$γ$-ray variability of changing-look blazars: FSRQ-like jet behaviour

Abstract

Changing-look blazars (CLBs) move between the flat-spectrum radio quasar (FSRQ) and BL Lacertae (BL Lac) optical classifications. Either the accretion flow switches regime, or a brightening jet continuum swamps the broad lines while the system itself is unchanged. The two pictures predict the same optical behaviour but differ in the jet, so the $γ$-ray emission could potentially separate them. Catalogue quantities averaged over the mission cannot, because a permanently intermediate source and one alternating between two states return the same mean. We therefore use 17.9\,yr of \textit{Fermi}-LAT light curves at 3-, 7- and 30-day cadence, comparing CLBs with BL Lac and FSRQ controls selected and analysed identically. CLBs and FSRQs have indistinguishable fractional variability, with medians of 0.483 and 0.437 at 30-day cadence ($p = 0.94$), while BL Lacs are far less variable at 0.262 ($p = 1.4\times10^{-4}$). The result survives restricting the controls to low-synchrotron-peaked sources and matching in flux and sampling. Several further diagnostics group CLBs with FSRQs, as does their Eddington-scaled disc luminosity. Followed through time, CLB light curves show no excess of bimodal flux distributions, and the direction of the optical transition does not predict how the $γ$-ray spectrum evolves. CLBs do reach hard, BL Lac-like spectra more often than FSRQs, but never for longer than chance allows. Only in this, and in how their spectra respond to brightening, do they fall between the two classes. The changing-look phenomenon therefore leaves little imprint on the $γ$-ray jet, favouring a variable jet continuum over a change of accretion regime.