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

## TL;DR

This study presents near-infrared spectra of the black hole binary A0620-00, revealing details about the donor star's spectral type, chemical composition, and the black hole's mass through spectral analysis.

## Contribution

The paper provides the first broadband NIR spectra of A0620-00 and constrains the donor star's spectral type and chemical abundances, refining the black hole mass estimate.

## Key findings

- Donor star is later than K3V in spectral type.
- Donor star's carbon abundance is significantly reduced.
- Black hole mass is estimated at 9.7±0.6 solar masses.

## Abstract

We present broadband 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 HI and HeII 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 K7V or earlier, demonstrating that the donor star, unless later than K7V, cannot be the sole NIR flux source in A0620-00. In addition, the atomic absorption lines in the K3V 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 K3V. 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 waveband is determined to be 82+-2%. Combined with previous published results from Froning & Robinson (2001) and Marsh et al. (1994), this gives a precise mass for the black hole in A0620-00 of M_BH = 9.7+-0.6 M_solar.