Minimalist approach to BL Lacertae: Explaining gamma-ray spectral and temporal variability with a single physical parameter

The eponymous BL Lac object BL Lacertae is one of the most well-monitored active galactic nuclei, frequently observed from radio to gamma rays. Its relatively soft γ-ray spectrum peaks near 500 MeV, and since 2020 it has undergone an exceptional series of flaring episodes. The observed emission is well described by synchrotron self-Compton (SSC) models, with negligible contribution from external seed photons. We investigate the physical origin of BL Lacertae's γ-ray temporal and spectral variability using data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and show that this variability can be explained by a single varying parameter, namely, the electrons' peak energy yp, under a single-zone SSC scenario with a log-parabolic electron distribution. We use a Markov chain Monte Carlo to estimate the spectral parameters of BL Lacertae over time, selected from an adaptive-binned gamma-ray light curve. We then study the correlation between the inverse Compton peak luminosity LIC and the position of this peak on the spectral energy distribution energy axis, Ep, and compare it with what is expected for a single-zone SSC scenario when only one parameter is free to vary. We find a correlation com.elsevier.xml.xocs.dtd.Math@7ac9aad3, consistent, within the errors, with the linear relation LIC k Ep, expected when yp is the only free parameter in the assumed SSC model. This result supports a minimalist SSC scenario in which changes in yp dominate the observed temporal and spectral variability of BL Lacertae.