A minimalist approach to BL Lacertae: explaining gamma-ray spectral and temporal variability with a single physical parameter

Abstract

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 (LAT) on board the \textit{Fermi} Gamma-ray Space Telescope, and show that this variability can be explained by a single varying parameter, namely the electrons' peak energy, γₚ, 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, Lᵢ꜀, and the position of this peak on the SED 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 Lᵢ꜀ = 10 superscript(42.33±0.15±0.18)ₛᵧₛ Eₚ superscript(0.98±0.05±0.06)ₛᵧₛ consistent, within the errors, with the linear relation Lᵢ꜀∝Eₚ , expected when γₚ is the only free parameter in the assumed SSC model. This result supports a minimalist SSC scenario in which changes in γₚ dominate the observed temporal and spectral variability of BL~Lacertae.