The MAGIC Cherenkov telescopes observed the Perseus cluster sky region in stereo mode for nearly 90 hr from October 2009 to February 2011. This campaign led to the discovery of very high energy Gamma-ray emission from the central radio galaxy NGC 1275 and the head-tail radio galaxy IC 310. Here we report the results on the most recent discovery of NGC 1275 which was detected at low energies in the 2010/2011 data. We also present latest results on IC 310, which had been detected in the 2009/2010 data.
Most of the previously known ∼45 extragalactic very high energy (VHE) γ-ray emitters are blazars, the few exceptions being two radio galaxies, M 87 [1] and Cen A [2], and two starburst galaxies, NGC 253 [3] and M82 [4]. The two galaxies NGC 1275 and IC 310 recently discovered by the MAGIC telescopes in the Perseus cluster (redshift z = 0.018) do not easily fit to any of the known VHE emitter classes. NGC 1275 is the central galaxy of the Perseus cluster and its classification varies between different papers and catalogues. It was included in the original Seyfert catalogue [5], but already flagged to be unusual because of its complex structure. After the introduction of Seyfert 1 and 2 subclasses by Khachikian and Weedman [6], NGC 1275 was labeled as Sy2. Few years later, Veron proposed it to be a BL Lac [7], but in his latest catalogue NGC 1275 is classified as Sy1.5 [8]. The complex structure of NGC 1275 including surrounding filaments leads to a peculiar morphology classification [9]. Additionally, another galaxy called High Velocity System is moving towards NGC 1275 along the line of sight. Recent observations show that a collision between the two galaxies has not started yet [10]. IC 310 is classified as head tail radio galaxy, a type of active galactic nuclei only occuring in dense galaxy clusters like the Perseus one. Due to the fast movement of IC 310 relative to the cluster, the friction between the jet with the intra-cluster medium (ICM) causes a strong bending of the jets [11,12]. In 1999 it was suggested that IC 310 could be a dim blazar because of the absence of strong emission lines and the spectral indices on radio and X-ray measurements [13]. Later on it was also shown that the X-ray emission may originate from the central active galactic nucleus of a BL Lac-type object [14]. The Fermi-LAT measured high energy γ-rays from NGC 1275 up to 25 GeV [15] and detected strong flaring activity in July 2010 [16]. Additionally, an analysis of Fermi data resulted in a detection of IC 310 for energies above 30 GeV [17].
The MAGIC (Major Atmospheric Gamma Imaging Cherenkov) experiment consists of two 17 meter Imaging Air Cherenkov telescopes located at the Canary Island of La Palma, at 2200 meters a.s.l., working in stereoscopic mode since the autumn 2009. The MAGIC telescopes are currently the largest world-wide existing Imaging Atmospheric Cherenkov Telescopes (IACTs), leading to a low energy threshold of ∼50 GeV. Depending on the energy of the primary particle, the system has an energy resolution of 15-25% and a angular resolution of 0.05-0.12 • . The telescopes have a field of view of 3.5 • and can turn to any position faster than 40 s. The sensitivity of the MAGIC telescopes is ∼0.8% of the Crab Nebula flux above ∼250 GeV in 50h of observation time [18]. The MAGIC experiment observed the central part of the Perseus cluster for ∼25 hr in 2008, when only the first telescope was operating (mono data). This survey resulted in upper limits on VHE γ-ray emission above 100 GeV from NGC 1275, intra-cluster cosmic rays and dark matter annihilations [19]. The stereo observation campaign on NGC 1275 and on the central part of the Perseus cluster took place from October 2009 to February 2011, during dark nights and observing the source at the lowest possible zenith angles, which guaranteed a low energy threshold. The data can be divided in two main samples, corresponding respectively to the observational seasons from October 2009 to February 2010, and from August 2010 to February 2011. During the first season, ∼45 hr of observation time were accumulated using two wobble [20] positions offset by 0.4 • in RA with respect to NGC 1275. Since the commissioning of the stereo trigger system was still ongoing, the observations were carried out in the so-called soft stereo mode, i.e. using the MAGIC-I trigger system and reading out the second telescope simultaneously. This results in slightly higher threshold, but allows a partially independent mono analysis. Conversely, for the second season, the full stereo trigger and four wobble pointing positions equally distributed around NGC 1275 were used. With this settings, a higher sensitivity and a better coverage of the central part of the Perseus cluster were achieved. Additional ∼45 hr of observation time were added to the previous 2009/2010 dataset.
The analysis of the data was performed with the standard MAGIC reconstruction and analysis software [21]. Due to the different trigger conditions used in the two seasons, the datasets had to be analyzed seperately. The overall stereo campaign resulted in the detection at VHE of both NGC 1275 and IC 310, as shown in the significance skymap above 150 GeV in figure 1. In the following, we report the main MAGIC results achieved so far on these two sources.
IC 310 was serendipitously discovered in the field of view of the NGC 1275 observation during the 2009/2010 survey. Due to angular acceptance reason, only one of the two wobbl
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