Probing the Inner Structure of Active Galactic Nuclei Through Reverberation Mapping
Abstract
In the centres of massive galaxies, active galactic nuclei (AGN) are supermassive black holes, surrounded by an accretion disk of ionized gas, that release tremendous energy in the form of electromagnetic radiation. Because AGN are unresolved through telescopes, we employ reverberation mapping (RM) to study their structure. RM capitalizes on the fact that AGN are variable – the continuum emission from the accretion disk varies, and surrounding gas (in the broad-line region, BLR) responds to those variations with a positive time lag. RM translates the measured time lag into a size of the BLR. Combined with gas velocities (measured from the width of broad emission lines in AGN spectra), RM yields dynamically measured black hole masses, essential for understanding How supermassive black holes grow over cosmic time?.
We conducted a detailed RM study of a highly accreting AGN, Markarian 142, for the first time with the Gemini North and Lijiang Observatories, with simultaneous ultraviolet (UV) continuum and spectroscopic BLR observations. We determined a Hydrogen-beta lag of 8.58 days relative to the UV continuum. Deriving UV lags is important as UV emission more closely tracks the ionizing continuum that dominates the energetics of the BLR than the optical continuum.
Constraining black hole masses in RM is difficult, partly due to uncertainties on emission-line widths. Higher-resolution and high signal-to-noise spectra allow separation of blended lines that originate in different regions of the BLR. In the particular case of Markarian 142, we determined that unresolved narrow lines and flexible narrow-line flux ratios result in excess flux attributed to the narrow-line component of the complex Hydrogen-beta line, broadening the apparent widths measured for these broad lines. We further present recommendations on methodology for line-width measurements in narrow-line AGN to extract more accurate values.
RM studies have improved over time with larger sample sizes that span a broader diversity of the AGN population. To successfully implement large-scale variability campaigns, optimized survey designs are required to maximize science returns. We illustrated the use of such a survey simulation pipeline to optimize AGN RM surveys with a next-generation UV telescope, CASTOR.