Near-Infrared Light Curves of the Black Hole Binary A0620−00

We measured the near-infrared orbital light curve of the black hole binary A0620-00 in 1995 and 1996. The light curves show an asymmetric, double-humped modulation with extra emission in the peak at orbital phase 0.75. There were no significant changes in the shape of the light curve over the 1 yr observation period. There were no sharp dips in the light curves nor reversals of the asymmetry between the two peaks as seen in earlier observations. The light curves are well fitted by models incorporating ellipsoidal variations from the mass-losing K-type star plus a beamed bright spot on the accretion disk around the compact star. The long-term stability of the light curve shape rules out superhumps and star spots as sources of asymmetry when we observed A0620-00. The ellipsoidal variations yield a lower limit i ≥ 38° on the orbital inclination. The light curves show no eclipse features, which places an upper limit, i ≤ 75°. This range of inclinations constrains the mass of the compact object to 3.3 ≤ M1 ≤ 13.6 M⊙. The light curves do not further constrain the orbital inclination because the contribution of the accretion disk to the observed flux is unknown. We argue that a previous attempt to measure the near-infrared flux from the accretion disk using the dilution of the 12CO (2, 0) band head in the spectrum of the K star is not reliable because the band strength depends strongly on surface gravity.