values. Given the eclipsing nature of the system these results allow us to determine the most likely masses of both components of the binary via Monte Carlo simulations. These suggest a mass for HD 153919 of M_*=58±11M_☉ - implying the initial mass of the companion was rather high (>60M_☉). The most likely mass for the compact companion is found to be M_x=2.44±0.27M_☉, with only 3.5 per cent of the trials resulting in a mass less than 2.0M_☉ and none less than 1.65M_☉. Such a value is significantly in excess of the upper observational limit to the masses of neutron stars of 1.45M_☉ found by Thorsett & Chakrabarthy (1999ApJ...512..288T), although a mass of 1.86M_☉ has recently been reported for the Vela X-1 pulsar (Barziv et al., 2001A&A...377..925B). Our observational data is inconsistent with the canonical neutron star mass and the lowest black hole mass observed (>4.4M_☉; Nova Vel). Significantly changing observational parameters can force the compact object mass into either of these regimes but, given the strong proportionality between M_* and M_x, the O-star mass changes by factors of greater than 2, well beyond the limits determined from its evolutionary state and surface gravity. The low mass of the compact object implies that it is difficult to form high mass black holes through both the Case A & B mass transfer channels and, if the compact object is a neutron star, would significantly constrain the high density nuclear equation of state.