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We monitored the flaring blazar 3C 454.3 during 2005 June-July with theSpitzer Infrared Spectrograph (IRS: 15 epochs), Infrared Array Camera (IRAC: 12epochs) and Multiband Imaging Photometer (MIPS: 2 epochs). We also made SpitzerIRS, IRAC, and MIPS observations from 2006 December-2007 January when thesource was in a low state, the latter simultaneous with a single Chandra X-rayobservation. In addition, we present optical and sub-mm monitoring data. The2005-2007 period saw 3 major outbursts. We present evidence that theradio-optical SED actually consists of two variable synchrotron peaks, theprimary at IR and the secondary at sub-mm wavelengths. The lag between theoptical and sub-mm outbursts may indicate that these two peaks arise from twodistinct regions along the jet separated by a distance of 0.07-5 pc. The fluxat 5-35 microns varied by a factor of 40 and the IR peak varied in frequencyfrom <1E13 Hz to 4E13 Hz between the highest and lowest states in 2005 and2006, respectively. Variability was well correlated across the mid-IR band,with no measurable lag. Flares that doubled in flux occurred on a time scale of3 days. The IR SED peak moved to higher frequency as a flare brightened, thenreturned to lower frequency as it decayed. The fractional variability amplitudeincreased with frequency, which we attribute to decreasing synchrotron-selfabsorption optical depth. Mid-IR flares may signal the re-energization of ashock that runs into inhomogeneities along the pre-existing jet or in theexternal medium. The synchrotron peak frequencies during each major outburstmay depend upon both the distance from the jet apex and the physical conditionsin the shocks. Variation of the Doppler parameter along a curved or helical jetis another possibility. Frequency variability of the IR synchrotron peak mayhave important consequences for the interpretation of the blazar sequence, andthe presence of a secondary peak may give insight into jet structure.

Blazar 3C 454.3 in Outburst and Quiescence During 2005-2007: Two Variable Synchrotron Emission Peaks