Revealing the structure of the lensed quasar Q 0957+561 II. Supermassive black hole mass via gravitational redshift

Abstract

Aims. We intend to use the impact of microlensing on the Fe III lambda lambda 2039 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 lambda lambda 2039 2113 spectral feature appears systematically redshifted in all epochs of observation by a value of Delta lambda similar to 17 angstrom on average. We find clear di fferences in the shape of the Fe III line blend between images A and B. Measuring the strength of those magnitude di fferences, we conclude that this blend may arise from a region of half-light radius of R-1/2 similar to 15 ltdays, which is in good agreement with the accretion disk dimensions for this system. We obtain a mass for the central SMBH of M-BH = 1.5(-0.5)(+0.5) x 10(9) M-circle dot, 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 lambda lambda 2039 2113 redshift based method with the virial, we estimate a virial factor in the f similar to 1:2 1:7 range for this system.