NEAR-INFRARED SPECTROSCOPY OF LOW-MASS X-RAY BINARIES: ACCRETION DISK CONTAMINATION AND COMPACT OBJECT MASS DETERMINATION IN V404 Cyg AND Cen X-4

Abstract

We present near-infrared (NIR) broadband (0.80-2.42 μm) 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 ± 0.05 and 0.97 ± 0.09, respectively. We remodeled the H-band light curve from Sanwal et al. 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 +0.2 −0.6 M . 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-M1 V. The donor star contribution in Cen X-4 is 0.94 ± 0.14 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., 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 +0.1 −0.4 M . 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.

The radio/X-ray domain of black hole X-ray binaries at the lowest radio luminosities

“…Then, the difference in radio normalisation attributed to Doppler beaming depends only on the orbital inclination of the system (assuming the jet axis is perpendicular to the orbital plane). Both XTE J1118+480 and V404 Cyg have well-determined orbital inclinations, of i = 68 • ± 2 • (Gelino et al 2006; see also Khargharia et al 2013 for a somewhat broader constraints and Farr et al 2011;Kreidberg et al 2012 for a comprehensive statistical analysis) and i = 67 Khargharia, Froning & Robinson 2010;Farr et al 2011;Kreidberg et al 2012), respectively. The inclination of GX 339-4 is only known to be < ∼ 60 • (Cowley et al 2002), and we assume a range from 45−60 • here (see Zdziarski et al 2004).…”

The black hole binary V404 Cygni: a highly accreting obscured AGN analogue

“…Shahbaz et al (1994) later determined a BH mass of about 12M⊙. More recently, near-infrared spectroscopy allowed a more precise determination of the compact object mass, MBH = 9.0 +0.2 −0.6 M⊙ (Khargharia et al 2010). On 2015 June 15 18:32 UT (MJD 57188.772), the Swift/BAT triggered on a bright hard X-ray flare from a source that was soon recognized to be the black hole low mass X-ray binary V404 Cyg back in outburst after 26 years of quiescence (Barthelmy et al 2015.…”

“…We left the Thomson optical depth τ electron temperature kTe and the normalization in COMPPS free to vary, while we fixed the seed photon temperature to kTbb = −0.1 keV (i.e. the seed photons are produced by a multi-color disk-blackbody with an inner disk temperature of 0.1 keV) and the inclination to 67 • (see Khargharia et al 2010).…”

Correlated optical, X-ray, andγ-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni

“…V404 Cyg (hereafter V404) is a low-mass X-ray binary (LMXB) consisting of a black hole (BH) with mass estimates ranging from ∼9 to 15 M , and a 0.7 +0.3 −0.2 M K3 III companion (Casares & Charles 1994;Shahbaz et al 1994;Khargharia et al 2010), located at a parallax distance 2.39 ± 0.14 kpc (Miller-Jones et al 2009). The inclination of the binary's rotational axis is 67 •+3 −1 (Shahbaz et al 1994;Khargharia et al 2010), the orbital period 6.5 d (Casares et al 1992).…”

“…The inclination of the binary's rotational axis is 67 •+3 −1 (Shahbaz et al 1994;Khargharia et al 2010), the orbital period 6.5 d (Casares et al 1992). This transient underwent three periods of outbursts during the twentieth century (Richter 1989), the last, in May 1989, leading to its discovery as an X-ray transient by the Ginga satellite (as GS 2023+338, Makino et al 1989).…”