Abstract

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 Î ? λ < 17 Eon 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'Ã - - 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.