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VHE Gamma Ray Astronomyan overview
John LearnedUniversity of HawaiiP711/A736 12 April 2005
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Origins of the Field
• Early ideas and attempts by Russians, English, particularly J.V.Jelly.
• CfA’s Trevor Weekes in 1970’s, Mt. Hopkins first dedicated (10m) telescope.
• Key idea from Micheal Hillas…. How to distinguish image of Gamma Shower from predominant proton (and nucleon) showers.
• Marginalized, almost no support, and largely considered a failure through 80’s.
• Blossoms in 90’s with first TeV sources (mainly the Crab Nebula), but really taking off now.
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TeV Gammas
• Lower energies (say <GeV) are observed with direct counting, few m2. Must be from balloons at few gm/cm2 or better satellites (or on moon).
• Energies ~MeV can be nuclear. Higher E’s involve particle acceleration, peculiarly common in the universe. Must be there, since CRs known to exist, but what flux and what origin?
• If seeking TeV energies, need areas in the range of >10,000 m2. Use showers.
• Was soon (80’s) known that there were no huge point sources in cosmic rays. Protons stirred by gal mag field, so should not point. So, no huge gamma fluxes, re P’s, and thus need means to sort out gamma showers.
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Aside: our UH role• No GR or any point sources detected.• Weekes at Mt. Hopkins, pursued imaging and
we were involved a bit (Gorham thesis, etc. JGL built calibrator, Stenger did data analysis). Not making much progress. Imaging a flop.
• Mid-80’s, we built GR telescope on Haleakala, with Wis, Purdue.
• Bet on wrong horse: simulations showed handle on GR showers by fast timing. Did not work.
• Finally, Weekes gets enough pixels, fast electronics and good calibrations…. Finds Crab.
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Methods of Detection• Cost: counters ~>$1000/m2, if need ~105 m2, then
~$100M.• Detect showers: at ground their size is about 105 m2 (less
in high mountains).• Use Cherenkov light in atmosphere. Too low an energy
for air fluorescence (EeV). Sort showers by image Mt. Hopkins, VERITAS, Cangaroo, HESS, Magic, and
around 6 others).• Use dense counter array, only water affordable
(MILAGRO, HAWC). • Use RPCs (Tibet).• New technology ACT, ASHRA in Hawaii.
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CGRO/EGRET
• Apr 1991 – Jun 2000• 30 MeV – 30 GeV 67%=5.85(100 MeV/E)0.534
Following slides from Mori, of ICRR and Cangaroo collab
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EGRET Allsky Map
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Diffuse gamma-ray spectrum
• Flatter than expected (E -2.75): why? Flatter proton/electron spectrum??⇒
S. Hunter, Heidelberg WS, 2000
EGRET Nishimura et al.
JACEE
Whipple0
Brems
IC
————————————————
uniform
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Third EGRET catalog
R.C. Hartman et al., ApJS, 1999
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EGRET point source summary
Pulsars 5
AGN (mostly blazars) 6627 (marginal)
Radio galaxy (Cen A) 1 (marginal)
Unidentified(Some may be SNRs)
170
Large Magellanic Cloud 1
Solar flare 1
Total 271
R.C. Hartman et al., ApJS, 1999
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Pulsars
GeV Thompson, Heidelberg WS, 2000
Radio Princeton catalog (706 pulsars), 1995
(GeV candidates: 1046-58, 0656+14, J0218+4232)
MeV only
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Gamma-ray pulsar light curves
GLAST proposal
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BL Lac’s and EGRET AGNs
TeV Whipple, HEGRA, CAT, 7TA, Durham
RED EGRET 3rd catalog AGNs
Green Padovani & Giommi MN 1995
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Gamma-ray blazars
• Mostly FSRQ and BL Lac’s
Lin et al. ApJ 1999Mukherjee et al. ApJ 1997
H(igh freq. peaked) BL X(-ray selected) BL
L(ow-freq. peaked) BL R(adio-selected) BL
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Multiwavelength spectrum of AGNs
• Double-peaked structure= synchrotron + inverse Compton
Kubo et al. ApJ 1998
PKS0528+134 (z=2.1, FSRQ)
Mrk 421 (z=0.03, XBL)
Kataoka, Ph.D 2000
↑
νsync
↑
νIC
=γ2νsync
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EGRET unidentified sources
• Low vs High latitude
• Persistent vs Variable
• Geminga-like pulsars?
• SNRs?• OB
associations?• Gould belt?
I. Grenier, GeV-TeV WS, 1999
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EGRET unIDs and SNRs
GeV Esposito et al. ApJ 461, 1996
TeV CANGAROO
RED EGRET 3rd catalog unID
Green D.A. Green’s catalog
TeV HEGRA
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Extragalactic diffuse gamma-rays
• Single power-law E –2.100.03 (30 MeV-100 GeV)
• Unresolved point sources (ex. Blazars etc.)?Upscattered CMB?
P. Sreekumnar et al., ApJ 1998
E –2.100.03
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Whipple1968
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Imaging Cherenkov technique
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Image parameters
●
D.J. Fegan, J.Phys.G, 1997(Simulation)
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Example of image cut analysis
• Hadron rejection power ~ 100
M. Punch et al., Nature, 1992 T. Yoshikoshi et al., ApJ, 1997
CANGAROO (Vela)
Whipple
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TeV catalog 2000
Classification Object Group Remark
Grade A
(>5σ,multiple)
Crab
PSR1706-44
Mrk 421
Mrk 501
Many
CANGAROO, Durham
Many
Many
Plerion
Plerion
AGN (BL Lac)
AGN (BL Lac)
Grade B
(>5σ)
SN1006
Vela
RXJ1713.7-3946
PKS2155-304
1ES1959+650
BL Lac
CANGAROO
CANGAROO
CANGAROO
Durham
Utah7TA
Crimea
SNR
Plerion
SNR
AGN (BL Lac)
AGN (BL Lac)
AGN (BL Lac)
Grade C
(strong but with somequalifications)
Cas A
Cen X-3
1ES2344+514
3C66A
Geminga
B1509-58
HEGRA CT
Durham
Whipple
Crimea
Crimea
CANGAROO
SNR
X-ray binary
AGN (BL Lac)
AGN (z=0.44)
Pulsar
Plerion
T.C. Weekes, Heidelberg WS, 2000
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TeV sky 2000
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TeV observations of AGNs
Krennrich, astro-ph/0101120
(Detection of 1ES1426+428 (z=0.13) is claimed by Whipple but not published)
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AGN: Mrk 421 variability
• Time scale < a few hours• Correlation with X-ray flux
Takahashi et al. ApJ 542, 2000Gaidos et al., Nature, 383, 1996
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AGN: Mrk 421 spectrum
• Synchrotron+ inverse Comptonmodel workswell
e⇒ origin• Proton model
still possible
Takahashi et al. ApJ 542, 2000
One-zone SSC model
δ=14, B=0.14G
synchrotron
inverse Compton
z=0.031
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AGN: TeV gamma-ray absorption by IR background
Protheroe et al. astro-ph/0005349
IR BackgroundMean free path for e+e- pair production
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AGN: Mrk 501 spectrum
Protheroe et al. astro-ph/0005349
Aharonian et al. A&Ap 349, 1999
CrisisCrisis?↓?↓
z=0.033
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Sensitivity of various detectors
(2000)
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MILAGRO PrincipleHAWC: High Altitude Water Cherenkov
• 200m x 200m water Cherenkov detector• Two layers of 8” PMTs on a 2.7 meter grid
– Top layer under 1.5m water (trigger & angle)– Bottom layer under 6m water (energy & particle ID)– ~11,000 PMTs total (5,000 top and 5000 bottom)– Trigger: >50 PMTs in top layer
• Two altitudes investigated– 4500 m (~Tibet, China)– 5200 m (Atacama desert Chile)
6 meters
e
200 meters
Following Milagro/HAWC slides from Gus Sinnis, LANL
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Event Reconstruction
Angular resolution ~0.75 degrees
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Gam
mas
Pro
tons
Background Rejection Bottom Layer30 GeV 70 GeV 230 GeV
20 GeV 70 GeV 270 GeV
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Background Rejection
Uniformity ParameternTop/cxPE > 4.3
Reject 70% of protons
Accept 87% of gammas
1.6x improvement in sensitivity
Gammas
Protons
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D.C. Sensitivity: Galactic Sources
• Crab Spectrum: dN/dE = 3.2x10-7 E-2.49
– Milagro 0.002 (0.001) Hz raw (cut) rate– HAWC 0.220 (0.19) Hz raw (cut) rate– Whipple 0.025 Hz– Veritas 0.5 (.12) Hz raw (cut) rate
• Background rate 80 (24) Hz raw (cut)• 4 /sqrt(day) raw data• 6 /sqrt(day) cut data
– 120 /sqrt(year)
• 40 mCrab sensitivity (all sky) in one year– Whipple: 140 mCrab per source– VERITAS: 7 mCrab per source (15 sources/year)
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Excess Coincident with EGRET source 3EG J0520+2556
Source Reported twice beforeby Milagro:
1) APS Meeting: April 2002Reported as a Hot Spot. ALarger than optimal bin size was used in that initial survey.
2) Location of one of the topexcesses in our published pointsource All Sky search.
Crab3EG J0520+2556
5.5 detection at (79.8o, 42o) using binsize= 2.9o
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Distribution of Excess in the Cygnus Region:
Gaussian Weighted Excess
l=80
l=85
l=75
b=0b=-5
b=+52 regions of excess give riseto the observed signal.
Cyg OB2 field
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Existing Arrays
Milagro
Dense sampling
Moderate altitude (2650m)
Background rejection
Tibet Array
Sparse sampling
High altitude (4300m)
No background rejection
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EAS Arrays• Provide synoptic view of the sky • See an entire hemisphere every day• Large fov & high duty cycle
– Gamma ray bursts– Transient astrophysics– Extended objects– New sources
• Excellent complement to GLAST– With >1000 sources need an all-sky instrument in VHE
• Current EAS arrays lack sensitivity to complement GLAST• What can be done?
– Need low threshold (GLAST overlap) < 100 GeV– High sensitivity
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EAS Arrays in the GLAST Era
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Effect of EBL on Distant Sources
z = 0.03z = 0.1z = 0.2
z = 0.3
z = 0.0
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Detection principle:Stereoscopic
imagingof Cherenkov light from air-showers
• Large collection area• Multiple views of the shower
– improved direction– improved energy– improved rejection of
background (cosmic rays!)
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Crab Nebula
Preliminary3-Telescope data (2003)
54 , (27 /hr0.5)10.8 +/- 0.2 /minute
@ 45 degree zenith angle
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Official detections by H.E.S.S. so far…
• Crab Nebula (2003, 3 Tel.) - 54 sigma
• PKS 2155 (2003, 2 Tel.) - 45 sigma
• Mrk 421 (2004, 4 Tel.) - 71 sigma
• PSR B1259 (2004, 4 Tel.) - 8 sigma
• RX J1713 (2003, 2 Tel.) - 20 sigma
• Sagittarius A* (2003. 2 Tel.) - 11 sigma
Very confident detections – all but Mrk 421 and PSR B1259 were confirmed independantly in datasets from two hardware configurations
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The Galactic Centre -rays detected by CANGAROO and Whipple but:
• Very complex region - lots of potential sources of -rays– Sagittarius A* - supermassive black hole - curvature
radiation of accelerated protons?– Several SNR, including Sag-A East, 'standard' CR
acceleration?– Dark matter annihilation?
• To resolve the ambiguity we need– precise spectrum– well determined position
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Sagittarius A*• H.E.S.S. 2003
– 2 telescopes, 16 hours
– Ethresh
= 160 / 250
GeV(2 data sets)
– 11 significance
Good source localisation
Hard energy spectrum
-ray candidates (hard cuts)
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Sagittarius A* - Source Location
Chandra GC surveyNASA/UMass/D.Wang et al.
CANGAROO (80%)
Whipple(95%)
H.E.S.S.
Contours from Hooper et al. 2004
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Point-like emission from Sgr A* direction
H.E.S.S.
ChandraF. Banagoff et al.
95%
68%
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Sgr A EastChandra & Radio NASA/G.Garmire (PSU)F.Baganoff (MIT)Yusef-Zadeh (NWU)
Sgr-A East not ruled out
H.E.S.S.limit on rmssource size
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Sagittarius A* - SpectrumDM annihilation: ?Curvature radiation: ?SNR Shocks: ?Shocks in winds: ?
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Galactic Centre – Dark Matter
• Neutralino annihilation?– Use DarkSusy– Expect two lines and continuum
• Power law index - 2.2 - 2.4• Cut off at roughly m / 3
• We see no lines and no cut off – exponential cut off is limited to < 4 TeV
Which implies m > 12 TeV
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RX J1713
• H.E.S.S. smoothed gamma-candidate map after image size cuts (> 800 GeV) - no background subtraction or acceptance correction
– Only two telescopes– 18 hours – 20 sigma
• c.f. ASCA (1-3 keV)
Flux = 70% of Crab
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THE MAGIC TELESCOPEE. LORENZ, for the MAGIC COLLABORATION
COLLABORATION:IFAE BARCELONA,UA BARCELONA,U. BERLIN, UC-DAVIS,U.LODZ, UC MADRID,MPI MUNICHINFN PADOVA, U. POTCHEFSTROOM, INFN SIENA, TUORLA OBSERVATORY, INFN UDINE, U. WUERZBURG,YEREVAN PHYSICS INST. , ETH ZURICH. 3 CANDIDATES, IN TOTAL 120 MEMBERS
17 mtr
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MUON ARC IMAGES
• THE MAGIC TRIGGER SUPPRESSES FULL MUON RINGS
(EX. CLOSE TO THRESHOLD)
• LIGHT YIELD FROM MUON ARCS AGREES WITHIN 10%
WITH OTHER METHODS (F-FACTOR ANALYSIS OF LIGHT
PULSER SIGNAL)
• FROM MC SIMULATION: RESIDUAL MUON BG ONLY A
FRACTION OF HADRONIC BG.
• IMPORTANT: MUONS DO NOT PEAK AT SMALL ALPHA
AND DO NOT FAKE A SOURCE
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
MC SIMULATION OF MUON IMPACTON GAMMA ANALYSIS AFTER STANDARD CUTS
GAMMAS
HADRONIC BACKGROUND
CONTRIBUTION FROM MUONS
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ECO - 1000
STUDY OF A 1000 m2 CHERENKOV TELESCOPE
34 m
17 m
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Summary
• VHE Gamma Ray Astronomy blossoming
• An interesting future with new telescopes and techniques
• Moving rapidly from explorations to regular observational astronomy.
• UH may have a role in ASHRA all sky observations.