We report the tentative detection of the modulation of a transient , redshifted Fe K \alpha 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 re-analysing the XMM-Newton data that the feature varies systematically in flux at intervals of 25 ks .
The peak moves in energy between 5.7 keV 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 timescale and the location of the orbiting flare , the mass of the black hole is estimated to be ( 1–5 ) \times 10 ^ { 7 } M _ { \odot } , which is in good agreement with values obtained from other techniques .