Issue |
A&A
Volume 617, September 2018
|
|
---|---|---|
Article Number | A91 | |
Number of page(s) | 9 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201832895 | |
Published online | 25 September 2018 |
Gamma-ray flaring activity of NGC1275 in 2016–2017 measured by MAGIC
1
Università di Udine, and INFN Trieste, 33100 Udine, Italy
2
National Institute for Astrophysics (INAF), 00136 Rome, Italy
3
Università di Padova and INFN, 35131 Padova, Italy
4
Technische Universität Dortmund, 44221 Dortmund, Germany
5
Croatian MAGIC Consortium: University of Rijeka, 51000 Rijeka, University of Split, FESB, 21000 Split, University of Zagreb, FER, 10000 Zagreb, University of Osijek, 31000 Osijek and Rudjer Boskovic Institute, 10000 Zagreb, Croatia
6
Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
7
Max-Planck-Institut für Physik, 80805 München, Germany
8
Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA Rio de Janeiro (RJ), Brasil
9
Unidad de Partículas y Cosmología (UPARCOS), Universidad Complutense, 28040 Madrid, Spain
10
Inst. de Astrofísica de Canarias, and Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
11
University of Łódź, Department of Astrophysics, 90236 Łódź, Poland
12
Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
13
ETH Zurich, 8093 Zurich, Switzerland
14
Institut de Física d’Altes Energies (IFAE), Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Spain
15
Università di Siena and INFN Pisa, 53100 Siena, Italy
16
Universität Würzburg, 97074 Würzburg, Germany
17
Finnish MAGIC Consortium: Tuorla Observatory and Finnish Centre of Astronomy with ESO (FINCA), University of Turku, Vaisalantie 20, FI-21500 Piikkiö, Astronomy Division, University of Oulu, 90014, Finland
18
Departament de Física, and CERES-IEEC, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain
19
Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
20
Japanese MAGIC Consortium: ICRR, The University of Tokyo, Chiba; Department of Physics, Kyoto University, 606-8502 Kyoto ;
Tokai University, Kanagawa; The University of Tokushima, Tokushima, Japan
21
Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
22
Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
23
Humboldt University of Berlin, Institut für Physik, 12489 Berlin, Germany
24
Universität Heidelberg, Zentrum für Astronomie, Landessternwarte, Königstuhl, 69117 Heidelberg, Germany
25
Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Finland
26
Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
27
Port d’Informació Científica (PIC), 08193 Bellaterra (Barcelona), Spain
28
INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Italy
29
Institute of Nuclear Physics Polish Academy of Sciences, 31342 Krakow, Poland
Received:
23
February
2018
Accepted:
4
June
2018
We report on the detection of flaring activity from the Fanaroff-Riley I radio galaxy NGC 1275 in very-high-energy (VHE, E > 100 GeV) gamma rays with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. The observations were performed between 2016 September and 2017 February, as part of a monitoring programme. The brightest outburst, with ∼1.5 times the Crab Nebula flux above 100 GeV (C.U.), was observed during the night between 2016 December 31 and 2017 January 1. The flux is fifty times higher than the mean flux previously measured in two observational campaigns between 2009 October and 2010 February and between 2010 August and 2011 February. Significant variability of the day-by-day light curve was measured. The shortest flux-doubling timescale was found to be of (611 ± 101) min. The spectra calculated for this period are harder and show a significant curvature with respect to the ones obtained in the previous campaigns. The combined spectrum of the MAGIC data during the strongest flare state and simultaneous data from the Fermi-LAT around 2017 January 1 follows a power law with an exponential cutoff at the energy (492 ± 35) GeV. We further present simultaneous optical flux density measurements in the R-band obtained with the Kungliga Vetenskaps Akademien (KVA) telescope and investigate the correlation between the optical and gamma-ray emission. Due to possible internal pair-production, the fast flux variability constrains the Doppler factor to values that are inconsistent with a large viewing angle as observed in the radio band. We investigate different scenarios for the explanation of fast gamma-ray variability, namely emission from magnetospheric gaps, relativistic blobs propagating in the jet (mini-jets), or an external cloud (or star) entering the jet. We find that the only plausible model to account for the luminosities here observed would be the production of gamma rays in a magnetospheric gap around the central black hole, only in the eventuality of an enhancement of the magnetic field threading the hole from its equipartition value with the gas pressure in the accretion flow. The observed gamma-ray flare therefore challenges all the discussed models for fast variability of VHE gamma-ray emission in active galactic nuclei.
Key words: galaxies: active / galaxies: jets / galaxies: individual: NGC 1275 / gamma rays: galaxies
© ESO 2018
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