Broadband Near-Infrared Spectroscopy of X-Ray Binaries

Abstract

We present broadband NIR spectroscopy of the X‐ray binaries A0620‐00, Cen X‐4, V404 Cyg, and GX 13+1. The NIR spectra consist of emission from the late‐type donor star and the accretion disk. In A0620‐00 and Cen X‐4, the donor star is the dominant component but there are emission lines from the accretion disk as well as evidence of a continuum disk component. In V404 Cyg, there are no signatures of the accretion disk in the NIR spectrum, whereas in GX13+1, the accretion disk is the dominant emitter. Using spectra of field stars of known spectral type, we have examined the donor star absorption spectra in these systems. The apparent spectral types of the donor stars in A0620‐00 and Cen X‐4 are those of late‐type dwarf stars (K7 or later). In A0620‐00, the weakness of the 12CO bandhead features relative to the atomic lines indicates a depleted carbon abundance in the system, [C/H] = −1.5. The spectrum of Cen X‐4 does not show this abundance anomaly. The donor stars in the V404 Cyg and GX13+1 are evolved, but while the donor star in V404 Cyg appears to the only NIR source, the donor in GX13+1 only accounted for about 1/4 of NIR flux at the time of our observations. For A0620‐00, we have used scaled fits of spectral type template stars to determine the donor star contribution in the H‐band, which is combined with previous work to determine the binary inclination and, in turn, the mass of the black hole: MBH = 9.7±0.6 M⊙.