Long-term optical spectroscopic variations in blazar 3C 454.3

Abstract

Aims. Characterisation of the long-term variations of the broad line region in a luminous blazar, where Comptonization 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 analyse 10 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. The optical spectra are dominated by a highly variable non-thermal synchrotron continuum with a prominent Mg II broad emission line. The line flux is obtained by spectral decomposition including significant contribution from the Fe II pseudo-continuum. Three methods are used to characterise variations of the line flux: (1) stacking of the continuum-subtracted spectra, (2) subtracting the running mean light curves calculated for different time scales, (3) Discrete Correlation Function (DCF) for evaluating potential time delays. Results. Despite very large variations of 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 of 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 increase in the line luminosity is delayed by ∼ 600 days. If this time delay results from reverberation of poorly constrained accretion disk 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 RBLR,MgII 0.28 pc and for the supermassive black hole mass MSMBH ∼ 8.5 × 10 8 M⊙. We have not identified additional examples of short-term "flares" of the line flux, besides the case of MJD 55512 reported by León-Tavares et al. (2013).

Flare-like Variability of the Mg ii λ2798 Å Emission Line and UV Fe ii Band in the Blazar CTA 102

“…In the SWIFT/UVOT grism monitoring of NGC 5548, Cackett et al (2015) showed that the Mg II emission line is not strongly correlated with the continuum variability, and there is no significant lag between the two. However, more recent studies of CTS C30.10 (radio quiet quasar) and 3C 454.3 (blazar) Czerny et al (2019) and Nalewajko et al (2019) show that there are time delays of 562 +116 −68 and ∼600 days, respectively, between the continuum variations and the response of the Mg II emission line. León-Tavares et al (2013) and Isler et al (2013) reported the statistically significant flare-like event in the Mg II emission line in the blazar 3C 454.3.…”

Broad line region and black hole mass of PKS 1510-089 from spectroscopic reverberation mapping

“…A positive correlation in the α − log L 5100 relation is found with r s = 0.43 and a p-value of 10 −13 . Therefore, a "redder when brighter" (RWB) trend is observed in PKS 1510, indicating the presence of an accretion disk in the continuum (e.g., Gu et al 2006;Nalewajko et al 2019).…”

“…For example, Zhang et al (2019) performed a reverberation study of FSRQ 3C273 using ten years of Steward Observatory monitoring data, which provide a highly reliable measurements of BLR size. Nalewajko et al (2019) performed Mg II reverberation studies of FSRQ 3C 454.3 using the Steward Observatory monitoring…”

Broad line region and black hole mass of PKS 1510-089 from spectroscopic reverberation mapping

“…10 −13 . Therefore, a "redder when brighter" (RWB) trend is observed in PKS 1510 indicating the presence of accretion disk in the continuum (e.g., Gu et al 2006;Nalewajko et al 2019).…”

“…For example, Zhang et al ( 2019) performed a reverberation study of the FSRQ 3C273 using 10-yr long Steward Observatory monitoring data providing a highly reliable measurement of BLR size. Nalewajko et al (2019) performed Mg II reverberation studies of FSRQ 3C 454.3 using the Steward Observatory monitoring data and Zajaček et al (2020) measured Mg II lag of HE 0413-4031 using SALT monitoring data.…”