Long-term optical spectroscopic variations in blazar 3C 454.3

TL;DR

This study analyzes ten years of optical and gamma-ray data of blazar 3C 454.3, revealing that broad line flux varies moderately despite large continuum changes, and suggests possible reverberation effects indicating the size of the broad-line region and black hole mass.

Contribution

It provides the first long-term spectroscopic analysis of 3C 454.3, exploring the relationship between line flux and continuum variability, and estimates the BLR size and black hole mass based on potential reverberation delays.

Findings

Line flux varies only moderately despite large continuum changes.

Possible ~600-day delay in line flux response suggests reverberation effects.

Estimated BLR radius > 0.28 pc and black hole mass ~8.5x10^8 solar masses.

Abstract

Characterisation of the long-term variations in the broad line region in a luminous blazar, where Comptonisation of broad-line emission within a relativistic jet is the standard scenario for production of gamma-ray emission that dominates the spectral energy distribution. We analysed ten years of optical spectroscopic data from the Steward Observatory for the blazar 3C 454.3, as well as gamma-ray data from the Fermi Large Area Telescope (LAT). The optical spectra are dominated by a highly variable non-thermal synchrotron continuum with a prominent Mg II broad emission line. The line flux was obtained by spectral decomposition including significant contribution from the Fe II pseudo-continuum. Three methods were used to characterise variations in the line flux: (1) stacking of the continuum-subtracted spectra, (2) subtracting the running mean light curves calculated for different timescales, and (3) evaluating potential time delays via the discrete correlation function (DCF). Despite very large variations in the gamma-ray and optical continua, the line flux changes only moderately (< 0.1 dex). The data suggest that the line flux responds to a dramatic change in the blazar activity from a very high state in 2010 to a deep low state in 2012. Two interpretations are possible: either the line flux is anti-correlated with the continuum or the increase in the line luminosity is delayed by ~600 days. If this time delay results from the reverberation of poorly constrained accretion disc emission in both the broad-line region (BLR) and the synchrotron emitting blazar zone within a relativistic jet, we would obtain natural estimates for the BLR radius [R_{BLR,MgII} >~ 0.28 pc] and for the supermassive black hole mass [M_SMBH ~ 8.5x10^8 M_sun]. We did not identify additional examples of short-term 'flares' of the line flux, in addition to the previously reported case observed in 2010.