Contemporaneous observations of the radio galaxy NGC 1275 from radio to very high energyγ-rays

Abstract

Aims. The radio galaxy NGC 1275, recently identified as a very high energy (VHE, >100 GeV) γ-ray emitter by MAGIC, is one of the few non-blazar active galactic nuclei detected in the VHE regime. The purpose of this work is to better understand the origin of the γ-ray emission and locate it within the galaxy. Methods. We studied contemporaneous multifrequency observations of NGC 1275 and modeled the overall spectral energy distribution. We analyzed unpublished MAGIC observations carried out between October 2009 and February 2010, and the previously published observations taken between August 2010 and February 2011. We studied the multiband variability and correlations by analyzing data of Fermi-LAT in the 100 MeV-100 GeV energy band, as well as Chandra (X-ray), KVA (optical), and MOJAVE (radio) data taken during the same period. Results. Using customized Monte Carlo simulations corresponding to early MAGIC stereoscopic data, we detect NGC 1275 also in the earlier MAGIC campaign. The flux level and energy spectra are similar to the results of the second campaign. The monthly light curve above 100 GeV shows a hint of variability at the 3.6σ level. In the Fermi-LAT band, both flux and spectral shape variabilities are reported. The optical light curve is also variable and shows a clear correlation with the γ-ray flux above 100 MeV. In radio, three compact components are resolved in the innermost part of the jet. One of these components shows a similar trend as the Fermi-LAT and KVA light curves. The γ-ray spectra measured simultaneously with MAGIC and Fermi-LAT from 100 MeV to 650 GeV can be well fitted either by a log-parabola or by a power-law with a subexponential cutoff for the two observation campaigns. A single-zone synchrotron-self-Compton model, with an electron spectrum following a power-law with an exponential cutoff, can explain the broadband spectral energy distribution and the multifrequency behavior of the source. However, this model suggests an untypical low bulk-Lorentz factor or a velocity alignment closer to the line of sight than the parsec-scale radio jet.

Spatially resolved origin of millimeter-wave linear polarization in the nuclear region of 3C 84

“…Assuming ν m = 86 GHz, S m = (5.6 ± 1.7) Jy, and θ m = 1.8 × (0.14 ± 0.01) mas 7 , we obtain B S S A ∼ (19 ± 13) × δ G. For the jet viewing angle θ ∼ 30 • − 60 • and the apparent jet speed ∼ 0.1c near the core, the Doppler factor is δ ∼ 1.1. With this, we obtain B S S A ∼ (21 ± 14) G. We note that Abdo et al (2009) and Aleksić et al (2014) suggest a rather large Doppler factor of δ ∼ 2 − 4 based on the theoretical modeling of the synchrotron self-Compton process in the jet. In this case, the resulting magnetic field strength would be ∼ (2 − 4) times larger.…”

Radio Galaxies at VHE Energies

“…NGC 1275 (3C 84), the central Perseus cluster RG at a distance of ∼ 70 Mpc, has been detected at VHE energies above 100 GeV by MAGIC, initially (based on data between 2009-2011) at moderate flux levels (∼ 3% of the Crab Nebula) and with a very steep VHE spectrum (photon index of Γ γ ∼ −4.1 if characterized by a power law) extending up to ∼ 650 GeV [18,19]. When HE (Fermi-LAT) and VHE data are combined, the average ("quiescent") γ-ray spectrum appears compatible with either a log-parabola or a power-law with a sub-exponential cut-off, suggestive of a common physical origin and of a peak or cut-off around several GeV.…”

Extreme and high synchrotron peak blazars beyond 4FGL: The 2BIGB γ-ray catalogue

“…The EHSP and HSP sources have -on average-a hard photon index Γ =1.85±0.01 as probed by Fermi-LAT (Arsioli et al 2015;Ackermann et al 2015b) in the MeV to GeV window. For this reason, EHSP and HSP blazars are considered promising candidates for VHE observations with the Cherenkov Telescope Arrays (Actis et al 2011;Cherenkov Telescope Array Consortium et al 2019), and the community dedicates significant attention to characterize their SEDs via multifrequency observation campaigns (Ahnen et al 2017a,b;Paiano et al 2017;Aleksić et al 2014;Takahashi et al 2000).…”