The Mass of the Black Hole in GS 2000+25

Abstract

We measured the I-band orbital light curve of the soft X-ray transient GS 2000+25 in 1992 August, and the Rand I-band light curves between 1998 September and 2000 August. The light curves are dominated by the ellipsoidal variations of the secondary star in the system. The light curves also showed transient distortions that persisted through individual observing runs, but were diluted and nearly absent in the mean light curve. A small, residual distortion in the mean light curve is consistent with extra flux from a bright spot where the stream of material from the secondary star interacts with the accretion disk around the compact star. The light curve does not show eclipses. The best fits to the mean light curve were achieved with a model that included ellipsoidal variations of the secondary star plus a bright spot on the edge of the accretion disk. The flux from the accretion disk is not well constrained but must be less than 32% of the total R-band flux. Combining our data with previously published spectroscopy of GS 2000+25, we find that the mass of the compact star lies in the range 5:5 P M 1 =M P 8:8, and therefore it is a black hole. The derived mass of the secondary star depends sensitively on the mass ratio, which is measured from the rotational broadening of the absorption lines in the secondary's spectrum. We find 0:16 P M 2 =M P 0:47, but we deem this result less secure than the mass for M 1 . The radius and spectral type (K5) of the secondary are not consistent with those of main-sequence stars with masses in this range but are consistent with an evolved star that has been stripped of mass.

Mass Measurements of Black Holes in X-Ray Transients: Is There a Mass Gap?

“…Both the Ioannou et al (2004) and Callanan et al (1996) results are consistent with the Beekman et al (1996) estimate 43 • < i < 60 • . For our final estimate, we adopt the Callanan et al (1996) result because it is less affected by binning than that of Ioannou et al (2004), given that the observations span only two days. We take i ∼ I(55.0 • , 65.0 • ) as our inclination estimate.…”

“…Another common practice when fitting inclination is to combine lightcurves from two or more observing runs (viz. Shahbaz et al 1994;Remillard et al 1996a;Gelino & Harrison 2003;Ioannou et al 2004;Orosz et al 2004;Gelino et al 2006;Casares et al 2009;Orosz et al 2011). However, due to aperiodic flickering and migratory hotspots, it is unlikely that combining and/or binning data reproduces the true underlying lightcurve shape.…”

The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

“…The computer program we use to calculate light curves for SS Cyg is an updated and improved version of the program we have used to calculate the light curves of black hole X-ray binaries ( Ioannou et al 2004). Although the program accepts many parameters and can synthesize light curves for complicated accretion geometries, just three components are needed to fit the light curve of SS Cyg:…”

Infrared contamination in Galactic X-ray novae

“…The low S/N of the individual spectra complicated the removal of the telluric features, with a number of residuals remaining. The resulting spectra were corrected for the Doppler shift of the spectral lines induced by the orbital motion of the secondary star using the ephemeris of Ioannou et al (2004) and median combined so as to negate the effect of cosmic ray hits and other spurious effects, while at the same time maximizing the available S/N. This spectrum was then dereddened using the extinction listed in Table 1.…”

“… a Gelino et al (2006); b Reynolds et al (2007); c Froning et al (2007); d Ioannou et al (2004); e Gelino et al (2001a); f D'Avanzo et al (2005); g Shahbaz et al (1994); h Torres et al (2004); i Harlaftis et al (1999); j Shahbaz et al (2004); k Marsh et al (1994); l Casares et al (1997); m Torres et al (2002); n Casares et al (1993). …”