A1 Refereed original research article in a scientific journal
Very-high-energy gamma-ray intranight variability from BL Lacertae during the extreme flaring state of 2022; 
Authors: Abe, K.; Abe, S.; Abhishek, A.; Acero, F.; Aguasca-Cabot, A.; Agudo, I.; Alispach, C.; Ambrosino, D.; Ambrosino, F.; Antonelli, L. A.; Aramo, C.; Arbet-Engels, A.; Arcaro, C.; Arnesen, T. T. H.; Aubert, P.; Baktash, A.; Balbo, M.; Bamba, A.; Baquero Larriva, A.; Barres de Almeida, U.; Barrio, J. A.; Barrios Jiménez, L.; Batkovic, I.; Baxter, J.; Becerra González, J.; Bernete, J.; Berti, A.; Bissaldi, E.; Blanch, O.; Bonnoli, G.; Bordas, P.; Briscioli, A.; Brunelli, G.; Buces, J.; Bulgarelli, A.; Burelli, I.; Burmistrov, L.; Cardillo, M.; Caroff, S.; Carosi, A.; Carraro, R.; Cassol, F.; Cerasole, D.; Cerviño Cortínez, A.; Chai, Y.; Chon, G.; Chytka, L.; Cicciari, G. M.; Contreras, J. L.; Cortina, J.; Costantini, H.; Croisonnier, M.; Dalchenko, M.; D’Amico, G.; Da Vela, P.; Dazzi, F.; De Angelis, A.; de Bony de Lavergne, M.; Del Burgo, R.; Delgado, C.; Delgado Mengual, J.; della Volpe, D.; De Lotto, B.; Del Peral, L.; de Menezes, R.; De Palma, G.; de Souza, V.; Díaz, C.; Di Bella, L.; Di Piano, A.; Di Pierro, F.; Di Tria, R.; Di Venere, L.; Dominis Prester, D.; Donini, A.; Dorner, D.; Eisenberger, L.; Elsässer, D.; Emery, G.; Feligioni, L.; Ferrarotto, F.; Fiasson, A.; Foffano, L.; Fukazawa, Y.; Gallozzi, S.; Garcia López, R.; Garcia Soto, S.; Gasbarra, C.; Gasparrini, D.; Giesbrecht Paiva, J.; Giglietto, N.; Giordano, F.; Godinovic, N.; Gradetzke, T.; Grau, R.; Green, J.; Grolleron, G.; Gunji, S.; Günther, P.; Hackfeld, J.; Hadasch, D.; Hashizume, M.; Hassan, T.; Hayashi, K.; Heckmann, L.; Heller, M.; Herrera Llorente, J.; Hiroshima, N.; Hoffmann, D.; Horns, D.; Houles, J.; Hrupec, D.; Imazawa, R.; Inada, T.; Inoue, S.; Ioka, K.; Iori, M.; Itokawa, T.; Iuliano, A.; Jahanvi, J.; Jimenez Martinez, I.; Jimenez Quiles, J.; Jorge Rodrigo, I.; Jurysek, J.; Kagaya, M.; Karas, V.; Katagiri, H.; Kerszberg, D.; Kiyomoto, T.; Kobayashi, Y.; Kohri, K.; Kornecki, P.; Kubo, H.; Kushida, J.; Lacave, B.; Lainez, M.; Lamastra, A.; Lemoigne, L.; Linhoff, M.; Lombardi, S.; Longo, F.; López-Coto, R.; López-Oramas, A.; Loporchio, S.; Lozano Bahilo, J.; Lucarelli, F.; Luciani, H.; Luque-Escamilla, P. L.; Makariev, M.; Mallamaci, M.; Mandat, D.; Mannheim, K.; Marini, F.; Mariotti, M.; Marquez, P.; Marsella, G.; Martí, J.; Martinez, O.; Martínez, G.; Martínez, M.; Massa, M.; Mazin, D.; Méndez-Gallego, J.; Menon, S.; Mestre Guillen, E.; Miceli, D.; Miener, T.; Miranda, J. M.; Molero Gonzalez, M.; Molina, E.; Montaruli, T.; Moralejo, A.; Morselli, A.; Moya, V.; Müller, A. L.; Muraishi, H.; Nagataki, S.; Nakamori, T.; Neronov, A.; Nieto Castaño, D.; Nievas Rosillo, M.; Nikolic, L.; Noda, K.; Novotny, V.; Nozaki, S.; Ohishi, M.; Ohtani, Y.; Okumura, A.; Orito, R.; Orsini, L.; Otero-Santos, J.; Ottanelli, P.; Palatiello, M.; Panebianco, G.; Paneque, D.; Paoletti, R.; Paredes, J. M.; Pech, M.; Pecimotika, M.; Peresano, M.; Perrotta, F.; Pfeifle, F.; Pihet, M.; Pirola, G.; Plard, C.; Podobnik, F.; Polo, M.; Pozo-Gonzaléz, C.; Prandini, E.; Rainò, S.; Rando, R.; Rhode, W.; Ribó, M.; Rodriguez Fernandez, G.; Rodríguez Frías, M. D.; Roy, A.; Ruina, A.; Ruiz-Velasco, E.; Saito, T.; Sakurai, S.; Sanchez, D. A.; Sano, H.; Santos Moura, E.; Šarić, T.; Sato, Y.; Saturni, F. G.; Savchenko, V.; Schiavone, F.; Schussler, F.; Schweizer, T.; Seglar Arroyo, M.; Silvestri, G.; Simongini, A.; Sitarek, J.; Sliusar, V.; Sofia, I.; Strišković, J.; Strzys, M.; Suda, Y.; Sunny, A.; Tajima, H.; Takahashi, M.; Takeishi, R.; Tanaka, S. J.; Tateishi, D.; Tavernier, T.; Temnikov, P.; Terada, Y.; Terauchi, K.; Terzic, T.; Teshima, M.; Tluczykont, M.; Tomura, T.; Torres, D. F.; Tramonti, F.; Travnicek, P.; Tripodo, G.; Tutone, A.; Vacula, M.; Vázquez Acosta, M.; Verna, G.; Viale, I.; Viana, A.; Vigliano, A.; Vigorito, C. F.; Visentin, E.; Vitale, V.; Voutsinas, G.; Vovk, I.; Vuillaume, T.; Walter, R.; Yamamoto, T.; Yamazaki, R.; Yao, Y.; Yoshida, T.; Yoshikoshi, T.; Zhang, W.; Zywucka, N.; CTAO-LST collaboration (the); Aceituno, F.; Acosta-Pulido, J. A.; Casanova, V.; Escudero Pedrosa, J.; Fallah Ramazani, V.; Jormanainen, J.; Jorstad, S.; Keating, G.; Kouch, P. M.; Gurwell, M.; Lähteenmäki, A.; Lindfors, E.; Marscher, A.; Morcuende, D.; Myserlis, I.; Nilsson, K.; Ortega Hunter, C. A.; Rao, R.; Sota, A.; Tornikoski, M.; Zhang, H.
Publisher: EDP Sciences
Publication year: 2026
Journal: Astronomy and Astrophysics
Article number: A41
Volume: 710
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202557924
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202557924
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/527082976
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
Context. BL Lacertae (BL Lac), the archetypal blazar of its subclass and one of the most studied blazars of recent decades, has undergone a series of major multi-wavelength outbursts since 2020, resulting in its highest recorded γ-ray flare to date between September and November 2022, together with those from August 2021 and October 2024.
Aims. We characterised 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 that occurred between September and November 2022.
Methods. We evaluated the variability of the flare, focusing on the nights of October 20 and November 13, 2022 when clear intranight very-high-energy (VHE, E > 100 GeV) γ-ray variability was observed. We modelled the γ-ray and broadband SEDs during periods of stable emission identified with a Bayesian block analysis and interpreted the flare’s evolution in terms of the variability in the relativistic particles and the jet’s physical parameters.
Results. During this flare, the VHE emission shows an average flux of 0.23 Crab units (C.U.) above 200 GeV and a variability amplitude of more than a factor of ten. We observe intranight flux-doubling variations as fast as ∼8 minutes during the nights of October 20 and November 13, 2022 with maximum fluxes of 4.4 C.U. above 100 GeV and 2.8 C.U. above 200 GeV. The spectral analysis reveals a transition of the X-ray emission from the high- to the low-energy SED peak and a shift in the γ-ray peak towards higher energies. We interpret the broadband emission within a leptonic two-zone model in which intranight variability is explained as magnetic reconnection in a compact region closely orientated with the line of sight, while variations in the relativistic electron distributions and the injection of freshly accelerated particles explain the weekly scale variations.
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We sincerely thank the anonymous referee for their thorough review of the manuscript and helpful comments. We gratefully acknowledge financial support from the following agencies and organisations: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Grant 309053/2022-6 and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) Grants E-26/200.532/2023 and E-26/211.342/2021, Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Fundação de Apoio à Ciência, Tecnologia e Inovação do Paraná - Fundação Araucária, Ministry of Science, Technology, Innovations and Communications (MCTIC), Brasil; Ministry of Education and Science, National RI Roadmap Project DO1-153/28.08.2018, Bulgaria; Croatian Science Foundation (HrZZ) Project IP-2022-10-4595, Rudjer Boskovic Institute, University of Osijek, University of Rijeka, University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Zagreb, Faculty of Electrical Engineering and Computing, Croatia; Ministry of Education, Youth and Sports, MEYS LM2023047, EU/MEYS CZ.02.1.01/0.0/0.0/16_013/0001403, CZ.02.1.01/0.0/0.0/18_046/0016007, CZ.02.1.01/0.0/0.0/16_019/0000754, CZ.02.01.01/00/22_008/0004632 and CZ.02.01.01/00/23_015/0008197 Czech Republic; CNRS-IN2P3, the French Programme d’investissements d’avenir and the Enigmass Labex, This work has been done thanks to the facilities offered by the Univ. Savoie Mont Blanc - CNRS/IN2P3 MUST computing center, France; Max Planck Society, German Bundesministerium für Forschung, Technologie und Raumfahrt (Verbundforschung / ErUM), the Deutsche Forschungsgemeinschaft (SFB 1491) and the Lamarr-Institute for Machine Learning and Artificial Intelligence, Germany; Istituto Nazionale di Astrofisica (INAF), Istituto Nazionale di Fisica Nucleare (INFN), Italian Ministry for University and Research (MUR), and the financial support from the European Union – Next Generation EU under the project IR0000012 - CTA+ (CUP C53C22000430006), announcement N.3264 on 28/12/2021: “Rafforzamento e creazione di IR nell’ambito del Piano Nazionale di Ripresa e Resilienza (PNRR)”; ICRR, University of Tokyo, JSPS, MEXT, Japan; JST SPRING - JPMJSP2108; Narodowe Centrum Nauki, grant number 2023/50/A/ST9/00254, Poland; The Spanish groups acknowledge the Spanish Ministry of Science and Innovation and the Spanish Research State Agency (AEI) through the government budget lines PGE2022/28.06.000X.711.04, 28.06.000X.411.01 and 28.06.000X.711.04 of PGE 2023, 2024 and 2025, and grants PID2019-104114RB-C31, PID2019-107847RB-C44, PID2019-105510GB-C31, PID2019-104114RB-C33, PID2019-107847RB-C43, PID2019-107847RB-C42, PID2019-107988GB-C22, PID2021-124581OB-I00, PID2021-125331NB-I00, PID2022-136828NB-C41, PID2022-137810NB-C22, PID2022-138172NB-C41, PID2022-138172NB-C42, PID2022-138172NB-C43, PID2022-139117NB-C41, PID2022-139117NB-C42, PID2022-139117NB-C43, PID2022-139117NB-C44, PID2022-136828NB-C42, PID2024-155316NB-I00, PDC2023-145839-I00 funded by the Spanish MCIN/AEI/10.13039/501100011033 and by ERDF/EU and NextGenerationEU PRTR; CSIC PIE 202350E189; the "Centro de Excelencia Severo Ochoa" program through grants no. CEX2020-001007-S, CEX2021-001131-S, CEX2024-001442-S; the "Unidad de Excelencia María de Maeztu" program through grants no. CEX2019-000918-M, CEX2020-001058-M; the "Ramón y Cajal" program through grants RYC2021-032991-I funded by MICIN/AEI/10.13039/501100011033 and the European Union “NextGenerationEU”/PRTR and RYC2020-028639-I; the "Juan de la Cierva-Incorporación" program through grant no. IJC2019-040315-I and "Juan de la Cierva-formación"’ through grant JDC2022-049705-I; the “Viera y Clavijo” postdoctoral program of Universidad de La Laguna, funded by the Agencia Canaria de Investigación, Innovación y Sociedad de la Información. They also acknowledge the "Atracción de Talento" program of Comunidad de Madrid through grant no. 2019-T2/TIC-12900; “MAD4SPACE: Desarrollo de tecnologías habilitadoras para estudios del espacio en la Comunidad de Madrid" (TEC-2024/TEC-182) project, Doctorado Industrial (IND2024/TIC34250) and Ayudas para la contratación de personal investigador predoctoral en formación (PIPF-2023/TEC-29694) funded by Comunidad de Madrid; the La Caixa Banking Foundation, grant no. LCF/BQ/PI21/11830030; Junta de Andalucía under Plan Complementario de I+D+I (Ref. AST22_0001) and Plan Andaluz de Investigación, Desarrollo e Innovación as research group FQM-322; Project ref. AST22_00001_9 with funding from NextGenerationEU funds; the “Ministerio de Ciencia, Innovación y Universidades” and its “Plan de Recuperación, Transformación y Resiliencia”; “Consejería de Universidad, Investigación e Innovación” of the regional government of Andalucía and “Consejo Superior de Investigaciones Científicas”, Grant CNS2023-144504 funded by MICIU/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR, the European Union’s Recovery and Resilience Facility-Next Generation, in the framework of the General Invitation of the Spanish Government’s public business entity Red.es to participate in talent attraction and retention programmes within Investment 4 of Component 19 of the Recovery, Transformation and Resilience Plan; Junta de Andalucía under Plan Complementario de I+D+I (Ref. AST22_00001), Plan Andaluz de Investigación, Desarrollo e Innovación (Ref. FQM-322). “Programa Operativo de Crecimiento Inteligente" FEDER 2014-2020 (Ref. ESFRI-2017-IAC-12), Ministerio de Ciencia e Innovación, 15% co-financed by Consejería de Economía, Industria, Comercio y Conocimiento del Gobierno de Canarias; the "CERCA" program and the grants 2021SGR00426 and 2021SGR00679, all funded by the Generalitat de Catalunya; and the European Union’s NextGenerationEU (PRTR-C17.I1). This work is funded/Co-funded by the European Union (ERC, MicroStars, 101076533). This research used the computing and storage resources provided by the Port d’Informació Científica (PIC) data center. State Secretariat for Education, Research and Innovation (SERI) and Swiss National Science Foundation (SNSF), Switzerland; The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreements No 262053 and No 317446; This project is receiving funding from the European Union’s Horizon 2020 research and innovation programs under agreement No 676134; ESCAPE - The European Science Cluster of Astronomy & Particle Physics ESFRI Research Infrastructures has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement no. 824064. H.Z. is supported by NASA under award number 80GSFC24M0006. Some of the data are based on observations collected at the Observatorio de Sierra Nevada; which is owned and operated by the Instituto de Astrofísica de Andalucía (IAA-CSIC). This article is based on observations made with the IAC80 operated on the island of Tenerife by the Instituto de Astrofísica de Canarias in the Spanish Observatorio del Teide. This work makes use of observations from the Las Cumbres Observatory global telescope network. This work makes use of data from the All-Sky Automated Survey for Supernovae (ASAS-SN). This work has made use of data from the Joan Oró Telescope (TJO) of the Montsec Observatory (OdM), which is owned by the Catalan Government and operated by the Institute for Space Studies of Catalonia (IEEC). Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grants No. AST-1440341 and AST-2034437 and a collaboration including current partners Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley, the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University, Cornell University, Northwestern University and Drexel University. Operations are conducted by COO, IPAC, and UW. This publication makes use of data obtained at the Metsähovi Radio Observatory, operated by the Aalto University. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. Maunakea, the location of the SMA, is a culturally important site for the indigenous Hawaiian people; we are privileged to study the cosmos from its summit.
