... detection

The radio-to-optical data presented in this paper are stored in the WEBT archive; for questions regarding their availability, please contact the WEBT President Massimo Villata.

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... (WEBT)

http://www.oato.inaf.it/blazars/webt/ see e.g. Böttcher et al. (2007); Villata et al. (2004); Raiteri et al. (2008).

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... Alto

Calar Alto data were taken as part of the MAPCAT (Monitoring AGN with Polarimetry at the Calar Alto Telescopes) project.

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... 4.6 h

With very steep slopes of about 0.006 mag per minute, i.e. 3 times steeper than the fastest variations observed in the strong IDV blazar 0716+714 (see Villata et al. 2008, and references therein).

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... band

More precisely, 1.25 mag in 64 min, i.e. a mean slope of 0.020 mag per minute (cf. Footnote 3).

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... Kea

These data were obtained as part of the normal monitoring program initiated by the SMA (Gurwell et al. 2007).

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... Veleta

The IRAM data are part of a standard AGN monitoring program of total flux and polarization, a complementary program to monitor additional sources selected for observations with GLAST (Fuhrmann et al., in prep.), and a dedicated polarimetry program to monitor the mm flare of 3C 454.3 in 2007 (Agudo et al., in prep.).

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... Database

http://www.vla.nrao.edu/astro/calib/polar/

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...##)

For comparison, the 0.83 mag brightening in 4.6 h observed on December 1 (JD $\sim$ 2 454 436.0, see Fig. 2) implies $T_{\rm b} \sim 4 \times 10^{8} \rm ~ K$.

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