Long-term mid-infrared colour variability in blazars

Abstract Utilizing a sample of 3,011 blazars with multi-epoch 3.4 μm (W1) and 4.6 μm (W2) data from the unTimely catalogue, we systematically investigate their long-term (~10yr) mid-infrared (MIR) colour variability. The long-term fractional flux variability is systematically higher in the W2 band than in the W1 band across the full sample and all subclasses. Both fractional variability (Fvar) and ensemble structure function (ESF) analyses reveal that BL Lac objects and non-Fermi blazars display systematically larger long-term MIR colour variability amplitudes than FSRQs and non-Fermi blazars, respectively. Employing a Bayesian Markov Chain Monte Carlo (MCMC) framework and the ΔCW1W2 method, we identify both bluer-when-brighter (BWB) and redder-when-brighter (RWB) spectral trends in every subclass. Neither trend dominates the population, contrasting with previous reports of BWB dominance. Furthermore, the long-term MIR colour variability shows no significant dependence on time lag across the probed timescales. Our findings indicate that long-term MIR colour variability in blazars is governed by a complex interplay between non-thermal jet synchrotron emission, thermal background components (accretion disc, dusty torus, and host-galaxy starlight), and Doppler boosting effects.