Paolo Coppi - Short Timescale Optical-Gamma Ray Variability in GeV Blazars: The Case of 3C 345.3

Friday 30 Jan 2026, 10:30 → 12:00 Europe/Berlin

To increase measurement statistics, the Fermi lightcurves of GeV blazars are routinely binned and plotted on day or weekly timescales.  Unfortunately, in those cases where the blazar is bright enough that shorter (~hour) timescales can be probed, significant variability on these short timescales is often found, complicating the standard model interpretation.  Variability on short timescales, for example, can challenge standard acceleration scenarios, and the presence of significant spectral variability can lead to misleading SED-fitting results when using spectra averaged over longer timescales.  Moreover, while a relatively good correlation is usually seen between optical and ~GeV gamma-ray fluxes on timescales longer than a few days, the correlation appears to break down on shorter timescales.  This is problematic since in the synchrotron-Compton model for these sources, the optical and GeV emission are supposed to be produced by the same electrons.  I will highlight some of these short-timescale variability issues by looking in detail at the optical-GeV lightcurve for 3C 454.3. This has been one of the brightest GeV blazars during the Fermi era and, over the 18 year lifetime of Fermi, has had approximately ten flaring episodes bright enough to allow probing variability down to hour timescales.  Several of these have not been studied in detail yet, and we are in the process of carrying out similar long-term studies for other bright blazars. The opportunities for such studies will only increase as large-scale optical monitoring programs such as LSST/Rubin come online, and the low-energy threshold of ground-based gamma-ray detectors, e.g., CTAO/LST,  starts to overlap significantly with the Fermi energy range. I will speculate on what may be responsible for 3C 454.3's complex variability, in particular the so-called Compton mirror scenarios.