- Email: [email protected]
Very high energy astrophysics with the SHALON Cherenkov telescopes
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Very high energy astrophysics with the SHALON Cherenkov telescopes V.G. Sinitsyna a ,∗, V.Y. Sinitsyna a , S.S. Borisov a , A.I. Klimov a,b , R.M. Mirzafatikhov a , N.I. Moseiko a,b a b
P.N. Lebedev Physical Institute, Russian Academy of Science, Leninsky prospect 53, 119991 Moscow, Russia National Research Center ‘‘Kurchatov Institute’’, Akademika Kurchatova square 1, 123182 Moscow, Russia
ARTICLE
INFO
Keywords: Imaging atmospheric Cherenkov telescope Cherenkov radiation Gamma-ray astronomy VHE
ABSTRACT The base of experiments using the imaging atmospheric Cherenkov telescopes is that the Cherenkov light emitted from the particles of extensive air shower created by the primary gamma-ray is collected by a mirror reflector and then detected by a pixelized PMT camera. SHALON are the imaging atmospheric Cherenkov telescopes created for the gamma-ray astronomy at the energies of 800 GeV to 100 TeV. Telescopes are located in the Tien-Shan mountains at the altitude of 3340 m a.s.l. The telescope systems have characteristics sufficient to record precise information about the shower structure in the energy range under consideration. SHALON experiment has been operating since 1992 and covers the wide astroparticle physics topics including an acceleration and origin of cosmic rays in supernova remnants, the physics of relativistic flaring objects like a black holes and active galactic nuclei as well as the long-term studies of the different type objects.
1. Introduction
2. SHALON telescopes
Cherenkov light emission is widely used in numerous astroparticle physics experiments where the success was reached with the detection technique using the imaging atmospheric Cherenkov telescopes. The base of such experiments is that the Cherenkov light emitted from the particles of extensive air shower created by the primary gamma-ray is collected by a mirror reflector and then detected by a pixelized PMT camera. SHALON are the imaging atmospheric Cherenkov telescopes (Fig. 1, [1]) created at the P.N. Lebedev Physical Institute for the gammaray astronomy and are located at Tien-Shan High Mountain Station. The telescope systems have characteristics sufficient to record precise information about the shower structure at the very high energies. SHALON experiment has been operating since 1992 and covers the wide astroparticle physics topics including an acceleration and origin of cosmic rays in supernova remnants [2], the physics of relativistic flaring objects like a black holes and active galactic nuclei [3] as well as the long-term studies of the different type objects [4]. Also, one of the issues of SHALON experiment is the development of an alternative method of neutrino detection at high altitudes [5,6]. In this method an earth matter or mountain is used as a target volume for conversion neutrinos to leptons which then initiate extensive air shower in the atmosphere, then showers can be detected by the Cherenkov telescope. Five candidates for neutrino showers have been detected in SHALON experiment [6]. During more than 25-year operation SHALON has studied in details the objects of different type at energies up to 100 TeV [7,8] and continues to produce the results in the field of very high energy astrophysics.
The SHALON mirror Cherenkov telescope system (Fig. 1) is designed to observe and investigate gamma-rays from cosmic sources in the energy range from 800 GeV to 100 TeV. Methodical experiments and observations at the first SHALON telescope have been started more twenty years ago [1,5,9–12]. SHALON gamma-ray telescopes are located in Tien-Shan mountains at an altitude of 3340 m above sea level. The idea of enhancement of angular resolution and sensitivity to the gammarays was realized in SHALON telescopes by the number of technical solutions presented here, including the construction of the widest field of view in the world. This high-altitude site has an optical quality optimal for the observation of Cherenkov light of extensive air showers. Each of SHALON telescopes is placed in an observatory building to prevent the destructive effects of precipitation, pollution, and illumination on the systems of the telescopes. Telescope has a composite spherical mirror with an area of 11.2 m2 and a dish-frame with ∼0.1% deviation from the sphericity in operation conditions [1]. Mirrors are characterized with >96% mirror reflectivity; mirror surface accuracy of 5𝜆 which defines as a point spot dispersion (𝜆 = 500 nm) and focal length dispersion of ≤1% [13]. Mirrors are focusing Cherenkov light of an air shower onto the detector camera that consists of 144 FEU-85 photomultiplier tubes assembled into a square array mounted at the mirror focus. Typical PMT85 amplitude and time spectra in a one- and multi-electron mode and thermoemission peaks are presented in [1,5,11]. PMT-85 have shown sustainability and stability of characteristics in the temperature range
∗ Corresponding author. E-mail address: [email protected] (V.G. Sinitsyna).
https://doi.org/10.1016/j.nima.2019.01.002 Received 15 October 2018; Received in revised form 7 December 2018; Accepted 2 January 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 3. Sensitivity of SHALON with modern and some historical Cherenkov telescopes in the energy range of 100 GeV–100 TeV (data are from [17]).
(𝐼𝑛𝑡0, 𝐼𝑛𝑡1 [2,9,12]) used the difference of Cherenkov photon fluxes within the small angles < 1◦ around shower axis with ones within the large angles of >2◦ for the gamma and hadron showers [1]. Fig. 2 shows the 𝛾-ray shower selection criteria distributions used in the SHALON experiment. The SHALON method of selecting gamma-ray showers from background cosmic-ray showers allows rejecting of 99.93% of the background showers [2,4]. The performance of Cherenkov telescope together with selection criteria is summarized by its angular resolution and 𝛾-ray flux sensitivity. The accuracy of the determination of the coordinates of the 𝛾-ray shower source in SHALON is ∼0.07◦ [3] and it is increased by a factor of ∼10 after additional processing [3,4] using deconvolution algorithm [19]. The sensitivity of the telescope is defined as the minimum flux of 𝛾-rays for a statistically significant detection. We estimated the flux sensitivity for 50 hrs of observation of a point-like source at a confidence level of 5𝜎. The significance is calculated according to the formulation of Li&Ma , formula 17 in the paper [20]. In the energy range from 0.8 to 50 TeV, due to the large field of view SHALON the sensitivity (Fig. 3) is limited by the cosmic ray protons which mimic 𝛾-ray showers with known spectrum 𝐹𝑝 (> 𝐸) ∝ 𝐸 −1.75 . The minimum detectable integral flux of 𝛾-rays at energy of 1 TeV is 2.1 × 10−13 𝑐𝑚−2 𝑠−1 . In the region 1–50 TeV the minimum detectable flux falls down to the value of 6 × 10−14 𝑐𝑚−2 𝑠−1 and then, at energies 𝐸 > 50 TeV, it grows because of limited telescopic field of view [2,4,11,14].
Fig. 1. SHALON mirror Cherenkov telescopes in the observatory.
from −20◦ to +40◦ . PMT’s modules are equipped with an equal divider with amplification on last two dynodes [11] to provide a gain resulting in the enhancement of the linearity of PMT range and the dynamic range ∼104 . Metal conic-to-square light-guides are used to improve light collection. The detector camera has characteristics sufficient to record shower structure information in the energy range under consideration, namely 800 GeV–100 TeV. The detector camera has the largest field of view in the world, >8◦ [1,2,4]. It allows monitoring the background from charged cosmic-ray particles together with the atmospheric transparency continuously during observations of the sources as well as expands the area of observation and, hence, the efficiency of observations [10]. The technique of simultaneously obtaining information about the cosmic-ray background and the showers initiated by gamma-rays from observed source is a unique and has been used in the SHALON experiment from the very beginning of its operation [5,9–11,14]. This technique serves to increase the efficiency of the source observation as the source and background observation conditions like thickness and other atmosphere parameters remain the same for the on-source and background data. Due to the larger field of view, larger choice of the background regions exist; it allows to collect more information on the background and to be sure that there is no influence of the studying source, especially in the case of an extended source. In addition, in the case of the large field of view the large-impact-distance showers, including of >100 m, are recorded completely and without significant distortions and showers more than 30 m impact distance account for more 95% of all of the showers recorded by the telescope. Multi-channel 12-bit analog-to-digital converters [15,16] are used to record the PMT signal from EAS Cherenkov lighting for further analysis. In SHALON experiment the analysis of the angular and lateral distributions of the recorded shower Cherenkov light for the selection of showers of primary 𝛾-rays is performed first, due to the optimized Hillas parameters [18] defined as moments of 2-dimensional shower image: 𝛼, 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 and ratio of 𝐿𝑒𝑛𝑔𝑡ℎ, 𝑊 𝑖𝑑𝑡ℎ and high efficiency parameters
3. Astrophysics with SHALON Observation and long-term study SHALON program includes different type galactic and extragalactic objects. Among the galactic sources are supernova remnants, binary systems, variable stars, pulsar wind nebulae. A list of extragalactic sources mostly consists of BL Lac-objects, FSRQ and Radio galaxies. Observations of the extragalactic sources can also be used for the study of Extragalactic Background Light. Since 1992 SHALON experiment has studied 19 galactic and 16 extragalactic objects in very details at energies up to 100 TeV [7,8] which have used in different topics of astroparticle physics.
Fig. 2. Distributions of the parameters for gamma-ray showers and background showers (black contour) used as selection criteria in the SHALON experiment [9]. Arrows show the cuts for background rejection.
2
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 5. left: 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 and models (see [3]). right: Chandra X-ray (1.5–3.5 keV) image of NGC 1275; the red contours indicate the SHALON image of NGC 1275 in the energy range 800 GeV–40 TeV.
shell-type SNRs Tychos SNR (1572 y), Cas A (1680 y), IC 443 (age 300–30000 y), 𝛾Cygni SNR (age 5000–7000 y), G166.0+4.3 (age 24000 y). For each of SNRs the observation results are presented with spectral energy distribution by SHALON in comparison with other experimental data and images by SHALON in together with data from X-ray by Chandra and radio-data by CGPS (Fig. 4). For the first time the location of TeV gamma-ray emission regions relative to the position SNRs remarkable features as a forward and reverse shocks, dense molecular clouds swept out by SNR explosion and shells due to the interaction of the supernova ejecta and the surrounding medium was shown (see. [2] for details). The collected experimental data confirmed the predictions of the theory about the hadronic generation mechanism of very high energy 800 GeV–100 TeV gamma-rays in Tycho’s SNR, Cas A, IC 443 and 𝛾Cygni SNR. 3.2. Radio galaxy NGC 1275 Long-term studies of the central galaxy in the cluster, NGC 1275, are being carried out in the SHALON experiment. Gamma-ray emission from NGC 1275 was detected by the SHALON telescope at energies 800 GeV– 40 TeV (Fig. 5, left). It was found that the TeV structure around NGC 1275 spatially coincides with the X-ray emission regions (Fig. 5, right). The brightness distribution of the X-ray emission and the observed TeV emission shows a sharp increase in intensity outside the bubbles blown by the central black hole and visible in the radio band. 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′′ . [3]. Also, days-time flux variability was detected. As result, it was found that 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 (black triangles) and the short-time flux variability 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.
Fig. 4. Characteristics of Shell-type SNRs: left: Spectral energy distributions of highand very high energy 𝛾-ray emission by SHALON (▴ or ▵) in comparison with other experiments and models [2]. right: Image of SNRs at energies of >0.8 TeV by SHALON (gray scale). Red contours are the X-ray emission by Chandra for Cas A, Tycho’s SNR and radio contours by CGPS for 𝛾Cygni SNR, IC 443 and G166+4.3. Also, X-ray data from ROSAT and location of high energy emission by Fermi LAT are shown.
3.1. Shell-type supernova remnants
3.3. GCRs from M-Dwarf star flares and ‘‘positron excess’’
Supernova Remnants (SNRs) have long been considered as unique candidates for cosmic-ray sources. Direct data on the distribution of cosmic rays in SNRs can be obtained from very high energy gammaray observations. As the presence of the electron cosmic-ray component is clearly seen by the emission generated by it in an SNR in a wide wavelength range, from radio to high-energy gamma-rays, while the nuclear cosmic-ray component can be detected only by very high energy 𝛾-ray emission. The SHALON observations have yielded the results on Galactic supernova remnants of different ages. Among them are the
Conventional sources of cosmic rays are believed to be supernovae and supernova remnants [21,22]. Recent observations of SNRs in TeV 𝛾-rays are making clear the origin of cosmic rays in these objects. However, the experimental data obtained by Pamela, Fermi, AMS-02, spectrometers requires the existence of nearby sources of cosmic rays at the distances < 1 kpc from the solar system. These sources could explain such experimental data as the growth of the ratio of galactic positrons to electrons with an increase of their energy [23,24], the complex dependence of the exponent of the proton and alpha spectra from the 3
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 6. Light curves and differential spectra of V388 Cas, V780 Tau, V962 Tau, V1589 Cyg at TeV energies from the long-term SHALON observations.
energy of these particles, the appearance of anomaly component in cosmic rays. Active dwarf stars are proposed as possible sources of galactic cosmic rays in energy range up to 1014 eV [25,26]. These stars produce powerful stellar flares. The generation of high-energy cosmic rays has to be accompanied by high-energy 𝛾-ray emission. The data obtained during more than 10 years observations of the dwarf stars V962 Tau, V780 Tau, V388 Cas and V1589 Cyg were analyzed. Very high energy gamma-ray emission in the range of 1–10 TeV mostly of flaring type from these red dwarfs is detected (Fig. 6, [26]). The energy released in stellar flares and its frequency is able to provide the necessary energy of GCRs in the disk of our Galaxy and make a significant contribution to the GCR spectrum up to energies of ∼1013 –1014 eV [25]. This result confirms that active dwarf stars are also the sources of the high energy cosmic rays. These sources could explain such experimental data of PAMELA and AMS spectrometers.
[3] V.G. Sinitsyna, V.Yu. Sinitsyna, Astron. Lett. 40 (2–3) (2014) 75. [4] V.G. Sinitsyna, V.Yu. Sinitsyna, Astron. Lett. 44 (3) (2018) 162. [5] V.G. Sinitsyna, in: R.C. Lamb (Ed.), Proc. Int. Workshop Towards a Major Atmospheric Cherenkov Detector - II, Canada, Iowa State University, Calgary, 1993, pp. 91–101. [6] V.G. Sinitsyna, et al., EPJ Web Conf. 52 (2013) 09010. [7] V.G. Sinitsyna, et al., EPJ Web Conf. 145 (2017) 19003. [8] V.G. Sinitsyna, et al., EPJ Web Conf. 145 (2017) 19004. [9] V.G. Sinitsyna, Nuovo Cim. 19C (1996) 965–971. [10] V.G. Sinitsyna, in: M. Cresti (Ed.), Proc. Int. Workshop Towards a Major Atmospheric Cherenkov Detector IV, Univ. of Padova, Padova, 1995, pp. 133–140. [11] V.G. Sinitsyna, in: O. De Jager (Ed.), Proc. Int Workshop Towards a Major Atmospheric Cherenkov Detector V, Kruger Park, South Africa, Westprint, Potchefstroom, 1997, pp. 190–195. [12] V.G. Sinitsyna, V.Yu. Sinitsina, Astron. Lett. 37 (9) (2011) 621–634. [13] V.G. Sinitsyna, et al., in: R. Schlickeiser (Ed.), Proc. 27th ICRC, Copernicus, Hamburg, 2001, pp. 2798–2801, OG 25. [14] V.G. Sinitsyna, in: J. Medina (Ed.), Proc. 16th ECRS, Universidad de Alcala, Alcala de Henares, 1998, pp. 383–387. [15] V.A. Kantcerov, V.B. Striguin, Inst. Exp. Tech. 3 (1987) 80–84. [16] N.I. Moseiko, et al., Nucl. Instr. Methods (RICH2018) https://doi.org/10.1016/j. nima.2018.12.070. [17] A.M. Hillas, Astropart. Phys. 43 (2013) 19. [18] A.M. Hillas, Proc. of the 19th ICRC, La Jolla, USA, NASA, Goddard Space Flight Center, 1985, pp. 445–448. [19] A.V. Goncharskii, A.M. Cherepashchuk, A.G. Yagola, Ill-Posed Problems in Astrophysics, Nauka, Fizmatlit, Moscow, 1985. [20] T.-P. Li, Y.-Q. Ma, Astrophys. J. 272 (1983) 317–324. [21] E.G. Berezhko, G.F. Krymskii, Usp. Fiz. Nauk 154 (1) (1988) 49. [22] S.P. Reynolds, Ann. Rev. Astron. Astrophys. 46 (2008) 89. [23] O. Adriani, et al., Nature 458 (7238) (2009) 607–609. [24] M. Aguilar, et al., PRL 110 (2013) 141102. [25] Yu.I. Stozhkov, Bull. RAS, ser. Physics 75 (3) (2011) 323. [26] V.G. Sinitsyna, V.Y. Sinitsyna, Yu.I. Stozhkov, Proc. 20th ISVHECRI, (Nagoya, Japan, 2018).
4. Conclusion SHALON experiment has been operating since 1992 and covers the wide field of astroparticle physics and gamma-ray astronomy at the energies of 800 GeV to 100 TeV and continues to produce the results in the field of very high energy astrophysics. Further deeper investigations in high and very high energies are needed for the better understanding on astroparticle VHE phenomena in Universe. References [1] S.I. Nikolsky, V.G. Sinitsyna, in: A.A. Stepanian, D.J. Fegan, W.F. Cawley (Eds.), Proc. Int Workshop of VHE Gamma-ray Astronomy, 1989, pp. 11–20. [2] V.G. Sinitsyna, et al., Adv. Space Res. 62 (10) (2018) 2845–2858.
4
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Very high energy astrophysics with the SHALON Cherenkov telescopes V.G. Sinitsyna a ,∗, V.Y. Sinitsyna a , S.S. Borisov a , A.I. Klimov a,b , R.M. Mirzafatikhov a , N.I. Moseiko a,b a b
P.N. Lebedev Physical Institute, Russian Academy of Science, Leninsky prospect 53, 119991 Moscow, Russia National Research Center ‘‘Kurchatov Institute’’, Akademika Kurchatova square 1, 123182 Moscow, Russia
ARTICLE
INFO
Keywords: Imaging atmospheric Cherenkov telescope Cherenkov radiation Gamma-ray astronomy VHE
ABSTRACT The base of experiments using the imaging atmospheric Cherenkov telescopes is that the Cherenkov light emitted from the particles of extensive air shower created by the primary gamma-ray is collected by a mirror reflector and then detected by a pixelized PMT camera. SHALON are the imaging atmospheric Cherenkov telescopes created for the gamma-ray astronomy at the energies of 800 GeV to 100 TeV. Telescopes are located in the Tien-Shan mountains at the altitude of 3340 m a.s.l. The telescope systems have characteristics sufficient to record precise information about the shower structure in the energy range under consideration. SHALON experiment has been operating since 1992 and covers the wide astroparticle physics topics including an acceleration and origin of cosmic rays in supernova remnants, the physics of relativistic flaring objects like a black holes and active galactic nuclei as well as the long-term studies of the different type objects.
1. Introduction
2. SHALON telescopes
Cherenkov light emission is widely used in numerous astroparticle physics experiments where the success was reached with the detection technique using the imaging atmospheric Cherenkov telescopes. The base of such experiments is that the Cherenkov light emitted from the particles of extensive air shower created by the primary gamma-ray is collected by a mirror reflector and then detected by a pixelized PMT camera. SHALON are the imaging atmospheric Cherenkov telescopes (Fig. 1, [1]) created at the P.N. Lebedev Physical Institute for the gammaray astronomy and are located at Tien-Shan High Mountain Station. The telescope systems have characteristics sufficient to record precise information about the shower structure at the very high energies. SHALON experiment has been operating since 1992 and covers the wide astroparticle physics topics including an acceleration and origin of cosmic rays in supernova remnants [2], the physics of relativistic flaring objects like a black holes and active galactic nuclei [3] as well as the long-term studies of the different type objects [4]. Also, one of the issues of SHALON experiment is the development of an alternative method of neutrino detection at high altitudes [5,6]. In this method an earth matter or mountain is used as a target volume for conversion neutrinos to leptons which then initiate extensive air shower in the atmosphere, then showers can be detected by the Cherenkov telescope. Five candidates for neutrino showers have been detected in SHALON experiment [6]. During more than 25-year operation SHALON has studied in details the objects of different type at energies up to 100 TeV [7,8] and continues to produce the results in the field of very high energy astrophysics.
The SHALON mirror Cherenkov telescope system (Fig. 1) is designed to observe and investigate gamma-rays from cosmic sources in the energy range from 800 GeV to 100 TeV. Methodical experiments and observations at the first SHALON telescope have been started more twenty years ago [1,5,9–12]. SHALON gamma-ray telescopes are located in Tien-Shan mountains at an altitude of 3340 m above sea level. The idea of enhancement of angular resolution and sensitivity to the gammarays was realized in SHALON telescopes by the number of technical solutions presented here, including the construction of the widest field of view in the world. This high-altitude site has an optical quality optimal for the observation of Cherenkov light of extensive air showers. Each of SHALON telescopes is placed in an observatory building to prevent the destructive effects of precipitation, pollution, and illumination on the systems of the telescopes. Telescope has a composite spherical mirror with an area of 11.2 m2 and a dish-frame with ∼0.1% deviation from the sphericity in operation conditions [1]. Mirrors are characterized with >96% mirror reflectivity; mirror surface accuracy of 5𝜆 which defines as a point spot dispersion (𝜆 = 500 nm) and focal length dispersion of ≤1% [13]. Mirrors are focusing Cherenkov light of an air shower onto the detector camera that consists of 144 FEU-85 photomultiplier tubes assembled into a square array mounted at the mirror focus. Typical PMT85 amplitude and time spectra in a one- and multi-electron mode and thermoemission peaks are presented in [1,5,11]. PMT-85 have shown sustainability and stability of characteristics in the temperature range
∗ Corresponding author. E-mail address: [email protected] (V.G. Sinitsyna).
https://doi.org/10.1016/j.nima.2019.01.002 Received 15 October 2018; Received in revised form 7 December 2018; Accepted 2 January 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 3. Sensitivity of SHALON with modern and some historical Cherenkov telescopes in the energy range of 100 GeV–100 TeV (data are from [17]).
(𝐼𝑛𝑡0, 𝐼𝑛𝑡1 [2,9,12]) used the difference of Cherenkov photon fluxes within the small angles < 1◦ around shower axis with ones within the large angles of >2◦ for the gamma and hadron showers [1]. Fig. 2 shows the 𝛾-ray shower selection criteria distributions used in the SHALON experiment. The SHALON method of selecting gamma-ray showers from background cosmic-ray showers allows rejecting of 99.93% of the background showers [2,4]. The performance of Cherenkov telescope together with selection criteria is summarized by its angular resolution and 𝛾-ray flux sensitivity. The accuracy of the determination of the coordinates of the 𝛾-ray shower source in SHALON is ∼0.07◦ [3] and it is increased by a factor of ∼10 after additional processing [3,4] using deconvolution algorithm [19]. The sensitivity of the telescope is defined as the minimum flux of 𝛾-rays for a statistically significant detection. We estimated the flux sensitivity for 50 hrs of observation of a point-like source at a confidence level of 5𝜎. The significance is calculated according to the formulation of Li&Ma , formula 17 in the paper [20]. In the energy range from 0.8 to 50 TeV, due to the large field of view SHALON the sensitivity (Fig. 3) is limited by the cosmic ray protons which mimic 𝛾-ray showers with known spectrum 𝐹𝑝 (> 𝐸) ∝ 𝐸 −1.75 . The minimum detectable integral flux of 𝛾-rays at energy of 1 TeV is 2.1 × 10−13 𝑐𝑚−2 𝑠−1 . In the region 1–50 TeV the minimum detectable flux falls down to the value of 6 × 10−14 𝑐𝑚−2 𝑠−1 and then, at energies 𝐸 > 50 TeV, it grows because of limited telescopic field of view [2,4,11,14].
Fig. 1. SHALON mirror Cherenkov telescopes in the observatory.
from −20◦ to +40◦ . PMT’s modules are equipped with an equal divider with amplification on last two dynodes [11] to provide a gain resulting in the enhancement of the linearity of PMT range and the dynamic range ∼104 . Metal conic-to-square light-guides are used to improve light collection. The detector camera has characteristics sufficient to record shower structure information in the energy range under consideration, namely 800 GeV–100 TeV. The detector camera has the largest field of view in the world, >8◦ [1,2,4]. It allows monitoring the background from charged cosmic-ray particles together with the atmospheric transparency continuously during observations of the sources as well as expands the area of observation and, hence, the efficiency of observations [10]. The technique of simultaneously obtaining information about the cosmic-ray background and the showers initiated by gamma-rays from observed source is a unique and has been used in the SHALON experiment from the very beginning of its operation [5,9–11,14]. This technique serves to increase the efficiency of the source observation as the source and background observation conditions like thickness and other atmosphere parameters remain the same for the on-source and background data. Due to the larger field of view, larger choice of the background regions exist; it allows to collect more information on the background and to be sure that there is no influence of the studying source, especially in the case of an extended source. In addition, in the case of the large field of view the large-impact-distance showers, including of >100 m, are recorded completely and without significant distortions and showers more than 30 m impact distance account for more 95% of all of the showers recorded by the telescope. Multi-channel 12-bit analog-to-digital converters [15,16] are used to record the PMT signal from EAS Cherenkov lighting for further analysis. In SHALON experiment the analysis of the angular and lateral distributions of the recorded shower Cherenkov light for the selection of showers of primary 𝛾-rays is performed first, due to the optimized Hillas parameters [18] defined as moments of 2-dimensional shower image: 𝛼, 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 and ratio of 𝐿𝑒𝑛𝑔𝑡ℎ, 𝑊 𝑖𝑑𝑡ℎ and high efficiency parameters
3. Astrophysics with SHALON Observation and long-term study SHALON program includes different type galactic and extragalactic objects. Among the galactic sources are supernova remnants, binary systems, variable stars, pulsar wind nebulae. A list of extragalactic sources mostly consists of BL Lac-objects, FSRQ and Radio galaxies. Observations of the extragalactic sources can also be used for the study of Extragalactic Background Light. Since 1992 SHALON experiment has studied 19 galactic and 16 extragalactic objects in very details at energies up to 100 TeV [7,8] which have used in different topics of astroparticle physics.
Fig. 2. Distributions of the parameters for gamma-ray showers and background showers (black contour) used as selection criteria in the SHALON experiment [9]. Arrows show the cuts for background rejection.
2
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 5. left: 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 and models (see [3]). right: Chandra X-ray (1.5–3.5 keV) image of NGC 1275; the red contours indicate the SHALON image of NGC 1275 in the energy range 800 GeV–40 TeV.
shell-type SNRs Tychos SNR (1572 y), Cas A (1680 y), IC 443 (age 300–30000 y), 𝛾Cygni SNR (age 5000–7000 y), G166.0+4.3 (age 24000 y). For each of SNRs the observation results are presented with spectral energy distribution by SHALON in comparison with other experimental data and images by SHALON in together with data from X-ray by Chandra and radio-data by CGPS (Fig. 4). For the first time the location of TeV gamma-ray emission regions relative to the position SNRs remarkable features as a forward and reverse shocks, dense molecular clouds swept out by SNR explosion and shells due to the interaction of the supernova ejecta and the surrounding medium was shown (see. [2] for details). The collected experimental data confirmed the predictions of the theory about the hadronic generation mechanism of very high energy 800 GeV–100 TeV gamma-rays in Tycho’s SNR, Cas A, IC 443 and 𝛾Cygni SNR. 3.2. Radio galaxy NGC 1275 Long-term studies of the central galaxy in the cluster, NGC 1275, are being carried out in the SHALON experiment. Gamma-ray emission from NGC 1275 was detected by the SHALON telescope at energies 800 GeV– 40 TeV (Fig. 5, left). It was found that the TeV structure around NGC 1275 spatially coincides with the X-ray emission regions (Fig. 5, right). The brightness distribution of the X-ray emission and the observed TeV emission shows a sharp increase in intensity outside the bubbles blown by the central black hole and visible in the radio band. 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′′ . [3]. Also, days-time flux variability was detected. As result, it was found that 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 (black triangles) and the short-time flux variability 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.
Fig. 4. Characteristics of Shell-type SNRs: left: Spectral energy distributions of highand very high energy 𝛾-ray emission by SHALON (▴ or ▵) in comparison with other experiments and models [2]. right: Image of SNRs at energies of >0.8 TeV by SHALON (gray scale). Red contours are the X-ray emission by Chandra for Cas A, Tycho’s SNR and radio contours by CGPS for 𝛾Cygni SNR, IC 443 and G166+4.3. Also, X-ray data from ROSAT and location of high energy emission by Fermi LAT are shown.
3.1. Shell-type supernova remnants
3.3. GCRs from M-Dwarf star flares and ‘‘positron excess’’
Supernova Remnants (SNRs) have long been considered as unique candidates for cosmic-ray sources. Direct data on the distribution of cosmic rays in SNRs can be obtained from very high energy gammaray observations. As the presence of the electron cosmic-ray component is clearly seen by the emission generated by it in an SNR in a wide wavelength range, from radio to high-energy gamma-rays, while the nuclear cosmic-ray component can be detected only by very high energy 𝛾-ray emission. The SHALON observations have yielded the results on Galactic supernova remnants of different ages. Among them are the
Conventional sources of cosmic rays are believed to be supernovae and supernova remnants [21,22]. Recent observations of SNRs in TeV 𝛾-rays are making clear the origin of cosmic rays in these objects. However, the experimental data obtained by Pamela, Fermi, AMS-02, spectrometers requires the existence of nearby sources of cosmic rays at the distances < 1 kpc from the solar system. These sources could explain such experimental data as the growth of the ratio of galactic positrons to electrons with an increase of their energy [23,24], the complex dependence of the exponent of the proton and alpha spectra from the 3
Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.
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Fig. 6. Light curves and differential spectra of V388 Cas, V780 Tau, V962 Tau, V1589 Cyg at TeV energies from the long-term SHALON observations.
energy of these particles, the appearance of anomaly component in cosmic rays. Active dwarf stars are proposed as possible sources of galactic cosmic rays in energy range up to 1014 eV [25,26]. These stars produce powerful stellar flares. The generation of high-energy cosmic rays has to be accompanied by high-energy 𝛾-ray emission. The data obtained during more than 10 years observations of the dwarf stars V962 Tau, V780 Tau, V388 Cas and V1589 Cyg were analyzed. Very high energy gamma-ray emission in the range of 1–10 TeV mostly of flaring type from these red dwarfs is detected (Fig. 6, [26]). The energy released in stellar flares and its frequency is able to provide the necessary energy of GCRs in the disk of our Galaxy and make a significant contribution to the GCR spectrum up to energies of ∼1013 –1014 eV [25]. This result confirms that active dwarf stars are also the sources of the high energy cosmic rays. These sources could explain such experimental data of PAMELA and AMS spectrometers.
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4. Conclusion SHALON experiment has been operating since 1992 and covers the wide field of astroparticle physics and gamma-ray astronomy at the energies of 800 GeV to 100 TeV and continues to produce the results in the field of very high energy astrophysics. Further deeper investigations in high and very high energies are needed for the better understanding on astroparticle VHE phenomena in Universe. References [1] S.I. Nikolsky, V.G. Sinitsyna, in: A.A. Stepanian, D.J. Fegan, W.F. Cawley (Eds.), Proc. Int Workshop of VHE Gamma-ray Astronomy, 1989, pp. 11–20. [2] V.G. Sinitsyna, et al., Adv. Space Res. 62 (10) (2018) 2845–2858.
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Please cite this article as: V.G. Sinitsyna, V.Y. Sinitsyna, S.S. Borisov et al., Very high energy astrophysics with the SHALON Cherenkov telescopes, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.002.