Aims: We intend to use the impact of microlensing on the Fe III λλ2039−2113 emission line blend along with a measure of its gravitational redshift to estimate the mass of the quasar's central supermassive black hole (SMBH).
Methods: We fit the Fe III feature in multiple spectroscopic observations between 2008 and 2016 of the gravitationally lensed quasar Q 0957+561 with relatively high signal-to-noise ratios (at the adequate wavelength). Based on the statistics of microlensing magnifications, we used a Bayesian method to derive the size of its emitting region.
Results: The Fe III λλ2039−2113 spectral feature appears systematically redshifted in all epochs of observation by a value of Δλ ∼ 17 Å on average. We find clear differences in the shape of the Fe III line blend between images A and B. Measuring the strength of those magnitude differences, we conclude that this blend may arise from a region of half-light radius of R1/2 ∼ 15 lt-days, which is in good agreement with the accretion disk dimensions for this system. We obtain a mass for the central SMBH of MBH = 1.5−0.5+0.5 × 109 M⊙, consistent within uncertainties with previous mass estimates based on the virial theorem. The relatively small uncertainties in the mass determination (< 35%) make this method a compelling alternative to other existing techniques (e.g., the virial plus reverberation mapping based size) for measuring black hole masses. Combining the Fe III λλ2039−2113 redshift based method with the virial, we estimate a virial factor in the f ∼ 1.2 − 1.7 range for this system.