Revealing the structure of the lensed quasar Q 0957+561

II. Supermassive black hole mass via gravitational redshift

¹ School of Physics and Astronomy and Wise Observatory, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv, Israel
e-mail: carinafian@mail.tau.ac.il
² Instituto de Astrofísica de Canarias, Vía Láctea s/n, La Laguna, 38200 Tenerife, Spain
³ Departamento de Astrofísica, Universidad de la Laguna, La Laguna, 38200 Tenerife, Spain
⁴ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain
⁵ Instituto Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
⁷ Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain
⁸ Observatorio Astronómico, Universidad de Valencia, 46980 Paterna, Valencia, Spain
⁹ Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa, Israel
¹⁰ Institute of Physics (IGAM), University of Graz, Universitätsplatz 5, 8010 Graz, Austria

Received: 1 April 2021
Accepted: 9 July 2021

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 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 R_1/2 ∼ 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 M_BH = 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.