Spectroscopy of A0620-00: The mass of the black hole and an image of its accretion disc

Abstract

We present spectra of the black hole binary A0620-00 taken at Hα and Hβ to study the distribution of emission-line flux from its accretion disc. Using the spectra of Hα, we have measured the K-type companion star's radial velocity semi-amplitude and rotational broadening to be K2 = 433±3 kms-1 and v sin i = 83±5 kms-1. Accounting for the Roche-lobe geometry of the companion, the rotational broadening gives the mass ratio q = M2/Ml = 0.067 ±0.01. The disc contributes 6 ± 3 per cent of the light at Ha and 17 ± 3 per cent at Hβ, with the K star providing the rest. The masses of the compact object and K star derived from our values of K2 and q are M1 = (3.09±0.09) sin-3i M⊙ and M2 = (0.21 ±0.04) sin-3 M⊙. Applying a constraint upon the orbital inclination,/, from an eclipse found in an earlier study, we obtain 2a ranges on the masses of 3.30 Mx 4.24M⊙ and 0.15 M2 0.38M⊙. The lower limit on the mass of the compact object is higher than the maximum mass of a rotating neutron star based upon equations of state for nuclear material. Subtraction of the correctly broadened template star removes all lines other than a line at 6708 A which we identify as Li i 6707.8 A, which has recently been found in another black hole binary, V404 Cyg. The equivalent width of this line in A0620-00 is 160 ± 30 mA compared to 290 ± 50 mA in V404 Cyg. Doppler images of the Balmer lines are very similar to those of quiescent dwarf novae, with emission from the region where the gas stream hits the disc and emission from the stream itself prior to this point. The Balmer decrement steepens towards the outer edge of the disc. The impact point gives a disc radius of ≈ 0.5 RL1, where RL1 is the inner Lagrangian point distance, consistent with the separation of the emission-line peaks. In general, the spectra appear to be of lower excitation than those of dwarf novae, with no He ii 4686 emission, weak He i emission and Balmer emission of large equivalent width. There is thus very little ionizing radiation in the system, and a rough analysis suggests that the accretion rate on to the compact object must be less than about 4 x 1012gs-1