The Experimental Status at TeV

Energies

Jim HintonUniversity of Leeds

Non-thermal radiationGround-based -ray TechniquesCurrent InstrumentsA quick introduction to the TeV source classes

Supernova remnantsPulsar wind nebulaeUnidentified galactic sourcesAGN

Outline

Energy Flux (F)

Stars

Dust

Detectors?

Radio Infra-red X-rays -rays

Tracers for ultrarelativistic electrons and hadrons

Non-thermal windowsRadio (low energy electrons)Hard X-ray

-ray

SatellitesCherenkov Telescopes

Inverse Compton Scattering

Synchrotron Emission

0 decay

Energy

Optical, UV, Soft X-ray – Heavily absorbed

The ‘Non-Thermal Windows’

Tracers

X-raysSoft X-rays still dominated by thermal emission2-10 keV band excellent resolution, very sensitive instruments

– but – Synchrotron emission gives information only on energetic electrons ( ×B2 )

Hard X-ray detectors not yet as sensitive

GeV -rays?Hard to launch large detectors, poor angular resolution (< a few GeV)

TeV Neutrinos?Small effective collection area, atmospheric background

TeV -rays?Large detection areas, better angular resolution

Pair production→e+ e-

Bremsstrahlunge- + () → e- +

Cascade develops

~ 120 m

Primary -ray

ParticleShower

~ 10 km

Air Cherenkov Technique

Pair production→e+ e-

Bremsstrahlunge- + () → e- +

Cascade develops

Charged relativistic particles emit Cherenkov light

1° angle at 10 km height → 100 m radius ‘light-pool’~10 ns light ‘flash’

~ 120 m

Primary -ray

ParticleShower

~ 100 m

~ 10 km

Air Cherenkov Technique

Primary -ray

Air-shower...

Detecting Very High Energy Gamma-Rays with Cherenkov Light

~ 120 m

Focal Plane

ParticleShower

Image Analysis gives Shower Energy Background rejection Shower Direction

~ 100 m

~ 10 km

Air Cherenkov Technique

Air-shower...

Detecting Very High Energy Gamma-Rays with Cherenkov Light

~ 120 m

Focal Plane

ParticleShower

Image Analysis gives Shower Energy Background rejection Shower Direction

Stereoscopic viewsImproved angular resolution and background rejection ~ 100 m

~ 10 km

Primary -ray

Air Cherenkov Technique

~ 120 m

ParticleShower

Total amount of Cherenkov light produced (approx) Shower Energy

Arrival times at photosensors (approx) Shower Direction

Distribution of particles on ground

(some) background rejection

Primary -ray

WaterTank

Photosensors

Water Cherenkov Technique

+ ~100% duty cycle+ Wide FoV- Background Sensitivity- Angular resolution- Energy resolution

IACT Systems

3 Major systems, all have~100 GeV energy threshold~0.1° angular resolution~4° Field of View1% Crab flux ( ~ 3 ×10-13 erg/cm2/s ) sensitivity

Major VHE Instruments

STACEE

MILAGRO

TIBETARGO-YBJ

PACT

TACTIC

MAGIC

HESS

MILAGRO

VERITAS

UK + Ireland

Four 13m diameter telescopes in the Khomas highlands of Namibia (southern Africa)

Latitude 23° south → galactic sources100 GeV – 100 TeV, 15% energy resolution5’ angular resolution, 5° field of view150 hours/year open to external observation proposals

e.g. HESS

completed early 2004

Four 13m diameter telescopes in the Khomas highlands of Namibia (southern Africa)

Latitude 23° south → galactic sources100 GeV – 100 TeV, 15% energy resolution5’ angular resolution, 5° field of view150 hours/year open to external observation proposals

e.g. HESS

VERITASVery similar system in Arizona, completed early 2007

Under Construction

MAGIC-IIA second 17 m tel.

HESS-IIA new 30 m tel.

Aiming at lower energies and better sensitivity

Performance: Sensitivity

Funk,

Reim

er,

Torr

es,

Hin

ton 2

007

Milagro

(VERITAS)+

Performance: Angular Resolution

Can reach 2 orders of magnitude better resolution than at 1 GeV

for much less money!

Resolution Science

source IDs, resolved systems…

Simulation: 36x 18m telescopes

Funk,

Reim

er,

Torr

es,

Hin

ton 2

007

1’

Source number versus time

Adapted from T. Kifune by R. White

The TeV Sky in 2007

TeV Source Populations

ExtragalacticActive galactic nuclei 20

( point-like emission, variability seen in all strong sources)

GalacticSupernova remnants ~10Pulsar wind nebulae ~20Unidentified galactic plane sources ~20

( all typically extended on 0.1-1 degree scales ) +Gamma-ray binaries 3 (4)

( all showing variable/periodic emission)

- 85°

+65°

Galactic Centre

~6°

Significance of -ray excess

2004-07, 40 sources, scale saturated at 20 σ

HESS Galactic Plane Survey

Milagro Northern Sky Survey

7 year exposure~20 TeV median energy0.5° angular resolution~0.5 Crab sensitivity3 significant new sources (all on galactic plane)

Abdo et al ICRC 2007

Milagro Northern Sky Survey

7 year exposure~20 TeV median energy0.5° angular resolution~0.5 Crab sensitivity3 significant new sources (all on galactic plane)

Abdo et al ICRC 2007

HESS ICRC 2007

Milagro Northern Sky Survey

7 year exposure~20 TeV median energy0.5° angular resolution~0.5 Crab sensitivity3 significant new sources (all on galactic plane)

Abdo et al ICRC 2007

Supernova remnants

Best candidates for acceleration of the bulk of the galactic cosmic rays

Well established mechanism (diffusive shock acceleration)Energetics are OK (10% kinetic energy into cosmic-rays)

Evidence for ultrarelativisitic electrons in young SNR

X-ray synchrotron emission:

x2 x2 x2

ROSAT – X-ray HESS – TeV -ray

Purely non-thermal X-ray source1000 year old, Distance ~ 1 kpc, dense environment?First TeV gamma-ray SNR (and first image, Nature 432, 75)Closely correlated keV/TeV morphology…

MoonFor Scale

ASCAcontours

TeV Shells e.g. RX J1713.7-3946

Close correlation with X-rays [+electrons]Spectral shape [+protons]IC interpretation implies (too) low B-field [+protons]No correlation with molecular material [+electrons]

Not yet clear…Need data at lower energies to be sure, e.g. GLAST

protons

electrons

Energy Spectrum

SNR/cloud interactions?

Correlations with available target materialIC 443 and W 28, Old (>104 yr) SNRs near mol. CloudsBoth have associated GeV sources pp → π0 → ?

Relativistic e-/e+ plasma wind driven by pulsar - confined by SNR of pulsar progenitor

Efficient conversion of rotation power into relativistic particlesAssociated with young pulsars - high ‘spin-down power’Expansion in non-uniform medium may lead to complex morphol.

High ISMdensity

Low ISMdensity

Reverse shock crushes PWN

Blondin et al. ApJ 563 (2001) 806

G21.5-0.9 Chandra / H.Matheson & S.Safi-Harb

Pulsar Wind Nebulae

Many known X-ray PWN now identified as TeV emitters and almost all of the highest spin-down power radio pulsars have associated TeV emission

Efficient particle accelerators

May be easier to detect in TeV than keV ?Integration over pulsar lifetime for TeV electrons (less cooling)TeV instruments sensitive to more extended objects no confusion with thermal emissionMany of our unidentified sources may be PWN

The PWN Population

Random Catalogues

Implied efficiency Spin-down → TeV

~ 1%

-ray PWN can be large, asymmetric and offset from the pulsar Need to assess chance coincidence HESS scan analysis shows that 70% of Edot/d2 > 1035 erg/s/kpc2 are TeV sources

Search for TeV PWN

HESS

PSR J1826-133431036 erg/s spin-down power, ~2104 years old

5’ X-ray PWNG 18.0-0.7 (Gaensler et al 2002)

1° TeV -ray sourceHESS J1825-137 (Aharonian et al 2005)

Energy dependent morphologyA first at TeV energies

Cooling of electrons away from pulsar? (tcool 1/E)

[ 2 keV synchrotron emission comes from 200 TeV electrons (if B 10 G)…, -rays come from lower energy electrons ]

HESS

HESS J1825-137

Gamma-ray binaries

Three (4) systems, two basic scenariosPSR B1259-63 / SS 2883, LSI +61 303, LS 5039 + (Cyg X-1)

Mirabel 2007

Gamma-ray binaries

High mass companions

O and B stars

PSR B1259-63 = NSLS 5039/LS I +61 303

Nature of compact object not clearBoth appear to have relativistic radio jets Gamma-ray spectral modulation (LS 5039), absorption, variation of acceleration with phase ??

Gamma-ray binaries

High mass companions

O and B stars

PSR B1259-63 = NSLS 5039/LS I +61 303

Nature of compact object not clearBoth appear to have relativistic radio jets Gamma-ray spectral modulation (LS 5039), absorption, variation of acceleration with phase ??

Unidentified Sources

Some sources have been (rather rapidly) identified through multiwavelength work

e.g HESS J1813-178 new radio SNR and new X-ray PWN

Some objects with compelling association but …E.g. Sgr A*, Stellar cluster Westerlund 2

Several rather extended objects where ID is difficultNeed more MWL work, and perhaps more sensitive TeV instruments (substructure, spectral clues, E-dep. morph. …)

Funk, Hinton et al 2007

XMM H.E.S.S. / VLA

The Galactic Centre

TeV source in Sgr A discovered using Whipple 10mConfirmed by CANGAROO, HESS + MAGICGravitational centre of our galaxy – dark matter annihilation?

Deep HESS observationsPrecise (10”) localisation of sourceSpectrum measured over two decades in energyDiscovery of diffuse emission in the central 200 pc

HESS

SNR/PWNG 0.9+0.1

Sgr A Sgr A

Supernova remnant Sgr A EastPulsar wind nebula G359.95-0.04Supermassive Black Hole Sgr A*Dark matter cusp?

H.E.S.S. 2005Contours - VLA radio

Sgr A East

100''

Sgr A*

Pulsar? - G359.95-0.04

Sagittarius A

Sagittarius AH.E.S.S. 2005preliminary

Contours - VLA radio

Sgr A East

100''

Sgr A*

Pulsar? - G359.95-0.04

Supernova remnant Sgr A EastPulsar wind nebula G359.95-0.04Supermassive Black Hole Sgr A*Dark matter cusp? 10''

Sag A*

G359.95

Chandra – X-ray

HESS2007

stat. +sys.

10''

G359.95

1 degree

CS Line Emission (dense clouds)smoothed to match H.E.S.S. PSF

HESS

pp → π0 → ?

Diffuse Emission

Point-source subtracted

TeV emission from Westerlund 2?

Extended TeV source coincident with the massive stellar cluster Westerlund 2 discovered using HESS in 2006

Collective effect of stellar winds?

Radio

PSF

TeV emission from Westerlund 2?

Extended TeV source coincident with the massive stellar cluster Westerlund 2 discovered using HESS in 2006

Collective effect of stellar winds?

Radio

PSF

All 20 known extragalactic VHE gamma-ray sources are active galactic nucleiAll but one (M 87) are blazars

Particle acceleration in relativistic jetsBeaming allows us to see distant objects… but,

The gamma-ray horizon is limited by absorption via pair-production on the extragalactic background light (EBL)

Lower gamma-ray energies → more distant objectsThe spectral shapes of VHE sources can be used to place limits on the EBL – important cosmologically

Extragalactic Sources

Relativistic AGN jet aligned within a few deg. of the line-of-sightHighly variable broad-band emission

typically correlated TeV/keV emission

keV TeV

Mrk 421

Whipple 10m tel.

Synchrotron Self Compton Fits

TeV Blazars

2-3 minute variability timescales Very constraining for models, implies Г > 50 can be used to probe Quantum Gravity

>2 order of magnitude flare, July 2006

Crab Nebula Flux

HESS28th July 2006

MAGIC30th June 2005

Crab Nebula Flux

×10

-9

Quiescent Flux

Mrk 501 (MAGIC), PKS 2155-304 (HESS)

e.g. Begelmann, Fabian, Rees 2007

e.g. Albert et al 2007

TeV Blazar Flares

New HESS and MAGIC AGN

xxx

VHEEBL e+e-

EBL

Approx.`Gamma-ray horizon’

3C 279(z=0.54)

RGB J0152+017

20 known TeV AGN

1ES 1011+496

TeV Blazars and the EBL

Absorption signature

EBL constraints

Mazin+Raue 2007

Combined limits from all VHE blazars

Galaxy Counts

Direct limits

M 87

Famous nearby radio galaxy16 Mpc, Jet angle ~30°

HESS 2 day variabilityEmission region

< 5 RS

Multi-year observations from HEGRA, HESS, VERITAS

Long timescale variability

Emission site?Knot HST1?Very close to SMBH?

HESS source pos.

M 87 - Variability

Colin et al 2007

M 87 – X-ray connection

2-10 keV emission (core dominated?) correlates well with TeV emission on long (6 month) timescales

04 05 0607 08 09 10 11 12 13

HESS

MAGIC

VERITAS

CTA

GLAST

AGILE

Phase 1

Phase 1 Phase 2

Phase 2

Move to permanent site

Design Study Construction

We may be entering the golden age of (>GeV) gamma-ray astronomy

GeV - TeV -Ray Projects

Conclusions

Enormous progress in the last few yearsLots of activity on new instrumentsGreat scientific potentialThe UK and Ireland have been involved since the beginning – and could play substantial roles in the future

Need new people / institutes

Funding?