The black hole binary A0620-00

Photometric and spectroscopic observations of the X-ray nova A 0620-00 in quiescence during 1981 - 1985 are presented. These data are used to derive a precise value for the binary period, the radial velocity curve of the K-dwarf companion, and a Doppler-corrected spectrum of the companion which reveals the presence of narrow absorption lines. The mass function of the system is f(M) = 3.18±0.16 M_sun;. A firm lower limit (3σ) to the mass of the compact X-ray source is 3.20 M_sun;; this value exceeds the maximum allowed mass of a stable neutron star.

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