The unprecedented optical outburst of the quasar 3C 454.3

Abstract

Context. The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was R = 12.0, which represents the most luminous quasar state thus far observed (M B ∼ −31.4). Aims. In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT). Methods. Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005. Results. The historical radio and optical light curves show different behaviours. Until about 2001.0 only moderate variability was present in the optical regime, while prominent and long-lasting radio outbursts were visible at the various radio frequencies, with higher-frequency variations preceding the lower-frequency ones. After that date, the optical activity increased and the radio flux is less variable. This suggests that the optical and radio emissions come from two separate and misaligned jet regions, with the inner optical one acquiring a smaller viewing angle during the 2004−2005 outburst. Moreover, the colour-index behaviour (generally redder-when-brighter) during the outburst suggests the presence of a luminous accretion disc. A huge mm outburst followed the optical one, peaking in June−July 2005. The high-frequency (37−43 GHz) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period (end of September 2005). VLBA observations at 43 GHz during the summer confirm the brightening of the radio core and show an increasing polarization. An exceptionally bright X-ray state was detected in May 2005, corresponding to the rising mm flux and suggesting an inverse-Compton nature of the hard X-ray spectrum. Conclusions. A further multifrequency monitoring effort is needed to follow the next phases of this unprecedented event.

The long-lasting activity of 3C 454.3

“…The total variation amplitude is similar in the two seasons and it increases with increasing wavelength, as already noticed in previous works (e.g. Villata et al 2006;Raiteri et al 2008b). This feature was interpreted as an effect of an additional, stabler, emission component, mainly contributing to the blue part of the optical spectrum.…”

“…We applied an absorbed single power-law model, where absorption is modelled according to Wilms et al (2000). Table 3 presents the result of the spectral analysis in the case where the hydrogen column is fixed to N H = 1.34 × 10 21 cm −2 , as determined by the Chandra observations in 2005 (Villata et al 2006). Column 1 gives the ObsID, Col. 2 the start time (UT), and Col. 3 the total exposure time, often distributed over several orbits.…”

“…Ghisellini & Maraschi 1989;Maraschi et al 1992), and curved and that the alignment of the various emitting regions with the line of sight changes in time (Villata et al 2006(Villata et al , 2009a. Because the Doppler factor δ = [Γ(1 − β cos θ)] −1 , where Γ is the bulk Lorentz factor of the jet plasma and β = v/c its bulk velocity, depends on the viewing angle θ, the flux at a given wavelength will be Doppler-enhanced according to the orientation of the corresponding emitting zone.…”

“…The outburst was simultaneously detected also in X-rays, while the millimetric radio flux peaked about one month after, and the 15-43 GHz flux much later Raiteri et al 2008b). According to Villata et al (2006Villata et al ( , 2007 the observed change in behaviour starting from 2001 was a geometric effect, i.e. the motion of a curved jet producing variations in the viewing angle of the different emitting regions.…”

“…Dermer et al 2009, and references therein). The quasar-type blazar 3C 454.3 has received particular attention by the international astronomical community since it underwent a major outburst in 2005 (Villata et al 2006;Giommi et al 2006;Pian et al 2006;Fuhrmann et al 2006). Indeed, A&A 534, A87 (2011) after many decades of intense radio, but only mild optical activity, the source began brightening in the optical band in 2001, until in May 2005 it reached the maximum optical level ever observed, R = 12.0.…”