GS 2000+25 is an X-ray binary system in the constellation Vulpecula, consisting of a late K-type star and a black hole. It is also an X-ray nova. Because the black hole is more massive than the companion star, it is the primary of the system. The black hole has a mass of about 5 solar masses while the companion has a mass of about 0.5 solar masses.

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We present new CCD photometric observations of QZ Vul, the optical counterpart of the very soft X-ray transient GS 2000 + 25, obtained during the quiescent state in 1990 and 1991 in the Cousins I and R bands and in a broad band (VR) covering the 4700 7200 A region.

QZ VULPECULAE P. A. Charles and J. Casares, Oxford University, write: "Optical spectroscopy of QZ Vul obtained with the 4.2-m William Herschel Telescope on La Palma shows broad (FWHM 2000 km/s), double-peaked H-alpha emission superposed on the late-type spectrum of the secondary star, detected here for the first time and constrained to be in ...

Nov 15, 1999 · We have used GRIM II on the ARC 3.5 meter telescope at Apache Point Observatory to obtain infrared photometry of GS2000+25 (QZ Vul). By modeling the SXT ellipsoidal variations with WD98, we can determine the orbital period and inclination of the system.

Feb 28, 1993 · We present new CCD photometric observations of QZ Vul, the optical counterpart of the very soft X-ray transient GS 2000 + 25, obtained during the quiescent state in 1990 and 1991 in the...

The mass of the black hole in GS 2000 + 25 ( = QZ Vul) Abstract: We report the first infrared light curve of GS 2000 + 25. By modelling the brightness variations as ellipsoidal modulations of the secondary star, we find the mass of the compact object to lie between 4.8 and 14.4 M ⊙ (90 per cent confidence) for secondary masses ≤ 0.7 M ⊙ ...

QZ Vul thus has the second largest mass function known amongst the blackhole x-ray binaries, after V404 Cyg (Casares and Charles 1994, MNRAS 271, L5)."

Oct 31, 1995 · We report the first spectroscopic detection of the secondary star in the X-ray transient QZ Vul (=GS 2000+25). Cross-correlation of individual spectra against K-type templates yields clear...

By modelling the brightness variations as ellipsoidal modulations of the secondary star, the authors find the mass of the compact object to lie between 4.8 and 14.4 M sun (90 per cent confidence) for secondary masses ≤0.7 M sun; this result is independent of the mass ratio, q.