Near-Infrared Spectra of the Black Hole X-Ray Binary A0620-00

We present broadband near-IR (NIR) spectra of A0620-00 obtained with SpeX on the IRTF. The spectrum is characterized by a blue continuum on which are superimposed broad emission lines of H I and He II and a host of narrower absorption lines of neutral metals and molecules. Spectral type standard star spectra scaled to the dereddened spectrum of A0620-00 in K exceed the A0620-00 spectrum in J and H for all stars of spectral type K7 V or earlier, demonstrating that the donor star, unless later than K7 V, cannot be the sole NIR flux source in A0620-00. In addition, the atomic absorption lines in the K3 V spectrum are too weak with respect to those of A0620-00 even at 100% donor star contribution, restricting the spectral type of the donor star in A0620-00 to later than K3 V. Comparison of the A0620-00 spectrum to scaled K star spectra indicates that the CO absorption features are significantly weaker in A0620-00 than in field dwarf stars. Fits of scaled model spectra of a Roche lobe-filling donor star to the spectrum of A0620-00 show that the best match to the CO absorption lines is obtained when the C abundance is reduced to [C/H] = -1.5. The donor star contribution in the H wave band is determined to be (82 ± 2)%. Combined with previous published results from Froning & Robinson and Marsh et al., this gives a precise mass for the black hole in A0620-00 of M1 = 9.7 ± 0.6 M☉.