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Gamma-Ray (+CR) Detection Principle of IACT(Imaging Atmospheric Cherenkov Telescope) Current IACT Systems and CTA Simul

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Gamma-Ray (+CR) Detection Principle of IACT(Imaging Atmospheric Cherenkov Telescope) Current IACT Systems and CTA Simul Outline Michiko Ohishi ( ICRR ) ⚫Gamma-ray (+CR) detection principle of IACT(Imaging Atmospheric Cherenkov Telescope) ⚫Current IACT systems and CTA ⚫Simulation studies related to CTA, which I involved - Definition of the “gamma-ray sensitivity” (in CTA) ➢ Effect of uncertainty of hadronic interaction models on the estimated CTA sensitivity (proton) ➢Cosmic-ray heavy nuclei composition (Fe, Si….) 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 1 ⚫ Detect Cherenkov photons (in visible light wavelength) emitted by charged particles in the air showers by a large telescope ⚫ Lower energy threshold than air shower array (if the 500GeV γ observation altitude is same) ⚫ If the primary is gamma (or electron), Cherenkov photons make a symmetric pattern called “light pool” But we can’t observe under • The Sun ☀ • The (bright) moon ☽ • Clouds ☁ →typical duty cycle of Light pool, radius of 140m* ~10% (current systems) 2 *depends on the observation altitude 2 Arrival direction reconstruction in the focal plane ⚫ We require angular resolution of <0.1 degree (full angle) for optics and focal plane instrument (camera) ⚫ We can determine • Arrival direction Aharonian 2008 • Core location • energy • Gamma-ray likeness *In the current analysis By the image information in simple weighted mean of the camera the intersection point is not used. We partlyuse machine learning regression analysis. We can determine × To achieve <0.1 deg arrival direction and resolution for we need a fine core location optics → FOV is small Light pool, radius of ~140m* (typically ~10-3 str) 3 *depends on the observation altitude ⚫ Core location reconstruction gamma → Impact parameter of each Edge of the light pool telescope is known ⚫ Look-up-table (LUT) is prepared from MC gamma-ray events; we can extract “expected” p.e. counts from this LUT for a reconstructed 空気シャワー impact parameter and Size (sum of p.e.s of the image) Aharonian 2008 ⚫ Take an average over telescopes → energy for the event is determined E *In energy determination 4 E2 × process we partly use machine learning E (regression) 3 E1 Light pool 4 • CR nuclei (background) are also easily detected and much more in number than gamma-rays (signal) →High background reduction ability is essential for the usage of gamma-ray detector • Indirect detection on the ground→we don’t know charge of the primary • Only shower image information is used to separate g from hadrons E=1 TeV gamma E=3 TeV proton y (degree) y (degree) 第三回 空気シャワー観測による宇宙線の起源探索勉強会,x upload (degree)版 x (degree) 5 Proton 100 GeV Gamma 100 GeV CORSIKA simulation Characteristics of the Tracks of Secondary particles shower is different from (J.Knapp) EM shower (mostly in transverse direction) →will lead to the difference in Cherenkov Distribution of Cherenkov photon pattern photons at the observation level muons 100GeV gamma 300GeV proton 6 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 • Extracting shower characteritics＝Hillas Parameters are well known • Most powerful parameter: WIDTH (transverse size of the shower) • Recently we use many other new parameters in addition to Hillas and put them in to machine learning MultiVariate Analysis (MVA, Boosted Decision Tree, RandomForest etc.) → Introduce a single index as “gamma-likeness (or hadroness) n 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 7 Parameters used in gamma-hadron separation MVA currently in CTA (not all) MRSCW Red: Proton Blue: gamma Height of shower maximum Maier, 2017 • Separation efficiency depends on energy（Upper figure corresponds to ~10 TeV） • Harder to distinguish in low energy “Beam test” is not available for us, air shower experiments → We are paying much effort to tune parameters in the simulation so that it is close to the reality 8 • We cannot remove background protons perfectly • So we estimate background level from “OFF-source data”, using regions where no known gamma-ray source exist • Subtract this background level How do you subtract background for OFF-source subtraction • Isotropic gamma-ray emission…? • CR electron…? We can’t subtract BG. So we have trust background MC simulation for that Maier, case. ISVHECRI2018 9 Next generation project Current systems (construction) CTA MAGIC (17mx2 , Spain) VERITAS (12mx４, US) H.E.S.S. (12m×４, 28mx1 Nambia) 完成イメージ ⚫ 99 telescopes, 3 types CANGAROO(10m×4台,Australia, terminated) ⚫ Cover wider energy range 20 GeV -300 TeV >1,400 members “10m-class reflector,stereo” generation Gigantic collaboration… Covers roughly 100 GeV – 20 TeV 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 10 North site South site LST 4/ MST 15 no SST Paranal, Chile 0.5 km 3 km Canary island, Spain LST 4/ MST 25 /SST 70 ⚫ We also defines a smaller array as “Implementation threshold” ⚫ North site→ Extragalactic sources are main targets → focusing on low energy threshold → no small-sized telescopes ⚫ South site → Galactic sources are main targets → >10 TeV high energy region is also important → large array with SSTs 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 11 ~ km-scale large array → increasing effective area of the gamma-ray and improving identification of the particle type ⚫ Current systems - Light-pool size > array size - Large zenith angle observation increase the effective area ⚫ CTA - Light-pool size << array size - Large zenith angle observation is not so effective 12 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 Large-Sized Telescope Medium-Sized Medium-Sized Telescope Telescope (SCT) 23m diameter FOV 4.3deg 11.5m diamter FOV 7.5/7.7deg 9.7m diameter FOV 7.6deg Small-Sized Telescope Diameter 4.0 m /4.3 m /4.0 m FOV 8.3 deg / 10.5 deg / 8.8 deg Detailed specification： https://www.cta- observatory.org/project/technology/ 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 13 Air shower description Detector response Cherenkov photon （optics・photon generation detector・electronics） (CORSIKA) ⚫Air shower description + Cherenkov photon generation → CORSIKA (H.E.S.S., MAGIC, VERITAS,+CTA) interaction models used (currently in CTA) in CORISKA6.990 Electromagnetic : EGS4 Hadronic : QGSJET-II-03 (high energy model) Switches at UrQMD（low energy model） 80 GeV/nucleon ⚫Detector response → original codes called sim_telarray (inherited from the one use in H.E.S.S.) ⚫ MC data mass production for sensitivity curve is (basically) done on EU-GRID ⚫ Computing resources in 20 institutes over 7 countries (as of 2018) ⚫ ~ 2 PB MC data were produced in the last 1.5 year ⚫ Most computing resource is consumed in Cherenkov photon generation 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 14 • https://www.cta-observatory.org/science/cta-performance we provide data files in FITS,ROOT, TXT too. 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 15 Acceptance in the analysis ⚫ Signal event statistics (Ng>10） 1 1 −1 푁훾 = 퐹훾퐴0훾휖훾푡 > 10 퐹훾 = 푁훾푡 퐴0훾 휖훾 User definition Simulation ⚫ Significance of signal to backgroundparameter(5σ) • This is approximated formula 1 푁훾 1 퐹훾퐴0훾휖훾푡 • Li&Ma (1983) Eq.(17) is the 푁휎 = = > 5 1+훼 푁퐵 1+훼 퐹퐵퐴0퐵휖퐵Ωt standard Literature value L 퐴0퐵Ω 휖퐵 ✓ Dependence−1/2 on the 퐹훾 = 퐹퐵 1 + 훼푁휎푡 퐴0훾 휖훾 observation time is different ✓ Strongly depends on the ⚫ Signal ratio to background(>5%) efficiency in the analysis which reject backgrounds 푁 퐹 퐴 휖 퐴 Ω 휖 푅 = 훾 = 훾 0훾 훾 >0.05 0퐵 퐵 훾퐵 푁 퐹 퐴 휖 Ω 퐹훾 = 퐹퐵 푅훾퐵 퐵 퐵 0퐵 퐵 퐴0훾 휖훾 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 16 ⚫ E>10 TeV CTA-South array, 50h observation sensitivity Signal event statistics dominates → We need to t0 enlarge effective area(or exposure) to increase sensitivity t-1 Significance of the signal events Signal event ⚫ E< 10 TeV to backgrounds dominates Signal to noise ratio t-1/2 statistics (S/B or S/√B) condition dominates → High resolution camera and relatively dense array helps to improve sensitivity 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 10TeV 17 If you have real telescopes…. “Image for illustration purposes only” We have real telescopes and real background data. Why do we need to rely on hadron simulations? ….we need proton IACT people Real data is enough! simulations until we basically don’t will get new simulate protons telescopes…. (except for special studies) We only simulate protons and electrons for 第三回 空気シャワー観測による宇宙線の起源探索勉強会, upload版 background 18 ⚫As for proton background, there are several interaction models ⚫ We apply a tight cut to select gamma-ray(-like) events in the sensitivity curve derivation. ⚫“Gamma-ray likeness” almost means “EM-shower likeness”. We can’t distinguish electrons from gamma-rays. ⚫In the sensitivity derivation, difference of “gamma-ray likeness” works in 2-stages: Difference in reconstructed energy Difference in g-hadron separation efficiency Detected proton rate BG proton Signal gamma • Models which • EM like showers generate more consume more noisy EM-like showers energy in e- cut show bad (major emitter separation from of Cherenkov) gamma-rays • More Cherenkov • After optimized Separation parameter yield→ cut, residual proton number higher recE noisy increases recE 19 • Very good summary at ISVHECIR https://indico.cern.ch/event/639198/contributions/2965268/attachments/1655072/2 649093/DESY-20180525-ISVHECRI.pdf We are VHE people, uncertainty in hadronic interaction is a matter of UHE. Maybe we don’t need to take it too seriously…… CTA full array image analysis, E>1 TeV focused MC data proton QGSJET-II-04 EPOS-LHC ..but • CTA has a better separation ability of g-hadron than QGSJET-II-04 g SIBYLL2.1 current systems • Difference in models may be seen more clearly QGSJET-II-03 # of events above a • Actually there seems to be certain BDT threshold factor2 difference in # of (QGSJETII04≡1) gamma-ray like proton QGSJET-II-04 events between recent g models… 20 Schematic diagram of a proton
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