Transcript 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.

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 (g):
CURRENTLY THE BY FAR BEST ‘MESSENGERS’ ABOUT
(ULTR)RELATIVISTIC PROCESSES IN THE UNIVERSE
THE OTHER IMPORTANT MESSENGER, THE n JUST AT THE DOOR
EXPERIMENTAL FACT: VHE/UHE g FLUXES VERY LOW
SATELLITE BORNE DETECTORS NOT ENOUGH DETECTION AREA
INSTRUMENTS WITH LARGE DETECTION AREA : GROUND-BASED
-->gs HAVE TO PASS EARTH ATMOSPHERE - > AIR SHOWERS
--->ALL TEV (FEW GeV-100TeV) g OBSERVATIONS INDIRECT
VIA SECONDARY PARTICLES
TIME INFO: OK
DIRECTION
FROM SECONDARY PARTICLES
ENERGY
IN THE 60th-90th: THE MAIN ‘WORKHORSE FOR
g 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, a,.. heavy ions
COMPILATION SIMON SWORDY
FRACTION OF gs UNKNOWN
< 10-4 from Galactic Plane
< 10-5 isotropic
Local g emission spots(stars) can
reach g fluxes of a few % of CR BG
For typ. angular resolution of 0.1°
BASICALLY NOTHING IS
KNOWN ABOUT THE
COSMIC n FLUX
Charged CR are ‘bad
messengers’ gs are ‘good
messengers’ but
-> g/hadron SEPARATION A
BIG EXPERIMENTAL
CHALLENGE
g LIMIT
Flux limits on cosmic
n, WIMP completely unknown
=========================
eV
2006
Mkn180
ALL SOURCES HAVE SPECTRA EXTENDING ABOVE 1 TEV
RARELY SPECTRA EXTEND ABOVE 10 TEV (CRAB->80 GEV
MANY AGNS HAVE A SOFT SPECTRUM
PG1553
NOT ALL SOURCES IN INNER GALACTIC PLAN
THE PHYSICS GOALS IN GROUND-BASED g ASTRONOMY (ABOVE A FEW GeV)
AGNs
Cosmological
ray horizon
g
Pulsars
GRBs
Tests
SNRs
Cold
Dark
Matter
on
Quantum
Gravity
effects
ARTIST VIEWOF A
PROTON INDUCED
AIR SHOWER +
OBSERVABLES
AIR MASS 1:
27 rad.length
11 hadronic abs. length
Zur Anzeige w ird der QuickTime™
Dekompressor “Foto - JPEG”
benötigt.
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 Theta) - (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 LIGHTTIGHT BOX
MESURES TIME: -> for direction MEASURES # PARTICLES -> for energy estimate
t ≈ 1-5 nsec. 10000 photons/MeV energy loss
Water Cherenkov Detector (AUGER)
• 12 m³ ultrapure water
• duty cycle: 100%
g
e
• angular resolution ≤ 1.1°
• energy resolution ≈
order (10%)
PMT signals:
• shape and
• time information
• 25 ns intervals
⇒ distinction between muonic and electromagnetic component
m
GENERAL ADVANTAGES AND DISADVANTAGES
OF ‘TAIL CATCHER ‘ CALORIMETERS
(NOTE: MOST IMPORTANT PHYSICS BELOW 1TEV TO AT MOST 100 TEV
i.e. CLOSE ABOVE THRESHOLD)
IACTS
GROUND-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 gs 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 g/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 g ASTRONOMY
• KASKADE
•
KASCADE GRANDE
• TUNKA
•
ICE-TOP
•
(ANI)
TUNKA 125
ARG
NOTE: OVER THE TIME THE DENSITY
(ACTIVE AREA FRACTION) OF ARRAY
WAS INCREASED TO LOWER THE
THREHOLD
QuickTime™ and a
TIFF (LZW) decompressor
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CRAB SPECTRUM (SED) COMPARISON OF TIBET AS DATA WITH OTHER EXPERIMENTS
C. D.HORNS
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
•(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 m2 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 PICKUP ELECTRODES ALLOW 2-DIM READOUT
FEW KHZ DEVICE
ARGO-YBJ layout
74 m
99 m
Detector layout
1 CLUSTER = 12 RPC
78 m
(43 m2)
10 Pads
(56 x 62 cm2)
for each RPC
8 Strips
(6.5 x 62 cm2)
for each Pad
111 m
Layer (92% active surface) of
Resistive Plate Chambers (RPC),
covering a large area (5600 m2)
+ sampling guard ring
+ 0.5 cm lead converter
BIG
PAD
ADC
RPC
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 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.)
▼
• 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°)
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-m 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
6s
DECL.
RIGHT ASC.
HOTSPOT AT
RA 79.6, DEC 25.8
CLOSE TO
EGRET 3EGJ0320+2556
4.5 s
CURRENT SITUATION:
• THE CURRENT TEV ARRAY CAN BARELY SEE THE STRONGEST
g SOURCES (5 s in 1 year), ->NOT MORE COMPETITIVE COMPARED
TO IACTS ON MOST g PHYSICS
•THEIR MAIN PHYSICS GOALS OUTSIDE TEV g ASTRONOMY
(CHEMICAL COMPOSITION OF CRs, TOTAL SPECTRUM OF CRs..)
•IS THERE SOME SERIOUS IMPROVEMENT POSSIBLE?
•IS THERE SOME SERIOUS PHYSICS NEED FOR TEV g ARRAYS?
WHERE AND HOW TO IMPROVE PERFORMANCE:
•LOWERING OF THE THRESHOLD (PHYSICS DRIVEN) ->
GO TO HIGH ALTITUDE
MAKE ALSO USE OF THE MORE ABUNDANT gs IN SHOWER TAIL
MAKE THE DETECTOR FULLY ACTIVE
•INCREASE IN SENSITIVITY -> VERY LARGE AREA
FINE, HIGH SENSITIVTY GRANULARITY
• IMPROVE ON g/h SEPARATION
DETECT MUON
ANALYSE HIT PATTERN OF TAIL PARTICLES
NEVERTHELESS g/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 g EMITTERS:
A) RARE FLARING AGNS (DURING DAYTIME)
B) SHORT GRBS (GRBS DURING DAYTIME)
C) UNKNOWN VARIABLE g 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 g 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 g RAYS
ABSORPTION OF ENERGETIC gs BY THE EBL
* A LOW THRESHOLD (<< 1 TEV) MANDATORY
* GOOD ENERGY RESOLUTION NEEDED << 1TEV
Absorption of extragalactic g - rays
Any g that crosses cosmological distances through the universe interacts with the EBL
g HE g EBL  e e

-
E1- cos   > 2me c

2 2
Attenuated flux function of g-energy and redshift z.
 (10 GeVFor the energy range of IACTs
10 TeV), the interaction takes place with
the infrared (0.01 eV-3 eV, 100 mm-1
mm).
Star formation, Radiation of stars,
EBL
Absorption and reemission by ISM
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
t (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
GRB Positions in Galactic Coordinates, BATSE
DURATION OF GRBs
GRB observation with MAGIC: GRB050713a
ApJ Letters 641, L9 (2006)
No VHE gs from GRBs seen
yet ...
(all observed GRB very
short or very high z)
MAGIC starts data-taking
GRB-alarm from SWIFT
PROPOSED BY PART OF THE MILAGRO GROUP HAWC: HIGH ALTITUDE WATER CHERENKOV DETECTOR
AN IMPROVED VERSION OF MILAGRO
HAWC Design
e
m
g
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




Large fov (~2 sr) / High duty cycle (~100%)




GRBs visible to redshift ~1
Near known GRB energy
AGN to redshift ~0.3
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 5s/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 g/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) g ASTRONOMY:
ARRAY DETECTORS: NO g SOURCES DISCOVERED
•
MAIN PROBLEMS: HIGH THRESHOLD, POOR g/h SEPARATION,
POOR RESOLUTION (Energy, Direction)
•
ABOUT 20 YEARS AGO: IMAGING AIR CHERENKOV TELESCOPES STARTED TO
DOMINATE g-ASTRONOMY. LOWER THRESHOLD, VERY HIGH g/h SEPARATION,
BETTER ANGULAR ACC. AND ENERGY DET., <10% DUTY CYCLE
UP TO NOW >30 VHE g SOURCES FOUND
•
ARRAY DETECTORS STEADILY IMPROVING BUT MOSTLY FOR OTHER PHYSICS
MAIN GOALS NOT MORE IN g-ASTRONOMY
•
THE UPCOMING DETECTORS FOR n ASTRONOMY REQUIRE PARALLEL OBS.
OF g 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 gs FROM SHORT (1 SEC) GRBs CAN ONLY BE DONE BY
SUCH TYPE OF DETECTOR