Minimalist approach to BL Lacertae: Explaining gamma-ray spectral and temporal variability with a single physical parameter
Year: 2025
Type: article
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, $γ_p$, 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, $L_{IC}$, and the position of this peak on the SED energy axis, $E_p$, 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 $L_{IC} = 10^{42.33\pm0.15\pm0.18_{sys}}E_p^{0.98\pm0.05\pm0.06_{sys}}$, consistent, within the errors, with the linear relation $L_{\mathrm{IC}} \propto E_p$, expected when $γ_p$ is the only free parameter in the assumed SSC model. This result supports a minimalist SSC scenario in which changes in $γ_p$ dominate the observed temporal and spectral variability of BL~Lacertae.
DOI: 10.1103/k6sx-5tcl
Authors: Raniere de Menezes, Francesco Massaro, Elisa Visentin, Federico Di Pierro, Haocheng Zhang
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: U.S. Department of Energy, National Aeronautics and Space Administration, Istituto Nazionale di Fisica Nucleare Sezione di Padova, Ministry of Education, Culture, Sports, Science and Technology, Swedish National Space Agency, Centre National d’Etudes Spatiales, Agenzia Spaziale Italiana, Knut och Alice Wallenbergs Stiftelse, Vetenskapsrådet, Centre National de la Recherche Scientifique, Istituto Nazionale di Astrofisica, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, High Energy Accelerator Research Organization, Istituto Nazionale di Fisica Nucleare, Japan Aerospace Exploration Agency, Institut National de Physique Nucléaire et de Physique des Particules
OpenAlex ID: W4417286670