GAMMA-RAY ASTRONOMY: IMAGING ATMOSPHERIC CHERENKOV TECHNIQUE
COSMIC RAYS Discovered in 1912 by Victor Hess (Nobel Prize) Messengers from the non-thermal part of the Universe E < 15 ev: galactic E > 17 ev: extra-gal.
COSMIC RAYS Cosmic Rays composition: ~89% PROTONS ~9% α-particles ~1% IONIZED HEAVIER ELEMENTS ~1% ELECTRONS ~0.1% PHOTONS The main part of the CRs are charged particles and hence interact with the interstellar magnetic fields, i.e. those particles arrive to the Earth isotropically, making impossible to reconstruct neither the original direction of the emitters nor an eventual time structure of the signals
GAMMA RAYS The particles keeping the directional information are the neutral ones: NEUTRONS (too short lifetime) NEUTRINOS (extreme low cross-section very large detectors) PHOTONS GAMMA RAYS trace back to the origin of their generator, carry energy information about it and preserve the time structure of the emission signal
GAMMA RAYS Medium Energy (< 30 MeV): completely absorbed by the atmosphere, only satellite telescopes can detect them High Energy (30 MeV 100 GeV): satellite telescopes and high-quote balloons are the main detectors Very High Energy (100 GeV 10 TeV): gamma-rays are so energetic that crossing Earth atmosphere they produce Electromagnetic Showers well detectable by ground-based telescopes (like IACTs) Ultra High Energy (> 10 TeV): these particles produce Extended Air Showers that could also be detected by groundbased telescopes, but with very low fluxes
SPACE-BASED TELESCOPES They detect gamma-rays directly from the space First gamma-rays observation by Explorer XI in 1965 The most important was EGRET hosted by the CGRO satellite (1991-2000)
SPACE-BASED TELESCOPES EGRET gave us the actual picture of the high-energy Universe with about 270 gamma-ray sources
SPACE-BASED TELESCOPES EGRET gave us the actual picture of the high-energy Universe with about 270 gamma-ray sources
SPACE-BASED TELESCOPES Now, we are entering in a new exciting era of gamma-ray astronomy: on June 11th (2008) FERMI/GLAST satellite was successfully launched on board of a Delta II rocket from Cape Canaveral
SPACE-BASED TELESCOPES Now, we are entering in a new exciting era of gamma-ray astronomy: on June 11th (2008) FERMI satellite was successfully launched on board of a Delta II rocket from Cape Canaveral Only about 100 hours!!! FERMI will revolutionize our view of the highenergy Universe!!!
GROUND-BASED TELESCOPES The most used technique is the IACT: Imaging Atmospheric Cherenkov Technique Collection of the Cherenkov light, emitted by electrons and positrons in atmospheric shower due to the CRs impinging the Earth atmosphere The first detector using this technique was the Whipple Telescope: in 1989 it discovered TeV emission from the Crab Nebula, that now is the standard candle for the very high-energy astronomy!
GROUND-BASED TELESCOPES Nowadays, there are four operative IACT: MAGIC HESS VERITAS CANGAROO Major Atmospheric Gamma-ray Imaging Cherenkov Telescope (Roque de los Muchachos, La Palma, Spain)
GROUND-BASED TELESCOPES Nowadays, there are four operative IACT: MAGIC HESS VERITAS CANGAROO High Energy Stereoscopic System (near Gamsberg, Namibia)
GROUND-BASED TELESCOPES Nowadays, there are four operative IACT: MAGIC HESS VERITAS CANGAROO Very Energetic Radiation Imaging Telescope Array System (Amado, Arizona, USA)
GROUND-BASED TELESCOPES Nowadays, there are four operative IACT: MAGIC HESS VERITAS CANGAROO Collaboration of Australia and Nippon for a GAmma Ray Observatory in the Outback (Woomera, Australia)
IACT TECHNIQUE Cosmic Rays hardly reach the Earth ground, but instead collide with the nucleons (nitrogen) present in the atmosphere: new particles are created which themselves interact with the atmosphere atoms, leading to the creation of an AIR SHOWER Depending on the incident particle, i.e. hadron or electromagnetic particle, distinction is made between ELECTROMAGNETIC and HADRONIC SHOWERS
IACT TECHNIQUE Electromagnetic Shower (γ, e-) Hadronic Shower
IACT TECHNIQUE Electromagnetic Shower (γ, e-) Hadronic Shower
IACT TECHNIQUE IACTs are based on the detection of the Cherenkov light emitted by atmospheric showers induced by cosmic rays The Cherenkov effect was discovered from the namesake Russian physicist in 1934
IACT TECHNIQUE CHERENKOV EFFECT: Cherenkov effect occurs when a charged particle travels into a dielectric medium of refractive index n, with a speed exceeding the light speed in the medium c/n. When a charge moves in a dielectric medium, a polarisation occurs (a). When the particle velocity is superluminal v>c/n, the particle is moving faster than the electromagnetic information which induce the polarisation. A coherent wave-front appears at an angle θ, and the emitted radiation is called CHERENKOV LIGHT
IACT TECHNIQUE Electrons are the main emitters of Cherenkov light The surface hit by the Cherenkov light is a circle of about 150 m
IACT TECHNIQUE Electrons are the main emitters of Cherenkov light Low energy primary particle implies a lower photon density, thus a larger detection area is needed The surface hit by the Cherenkov light is a circle of about 150 m
IACT TECHNIQUE IACTs do not detect directly the gamma-rays emitted by an observed astrophysical object, they detect Cherenkov light emitted by air showers electron-positron pairs after the gamma-rays interactions with the Earth atmosphere
IACT TECHNIQUE IACTs do not detect directly the gamma-rays emitted by an observed astrophysical object, they detect Cherenkov light emitted by air showers electron-positron pairs after the gamma-rays interactions with the Earth atmosphere primary task for an IACT is the identification of the difference between electromagnetic and hadronic cascades: since the gamma-ray to charged cosmic ray ratio is of about 10-4 a very powerful technique is needed in order to distinguish the gamma events from the dominating hadronic ones Imaging Technique: study the atmospheric shower by analysing the images produced by Cherenkov photons when they are focused on a plane
IACT TECHNIQUE IACTs do not detect directly the gamma-rays emitted by an observed astrophysical object, they detect Cherenkov light emitted by air showers electron-positron pairs after the gamma-rays interactions with the Earth atmosphere primary task for an IACT is the identification of the difference between electromagnetic and hadronic cascades: since the gamma-ray to charged cosmic ray ratio is of about 10-4 a very powerful technique is needed in order to distinguish the gamma events from the dominating hadronic ones Imaging Technique: study the atmospheric shower by analysing the images produced by Cherenkov photons when they are focused on a plane IACT instruments can be considered as operators transforming arrival directions of the detected photons into distance from the centers forming a shower image: a parabolic mirror surface reflects the incoming light and concentrates it into a pixelled camera which converts the electromagnetic radiation into electronic signal
IACT TECHNIQUE Cherenkov photons ~ 1o image on telescope camera typical elliptic shape
IACT TECHNIQUE head Cherenkov photons ~ 1o image on telescope camera typical elliptic shape
IACT TECHNIQUE head Cherenkov photons ~ 1o image on telescope camera tail typical elliptic shape
IACT TECHNIQUE images analysis ~ 1o INTENSITY: primary particle energy
IACT TECHNIQUE images analysis gamma event image points toward the camera center ~ 1o ~ 1o INTENSITY: primary particle energy ORIENTATION: incoming isotropic distributions of events direction direction hadronic event
IACT TECHNIQUE images analysis gamma event elliptic shape very compact ~ 1o ~ 1o ~ 1o roundish shape fragmentation INTENSITY: primary particle energy ORIENTATION: incoming direction SHAPE: primary particle nature hadronic event
IACT TECHNIQUE Image Parametrisation IMAGE PARAMETERS (or Hillas parameters) Main Image Parameters: ~ 1o ~ 1o ~ 1o ALPHA: angle between major axis and the center of gravity-camera center direction SIZE: total number of collected photons
OBSERVATION AND ANALYSIS Sample Selection: ON, OFF and Monte Carlo data Calibration: charge number of photons Image Cleaning : ~ 1o ~ 1o ~ 1o Image parameters calculation: parametrisation parameters
OBSERVATION AND ANALYSIS GAMMA-HADRON separation RANDOM FOREST (multidimensional classification method): gamma sample (MC) hadron sample (ON/OFF) a set of discriminating image parameters ~ 1o ~ 1o ~ 1o global parameter HADRONNESS: real number [0,1] 0 gamma-like event 1 hadron-like event
OBSERVATION AND ANALYSIS GAMMA-HADRON separation global parameter HADRONNESS: real number [0,1] 0 gamma-like event 1 hadron-like event ~ 1o ~ 1o ~ 1o Rejection hadron-like events gamma (Hadronnes Cut) hadrons
OBSERVATION AND ANALYSIS Gamma events are selected from an Alpha distribution α-plot CRAB NEBULA DATA SAMPLE ~ 1o ~ 1o ~ 1o
OBSERVATION AND ANALYSIS Gamma events are selected from an Alpha distribution α-plot CRAB NEBULA DATA SAMPLE Hadronness cut: Excess events (gamma ~ 1o ~ 1o ~ 1o events characterised by small alpha values) Background events (hadronic events have a random Alpha distribution)
OBSERVATION AND ANALYSIS Gamma events are selected from an Alpha distribution α-plot CRAB NEBULA DATA SAMPLE A source is detected if the signal significance is greater than 5σ ~ 1o ~ 1o ~ 1o
OBSERVATION AND ANALYSIS Gamma events are selected from an Alpha distribution α-plot CRAB NEBULA DATA SAMPLE A source is detected if the signal significance is greater than 5σ ~ 1o ~ 1o ~ 1o
THE MAGIC TELESCOPE IACT located in the Canary Island La Palma (2225m a.s.l.) field of view: 3.5 angular resolution: 0.1 energy range: 50 GeV 10 TeV ~ 1o ~ 1o ~ 1oenergy resolution: 20 30% flux sensitivity: 1.6% Crab Nebula flux (5σ in 50 hours) fast repositioning (< 40 sec) for GRB observations parabolic reflecting surface of 17 m diameter (250 m2) camera composed by 577 photo-multipliers multilevel trigger 2 GHz MuX FADC LARGEST SINGLE-DISH LOWEST ENERGY THRESHOLD
TARGETS ~ 1o ~ 1o ~ 1o
GRACIAS ~ 1o ~ 1o ~ 1o