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 LOWSATELLITE BORNE DETECTORS NOT ENOUGH DETECTION AREA

INSTRUMENTS WITH LARGE DETECTION AREA : GROUND-BASEDs HAVE TO PASS EARTH ATMOSPHERE - > AIR SHOWERS--->ALL TEV (FEW GeV-100TeV) OBSERVATIONS INDIRECT VIA SECONDARY PARTICLES

TIME INFO: OKDIRECTION FROM SECONDARY PARTICLESENERGY

IN THE 60th-90th: THE MAIN ‘WORKHORSE FOR ASTRONOMY: GROUND-BASED ARRAY DETECTORSTO 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 THESHOWER TAIL PARTICLES)

THE COSMIC RAY SPECTRUMMostly protons, ,.. heavy ions

FRACTION OF s UNKNOWN< 10-4 from Galactic Plane< 10-5 isotropicLocal emission spots(stars) canreach fluxes of a few % of CR BGFor 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

=========================

eV

COMPILATION SIMON SWORDY

LIMITLIMIT

Flux limits on cosmic, WIMP completely unknown

Mkn180PG1553

2006

NOT ALL SOURCES IN INNER GALACTIC PLANE SHOWN

KIFUNE PLOT

ALL SOURCES HAVE SPECTRA EXTENDING ABOVE 1 TEVRARELY SPECTRA EXTEND ABOVE 10 TEV (CRAB->80 GEVMANY AGNS HAVE A SOFT SPECTRUM

AGNsAGNs

SNRsSNRs Cold Dark Cold Dark MatterMatter

PulsarsPulsarsGRBsGRBs

Tests on Tests on Quantum Quantum Gravity Gravity effectseffects

Cosmological Cosmological ray horizon ray horizon

THE PHYSICS GOALS IN GROUND-BASED ASTRONOMY (ABOVE A FEW GeV)

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AIR MASS 1:27 rad.length11 hadronic abs. length

ARTIST VIEWOF A PROTON INDUCEDAIR SHOWER +OBSERVABLES

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

THE MAIN PROBLEM WITH TAIL CATCHER CALORIMETERS : THE HIGH THRESHOLD

CARTOON

SHOWER FRONT (FLASHPHOTO BEFORE HITTING GROUND)

DETECTOR CONCEPTS

MAY BE IN FUTURE:DETECTION BY RADIOSIGNALS??(24 h, ALL SKY??)

THE CLASSICAL ‘WORKHORSE’ FOR LARGE GROUND BASED ARRAYS

PLASTIC (LIQUID) SCINTILLATOR VIEWED BY A PHOTOMULTIPLIER(S) IN A LIGHT-TIGHT BOXMESURES TIME: -> for direction MEASURES # PARTICLES -> for energy estimate ≈ 1-5 nsec. 10000 photons/MeV energy loss

• 12 m³ ultrapure water

• duty cycle: 100%

• angular resolution ≤ 1.1°

• energy resolution ≈ order (10%)

PMT signals:

• shape and

• time information

• 25 ns intervals

⇒ distinction between muonic and electromagnetic component

Water Cherenkov Detector(AUGER)

e

GENERAL ADVANTAGES AND DISADVANTAGESOF ‘TAIL CATCHER ‘ CALORIMETERS(NOTE: MOST IMPORTANT PHYSICS BELOW 1TEV TO AT MOST 100 TEVi.e. CLOSE ABOVE THRESHOLD)

IACTSGROUND-BASED TAIL CATCHER ARRAYS HAVE 24 H UP-TIME, ALL YEAR 10% duty cycle

ALL SKY DETECTION (up to 2-3 sterad possible) 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)

ARGO

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NOTE: OVER THE TIME THE DENSITY (ACTIVE AREA FRACTION) OF ARRAYWAS INCREASED TO LOWER THE THREHOLD

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CRAB SPECTRUM (SED) COMPARISON OF TIBET AS DATA WITH OTHER EXPERIMENTSC. D.HORNS

•(Tibet), 4300 m a.s.l.(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[4300m ASL6,000 m2 RPC detectorScalers sensitive ~GeV energies.95% active area coverageGood for GRB detectionThreshold below 100 GeVNear Tibet AS

GEIGERTUBE (PARENT OF THERPC (Resistive plate chamber)

IN AN RPC ONE USES HIGH RESISTIVEOUTER WALLS, THAT LIMIT DISCHARGEAND CONFINE IT LOCALLY, OUTER PICK-UP ELECTRODES ALLOW 2-DIM READOUTFEW KHZ DEVICE

ARGO-YBJ layoutDetector layout

Layer (92% active surface) of Resistive Plate Chambers (RPC),

covering a large area (5600 m2)+ sampling guard ring+ 0.5 cm lead converter

10 Pads (56 x 62 cm2)for each RPC

8 Strips (6.5 x 62 cm2) for each Pad

1 CLUSTER = 12 RPC

78 m111 m

99 m

74 m

BIGPAD

ADC

RPC

Read-out of the charge

induced on“Big Pads”

(43 m2)

Main detector features and performances

▼• good pointing accuracy (≤0.5°)• detailed space-time image of the shower front• capability of small shower detection ( low E threshold)• large aperture (2π) and high “duty-cycle” (100%)

continuous monitoring of the sky (-10°< <70°)

Active element: Resistive Plate Chamber time resolution 1 ns Time information from Pad (56 x 62 cm2)

Space information from Strip (6.5 x 62 cm2)

Full coverage and large area ( 10,000 m2) High altitude (4300 m a.s.l.)

First Results with 42 clusters.

0.6 billion events in 1000 hours live time

Mkn 421 Mkn 501Crab

Sky survey with the ARGO-YBJ detectorSky survey with the ARGO-YBJ detector. . S. Vernetto et al. for the ARGO-YBJ CollaborationS. Vernetto et al. for the ARGO-YBJ Collaboration

Predicted sensitivity, full detector

No source seen with partially completed detector (2005)

CONCEPT OF A WATER TAILCATCHER ARRAY WITHe- 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

RIGHT ASC.

DECL.

6

HOTSPOT ATRA 79.6, DEC 25.8CLOSE TOEGRET 3EGJ0320+25564.5

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 BECOVERED 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

Any that crosses cosmological distances through the universe interacts with the EBL

Absorption of extragalactic - rays

−+→ eeEBLHE γγ

€

Eε 1− cosθ( ) > 2 mec2( )

2

Attenuated flux function of -energy and redshift z.

For the energy range of IACTs (10 GeV-10 TeV), the interaction takes place with the infrared (0.01 eV-3 eV, 100 m-1 m). Star formation, Radiation of stars, Absorption and reemission by ISM

Acc. by new detectorsBy measuring the cutoffs in the spectra of AGNs, any suitable type of detector can help in determining the IR background-> needs good energy resolution

EBL

GAMMA-RAY HORIZON

FAZIO-STECKER RELATION (E,z) =1

Extragalactic: Markarian501 (AGN)

In flare July 9/10:Evidence for fast variability (< 10 min), doubling time O(5min) ...

(MAGIC preliminary)(MAGIC preliminary)

(preliminary)(preliminary)

GRB Positions in Galactic Coordinates, BATSE

THE CHALLENGE TO OBSERVE GRBs

Acc. by IACTs, onlyDuring clear nights

More energetic GRBsOnly to be seen by all sky monitor detectors

DURATION OF GRBs

GRB observation with MAGIC: GRB050713a ApJ Letters 641, L9 (2006)ApJ Letters 641, L9 (2006)

GRB-alarm from SWIFTGRB-alarm from SWIFTMAGIC starts data-takingMAGIC starts data-taking

No VHE No VHE s from GRBs seens from GRBs seenyet ...yet ...

(all observed GRB very (all observed GRB very short or very high z)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

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)

7 meters

e

200 meters

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 CONCEPTDO NOT USE TAIL CATCHER PRINCIPLEDETECT CHERENKOV LIGHT FROM SHOWERS STOPPING HIGH IN THE ATMOSPHEREOPTIONS: 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 DETECTOR HUT WITH A PM VIEWINGDIRECTLY THE SKY. ENHANCE COLLECTION AREA BY WINSTONCONE BUT LIMITS ANGULAR ACCEPTANCE(LIOUVILLE THEOREM)HUGHE NIGHT SKY LIGHT INDUCED BG

CHERENKOV LIGHT DISC FROM AIRSHOWER. TYP 250 mØ, VERY SHARPIN TIME , CONICAL

ARRAY OF OPEN PMTS LOOKING INTO NIGHT SKY

A Cherenkov light wave front sampling array with all sky monitoring (1sterad)(IMPROVED VERSION OF AIROBICC,BLANCA, TUNKA ARRAY)

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 AREAOPERATION MODE EITHER HIGH SENSITIVITY WHEN POINTING OR ALL SKY MONITOR

IACT PARAMETERSMirror 18 m2F/D≈ 1,2-1.4Camera FOV: 5-7°Pixels 0.25°Pmts: hemispherical 32%QE at 400 nm500 Mhz ringsampling FADCThreshold 250-300 GeVCost/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