Flux and energy modulation of redshifted iron emission in NGC 3516: implications for the black hole mass

We report on the tentative detection of the modulation of a transient, redshifted Fe Kalpha emission feature in the X-ray spectrum of the Seyfert galaxy NGC 3516. The detection of the spectral feature at 6.1 keV, in addition to a stable 6.4-keV line, has been reported previously. We find, on reanalysing the XMM-Newton data, that the feature varies systematically in flux at intervals of 25 ks. The peak moves in energy between 5.7 and 6.5 keV. The spectral evolution of the feature agrees with Fe K emission arising from a spot on the accretion disc, illuminated by a corotating flare located at a radius of (7-16) r(g), modulated by Doppler and gravitational effects as the flare orbits around the black hole. Combining the orbital time-scale and the location of the orbiting flare, the mass of the black hole is estimated to be (1-5) x 10(7) M-circle dot, which is in good agreement with values obtained from other techniques.