Gamma-Ray Astronomy With Ground Based Arrays: Results and Future Perspectives Eckart Lorenz (MPI-Munich) OVERVIEW INTRODUCTION THE GENERAL CONCEPT CURRENT EXPERIMENTS AND RESULTS COMPARISON WITH OTHER DETECTION METHODS IMPROVEMENTS OF CURRENT DETECTORS AND POSSIBLE NEXT GENERATION DETECTORS CONCLUSIONS
HIGH ENERGY GAMMA-RAYS (!): CURRENTLY THE BY FAR BEST MESSENGERS ABOUT (ULTR)RELATIVISTIC PROCESSES IN THE UNIVERSE THE OTHER IMPORTANT MESSENGER, THE " JUST AT THE DOOR EXPERIMENTAL FACT: VHE/UHE! FLUXES VERY LOW SATELLITE BORNE DETECTORS NOT ENOUGH DETECTION AREA INSTRUMENTS WITH LARGE DETECTION AREA : GROUND-BASED ##>!s HAVE TO PASS EARTH ATMOSPHERE - > AIR SHOWERS --->ALL TEV (FEW GeV-100TeV)! OBSERVATIONS INDIRECT VIA SECONDARY PARTICLES TIME INFO: OK DIRECTION FROM SECONDARY PARTICLES ENERGY
IN THE 60th-90th: THE MAIN WORKHORSE FOR! ASTRONOMY: GROUND-BASED ARRAY DETECTORS TO DETECT SHOWER TAIL PARTICLES REACHING GROUND IN MODERN HEP DETECTOR LANGUAGE: TAIL CATCHER CALORIMETERS (ATMOSPHERE THE ABSORBER, DETECTOR AT GROUND THE DEVICE TO MESURE A (POOR) CALORIMETRIC SIGNAL --> SIGNAL ABOUT DIRECTION AND ENERGY FROM THE SHOWER TAIL PARTICLES)
THE COSMIC RAY SPECTRUM Mostly protons, $,.. heavy ions COMPILATION SIMON SWORDY FRACTION OF!s UNKNOWN < 10-4 from Galactic Plane < 10-5 isotropic Local! emission spots(stars) can reach! fluxes of a few % of CR BG For typ. angular resolution of 0.1 BASICALLY NOTHING IS KNOWN ABOUT THE COSMIC " FLUX Charged CR are bad messengers!s are good messengers but ->!/hadron SEPARATION A BIG EXPERIMENTAL CHALLENGE! LIMIT Flux limits on cosmic ", WIMP completely unknown ========================= ev
2006 Mkn180 PG1553 ALL SOURCES HAVE SPECTRA EXTENDING ABOVE 1 TEV RARELY SPECTRA EXTEND ABOVE 10 TEV (CRAB->80 GEV MANY AGNS HAVE A SOFT SPECTRUM NOT ALL SOURCES IN INNER GALACTIC PLAN
THE PHYSICS GOALS IN GROUND-BASED! ASTRONOMY (ABOVE A FEW GeV)!AGNs!Cosmological! ray horizon!pulsars!snrs!cold Dark Matter!GRBs!Tests on Quantum Gravity effects
ARTIST VIEWOF A PROTON INDUCED AIR SHOWER + OBSERVABLES AIR MASS 1: 27 rad.length 11 hadronic abs. length
THE MAIN PROBLEM WITH TAIL CATCHER CALORIMETERS : THE HIGH THRESHOLD DETECTOR AT 5000 M ASL 0 45 ZENITH ANGLE MIN 50-100 e AT DETECTOR ACTIVE LEVEL FOR BARE DETECTION, FULLY ACTIVE SURFACE 10** 3 e FOR GOOD SHOWER PARAMETERE DETERMINATION, FULLY ACTIVE SURFACE!Threshold scales with (cos %heta) - (6-7),!Converting gammas in shower tail (5-7 times more than e) helps if electrons are not lost in converter
CARTOON SHOWER FRONT (FLASH PHOTO BEFORE HITTING GROUND) DETECTOR CONCEPTS MAY BE IN FUTURE: DETECTION BY RADIO SIGNALS?? (24 h, ALL SKY??)
THE CLASSICAL WORKHORSE FOR LARGE GROUND BASED ARRAYS PLASTIC (LIQUID) SCINTILLATOR VIEWED BY A PHOTOMULTIPLIER(S) IN A LIGHT- TIGHT BOX MESURES TIME: -> for direction MEASURES # PARTICLES -> for energy estimate &! 1-5 nsec. 10000 photons/mev energy loss
Water Cherenkov Detector (AUGER) 12 m_ ultrapure water duty cycle: 100%! e µ angular resolution! 1.1 energy resolution " order (10%) PMT signals: shape and time information 25 ns intervals distinction between muonic and electromagnetic component
GENERAL ADVANTAGES AND DISADVANTAGES OF TAIL CATCHER CALORIMETERS (NOTE: MOST IMPORTANT PHYSICS BELOW 1TEV TO AT MOST 100 TEV i.e. CLOSE ABOVE THRESHOLD) GROUND-BASED TAIL CATCHER ARRAYS HAVE 24 H UP-TIME, ALL YEAR ALL SKY DETECTION (up to 2-3 sterad possible) IACTS 10% duty cycle 0.01 sterad ROBUST NEARLY NEVER MOVING MECHANICAL PARTS HIGH THRESHOLD, VERY STRONG ZENITH ANGLE DEPENDENCE! (cos theta) -(6 to 7)! (cos theta )-2.7 VERY DIFFICULT TO DETECT!s BELOW 1 TEV <100 GeV VERY MODEST ENERGY RESOLUTION CLOSE TO THRESHOLD 10-20% MODEST ANGULAR RESOLUTION. 0.1 PROBLEMS TO FIX ANGULAR REFERENCE POSITION (SHADOW OF THE MOON) ALSO A MAIN WEAKNESS: BASICALLY NO!/HADRON SEPARATION 90-99% DETCTION AREA SHRINKS WITH LARGE ZENITH ANGLE INCREASE w. theta
CURRENT ARRAY DETECTORS NEW PROJECTS TIBET AS ARGO AT YANJABING MILAGRO MINI HAWC/HAWC HE-ASTRO CTA-ULTRA II DETECTORS WITH MAIN GOAL NOT FOR! ASTRONOMY KASKADE KASCADE GRANDE TUNKA TUNKA 125 ICE-TOP (ANI)
ARG
NOTE: OVER THE TIME THE DENSITY (ACTIVE AREA FRACTION) OF ARRAY WAS INCREASED TO LOWER THE THREHOLD
CRAB SPECTRUM (SED) COMPARISON OF TIBET AS DATA WITH OTHER EXPERIMENTS C. D.HORNS
(Tibet), 4300 m a.s.l. High Altitude Cosmic Ray Laboratory @ YangBaJing (Site Coordinates: longitude 90 31 50 E, latitude 30 06 38 N)
1. ARGO-YBJ [Girolamo[ GEIGERTUBE (PARENT OF THE RPC (Resistive plate chamber) 4300m ASL 6,000 m 2 RPC detector Scalers sensitive ~GeV energies. 95% active area coverage Good for GRB detection Threshold below 100 GeV Near Tibet AS IN AN RPC ONE USES HIGH RESISTIVE OUTER WALLS, THAT LIMIT DISCHARGE AND CONFINE IT LOCALLY, OUTER PICK- UP ELECTRODES ALLOW 2-DIM READOUT FEW KHZ DEVICE
ARGO-YBJ layout Detector layout 99 m 74 m 78 m 1 CLUSTER = 12 RPC ('43 m 2 ) 10 Pads (56 x 62 cm 2 ) for each RPC 8 Strips (6.5 x 62 cm 2 ) for each Pad 111 m Layer ('92% active surface) of Resistive Plate Chambers (RPC), covering a large area (5600 m 2 ) + sampling guard ring + 0.5 cm lead converter BIG PAD RPC ADC Read-out of the charge induced on Big Pads
Main detector features and performances " Active element: Resistive Plate Chamber ( time resolution '1 ns " Time information from Pad (56 x 62 cm 2 ) " Space information from Strip (6.5 x 62 cm 2 ) " Full coverage and large area (' 10,000 m 2 ) " High altitude (4300 m a.s.l.) _ good pointing accuracy ("0.5 ) detailed space-time image of the shower front capability of small shower detection (( low E threshold) large aperture ()2#) and high duty-cycle ()100%) ( continuous monitoring of the sky (-10 <* <70 )
Sky survey with the ARGO-YBJ detector. S. Vernetto et al. for the ARGO-YBJ Collaboration First Results with 42 clusters. 0.6 billion events in 1000 hours live time Predicted sensitivity, full detector Crab Mkn 421 Mkn 501 No source seen with partially completed detector (2005)
CONCEPT OF A WATER TAIL CATCHER ARRAY WITH e-µ DISCRIMINATION 100% ACTIVE AREA
TAIL CATCHER WATER CHERNKOV DETECTOR ARRAY!100% ACTIVE COVERAGE AT SHOWER END HIGH CONVERSION PROB. FOR GAMMAS IN SHOWER TAIL
SCAN OF THE NORTHERN TEV SKY BY MILAGRO 6 + DECL. RIGHT ASC. HOTSPOT AT RA 79.6, DEC 25.8 CLOSE TO EGRET 3EGJ0320+2556 4.5 +
CURRENT SITUATION: THE CURRENT TEV ARRAY CAN BARELY SEE THE STRONGEST! SOURCES (5 + in 1 year), ->NOT MORE COMPETITIVE COMPARED TO IACTS ON MOST! PHYSICS THEIR MAIN PHYSICS GOALS OUTSIDE TEV! ASTRONOMY (CHEMICAL COMPOSITION OF CRs, TOTAL SPECTRUM OF CRs..) IS THERE SOME SERIOUS IMPROVEMENT POSSIBLE? IS THERE SOME SERIOUS PHYSICS NEED FOR TEV! ARRAYS?
WHERE AND HOW TO IMPROVE PERFORMANCE: LOWERING OF THE THRESHOLD (PHYSICS DRIVEN) -> GO TO HIGH ALTITUDE MAKE ALSO USE OF THE MORE ABUNDANT!s IN SHOWER TAIL MAKE THE DETECTOR FULLY ACTIVE INCREASE IN SENSITIVITY -> VERY LARGE AREA FINE, HIGH SENSITIVTY GRANULARITY IMPROVE ON!/h SEPARATION DETECT MUON ANALYSE HIT PATTERN OF TAIL PARTICLES NEVERTHELESS!/h SEPARATION OF IACTS OUT OF REACH KEY OTHER ISSUES EXTREME HIGH TRIGGER RATE-> HUGE READOUT SYSTEM REDUCTION IN COST NEEDED IMPROVE ANGULAR RESOLUTION CLOSE ABOVE THRESHOLD IMPROVE ENERGY RESOLUTION (TRICKY BECAUSE OF FLUCTUATIONS) THERE IS NO PRACTICAL METHOD TO REDUCE STRONG THETA DEPENDENCE OF THRESHOLD TAIL CATCHER CALORIMETERS HAVE SOME FUNDAMENTAL DIFFICULTIES THAT CANNOT BE OVERCOME!!
IS THERE A PHYSICS NICHE THAT CANNOT BE COVERED BY EVEN IMPROVED IACTS OR GLAST? (UNPREDICTABLE) FLARING OR VARIABLE VHE/UHE! EMITTERS: A) RARE FLARING AGNS (DURING DAYTIME) B) SHORT GRBS (GRBS DURING DAYTIME) C) UNKNOWN VARIABLE! EMISSION IN OTHER GALAXIES (M87) D) EFFECTS LINKED TO THE SUN(MOON) THE START OF NEUTRINO ASTRONOMY DETECTORS: NEED FOR MAXIMUM SOURCE MONITORING (24 h,all-sky) OF VARIABLE! SOURCES TO EXTRACT PHYSICS FROM THESE SOURCES (IACTS COULD DO THIS IN PART (example source flares during daytime), SINGLE SOURCE OBSERVATION OBSERVATION TIME AT LEADING IACTS VERY PRECIOUS NEED FOR DEDICATED IACTS.) IACT COMMUNITY IS VERY ACTIVE TO IMPROVE DETECTORS
A SEVERE PROBLEM WHEN OBSERVING DISTANT OBJECTS(AN,GRB) IN! RAYS ABSORPTION OF ENERGETIC!s BY THE EBL * A LOW THRESHOLD (<< 1 TEV) MANDATORY * GOOD ENERGY RESOLUTION NEEDED << 1TEV
Absorption of extragalactic! - rays Any! that crosses cosmological distances through the universe interacts with the EBL +! # HE # EBL " e e E" 1# cos$ ( ) > 2 m e c 2 Attenuated flux function of!-energy and redshift z. ( ) 2 For the energy range of IACTs (10 GeV- 10 TeV), the interaction takes place with the infrared (0.01 ev-3 ev, 100 µm-1 µm). Star formation, Radiation of stars, Absorption and reemission by ISM EBL By measuring the cutoffs in the spectra of AGNs, any suitable type of detector can help in determining the IR background-> needs good energy resolution Acc. by new detectors
GAMMA-RAY HORIZON FAZIO-STECKER RELATION & (E,z) =1
Extragalactic: Markarian501 (AGN) (MAGIC preliminary) In flare July 9/10: Evidence for fast variability (< 10 min), doubling time O(5min)... (preliminary)
THE CHALLENGE TO OBSERVE GRBs More energetic GRBs Only to be seen by all sky monitor detectors Acc. by IACTs, only During clear nights DURATION OF GRBs GRB Positions in Galactic Coordinates, BATSE
GRB observation with MAGIC: GRB050713a ApJ Letters 641, L9 (2006) No VHE!s from GRBs seen yet... MAGIC starts data-taking GRB-alarm from SWIFT (all observed GRB very short or very high z)
PROPOSED BY PART OF THE MILAGRO GROUP HAWC: HIGH ALTITUDE WATER CHERENKOV DETECTOR AN IMPROVED VERSION OF MILAGRO
HAWC Design e µ! 7 meters 200 meters! 200m x 200m water Cherenkov detector! Two layers of 8 PMTs on a 3 meter grid " Top layer under 1.5m water (trigger & angle) " Bottom layer under 6m water (energy & particle ID)! Two altitudes investigated " 4500 m (Tibet, China) " 5200 m (Altacama desert Chile)
HAWC! A large area, high altitude all sky VHE detector will: " Detect the Crab in a single transit " Detect AGN to z = 0.3 " Observe 15 minute flaring from AGN " Detect GRB emission at ~50 GeV / redshift ~1 " Detect 6-10 GRBs/year (EGRET 6 in 9 years) " Monitor GLAST sources " Have excellent discovery potential! Continuing work " Improve background rejection & event reconstruction # Increase sensitivity by ~50% - 100%? # Develop energy estimator " Detailed detector design (electronics, DAQ, infrastructure) " Reliable cost estimate needed (~$30M???) " Site selection (Chile, Tibet, White Mountain)! Time Line " 2004 R&D proposal to NSF " 2006 full proposal to NSF " 2007-2010 construction
HAWC Performance Requirements! Energy Threshold ~20 GeV # GRBs visible to redshift ~1 # Near known GRB energy # AGN to redshift ~0.3! Large fov (~2 sr) / High duty cycle (~100%) # GRBs prompt emission # AGN transients! Large Area / Good Background Rejection " High signal rate " Ability to detect Crab Nebula in single transit! Moderate Energy Resolution (~40%) " Measure GRB spectra " Measure AGN flaring spectra GUS SINNIS, ARGONNE NAT. LAB
Effective Area vs. Energy IACT
Point Source Sensitivity! HESS, MAGIC 5+/50 h
A POSSIBLE ALTERNATIVE DETECTOR CONCEPT DO NOT USE TAIL CATCHER PRINCIPLE DETECT CHERENKOV LIGHT FROM SHOWERS STOPPING HIGH IN THE ATMOSPHERE OPTIONS: USE ARRAYS OF LIGHT SENSORS A) ARRAY OF OPEN PMTS LOOKING DIRECTLY INTO THE SKY B) ARRAY OF IACTS EACH POINTING TO A SMALL AREA OF THE SKY (<0.025 sterad/iact) ADVANTAGES CAN COVER LARGE ANGLE->ALL SKY MONITOR LOW THRESHOLD (IACTS), THRESHOLD LESS THETA DEP. BEST ENERGY RESOLUTION GOOD ANGULAR RESOLUTION MUCH BETTER!/h SEPARATION->HIGH SENSITIVITY OPEN PMT ARRAY RELATIVELY CHEAP IACT ARRAYS: CAN FOCUS ON ONE OBJECT DISADVANTAGES LOSS OF 24 H DUTY CYCLE (> 3 ARRAYS AROUND EARTH) LOOSE OFTEN OPPORTUNITY TO MONITOR SKY AREA FOR MORE THAN HALF A YEAR (NORTH/SOUTH ARRAYS) WEATHER DEPENDENT/CLEAR NIGHT SKY(Moon less probl.) SERVICE DEMANDING IACT ARRAYS QUITE EXPENSIVE
A Cherenkov light wave front sampling array with all sky monitoring (1sterad) (IMPROVED VERSION OF AIROBICC,BLANCA, TUNKA ARRAY) CHERENKOV LIGHT DISC FROM AIR SHOWER. TYP 250 mø, VERY SHARP IN TIME, CONICAL ARRAY OF OPEN PMTS LOOKING INTO NIGHT SKY A DETECTOR HUT WITH A PM VIEWING DIRECTLY THE SKY. ENHANCE COLLECTION AREA BY WINSTON CONE BUT LIMITS ANGULAR ACCEPTANCE (LIOUVILLE THEOREM) HUGHE NIGHT SKY LIGHT INDUCED BG
A PROJECT STUDY: HE-ASTRO (astro-ph /0511342)
ULTRA II (ULTRA LARGE TELESCOPE ARRAY) A POSSIBLE PART OF THE EUROPEAN LARGE CHERENKOV OBSERVATORY CTA 100 IACTS DISTRIBUTED OVER 2 km2 AREA OPERATION MODE EITHER HIGH SENSITIVITY WHEN POINTING OR ALL SKY MONITOR IACT PARAMETERS Mirror 18 m2 F/D! 1,2-1.4 Camera FOV: 5-7 Pixels 0.25 Pmts: hemispherical 32%QE at 400 nm 500 Mhz ringsampling FADC Threshold 250-300 GeV Cost/telescope < 200 k" Construction! as HEGRA IAC 70-100 m
CONCLUSIONS UP TO!20 YEARS AGO: ARRAY DETECTORS WERE THE MAIN WORKHORSE FOR UHE,(VHE)! ASTRONOMY: ARRAY DETECTORS: NO! SOURCES DISCOVERED MAIN PROBLEMS: HIGH THRESHOLD, POOR!/h SEPARATION, POOR RESOLUTION (Energy, Direction) ABOUT 20 YEARS AGO: IMAGING AIR CHERENKOV TELESCOPES STARTED TO DOMINATE!-ASTRONOMY. LOWER THRESHOLD, VERY HIGH!/h SEPARATION, BETTER ANGULAR ACC. AND ENERGY DET., <10% DUTY CYCLE UP TO NOW >30 VHE! SOURCES FOUND ARRAY DETECTORS STEADILY IMPROVING BUT MOSTLY FOR OTHER PHYSICS MAIN GOALS NOT MORE IN!-ASTRONOMY THE UPCOMING DETECTORS FOR " ASTRONOMY REQUIRE PARALLEL OBS. OF! SOURCES WITH 24 H UP-TIME AND ALL SKY MONITORING FEATURES IACTS CAN ONLY DO IT PARTIALLY. NEW REQUIREMENT FOR LARGE AREA, LOW THRESH., 24 h UPTIME DETECTORS # 100%ACTIVE AREA, ALL SKY MONITORING.-> HAWC TYPE DETECTORS?? # LARGE ARRAY OF IACTS (HE-ASTRO, ULTRA-II..) * OBSERVATION OF VHE,UHE!s FROM SHORT (1 SEC) GRBs CAN ONLY BE DONE BY SUCH TYPE OF DETECTOR