VHE gamma-ray intranight variability from BL Lacertae during the extreme flaring state of 2022

Abstract

BL Lacertae (BL Lac), the archetype blazar of its subclass and one of the most studied blazars in the last decades, has gone through a series of major multi-wavelength outbursts since 2020, resulting in its highest recorded $γ$-ray flare up to date between September and November 2022 together with those from August 2021 and October 2024. We characterise the $γ$-ray and multi-wavelength emission and spectral energy distribution (SED) of BL Lac, as well as their evolution during the major and extended $γ$-ray and multi-wavelength flare occurring between September and November 2022. We evaluate the variability of the flare, with focus on the nights of October 20 and November 13, when clear intranight very-high-energy (VHE, $E>100$ GeV) $γ$-ray variability is observed. We model the $γ$-ray and broadband SEDs during periods of stable emission identified with a Bayesian block analysis, interpreting their evolution of the flare from the variability of the relativistic particles and physical parameters of the jet. The VHE emission shows an average flux of 0.23 Crab Units (C.U.) above 200 GeV during this flare and a variability amplitude of more than a factor 10. Intranight doubling-flux variations as fast as $\sim$8 minutes are observed during the nights of October 20 and November 13, when maximum fluxes of 4.4 C.U. above 100 GeV and 2.8 C.U. above 200 GeV are reached. The spectral analysis reveals a transition of the X-ray emission from the high- to the low-energy SED peak, and a shift of the $γ$-ray peak towards higher energies. The broadband emission was interpreted within a leptonic two-zone model in which intranight variability is explained as magnetic reconnection in a compact region closely oriented with the line of sight while weekly-scale variations can be explained as variations of the electron distributions and the injection of accelerated particles.

Figures (9)

• Figure 1: Multi-wavelength light curves of BL Lacertae between September 21 and November 30, 2022. From top to bottom: LST-1 night-wise VHE $\gamma$-ray light curve above 200 GeV, Fermi-LAT 12-hour binned HE $\gamma$-ray light curves between 100 MeV and 300 GeV, XRT X-ray light curve between 0.3 and 10 keV, UVOT optical-UV light curves in the different filters available, optical light curves in the $BVRI$ bands, 37-GHz and 225-GHz radio light curves. The blue dashed line in the top panel corresponds to the Crab Nebula flux above 200 GeV aleksic2016. The blue contours delimited by black dashed lines highlight the Bayesian blocks identified in the LST-1 light curve.
• Figure 2: Intranight light curves of BL Lac. Left: 5-minute binned light curve during the night of October 20 above 100 GeV. Right: 5-minute binned light curve during the night of November 13 above 200 GeV. Black points correspond to LST-1's measurements. Black solid lines represent the light curve models. The horizontal dashed red lines show the flux of the Crab Nebula as measured by the MAGIC telescopes aleksic2016.
• Figure 3: Joint LAT-LST-1 SED for BB8 using a log-parabola plus EBL model. White circles and squares correspond to the LAT and LST-1 spectral points, respectively. Down-pointing arrows correspond to flux upper limits. The blue and red lines and bowtie shapes represent the best fits of the joint LAT+LST-1 and LST-1-only data, respectively.
• Figure 4: Broadband SED of BL Lac for BB8. Black points and arrows correspond to flux points and upper limits obtained from the different instruments. The black solid line represents the fitted co-spatial two-zone SSC model. The blue dotted and dashed-dotted lines show the synchrotron and SSC emission of the core, respectively. The orange dotted and dashed-dotted lines indicate the synchrotron and SSC emission of the blob. The magenta dashed-dotted and dashed lines are the EC emission from the core and the blob, respectively. The dark red dashed line is the emission resulting from the interaction between the core and the blob. The grey solid line is the SED of the previous block. The historical data retrieved from the SSDC database (https://tools.ssdc.asi.it/SED/) are shown with grey open points.
• Figure 5: Results of the X-ray and UV Swift data analysis. Left: X-ray and optical-UV SEDs obtained from the Swift-XRT/UVOT analyses between September and November 2022. The colour scale corresponds to the MJD of the observation. Right: X-ray power-law spectral index as function of the 0.3-10 keV flux. Observations for which a log-parabola shape was preferred over a power law with a significance $\geq$3$\sigma$ are shown with open markers.