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Astrophysics and Astronomy

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Published in the journal Astronomy & Astrophysics in the section Extragalactic Astronomy.

Aleksić, J. et al., A&A, 578., A22, 2015, reproduced with permission, © ESO

The authors thank the anonymous referee for providing a very detailed and constructive list of remarks that helped us to improve the manuscript. The MAGIC collaboration would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF under the Spanish MINECO, and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Centro de Excelencia Severo Ochoa SEV-2012-0234, CPAN CSD2007- 00042, and MultiDark CSD2009-00064 projects of the Spanish ConsoliderIngenio 2010 programme, by grant 268740 of the Academy of Finland, by the Croatian Science Foundation (HrZZ) Project 09/176 and the University of Rijeka Project, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESSMAGIC/2010/0. The VERITAS collaboration acknowledges supports from the grants from the US Department of Energy Office of Science, the US National Science Foundation and the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland (SFI 10/RFP/AST2748) and by STFC in the UK. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. The Fermi-LAT collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l’Énergie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’Études Spatiales in France. The research at Boston University was funded in part by NASA Fermi Guest Investigator grant NNX11AQ03G. 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. The OVRO 40-m monitoring program is supported in part by NASA grants NNX08AW31G and NNX11A043G, and NSF grants AST-0808050 and AST-1109911. The Metsähovi team acknowledges the support from the Academy of Finland to our observing projects (numbers 212656, 210338, 121148, and others). This work was partly supported by Russian RFBR grant 12-02-00452 and St.Petersburg University research grants, The Abastumani Observatory team acknowledges financial support by the Georgian National Science Foundation through grant GNSF/ST07/4-180 and by the Shota Rustaveli National Science Foundation through the grant FR/577/6-320/13. We acknowledge the use of public data from the Swift and RXTE data archive.


Context. Because of its proximity, Mrk 421 is one of the best sources on which to study the nature of BL Lac objects. Its proximity allows us to characterize its broadband spectral energy distribution (SED).

Aims. The goal is to better understand the mechanisms responsible for the broadband emission and the temporal evolution of Mrk 421. These mechanisms may also apply to more distant blazars that cannot be studied with the same level of detail.

Methods. A flare occurring in March 2010 was observed for 13 consecutive days (from MJD 55 265 to MJD 55 277) with unprecedented wavelength coverage from radio to very high energy (VHE; E> 100 GeV) γ-rays with MAGIC, VERITAS, Whipple, Fermi-LAT, MAXI, RXTE, Swift, GASP-WEBT, and several optical and radio telescopes. We modeled the day-scale SEDs with one-zone and two-zone synchrotron self-Compton (SSC) models, investigated the physical parameters, and evaluated whether the observed broadband SED variability can be associated with variations in the relativistic particle population.

Results. The activity of Mrk 421 initially was high and then slowly decreased during the 13-day period. The flux variability was remarkable at the X-ray and VHE bands, but it was minor or not significant at the other bands. The variability in optical polarization was also minor. These observations revealed an almost linear correlation between the X-ray flux at the 2–10 keV band and the VHE γ-ray flux above 200 GeV, consistent with the γ-rays being produced by inverse-Compton scattering in the Klein-Nishina regime in the framework of SSC models. The one-zone SSC model can describe the SED of each day for the 13 consecutive days reasonably well, which once more shows the success of this standard theoretical scenario to describe the SEDs of VHE BL Lacs such as Mrk 421. This flaring activity is also very well described by a two-zone SSC model, where one zone is responsible for the quiescent emission, while the other smaller zone, which is spatially separated from the first, contributes to the daily variable emission occurring at X-rays and VHE γ-rays. The second blob is assumed to have a smaller volume and a narrow electron energy distribution with 3 × 104<γ< 6 × 105, where γ is the Lorentz factor of the electrons. Such a two-zone scenario would naturally lead to the correlated variability at the X-ray and VHE bands without variability at the optical/UV band, as well as to shorter timescales for the variability at the X-ray and VHE bands with respect to the variability at the other bands.

Conclusions. Both the one-zone and the two-zone SSC models can describe the daily SEDs via the variation of only four or five model parameters, under the hypothesis that the variability is associated mostly with the underlying particle population. This shows that the particle acceleration and cooling mechanism that produces the radiating particles might be the main mechanism responsible for the broadband SED variations during the flaring episodes in blazars. The two-zone SSC model provides a better agreement with the observed SED at the narrow peaks of the low- and high-energy bumps during the highest activity, although the reported one-zone SSC model could be further improved by varying the parameters related to the emitting region itself (δ, B and R), in addition to the parameters related to the particle population.