Browsing by Author "Petropoulou, M."

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  • A combined radio and GeV gamma-ray view of the 2012 and 2013 flares of Mrk 421
    (2015) Hovatta, T.; Petropoulou, M.; Richards, J.L.; Giannios, D.; Wiik, K.; Balokovic, M.; Lähteenmäki, A.; Lott, B.; Max-Moerbeck, W.; Ramakrishnan, V.; Readhead, A.C.S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Confronting observations of VHE gamma-ray blazar flares with reconnection models
    (2022-03-18) Jormanainen, J.; Hovatta, T.; Lindfors, E.; Christie, I.; Petropoulou, M.; Liodakis, I.
     A4 Artikkeli konferenssijulkaisussa
    Several models have been suggested to explain the fast gamma-ray variability observed in blazars, but its origin is still debated. One scenario is magnetic reconnection, a process that can efficiently convert magnetic energy to energy of relativistic particles accelerated in the reconnection layer. In our study, we compare results from state-of-the-art particle-in-cell simulations with observations of blazars at Very High Energy (VHE, E > 100 GeV) gamma-rays. Our goal is to test our model predictions on fast gamma-ray variability with data and to constrain the parameter space of the model, such as the magnetic field strength of the unreconnected plasma and the reconnection layer orientation in the blazar jet. For this first comparison, we used the remarkably well-sampled VHE gamma-ray light curve of Mrk 421 observed with the MAGIC and VERITAS telescopes in 2013. The simulated VHE light curves were generated using the observable parameters of Mrk 421, such as the jet power, bulk Lorentz factor, and the jet viewing angle, and sampled as real data. Our results pave the way for future model-to-data comparison with next-generation Cherenkov telescopes, which will help further constrain the different variability models.
  • The hunt for extraterrestrial high-energy neutrino counterparts
    (2022-09-30) Liodakis, I.; Hovatta, T.; Pavlidou, Vassiliki; Readhead, A. C. S.; Blandford, R. D.; Kiehlmann, S.; Lindfors, E.; Max-Moerbeck, W.; Pearson, T. J.; Petropoulou, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and we lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, to relativistic jets and galaxy clusters with blazar TXS 0506+056 the most compelling association thus far. Since that association, immense effort has been put into proving or disproving that jets are indeed neutrino emitters, but to no avail. By generating simulated neutrino counterpart samples, we explore the potential of detecting a significant correlation of neutrinos with jets from active galactic nuclei. We find that, given the existing challenges, even our best experiments could not have produced a > 3σ result. Larger programs over the next few years will be able to detect a significant correlation only if the brightest radio sources, rather than all jetted active galactic nuclei, are neutrino emitters. We discuss the necessary strategies required to steer future efforts into successful experiments.
  • Quantitative comparisons of very-high-energy gamma-ray blazar flares with relativistic reconnection models
    (2023-10-01) Jormanainen, J.; Hovatta, T.; Christie, I. M.; Lindfors, E.; Petropoulou, M.; Liodakis, I.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The origin of extremely fast variability is one of the long-standing questions in the gamma-ray astronomy of blazars. While many models explain the slower, lower energy variability, they cannot easily account for such fast flares reaching hour-to-minute timescales. Magnetic reconnection, a process where magnetic energy is converted to the acceleration of relativistic particles in the reconnection layer, is a candidate solution to this problem. In this work, we employ state-of-the-art particle-in-cell simulations in a statistical comparison with observations of a flaring episode of a well-known blazar, Mrk 421, at a very high energy (VHE, E>100 GeV). We tested the predictions of our model by generating simulated VHE light curves that we compared quantitatively with methods that we have developed for a precise evaluation of theoretical and observed data. With our analysis, we can constrain the parameter space of the model, such as the magnetic field strength of the unreconnected plasma, viewing angle and the reconnection layer orientation in the blazar jet. Our analysis favours parameter spaces with magnetic field strength 0.1 G, rather large viewing angles (6-8°), and misaligned layer angles, offering a strong candidate explanation for the Doppler crisis often observed in the jets of high synchrotron peaking blazars.