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

• Published in: Monthly notices of the Royal Astronomical Society 1994-01, Vol.266 (1), p.137-154
• Main Authors: Marsh, T. R., Robinson, E. L., Wood, J. H.
• Format: Article
• Language: English
• Subjects: accretion; Accretion and accretion disks; accretion discs; Astronomy; binaries: close; black hole physics; Black holes; Earth, ocean, space; Exact sciences and technology; Late stages of stellar evolution (including black holes); Stars; stars: individual: A0620–00; stars: rotation; Stellar characteristics and properties; x-rays: stars
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/266.1.137

We present spectra of the black hole binary $A0620-00$ taken at ${\rm H}\alpha$ and ${\rm H}\beta$ to study the distribution of emission-line flux from its accretion disc. Using the spectra of ${\rm H}\alpha$, we have measured the K-type companion star's radial velocity semi-amplitude and rotational broadening to be $K_2=433 \pm 3 \ {\rm km} \ {\rm s}^{-1}$ and $v \ {\rm sin} \ i=83 \pm 5 \ {\rm km} \ {\rm s}^{-1}$. Accounting for the Roche-lobe geometry of the companion, the rotational broadening gives the mass ratio $q=M_2/M_1=0.067 \pm 0.01$. The disc contributes $6 \pm 3$ per cent of the light at ${\rm H}\alpha$ and $17 \pm 3$ per cent at ${\rm H}\beta$, with the K star providing the rest. The masses of the compact object and K star derived from our values of $K_2$ and q are $M_1=(3.09 \pm 0.09) \ {\rm sin}^{-3} i \ {\rm M}_\odot$ and $M_2=(0.21 \pm 0.04) \ {\rm sin}^{-3} i \ {\rm M}_\odot$. Applying a constraint upon the orbital inclination, i, from an eclipse found in an earlier study, we obtain $2\sigma$ ranges on the masses of $3.30 < M_1 < 4.24 \ {\rm M}_\odot$ and $0.15 < M_2 < 0.38 \ {\rm M}_\odot$. 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 Å which we identify as Li i 6707.8 Å, which has recently been found in another black hole binary, V404 Cyg. The equivalent width of this line in $A0620-00$ is $160 \pm 30 \ {\rm m}$ Å compared to $290 \pm 50 \ {\rm m}$ Å 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 $\approx 0.5 R_{{\rm L1}}$, where $R_{{\rm Ll}}$ 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 $4 \times 10^{12} \ {\rm g} \ {\rm s}^{-1}$.