TeV gamma-rays from the region of Perseus Cluster

The results of 20-year observations of the Perseus cluster centering on the NGC 1275 including IC 310 radio galaxy and extragalactic supernova SN 2006gy at energies 800 GeV - 45 TeV by the SHALON telescope are presented. It was found, that the TeV γ-ray emission at energies > 800 GeV has an extended structure with a distinct core centered at the NGC 1275 nucleus and well correlates with the photon emission regions viewed in X-rays by Chandra and anti-correlates with radio-structures. Also, the variations of TeV γ-ray flux both at year- and day- scales were found. The obtained data indicate that the part of TeV γ-ray emission is generated by relativistic jets in the nucleus of NGC 1275. Whereas, the presence of an extended structure around NGC 1275 and the slow rise of the γ-ray flux is the evidence of the interaction of cosmic rays and magnetic fields generated in the jets at the galactic center with the gas of the Perseus cluster.


Introduction
The Perseus galaxy cluster with the central galaxy NGC 1275 has long been considered as an ideal candidate both for studying the physics of relativistic jets from Active Galactic Nuclei and for revealing the feedback role of the central galaxy. Here, we present the results of twentyyear investigations of the NGC 1275 and its surroundings, including IC 310 radio galaxy and extragalactic supernova SN 2006gy, obtained at 0.8 -50 TeV with SHALON telescope [1,2,3,4].
The SHALON instrument consists of two imaging atmospheric Cherenkov telescopes located in Tien-Shan mountains at an altitude of 3340 m above the sea level [1,2,3,4]. It is designed for observations of γ-ray sources in the energy range from 800 GeV to 100 TeV. Each of the telescopes has a composite mirror with an area of 11.2 m 2 . The detector has the largest field of view > 8 • among similar instruments. It allows the background from charged cosmic-ray particles and the atmospheric transparency to be continuously monitored simultaneously with the observations of γ-ray sources. It gives the reduction of systematic uncertainties, related to the changes in atmospheric parameters, below 10% and increases the rejection factor of background events separation from γ-rays. [1,5]. The accuracy of the determination of the coordinates of the γ-ray shower source in SHALON experiment is ∼ 0.07 • , and it is improved by a factor of 10 after additional processing (see [3,5]) using deconvolution algorithm [6]. The SHALON method of selection of γ-ray showers from the background cosmic-ray showers allows the rejection of 99.93% of the background events. The minimum detectable integral flux of γ-rays at 1 TeV is 2.1 × 10 −13 cm −2 s −1 . In the energy range from 1 -50 TeV the minimum detectable flux falls to the value of 6 × 10 −14 cm −2 s −1 [1,2]. The SHALON experiment has been in operation since 1992 providing the long-term observations of many different types of sources that are of interest for many areas of astroparticle physics.

NGC 1275 at very high energies
NGC 1275 is a powerful source of radio and X-ray emission, surrounded by extended filamentary structures historically aroused great interest owing to both its position at the center of the Perseus cluster and its possible "feedback" role [7] which revealed with X-ray detection of hot gas shells and cavities that spatially coincide with the radio structures ( Fig. 1) extending from the central, active part of the AGN.
In order to find the mechanisms of generation of very high energy emission in the source the correlation of the emission regions in the wide energy range including radio, X-rays and TeV γ-rays should be established.
In Fig. 1 an TeV emission map of NGC 1275 by SHALON is overlaid with ones in X-rays and radio. In X-rays and TeV γ-rays, both, the images of Perseus cluster center demonstrate a circular symmetric structure with the distinct emission from the position of NGC 1275 core. The X-ray surface brightness maxima around the core are coincides with maxima of TeV flux to the east and west of the nucleus of NGC 1275. The clearly seen minima in the X-rays and in the TeV γ-rays to the north and south from NGC 1275 coincides with outer radio lobes (black lines) which are further surrounded with bright X-ray arc regions. The interpretation is that the intense emission from these rims comes from the shells surrounding the radio lobes [9].
The data about TeV γ-rays from NGC 1275 have been collected in SHALON experiment  Figure 2. left: IC 310 images. Grey scale -SHALON data in 0.8 -50 TeV; red contours -X-ray data by ROSAT; black contours are the radio structures from WENSS sky survey. right: Spectral energy distribution of IC 310.
-the data from the SHALON.
since 1996, and the intensity of NGC 1275 was found to be variable in the very high energies (see Fig.1). The SHALON telescope has detected four short-time (within five days) increases of the TeV γ-ray flux [3] in the entire time of observations of NGC 1275, and a light curve shows a slow TeV γ-ray flux increase after the 2001 year. The archive light curves at radio [13], X-ray data [14,15] and Fermi LAT year fluxes at E > 100 MeV [16] are presented in Fig. 1

IC 310
The radio galaxy IC 310 is located in the Perseus Cluster at 0.6 • from the its central galaxy, NGC 1275. IC 310 is known as a head-tail radio galaxy which radio morphology consists of a bright "head", located at the core of the galaxy, and "tail" of a radio lobe pointing away from the center of the cluster (Fig. 2, left). In X-rays IC 310 looks as a point source at the position of the radio "head" [18] and its luminosity suggests that this galaxy contains an active nucleus. IC 310 has been systematically observed by SHALON telescope in 1996 -2016 years for a total of 170 hours and it was detected at > 0.8 TeV at level of 19.8σ [8] with the average integral flux (0.89 ± 0.09) × 10 −13 cm −2 s −1 (see [17] for details). The differential spectrum of γ-rays from IC 310 in the 0.8 -40 TeV energy range is well fitted with a hard power law with an exponential cutoff dF/dE = (0.83±0.09)×10 −12 ×E −1.56±0.16 ×exp(−E γ /11.5±3 TeV) TeV −1 cm −2 s −1 (see Fig. 2 Fig. 2 with radio emission map from WENSS [19] and X-ray image from ROSAT [18]. The main TeV γ-ray emission region of IC 310 corresponds to the galaxy core visible in X-rays and coincides with the "head" of the radio structure. The detection of TeV γ-rays from the core of IC 310 galaxy and the day scale flux variability point out the origin of this emission in the relativistic outflow from the active nucleus.

Conclusion
The results of twenty-year-long observations of the Perseus cluster centering on NGC 1275 at energies 800 GeV -45 TeV by the SHALON are presented. The characteristics of IC 310 radio galaxy and extragalactic supernova SN 2006gy accompanying the investigation of NGC 1275 are obtained. The emission regions of TeV γ-rays observed by SHALON from NGC 1275 well correlate with ones viewed in X-rays by Chandra and anti-correlate with radio-structures. This TeV γ-ray emission recorded by SHALON has an extended structure with a distinct core centered at the source's position. The emission component corresponding to the core of NGC 1275 was fully identified. Also, the variations of TeV γ-ray flux both at year-and day-scales were found. The data obtained at very high energies indicate that a part of this emission is generated by relativistic jets in the nucleus of NGC 1275. But, the presence of an extended structure around NGC 1275 and the slow rise of the γ-ray flux is evidence of the interaction of cosmic rays and magnetic fields generated in the jets at the galactic center with the gas of the Perseus cluster.