Very high energy gamma-ray emission of Perseus Cluster and NGC 1275

The Perseus cluster of galaxies with the central galaxy NGC 1275 is ideally suitable both for studying the physics of relativistic jets from Active Galactic Nuclei and for revealing the feedback role of the central galaxy. The data obtained at very high energies by SHALON, namely the images of the galaxy and its surroundings, and the flux variability indicate that the TeV γ-ray emission is produced by a number of processes: in partraular, part of this emission is generated by relativistic jets in the nucleus of NGC 1275 itself. Unique data on GK Per(Nova 1901) TeV γ-ray emission were obtained with SHALON experiment for the first time.


Introduction
Clusters of galaxies have long been considered as possible candidates for the sources of TeV gamma rays emitted by protons and electrons accelerated at large-scale shocks or by a galactic wind or active galactic nuclei [1,2,3,4,5,6,7,8,9]. The cluster of galaxies in Perseus is one of the best-studied clusters owing to its relative proximity (its distance ∼ 100 Mpc or redshift z = 0.0179) and brightness. The dominant galaxy in the Perseus cluster is NGC 1275 (Figs. 1,2).
NGC 1275 is a powerful source of radio and X-ray emission. The NGC 1275 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 [10]. Evidence for the "feedback" role of NGC 1275 can be obtained from ROSAT and Chandra observations, which reveal shells of hot gas and cavities that spatially coincide with the radio structures ( Fig. 2) extending from the central, active part of the AGN. NGC 1275 at redshift z = 0.0179 [11] also arouses interest owing to its close proximity to the Earth, making it possible to study the physics of relativistic jets.
NGC 1275 at very high energies Metagalactic sources of very high energy gamma-rays have been searched for in the SHALON experiment from the very beginning of its operation [12,13]. In 1996, the observations with the SHALON mirror Cherenkov telescope revealed a new metagalactic source of γ-ray emission at very high energies E > 800 GeV [14,15] (Figs. 1, 2). The position of emission source detected in our experiment coincides in its coordinates with the Seyfert galaxy NGC 1275 [14,15,16,17,18,19,20,21,22]. NGC 1275 was observed by the SHALON telescope for 271.2 h in different years (from 1996 to 2012) during the clear moonless nights at zenith angles from 3 • to 33 • . Gamma-ray emission from NGC 1275 was detected by the SHALON telescope at energies above 800 GeV at the 31.4σ confidence level determined according to Li&Ma [23].  [26]; the red contours indicate the SHALON image of NGC 1275 in the energy range 800 GeV − 40 TeV. Right: Integral spectrum of γ-rays from NGC 1275 obtained by SHALON in comparison with other experimental data (see text and [22]).
Possible correlations between the emission regions of TeV γ-rays and low-energy (radio and X-ray) photons should be established to elucidate the mechanisms of the generation of very  [8]. The contours represent the source's radio structure from VLA radio observations. Right: Spectral energy distribution of the γ-ray emission from NGC 1275. and represent the data from the SHALON Cherenkov telescope in comparison with experiment data (see [22]).The dashed, dash-dotted, and dashdotted with two dots curves indicate the spectral energy distributions of NGC 1275 obtained in the CM model [27] and see text below. high energy emission in the source and to test the models describing them. Fig. 2 left shows a ROSAT X-ray image of NGC 1275; the contours represent the source's radio structure from VLA radio observations. The radio and X-ray emission maxima coincide with the AGN NGC 1275 (Figs. 1, 2). But, the X-ray emission disappears almost completely near the bright areas of the radio components located in the north and the south symmetrically relative to the core [8].
We also combined the SHALON-1 (0.8 − 40 TeV) and Chandra (1.5 − 3.5 keV X-ray) images. Fig. 1 (black-and-white scale) presents a Chandra X-ray (1.5 − 3.5 keV) image for the central part of the Perseus cluster centered on NGC 1275 with a size of ∼ 5.5 arcmin [26]. In the X-ray energy range, the core of the Perseus cluster, on the whole, appears as a clear circularly symmetric structure with a distinct maximum on NGC 1275 (Fig. 1).
The clearly seen dimming in X-ray flux, along with the dip NW of the center correlates with the components of the extended double radio structure 3C 84 (Fig. 1). These dips are surrounded by bright (at energies 1.5 − 3.5 keV) arc regions from the north and the south. The simplest interpretation is that the intense emission from these rims comes from the shells surrounding the radio lobes [26]. A bright emission spot is also observed to the east.
The emission regions of very high energy γ-rays observed by SHALON from NGC 1275 have a structure similar to that described above (see [26]) and well correlates with the photon emission regions in the energy range 1.5 − 3.5 keV (Fig. 1). A correlation of the emission with energies 0.8 − 40 TeV [17,18,19] and the X-ray emission in the range 0.3 − 7 keV [26] was also found. Thus, the TeV γ-ray emission recorded by SHALON from NGC 1275 has an extended structure with a distinct core centered at the source's position.
To analyze the emission related to this core, we additionally identified the emission component corresponding to the central region of NGC 1275 with a size of 32". The emission from the central region of NGC 1275 was detected at energies above 0.8 TeV at a 13.5σ confidence level determined by the Li&Ma method [23] with a average integral flux (3.26±0.30)×10 −13 cm −2 s −1 . The γ-ray energy spectrum of the central component in the entire energy range from 0.8 to 40 TeV is described by a power law with an exponential cutoff, I(> E γ ) = (2.92 ± 0.11) × 10 −13 × E −1.55±0.10 ×exp(−E γ /10T eV )cm −2 s −1 . The SHALON spectrum corresponding to the emission from the central region of NGC 1275 is represented in Fig. 1 with the black triangles.

GK Per (Nova 1901)
During the observations of NGC 1275 the SHALON field of view contains the source of nonthermal radio and X-ray emission GK Per (Nova 1901) of classical nova type as it located at ∼ 3 • SW from NGC 1275. So due to the large telescopic field of view (≥ 8 o ) the observations of NGC 1275 is naturally followed by the tracing of GK Per. GK Per as a source accompanying to NGC 1275 was observed with SHALON telescope during the period from 1996y to 2012y till now for a total of 111 hours. The γ-ray source associated with the GK Per was detected above 2 TeV with average γ-flux (2, 9 ± 1, 3) × 10 −13 cm −2 s −1 . The γ-ray source associated with the GK Per was detected above 800 GeV with a statistical significance [23] of 9.2σ. The signal significance for this SNR is less then one for the source with similar flux and spectrum index obtained in the same observation hours because of less collection field of view relative to the standard procedure of SHALON experiment. The corrections for the effective field of view were made to calculate source flux and energy spectrum.The energy spectrum of γ-rays in the observed energy region from 2 to 15 TeV (Fig. 3, left) is well described by the power law F (> E O ) ∝ E kγ , with k γ = −1.90 ± 0.35 (also see these Proceedings). The image of GK Per at TeV-energies by SHALON are shown with Fig. 3, right. The analysis of γ-ray shower arrival direction revealed the main TeV-emission region coinciding with the position of central source of GK Per and the weak emission of shell, that is also observed in X-ray by Chandra [28].  [25]. A slight local flux increase can be seen in the period of mid-October 2009 [25], which corresponds to the above-mentioned γ-ray flux increase observed by SHALON.

NGC 1275 as a point and extended source
The extended structure around NGC 1275 (Fig. 1) that spatially coincides with the X-ray emission regions (Fig. 1) can be produced by mechanisms related to the generation of an X-ray structure [9,26,29]. The brightness distribution of the X-ray emission and the observed TeV emission shows a sharp increase in intensity right outside the bubbles blown by the central black hole and visible in the radio band. This suggests that the X-ray-generating particles are swept up from the region of the radio lobes under the pressure of cosmic rays and magnetic fields generated in the jets at the center of NGC 1275 [9,29]. The structures visible in TeV γ-rays are formed through the interaction of very high energy cosmic rays with the gas inside the Perseus cluster and interstellar gas heating at the boundary of the bubbles blown by the central black hole in NGC 1275. The presence of emission in the energy range 1 -40 TeV from a central region of ∼ 32" in size around the nucleus of NGC 1275 (see Fig. 1, the black triangles) and the short-time flux variability [22] point to the origin of the very high energy emission as a result of the generation of jets ejected by the central supermassive black hole of NGC 1275.
The multifrequency spectral energy distribution for the nucleus of NGC 1275, up to high and very high energies, was described in the CM model [27] and is a composition of the components of inverse Compton scattering of the intrinsic synchrotron radiation from relativistic electrons (synchrotron self-Compton) of three separate plasma blobs ejected from the inner regions of the NGC 1275 nucleus (Fig. 2, the dashed, dash-dotted, and dash-dotted with two dots curves). The available Fermi LAT data at high energies and the SHALON observations at very high energies in a region < 32" around NGC 1275 are described in terms of this model with one of the components produce synchrotron self-Compton emission of the relativistic jets from the nucleus itself (Fig. 2, the dash-dotted with two dots curve).

Conclusion
Long-term studies of the central galaxy in the cluster, NGC 1275, are being carried out in the SHALON experiment. We presented the results of fifteen-year-long observations of the AGN NGC 1275 at energies 800 GeV -40 TeV discovered by the SHALON telescope in 1996 [14,15,16,17,18,19,20]. The data obtained at very high energies, namely the images of the galaxy and its surroundings, and the flux variability indicate that the TeV gamma-ray emission is generated by a number of processes: in particular, part of this emission is generated by relativistic jets in the nucleus of NGC 1275 itself. Whereas, the presence of an extended structure around NGC 1275 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. Also, in the longterm observations of the Perseus Cluster revealed the γ-ray emission from the nearby object -GK Per of classical nova type, located at ∼ 3 • SW from NGC 1275. The TeV γ-ray emission of classical nova GK Per, that could be a shell-type supernova remnant on early evolution stage, was detected for the first time by SHALON. Also, very high energy γ-rays from the shell of GK Per, visible in the X-rays, were detected with SHALON experiment for the first time.