The peak density for the 1-TeV gamma-rays' light coincides approximately with that of 3-TeV protons. The telescope positions are indi- cated by the black circles ...
Frascati Physics Series Vol. nnn (2001), pp. 000-000 IX Int. Conf. on Calorimetry in Part. Phys. - Annecy, Oct. 9-14, 2000
IMAGING ATMOSPHERIC CHERENKOV TELESCOPE Ryoji Enomoto ICRR, Univ. of Tokyo, 5-1-5 Kashiwa-no-ha, Chiba 277-8582, Japan
ABSTRACT ˇ The principle of Air Cherenkov techniques used to detect very high-energy astronomical gamma-rays is presented. The CANGAROO experiment in the southern hemisphere as well as other experiments in the world are introduced. The results of past and present experiments are briefly reviewed and also future prospects of this technique are mentioned.
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Introduction
There have been many measurements of the universe in the optical, micro-wave, infrared, and gamma-ray (∼100 MeV) regions. However, we are now observing the universe by a new window, i.e., TeV (sub-TeV) region gamma-rays using Imaging Atmospheric Cherenkov Telescopes (IACT). Their original purpose was to reveal the origin of cosmic-rays.
Figure 1: Atmospheric shower profiles; the left is a 1-TeV gamma-ray and the right is a 3-TeV proton. In each plot, the east-west projection is plotted left and the north-south projection is right. 2
Imaging Atmospheric Cherenkov Telescope
The basic aspects of the IACT technique are the followings. The atmosphere thickness is 30-40km. In total, we have 1Kg/cm2 air, which corresponds to 30X0 and 10λ (inelastic interaction length). The height of the shower max for 1-TeV gamma-rays is about 10km, where the air pressure is 0.3 atm. The Cherenkov threshold for secondary electrons there is 40 MeV and the Cherenkov angle is 0.8◦ . Air shower profiles for 1-TeV gamma-rays and 3-TeV protons are shown in Figure 1. Gamma-ray showers have thin, high-density structures. On the other hand, protons are broad with low density. A Cherenkov light is emitted along the secondary particle trajectories with a small Cherenkov angle. Figure 2 shows light pools on the earth surface for both a gamma-ray and
Figure 2: Light pools on the earth surface; left for a 1-TeV gamma-ray and right for a 3-TeV proton. The inserted figures are focal-plane images of telescopes. a proton shower. The peak density for the 1-TeV gamma-rays’ light coincides approximately with that of 3-TeV protons. The telescope positions are indicated by the black circles in Figure 2. The inserted graphs are the focal-plane images of this Cherenkov light viewed from a slightly different direction from the shower axis by a 10-m telescope at the ground. The thin image of the gamma-ray shower can be clearly seen, contrary to that of hadrons. In order to know the incident direction of the initial gamma-ray, we carried out the following image analysis. 1) First “distance” is the angular difference between the source point (typically center of field of view (FOV)) and the shower centroid. It should be located within a 1-degree circle due to the electromagnetic(EM) shower development. We, thus, assume that the shower shape is an ellipse. The lengths of the long and short axes are “length” and “width”, respectively. Both of these parameters for gamma-rays should be smaller than
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Figure 3: Shower shape distributions: a) distance, b) length, c) width, and d) alpha. those for protons. These are shown in Figure 3. After subjecting cuts on these parameters, we calculated α, which is the angle between the long axis of the shower and the direction from the centroid to the source point. When we are looking at point sources, such as pulsars, there should be a sharp peak around zero degree, as shown in Figure 3d). There is an interesting technique used to measure higher energy gammarays, which is called “large-zenith-angle observation”. 2) The gamma-rays from the large zenith angle develop a shower earlier. From telescopes, it is located further. They make larger area light pools on the surface, as shown in Figure 4. With this we can increase the effective area drastically, although, the light densities are smaller, i.e., the energy threshold becomes higher. In spite of the smaller shower images (see the inserted graph of Figure 4), we can analyze these events when the telescope is located in an appropriate position in the light
Figure 4: Light pool for a large zenith angle event when gamma-ray incidents occur from west to east. The insert is the focal-plane image. pool. Usually, a gamma-ray source which can be seen in the zenith from the north hemisphere of the earth can be seen at a large angle from the southern hemisphere. Measurements using this technique are complimentary to each other (northern and southern hemisphere experiments). 3
CANGAROO Experiment and Others
IACT was pioneered by WHIPPLE observatory 3) located on Mt. Hopkins, USA. By now, they have established steady emission from the Crab pulsar, two of Active Galactic Nucleis (AGN; Mkn501 4) and Mkn421 5) ), and etc. The CANGAROO experiment started in 1992 at Woomera, South Australia with a relatively smaller optical mirror (3.8m) along with a high-resolution camera system. We had detected gamma-rays from some kinds of supernova remnants (PSR1706-44, 6) SN1006, 7) and etc) and also measured the energy
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Figure 5: Two telescope images produced by stereoscopic observations. spectrum of the Crab pulsar in the range 7-50 TeV. 2) From 1999, a bigger telescope system (D=10m) has been under operation. There are, also, many projects, such as HEGRA (3m × 7) 8) and CAT (5m). 9) 4
Stereoscopic Observation
The near-future trend of IACT is stereoscopic observations. The optimum spacing of the telescopes was calculated to be about 100m. 10) Simulation results are shown in Figure 5. There are two camera images obtained by two telescopes separated by 100m. The source point is located at the center of FOV. From the intersection of the long axes of the two images, we can obtain the gamma-ray’s emission point without assuming the source direction. We can detect even a diffuse source on an event-by-event basis. The CANGAROO-III experiment, aiming at stereoscopic observations, will start from sometime in
2001 after installing a second telescope. 11) Similar projects are VERITAS (successor of WHIPPLE) on Mt. Hop12) kins and HESS with larger telescope (D=12-m) system at Namibia. 10) A project with a very large mirror (D=17m) is also underway (MAGIC 13) on Canary Island). 5
Future Prospect
In the 21th century, we will have two observatories in the southern hemisphere and two in the north. They should work complimentary to each other. The research and development for future possibilities of IACT is continuing. For example, IACT at a high altitude, such as on mountains of 5000m, will be tested. Some of CANGAROO collaborators are implementing a gamma-ray camera onto the SUBARU optical telescope in Hawaii. 14) At a higher altitude, we can observe gamma-ray showers from the closest available point, i.e., hoping to greatly reduce the energy threshold (such as down to 5 GeV). 6
Summary
The technology of an Imaging Atmospheric Cherenkov Telescope has been reviewed in this article. The on-going and future projects are introduced. References 1. A.M. Hillas, 1985, Proc. ICRC (La Jolla), 3, 445. 2. T. Tanimori et al, Astrophys. J. Lett., 429, L61-64 (1994); T. Tanimori et al, Astrophys. J. Lett., 492, L33-36 (1998). 3. T.C. Weekes et al, 1989, ApJ, 341, 379. 4. J. Quinn et al, 1996, ApJ, 456, L83. 5. M. Punch et al, 1992, Nature, 358, 477. 6. T. Kifune et al, 1994, ApJ, 438, L91. 7. T. Tanimori et al, 1998, ApJ, 497, L25. 8. A. Daum et al, 1997, Astropart. Phys., 8, 1.
9. A. Barrau et al, 1998, Nucl. Instrum. Meth., A416, 278. 10. W. Hofmann et al, 2000, Astropart. Phys., 13, 253. 11. M. Mori et al, 2000, Proc. on “Towards a Major Atmospheric Cherenkov Detector VI” (Snowbird, Utah, USA, 1999), 485. 12. VERITAS proposal. http://veritas.sao.arizona.edu/ 13. http://hegra1.mppmu.mpg.de/. 14. http://www.hp.phys.titech.ac.jp/subaru/index e.html.
