Insights from the first flaring activity of a high synchrotron peaked blazar with X-ray polarization and VHE gamma rays
Year: 2025
Type: article
Abstract: Context. Blazars exhibit strong variability across the entire electromagnetic spectrum, including periods of high-flux states commonly known as flares. The physical mechanisms in blazar jets responsible for flares remain poorly understood to date. Aims. Our aim is to better understand the emission mechanisms during blazar flares using X-ray polarimetry and broadband observations from the archetypical TeV blazar Mrk 421, which can be studied with higher accuracy than other blazars that are dimmer and/or located farther away. Methods. We studied a flaring activity from December 2023 that was characterized from radio to very high-energy (VHE; E > 0.1 TeV) gamma rays with MAGIC, Fermi-LAT, Swift, XMM-Newton, and several optical and radio telescopes. These observations included, for the first time for a gamma-ray flare of a blazar, simultaneous X-ray polarization measurements with IXPE, in addition to optical and radio polarimetry data. We quantify the variability and correlations among the multi-band flux and polarization measurements, and describe the varying broadband emission within a theoretical scenario constrained by the polarization data. Results. We find substantial variability in both X-rays and VHE gamma rays throughout the campaign, with the highest VHE flux above 0.2 TeV occurring during the IXPE observing window, and exceeding twice the flux of the Crab Nebula. However, the VHE and X-ray spectra are on average softer, and the correlation between these two bands is weaker than those reported in the previous flares of Mrk 421. IXPE reveals an X-ray polarization degree significantly higher than that at radio and optical frequencies, similar to previous results for Mrk 421 and other high synchrotron peaked blazars. Differently to past observations, the X-ray polarization angle varies by ∼100° on timescales of days, and the polarization degree changes by more than a factor of 4. The highest X-ray polarization degree, analyzed in 12 h time intervals, reaches 26 ± 2%, around which an X-ray counter-clockwise hysteresis loop is measured with XMM-Newton. It suggests that the X-ray emission comes from particles close to the high-energy cutoff, hence possibly probing an extreme case of the Turbulent Extreme Multi-Zone model for which the chromatic trend in the polarization may be more pronounced than theoretically predicted. We model the broadband emission with a simplified stratified jet model throughout the flare. The polarization measurements imply an electron distribution in the X-ray emitting region with a very high minimum Lorentz factor ($ \gamma\prime_{\mathrm{min}}\gtrsim10^4 $), which is expected in electron-ion plasma, as well as a variation of the emitting region size of up to a factor of 3 during the flaring activity. We find no correlation between the fluxes and the evolution of the model parameters, which indicates a stochastic nature of the underlying physical mechanism that likely explains the lack of a tight X-ray/VHE correlation during this flaring activity. Such behavior would be expected in a highly turbulent electron-ion plasma crossing a shock front.
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, Smithsonian Institution, U.S. Department of Energy, National Aeronautics and Space Administration, W. M. Keck Foundation, Centres de Recerca de Catalunya, European Commission, Deutsche Forschungsgemeinschaft, Ministry of Education, Culture, Sports, Science and Technology, Academia Sinica, Generalitat de Catalunya, Conselho Nacional de Desenvolvimento Científico e Tecnológico, Agenzia Spaziale Italiana, Liverpool John Moores University, Vetenskapsrådet, Hrvatska Zaklada za Znanost, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Centre National de la Recherche Scientifique, Istituto Nazionale di Astrofisica, High Energy Accelerator Research Organization, Junta de Andalucía, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Istituto Nazionale di Fisica Nucleare, Instituto de Astrofísica de Andalucía, Marshall Space Flight Center, Goddard Space Flight Center, Smithsonian Astrophysical Observatory, Science and Technology Facilities Council, Consejo Superior de Investigaciones Científicas, Japan Aerospace Exploration Agency, Institut National de Physique Nucléaire et de Physique des Particules, Integrated Electronics Engineering Center, Binghamton University
Awards: G1141912172, G1171257612, A New Probe of the Magnetic Interstellar Medium, G1382750735, G1897041155, G1988743345, G2209488408, G2371548206, G278595929, G2950570888, G303061748, G349566373, G3515111791, G3528041363, G3696727022, G380733860, G3978510197, G4066435496, G4096230270, G4103929414, EXC 2094: ORIGINS: Vom Ursprung des Universums bis zu den ersten Bausteinen des Lebens, G4473368111, G4654690619, G4703727483, G4711507462, Opticon RadioNet Pilot, G5123872564, G5241832549, WoU-MMA: Linear and Circular Polarization as a Probe of the Relativistic Jets of Blazars, Very-high-energy Gamma Ray Astronomy in the Multi-messenger Era, G5438923238, G5570207115, G5571629311, G5639178410, G5881647929, G5957713366, G5968124160, G6009609052, G6067053368, G6313143952, G6791753710, G702601109, G727539915, G7306159782, G7411181859, G7518687652, G754392931, G7642730041, G7743295225, G7840098671, G796551035, G8314674143, G8362730697, G8726098666, G8920101981, G8945569637, G914374994, G969889393
OpenAlex ID: W7111122259
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