Dynamical evidence for a black hole in the X-ray transient QZ VUL (=GS 2000+25)

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 velocity modulations with the 8.3-h orbital period. From the amplitude of the radial velocity curve (520+/-16 km s^-1) we derive a mass function f(M)=5.02+/-0.46 M_solar, the second largest after that of V404 Cygni. We classify the companion star as a K5+/-2V which is the dominant light source in our spectral range (lambdalambda6000-8800). Doppler imaging of the broad Hα emission shows evidence for an intense hotspot from which we estimate an accretion disc radius of (0.47+/-0.03)R_Lx.