The TeV Gamma-ray Universe
Trevor C. Weekes
Harvard-Smithsonian
Center for Astrophysics
Motivation/Techniques
The TeV Sky
Future Prospects
A Lonely TeV Cosmic Ray
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The Lonely TeV Proton takes a mate and produces a family;
many of the off spring go astray but dutiful gamma rays
carry on the family tradition and relay its message.
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The Relativistic Universe
The Relativistic Universe is defined by the presence of high energy particles,
the sites where the particles are accelerated, the mechanisms by which they
are accelerated, and the regions through which they propagate. Their presence
is indicated by the emission of TeV gamma rays.
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EGRET
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Collection Area = Size
of Football Field
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Simple Technique,
Simple Detectors,
Low Budget
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Development of GeV-TeV
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First Generation Systems 1960 – 1985
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Weak or no discrimination
Lebedev, Glencullen, Whipple, Narrabri, Crimea
New Technology
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Atmospheric Cherenkov Imaging Telescopes
Whipple, Crimea, CAT, HEGRA, Durham, CANGAROO
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Increase in Scale
Third Generation Systems 2004 – 2010
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Arrays of Large ACITs
MAGIC, HESS, CANGAROO-III, VERITAS, MACE
New Technology?
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Zero
Second Generation Systems 1985 – 2004
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TeV
Sources
Fourth Generation Systems 2010 
TBD
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~ 12
> 100
1000?
Development of MeV-GeV
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First Generation Systems 1960 – 1972
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Spark Chambers
Small Satellites
SAS-II, COS-B
Increase in Size
One
30 (15)
Third Generation Systems 1991-2007
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New Technology
Second Generation Systems 1972-1991
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Spark Chambers
Balloons
Controversy
100 MeV
Sources
Spark Chamber
Bigger
EGRET on CGRO
New Technology
270
Fourth Generation Systems 2007-2012+
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New Technology: Solid State
AGILE, GLAST
10,000?
What?
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Early Expectations of TeV Gamma-ray
Astronomy
Find the Origin of the Cosmic Radiation:
* Single source or class of sources
* Unambiguous detection of the 70 MeV bump in
the spectrum
* Source(s) would be in the Galaxy
Locate the “Smoking Gun” of Cosmic Ray Origins!
The reality has been quite different!
* Many different sources (too many!)
* No unambiguous proton source detection
* Many sources are Extragalactic
Atmospheric Cherenkov Imaging Technique
(ACIT)
Proposed in 1977
*Imaging systems came into operation 1984
(Whipple, Crimea)
*First TeV Source detected (Crab Nebula/
Whipple Observatory) 1989
 Standard Candle for TeV Gamma-ray
Astronomy
Strongest Steady Source in TeV Sky
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TeV Image of Crab
(not resolved)
Synchrotron
Compton Synchrotron Model
for TeV Gamma-ray emission
(first proposed by Gould, 1964)
Electron Progenitor
Prototype Model for most TeV
gamma-ray sources
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Compton
Detection of TeV Gamma-ray AGN
Markarian 421
Markarian 421 Cross = X-ray source
Dotted line : EGRET
error circle
Contours: TeV source
intensity (29 sigma)
Weak Source in EGRET but strong at TeV energies
Variation in Nightly Rates from
Markarian 421
Hours-days-months
TeV Catalog of AGN
Catalog Name
Source
Date/Group
Type
Redshift
TeV 1104+3813
Mrk 421
1992/Whipple
HBL
0.031
TeV 1429+4240
H1426+428
2002/Whipple
HBL
0.129
TeV 1654+3946
Mrk 501
1995/Whipple
HBL
0.033
TeV 2000+6509
1ES1959+650
1999/TA
HBL
0.048
TeV 2159-3014
PKS2155-304
1999/Durham
HBL
0.116
TeV 2347+5142
1ES2344+514
1997/Whipple
HBL
= High frequency
BL Lac HBL
0.044
Horan, Weekes, 2003
All confirmed sources
Spectra measured
Light-curves determined
Multi-wavelength Correlations
Only two in EGRET Catalog
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Multiwavelength Results: Power Spectra
Mrk 501
Synchrotron
Compton
Similar double peaked Power Spectra seen in other AGN
AGN Jet Emission Mechanisms
Electron Progenitors:
Synchrotron Self
Compton
External Compton
Proton Progenitors:
Proton Cascades
Proton Synchrotron
Electron Synchrotron Self Compton Models most consistent with TeV
AGN…..but observations are complex and require more sophisticated
Modelling of Jets.
Limitations of ACIT Telescopes
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Second Generation Telescopes successful
but….
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Limited Flux Sensitivity
Hitting the “Muon Wall”
Need Lower Energy for GLAST Overlap
Array Concept demonstrated by HEGRA
ARRAYS
(Third Generation)
Arrays of Cherenkov telescopes viewing the same shower and
improving the energy threshold, the angular resolution and the
energy resolution; muon background removed.
F
Factor
of 10-20
a
improvement
in flux
c
sensitivity
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The Big 5 TeV ACIT Observatories
MACE (India)
2 tel. 2008
VERITAS, (Arizona)
4 tel. 2006
7 tel. 2008?
HESS, (Namibia) 4 tel., 2003
5 tel., 2007
MAGIC (La Palma),
1 tel., 2004
2 tel., 2008
CANGAROO III, 4 tel., 2006
(Australia)
VERITAS: Very Energetic Radiation Imaging
Telescope Array System
The VERITAS Collaboration
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Iowa State University
Leeds University
McGill University
National University of Ireland, Dublin
Purdue University
Smithsonian Astrophysical Observatory
University of California, Los Angeles
University of Chicago
University of Utah
Washington University, Saint Louis
Adler Planetarium
Barnard College
DePauw University
Grinnell College
U.C. Santa Cruz
U. Mass.
N.U.I., Galway
Cork I.T.
Galway-Mayo I.T.
First two 12 m telescopes of
VERITAS now in operation at
temporary site at Whipple
Observatory Basecamp,
December, 2005
Four telescopes in operation
in 2006
Seven telescopes in 2008?
Funding from NSF/DOE/Smithsonian/PPARC/SFI/NSERC
Differential
Flux Sensitivity
D
i
1 GeV
100 GeV
VERITAS, HESS and MAGIC will overlap
and complement GLAST
Whipple 10 m
(3s in 50 hrs)
GLAST
(2 Years)
VERITAS-4
(3s in 50 hrs)
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HESS
European Collaboration;
M.P.I (Heidelberg)
4 x 12 m Telescopes
Completed in Dec. 2003
Located in NAMIBIA
First of the Big 5 to
come on-line
Direction ~ arc-min
Energy Resolution ~ 10%
Background ~ 0
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The TeV Sky - 2005
Diverse Categories of TeV
Gamma-ray
1ES 1218
H1426
sources:
M87
Mrk501
AGN
1ES1959
SNR G0.9
Radio Galaxy
Cas A
1ES 2344
PSR B1259
1ES
1101Shell)
SNR
(Plerion
and
RXJ 1713
Microquasar
LS 5039
GC
TeV 2032
Galactic Plane
Binary
Cygnus
Diffuse
PKS 2155
Extended Sources
Galactic Center
RXJ 0852
Vela X
Crab
HessJ1303
15-52
DarkMSH
Sources
H2356
PKS 2005
Pulsar Nebula
AGN
SNR
Other, UNID
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R.A.Ong
Aug 2005
Catalog of TeV AGN c. 2005
Name
z
Class
Discovery
Markarian 421
0.031
HBL
Whipple (Punch, 1992)
Markarian 501
0.034
HBL
Whipple (Quinn, 1996)
1ES 2344+514
0.044
HBL
Whipple (Catanese, 1998)
1ES 1959+650
0.048
HBL
T. A. (Nishiyama, 2000)
BL Lacertae
0.069
LBL
Crimea (Neshpor, 2001)
PKS 2005-489
0.071
HBL
H.E.S.S. (Aharonian, 2005)
PKS 2155-304
0.117
HBL
Durham (Chadwick, 1999)
H 1426+428
0.129
HBL
Whipple (Horan, 2002)
H 1256-309
0.165
HBL
H.E.S.S. (Aharonian 2005)
BL 1219+305
0.182
HBL
MAGIC (MAGIC 2005)
BL 1101-232
0.186
HBL
H.E.S.S. (Aharonian 2005)
3C66A
0.444*
LBL
Crimea (Neshpor, 1998)
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Gamma-ray Meets IR-Photon
Source:
dN/dE ~ E-2
Absorption:
exp(-t(E))
Spectrum at earth:
E-2 exp(-t(E))
e+
g-ray
IR-photon
e-
• Extragalactic Background Light (EBL) causes
spectral distortion due to g + g  e+ + e• Optical depth depends on integral over the EBL spectrum
from the threshold for pair creation up to higher energies
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EBL Detections & Limits
From Dwek & Krennrich 2004, ApJ
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HESS Survey: New Sources
HESS J1834-087
Gal. Center
HESS J1804-216
HESS J1825-137
HESS J1837-069
HESS J1813-178
G0.9+0.1
30°
0°
LS 5039
HESS J1745-303
HESS J1713-381
HESS J1702-420
HESS J1708-410
HESS 1632-478
HESS J1634-472
330°
359°
Sources > 6 sigma (9 new, 11 total)
Sources > 4 sigma (7 new) RX J1713.7-3946
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HESS J1614-518
HESS J1640-485
HESS J1616-508
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Microquasar: LS 5039
7 sigma detection by HESS
Identification based on position
Consistent with EGRET Source
No time variability
Hard spectum
Microblazar?
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Relativistic Jets and TeV Sources
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Galactic Center
HESS and MAGIC Spectrum
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Hard spectrum G = 2.2.
No evidence for variability on
a variety of time scales.
Unlikely to be dark matter because
of energy spectrum.
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Good agreement between HESS and
MAGIC.
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RX J1713-394 (1)
CANGAROO detection ~7s.
Shell Supernova Remnant
HESS confirmation ~ 40s.
Extended Bright Source
Close Correlation with X-rays
Spectrum
Cosmic Ray Source?
HESS Gamma: color
ASCA X-ray: Lines
Hard spectrum G ~ 2
Not a simple power-law.
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RX J1713-394 (2)
Weak Radio
CO Distributions: Target Material?
Progenitors: Electrons or Protons
“No decisive conclusions can yet be drawn regarding the parent population
dominantly responsible for the gamma-ray emission from RX J1713.7-3946”
Not the Smoking Gun!
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Future of GeV/TeV Gamma-ray Astronomy
GLAST:
the Next Generation
Gamma-ray Space
Telescope: 2007-2012
Also smaller version: AGILE (2006)
Not clear what GeV space telescope might come after GLAST
Future of GeV/TeV Gamma-ray Astronomy
(ground-based)
Third generation Observatories coming on-line (<2008)
It is easy to extend/scale-up ground-based observatories
HESS-2: Add 28m telescope: improved sensitivity at lower threshold (50 GeV) in
coincidence mode (stereo)
Fourth generation Observatories under discussion (>2010)
e.g. HE-ASTRO proposed by Vladimir Vassiliev
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HE-ASTRO
Because the size of
the HE-ASTRO, ~1
km2, is much larger
than the size of the
Cherenkov light pool,
~108 cm2, the
number of telescopes
required is > 200
A
Coupling distance: d=80m
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HE-ASTRO (specifications)
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Array of 217 telescopes
Elevation 3.5km
Telescopes’ coupling distance 80m
Area ~1.0km2 (~1.6km2)
o
Single Telescope Field of View ~15
FoV area ~177 deg2
Reflector Diameter ~7m
Reflector Area ~40 m2
QE 50% (200-400 nm)
Trigger sensor pixel size 0.146o
Trigger Sensor Size ~31.2cm
NSB rate per Trigger pixel ~3.2 pe
per 20 ns
Single Telescope NSB Trigger Rate
1KHz
Energy Range 20–200 GeV
Differential Detection Rate Peak
~30 GeV
Single Telescope CR trigger rate
~30 kHz
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Image pixel size – 0.0146o
Readout image – 128 x 128 pixels
Readout Image size –
1.875o x 1.875o
NSB per pixel – 0.032 (20 nsec gate)
ADC – 8 bit (S/N improved,
10– >8)
Pixel dimension 12mm x 12mm
Sensor area – 12.3 mm x 12.3 mm
Shutter exposure – a few msec
Image integration time - 20 ns
Optical system TBD
Array trigger protocol TBD
Data Rates ~80 Mb/secper node
Online data processing TBD
TeV Astrophysics Workshop,
Palaiseau, April, 2005 (Vassiliev)
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Science coming soon (from
a TeV Source near you)
Astronomy and Astrophysics
> 300 sources
Old:
SNR, AGN, Microquasars, Binaries, Dark Sources
New:
Clusters, Starburst, Pulsars, Others
Cosmological Questions
EBL Measured
Distant Transients detected
Magnetic Fields
Lorentz Invariance
Sources (ACIT Observatories)
Distribution (EAS Arrays)
UHE Sources
Galactic Plane
Dark Matter??
GRBs ?? Prompt: ( Arrays EAS)
Delayed: (ACIT Observatories)
PBHs
Origin of Cosmic Rays
Physics
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Summary (1): The TeV Sky (present)
Diverse Categories of TeV Gamma-ray sources:
AGN
Radio Galaxies
Galactic Plane
Extended Sources
Galactic Center
SNR (Plerions and Shell)
Microquasar
Binary
Dark Sources
but no confirmed detections (yet!) of:
Pulsars
GRBs
Clusters of Galaxies
Starburst Galaxies
UHE Sources
No Smoking Gun for Origin of the Cosmic Radiation …but
Cosmic Particle Acceleration is Ubiquitous
Summary (2): The TeV Sky (future)
Within a few years there will be five major ground-based
gamma-ray observatories using the ACIT in operation.
These will be complemented by:
Space Telescopes: AGILE, GLAST (lower E, wide field)
Air Shower Arrays: Milagro, Tibet(high E, wide fields)
Neutrino Telescopes: IceCube, KM3
The Next Generation of TeV Gamma-ray Observatories
using the ACIT are now under discussion:
(lower energy, wider fields, large collection area)
Watch this space!
Why study TeV Gamma-rays?
Why are Elephants the most
popular animals in the zoo?
They are easy to see and
they tell us much!
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Cosmological studies of High
Energy Transient Phenomena
to determine:
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Redshift evolution of these objects
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Population properties of AGN and GRBs
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Redshift evolution of EBL (z=0-6)
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Major contributors to EBL (stars, dust, AGN, Population III objects,
relic particles, SFR, GFR, IMF, BH accretion histories, supernovae
feedback, merger history)
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Cosmological magnetic fields and their evolution
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High energy properties of space-time
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