We present near-infrared ( NIR ) broadband ( 0.80–2.42 \mu m ) spectroscopy of two low mass X-ray binaries : V404 Cyg and Cen X-4 .
One important parameter required in the determination of the mass of the compact objects in these systems is the binary inclination .
We can determine the inclination by modeling the ellipsoidal modulations of the Roche-lobe filling donor star , but the contamination of the donor star light from other components of the binary , particularly the accretion disk , must be taken into account .
To this end , we determined the donor star contribution to the infrared flux by comparing the spectra of V404 Cyg and Cen X-4 to those of various field K-stars of known spectral type .
For V404 Cyg , we determined that the donor star has a spectral type of K3 III .
We determined the fractional donor contribution to the NIR flux in the H- and K-bands as 0.98 \pm . 05 and 0.97 \pm . 09 , respectively .
We remodeled the H-band light curve from Sanwal et al .
( 52 ) after correcting for the donor star contribution to obtain a new value for the binary inclination .
From this , we determined the mass of the black hole in V404 Cyg to be M _ { BH } = 9.0 ^ { + .2 } _ { - .6 } M _ { \odot } .
We performed the same spectral analysis for Cen X-4 and found the spectral type of the donor star to be in the range K5 – M1V .
The donor star contribution in Cen X-4 is 0.94 \pm . 14 in the H-band while in the K-band , the accretion disk can contribute up to 10 % of the infrared flux .
We remodeled the H-band light curve from Shahbaz et al .
( 54 ) , again correcting for the fractional contribution of the donor star to obtain the inclination .
From this , we determined the mass of the neutron star as M _ { NS } = 1.5 ^ { + .1 } _ { - .4 } M _ { \odot } .
However , the masses obtained for both systems should be viewed with some caution since contemporaneous light curve and spectral data are required to obtain definitive masses .