Long-term optical spectroscopic variations in blazar 3C 454.3

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
e-mail: knalew@camk.edu.pl
² Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263002, India
³ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China
⁴ University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, PR China

Received: 16 May 2019
Accepted: 1 September 2019

Abstract

Aims. 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 γ-ray emission that dominates the spectral energy distribution.

Methods. We analysed ten years of optical spectroscopic data from the Steward Observatory for the blazar 3C 454.3, as well as γ-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).

Results. Despite very large variations in the γ-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 ∼ 8.5 × 10⁸ M_⊙. We did not identify additional examples of short-term “flares” of the line flux, in addition to the previously reported case observed in 2010.