The Great Slump: Mrk 926 reveals discrete and varying Balmer line satellite components during a drastic phase of decline^⋆,⋆⋆

¹ Institut für Astrophysik, Universität Göttingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany
e-mail: wkollat@astro.physik.uni-goettingen.de
² Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
³ School of Physics & Astronomy and the Wise Observatory, The Raymond and Beverly Sackler Faculty of Exact Sciences Tel-Aviv University, Tel-Aviv 69978, Israel
⁴ Technion, Institute of Technology, Haifa 3200003, Israel
⁵ Physics Department and the Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa 3498838, Israel
⁶ SUPA School of Physics and Astronomy, University of St. Andrews, St. Andrews, Fife KY16 9SS, UK
⁷ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁸ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile

Received: 12 August 2021
Accepted: 9 October 2021

Abstract

Aims. Mrk 926 is known to be a highly variable active galactic nucleus. Furthermore, it is known to show very broad line profiles. We intended to study the continuum and line profile variations of this object with high temporal resolution in order to determine its broad-line region structure and to derive its black hole mass.

Methods. We carried out a high-cadence spectroscopic variability campaign of Mrk 926 with the 10m HET telescope, aided by photometric V-band data taken with the C18 telescope at the Wise Observatory, over a period of about five months. We extracted spectroscopic continuum and line light curves, and computed cross-correlation functions (CCFs) as well as velocity-resolved CCFs with respect to the combined spectroscopic and photometric V-band light curve.

Results. The continuum luminosity of Mrk 926 showed a drastic decrease during our campaign. The luminosity dropped to less than 50% of its original luminosity within only 2.5 months. Furthermore, the spectra of Mrk 926 show complex and very broad Balmer line profiles, including outer Balmer satellites ranging from ±5000 to ±13 000 km s⁻¹. The integrated Hα, Hβ, and He Iλ5876 line light curves are delayed relative to the continuum light curve. The Hα and Hβ lines show two velocity-delay structures in the central part of their line profile (within ±5000 km s⁻¹), at ∼10 and ∼57 light-days and at ∼5 and ∼48 light-days, respectively. These structures might be interpreted as the upper and lower halves of an ellipse in the velocity-delay plane, which might be the signature of a line-emitting ring, inclined by ∼50° to the line of sight and orbiting the black hole at radii, R, of 33.5 and 26.5 light-days. We determined continuum luminosities, log(λ L_λ/erg s⁻¹), of 43.68–44.13, which are in good agreement with the established R_BLR − L_AGN relation. Adopting delays of 33.5 and 26.5 days for Hα and Hβ, respectively, we derive a black hole mass of (1.1 ± 0.2)×10⁸ M_⊙; this indicates a low Eddington ratio, which decreased from 8 to 3 percent during our campaign. The Balmer satellite components show a higher correlation coefficient with respect to the continuum than the central line profile, and their response to the continuum variations is on the order of only 3 − 5 days. We attribute this to the central line segment and the Balmer satellites having different, spatially distinct regions of origin.