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

Abstract

We present broadband near-IR (NIR) spectra of A0620-00 obtained withSpeX on the IRTF. The spectrum is characterized by a blue continuumon which are superimposed broad emission lines of H I and He II anda host of narrower absorption lines of neutral metals and molecules.Spectral type standard star spectra scaled to the dereddened spectrumof A0620-00 in K exceed the A0620-00 spectrum in J and H for allstars of spectral type K7 V or earlier, demonstrating that the donorstar, unless later than K7 V, cannot be the sole NIR flux sourcein A0620-00. In addition, the atomic absorption lines in the K3 Vspectrum are too weak with respect to those of A0620-00 even at 100%donor star contribution, restricting the spectral type of the donorstar in A0620-00 to later than K3 V. Comparison of the A0620-00 spectrumto scaled K star spectra indicates that the CO absorption featuresare significantly weaker in A0620-00 than in field dwarf stars. Fitsof scaled model spectra of a Roche lobe-filling donor star to thespectrum of A0620-00 show that the best match to the CO absorptionlines 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 blackhole in A0620-00 of M1=9.7+/-0.6 Msolar.