Telescopi γ-rays. Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

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1 Telescopi γ-rays LE-HE (10s MeV/100 Gev) Space-based Observations Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range VHE-HE (>10s GeV) Ground-based Observations Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

2 Cherenkov Telescopes Instruments to detect photons derived from the interactions of the primary γ-ray with the upper layers of the troposphere. The atmosphere is a key part of the detection chain. Above 50 GeV the production of atmospheric electromagnetic showers by primary γ-rays is dominated by the pair production and bremsstrahlung mechanisms n STP

3 Cherenkov Telescopes N photons emitted by a charged particle of charge ze per unit path length x and unit wavelength λ UV ultra-relativistic electron through a layer of 10 km G. Rodeghiero 2015 Observations mainly at UV-VIS waves in a photon starved regime. What about the atmospheric diffuse background during a night of the new moon? Bckg (350 nm nm) =10 12 photons*sr/m 2 /s Cherenkov flash flux 50 photons/m 2 in 5 ns(*) in a radius of 100 m (light pool), angle subtended almost 1 deg (3*10-4 sr) with these numbers the residual bckg is of the order of 1-2 photons/m 2 (*) differently from conventional optical astronomy, the image can t be improved increasing the exposure.

4 Cherenkov Telescopes What about the atmospheric diffuse background during a night of the new moon? Bckg (350 nm nm) =10 12 photons*sr/m 2 /s Cherenkov flash flux 50 photons/m 2 in 5 ns in a radius of 100 m, angle subtended almost 1 deg (3*10-4 sr) with these numbers the residual bckg is of the order of 1-2 photons/m 2 Direction reconstruction: from single telescope towards array of telescopes to allow geometric reconstruction of shower direction and impact in stereoscopic mode de Naurois & Mazin, 2015

5 Ground-based γ-ray astronomy with Imaging Atmospheric Cherenkov Technique was pioneered with the Whipple, CAT and HEGRA telescopes. In the field of very high energy gamma-ray astronomy (VHE, energies >100 GeV) recent IACTs are: H.E.S.S. MAGIC VERITAS CANGAROO Cherenkov Telescopes and the future is CAT Cherenkov Telescope Array

6 Cherenkov Telescopes H.E.S.S. High Energy Stereoscopic System four 12 m diameter IACTs + one 28 m IACT Phase 2 One single huge dish with about 600 m 2 mirror area (28m in diameter) Phase 1 Four 12m in diameter telescopes on a square with 120m side length H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality.

7 Cherenkov Telescopes MAGIC Major Atmospheric Gamma Imaging Cherenkov two 17 m diameter IACTs F/1.03 separated by 85 m VHE (30 GeV TeV) 2200 m a.s.l. on the Roque de los Muchachos European Northern Observatory on the Canary Island of La Palma (28 N, 18 W)

8 Cherenkov Telescopes VERITAS Very Energetic Radiation Imaging Telescope Array System four 12 m optical reflectors VHE (50 GeV - 50 TeV) Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, USA

9 Cherenkov Telescopes CANGAROO Collaboration of Australia and Nippon (Japan) for a GAmma Ray Observatory in the Outbackfour four 10 m optical reflectors VHE (100 GeV) Woomera, Australia

10 Cherenkov Telescopes CAT Cherenkov Telescope Array Two array sites = CTAObservatory Energy range (20 GeV TeV) La Palma Canarie (see MAGIC) Large-Size Telescope (LST) 23 m diameter parabolic shape Medium-Size Telescope (MST) 12 m diameter Small-Size Telescope (SST) 4 m diameter Northern Hemisphere Site rendering; credit Gabriel Pérez Diaz, IAC SMM

11 Cherenkov Telescopes CAT Cherenkov Telescope Array Two array sites = CTAObservatory Energy range (20 GeV TeV) Atacama desert - Chile Large-Size Telescope (LST) 23 m diameter parabolic shape Medium-Size Telescope (MST) 12 m diameter Small-Size Telescope (SST) 4 m diameter Southern Hemisphere Site rendering; credit Gabriel Pérez Diaz, IAC SMM

12 Main telescope optics parameters: Reflective area to define the amount of collected light (large aperture and only a few optical elements) PSF/large FoV to be compared with pixel diameter (see photomultiplier tube), i.e. low angular resolution (arcmin) Time dispersion different light paths through the telescope smaller than c*3 ns (average Cherenkov light pulse from a gamma-ray shower) Large throughput Large FoV ( 15 deg) -> 10 3 m 2 deg 2 > 10 2 m 2 deg 2 optical telescopes Optics configurations: Cherenkov Telescopes single-mirror two-mirrors Parabolic reflector Davies-Cotton design Schwarzschild Couder telescopes Aplanatic Telescope

13 Cherenkov Telescopes Davies-Cotton telescope (originally proposed as a solar concentrator) Each facet has a spherical shape with a curvature radius = 2f and is distributed along a sphere of radius f VERITAS telescope (Davies-Cotton design) 350 identical hexagonal spherical mirrors (of area m2 and radius-of-curvature of approximately 24m) giving a total reflector area of 110 m 2

14 Cherenkov Telescopes Parabolic mirror advantage: minimum time dispersion (isochronous) Davies Cotton design advantages: better off-axis imaging (large FoV) many small, identical and spherical facets trivial optical alignment easy mirror replacement and mirror re-coating Both single-mirror solutions are affected by off-axis aberrations, limiting the maximum aperture. Increasing the aperture, decreases the focal ratio (f) while primary aberrations are amplified with the field of view (δ): only two-mirrors could also correct coma.

15 PSF radial component [azimuth is 80%] Parabolic mirror advantage: minimum time dispersion (isochronous) Davies Cotton design advantages: better off-axis imaging (large FoV) many small, identical and spherical facets trivial optical alignment easy mirror replacement and mirror re-coating f/# Cherenkov Telescopes H.E.S.S. Bernlohr et al Davies Cotton layout Increased cost, lower resonant freqs + mechanical complications (see spider for camera) Davies Cotton layout f/1.2 (bottom) Parabolic layout (top) i.e. spherical facets on parabola Davies Cotton layout f/1.2 with mirror facets with different PSF

16 Cherenkov Telescopes H.E.S.S. VERITAS Mirror mountings Mirror facet support unit The 499 PMT pixel camera. The focus box is 1.8 m square. A remotely operated shutter usually covers the camera during daylight hours. An image of Polaris in the focal plane of the telescope recorded with a CCD camera. The point spread function is 0.06 FWHM. The circle indicates the size of a VERITAS PMT (0.15 diameter).

17 Cherenkov Telescopes W. Hofmann

18 Cherenkov Telescopes W. Hofmann

19 Cherenkov Telescopes H.E.S.S. Mirror facets alignment Images of a star on the camera lid before alignment. Each spot corresponds to the star image of a mirror facet. Spots spread radius ~1 deg -> 0.5 facet tilt (doubled due to reflection) Cornils et al. 2003

20 Cherenkov Telescopes W. Hofmann

21 Cherenkov Telescopes Schwarzschild Couder design advantage: large FoV The ASTRI (*) prototype, the first Schwarzschild-Couder telescope to be built and tested, was inaugurated in September 2014 and has been undergoing testing at the Serra La Nave observing station on Mount Etna in Sicily. The ASTRI is one of three proposed Small-Size Telescope designs for CTA. Credit: Cherenkov Telescope Observatory Vercellone S. et al (*) Astrofisica con Specchi a Tecnologia Replicante Italiana

22 Cherenkov Telescopes Schwarzschild Couder design advantage: large FoV Schwarzschild aplanatic telescope (1905) f/3 with FoV = ±1.4 large obscuration ratio (0.5) and focus between M1 and M2 M1=hyperboloid and M2=oblate spheroid difficult to manufacture and test. modified by Couder aplanatic anastigmatic telescope (1926) M1-to-M2 distance exactly twice the EFL (f ), i.e. a less compact design but a lower obscuration ratio. Wilson R.N. 2007

23 Cherenkov Telescopes Schwarzschild Couder design advantage: large FoV Wilson R.N. 2007

24 Cherenkov Telescopes Schwarzschild Couder design advantage: large FoV SC aplanatic telescopes for application in γ-ray astronomy with IACTs was developed by V. Vassiliev et al. (see Refs). Significant reduction of the plate scale with respect to single-mirror solutions making it compatible with cameras of PMTs and Winston cones that have dimensions in the order of centimeters. ASTRI focal length value (f = mm), the derived plate scale is PS = 96"/mm. Angular resolution in sky of J=0.166.

25 Telescopi γ-rays LE-HE (10s MeV/100 Gev) Space-based Observations Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range VHE-HE (>10s GeV) Ground-based Observations Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

26 Water Cherenkov Telescopes HAWC High-Altitude Water Cherenkov observatory (from MILAGRO s heritage) VHE (100 GeV and 100 TeV) HAWC is located on Sierra Negra volcano near Puebla, Mexico at an altitude of 4100 meters. The detector has an instantaneous FoV 15% of the sky and during each 24 hour period HAWC observes two-thirds of the sky. The detector contains 300 tanks with 1200 PMTs in total

27 Water Cherenkov Telescopes HAWC corrugated steel tanks 4 m high and 7.3 m in dia with 4 PMTs sensitive at UV wavelengths The water is dense (relative to air), and so a γ-ray produces an e+e- pair once it enters the tank. These charged particles then emit Cherenkov radiation as they speed through the water. n water = G. Rodeghiero 2015

28 Telescopi γ-rays LE-HE (10s MeV/100 Gev) Space-based Observations Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range VHE-HE (>10s GeV) Ground-based Observations Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

29 Extensive Air Shower detectors Detectors that measure particles of the shower tail reaching the ground. This method provides a snapshot of the shower at the moment it reaches the ground and constitutes the so-called particle sampler" technique. Those detectors have a very large duty cycle (potentially 100%), but rather high energy threshold (as high energy showers are more penetrating and produce charged particles at lower altitude than lower energy showers). Moreover, as they only have access to shower tails, they usually have a rather poor capability to discriminate the showers induced by γ-rays from the much more numerous showers induced by protons and charged nuclei. Such detectors are usually installed at high altitude to collect more charged particles.

30 Extensive Air Shower detectors KASKADE A calorimeter is one of the main components of KASCADE: the hadrons and their interactions are important for the understanding of the shower development within the atmosphere The field array (200m x 200m) consists of 252 detector stations arranged on a rectangular grid with a distance of 13 meters to each other. In each station there are up to four electron/gamma-detectors and one muon-detector under a iron-lead-absorber

31 Telescopi γ-rays LE-HE (10s MeV/100 Gev) Space-based Observations Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range VHE-HE (>10s GeV) Ground-based Observations Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

32 Coded-mask telescopes The telescope is a coded mask: each direction in the sky results in a distinctive shadow pattern (shadowgram, see PSF). The image is recovered by deconvolution. IBIS imager aboard INTEGRAL, an example The coded mask (a) partially covers the operture of the telescope. It is made of opaque plates and holes optimally distributed. The detector (b) records the shadow of the mask projected by the gamma-ray sources located within the field of view (c). In the case of two sources, the superimposition of the shadows of the mask onto the detector is more complex. Credits: ISDC/M. Türler a square of size mm3 made up of individual square cells of size mm2 ESA IBIS-INTEGRAL coded mask

33 Compton telescopes Coded mask patterns can be apparently random (Beppo-SAX/WFC) or highly structured (INTEGRAL/IBIS) but always satisfying established principles. Different mask patterns exhibit different image resolutions, sensitivities and backgroundnoise rejection, and computational simplicities and ambiguities, aside from their relative ease of construction. FZP = Fresnel Zone Plate ORA = Optimized RAndom pattern URA = Uniformly Redundant Array HURA = Hexagonal Uniformly Redundant Array MURA = Modified Uniformly Redundant Array Levin Veeraraghavan

34 Coded-mask telescopes Schematic view of the SPectrometer of INTEGRAL (SPI) energy range 20 kev - 8 MeV coded mask complex AntiCoincidence System (ACS) to actively shield the tube and the bottom of the telescope Photo courtesy INTA, Spain 127 hexagonal elements: 63 = 3 cm thick tungsten (absorption efficiency >95% at 1 MeV) 64 = nearly free of matter 19 germanium detectors Credits: ISDC Credits: SPI Team A 3-D plot of the shadowgram of SPI showing the counts received in each of the 19 hexagonal Ge detectors during one pointed observation of the INTEGRAL spacecraft.

35 Telescopi γ-rays LE-HE (10s MeV/100 Gev) Space-based Observations Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range VHE-HE (>10s GeV) Ground-based Observations Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

36 Compton telescopes Telescope design based on Compton scattering: a photon hits an electron and some of the photon energy is transferred to the charged particle. the angle of the incoming gamma ray is inferred using the data from both sets of phototube detectors. This angle defines a ring (event circle) in the sky where the gamma ray could have come from.

37 Compton telescopes Compton telescopes are usually constructed in two layers. A cosmic gamma ray enters and scatters on an atom in the first layer. The resulting electron is detected through scintillation in the top layer and is observed by phototubes. The photon that results from the interaction passes through to the second layer of the telescope where it is absorbed and detected by another set of phototubes. The phototubes viewing the two levels can approximately determine the interaction points at the two layers and the amount of energy deposited in each layer. Credit: NASA's Goddard Space Flight Center COMPTEL Compton Telescope that flew on the Compton Gamma-Ray Observatory (CGRO)

38 Telescopi γ-rays LE-HE (10s MeV/100 Gev) Space-based Observations Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range VHE-HE (>10s GeV) Ground-based Observations Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

39 Pair-conversion telescopes Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi), formerly the Gamma-ray Large Area Space Telescope (GLAST). Observation in the energy range between 20 MeV and 300 GeV. LAT is a pair-conversion telescope with: W.B. Atwood et al The telescope s dimensions are 1.8 m 1.8 m 0.72 m. with a mass of 2.7 tons a precision converter-tracker, The converter-tracker has 16 planes of high-z material in which γ-rays can convert to an e+e pair. The converter planes are interleaved with positionsensitive detectors that record the passage of charged particles, thus measuring the tracks of the particles resulting from pair conversion to reconstruct the directions of the incident γ-rays. a calorimeter to measure the energy deposition due to the electromagnetic particle shower that results from the e+e pair produced by the incident photon and to image the shower development profile. Each calorimeter module has 96 CsI(Tl) crystals. an anti-coincidence detector to provide charged-particle background rejection; therefore its main requirement is to have high detection efficiency for charged particles

40 Completed tracker array Pair-conversion telescopes A flight tracker tray W.B. Atwood et al The probability distribution for the reconstructed direction of incident γ-rays from a point source is referred to as the Point Spread Function (PSF). Multiple scattering of the e+e limits the obtainable resolution. To get optimal results requires that the e+e directions be measured immediately following the conversion. layers of silicon strip (Si) detectors interleaved with layers of tungsten foil (W) to facilitate the pair-creation

41 Pair-conversion telescopes W.B. Atwood et al Anticoincidence Detector (ACD) LAT calorimeter module

42 Pair-conversion telescopes What about the field of view of such a telescope? 2.4 sr at 1 GeV, where A eff is the effective area of the LAT (~1m 2 ) Effective area versus energy at normal incidence for Diffuse (dashed curve), Source (solid curve), and Transient (dotted curve) analysis classes. Effective area versus energy at normal incidence (solid curve) and at 60 off-axis (dashed curve) for Source analysis class W.B. Atwood et al. 2009

43 γ-ray observations comparison To summarise. G. Rodeghiero 2015

44 References H.E.S.S. - MAGIC - VERITAS - CANGAROO - CTA - HAWC - KASKADE - COMPTEL - The CTA Consortium, Exp Astron (2011) 32: Vassiliev, V.V., et al., Astropart. Phys. 28, 10 (2007) [astro-ph/ ] Holder J. et al. Astropart. Phys. 25, 391 (2006) Wilson R.N., Reflecting telescopes, Springer Ed Longair M. S. High Energy Astrophysics, Cambridge University Press