The Experimental Status at TeV Energies Jim Hinton University of Leeds

Non-thermal radiation Ground-based  -ray Techniques Current Instruments A quick introduction to the TeV source classes Supernova remnants Pulsar wind nebulae Unidentified galactic sources AGN Outline

Energy Flux (F ) Stars Dust Detectors? Radio Infra-red X-rays  -rays Tracers for ultrarelativistic electrons and hadrons Non-thermal windows Radio (low energy electrons) Hard X-ray  -ray Satellites Cherenkov Telescopes Inverse Compton Scattering Synchrotron Emission  0 decay Energy Optical, UV, Soft X-ray – Heavily absorbed The ‘Non-Thermal Windows’

Tracers X-rays Soft X-rays still dominated by thermal emission 2-10 keV band excellent resolution, very sensitive instruments – but – Synchrotron emission gives information only on energetic electrons ( ×B 2 ) 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 - Bremsstrahlung e - + (  ) → e - +  Cascade develops ~ 120 m Primary  -ray Particle Shower ~ 10 km Air Cherenkov Technique

Pair production  →  e + e - Bremsstrahlung e - + (  ) → 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 Particle Shower ~ 100 m ~ 10 km Air Cherenkov Technique

Primary  -ray Air-shower... Detecting Very High Energy Gamma-Rays with Cherenkov Light ~ 120 m Focal Plane Particle Shower 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 Particle Shower Image Analysis gives  Shower Energy  Background rejection  Shower Direction Stereoscopic views  Improved angular resolution and background rejection ~ 100 m ~ 10 km Primary  -ray Air Cherenkov Technique

~ 120 m Particle Shower 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 Water Tank 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 View 1% Crab flux ( ~ 3 × erg/cm 2 /s ) sensitivity

Major VHE Instruments STACEE MILAGRO TIBET ARGO-YBJ PACT TACTIC MAGICHESSMILAGROVERITAS UK + Ireland

Four 13m diameter telescopes in the Khomas highlands of Namibia (southern Africa) Latitude 23° south → galactic sources 100 GeV – 100 TeV, 15% energy resolution 5’ angular resolution, 5° field of view 150 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 sources 100 GeV – 100 TeV, 15% energy resolution 5’ angular resolution, 5° field of view 150 hours/year open to external observation proposals e.g. HESS VERITAS Very similar system in Arizona, completed early 2007

Under Construction MAGIC-II A second 17 m  tel. HESS-II A new 30 m  tel. Aiming at lower energies and better sensitivity

Performance: Sensitivity Funk, Reimer, Torres, Hinton 2007 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, Reimer, Torres, Hinton ’

Source number versus time Adapted from T. Kifune by R. White

The TeV Sky in 2007

TeV Source Populations Extragalactic Active galactic nuclei 20 ( point-like emission, variability seen in all strong sources) Galactic Supernova remnants ~10 Pulsar wind nebulae ~20 Unidentified galactic plane sources ~20 ( all typically extended on degree scales ) + Gamma-ray binaries 3 (4) ( all showing variable/periodic emission)

- 85° +65° Galactic Centre ~6° Significance of  -ray excess , 40 sources, scale saturated at 20 σ HESS Galactic Plane Survey

Milagro Northern Sky Survey 7 year exposure ~20 TeV median energy 0.5° angular resolution ~0.5 Crab sensitivity 3 significant new sources (all on galactic plane) Abdo et al ICRC 2007

Milagro Northern Sky Survey 7 year exposure ~20 TeV median energy 0.5° angular resolution ~0.5 Crab sensitivity 3 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 energy 0.5° angular resolution ~0.5 Crab sensitivity 3 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

ROSAT – X-ray HESS – TeV -ray Purely non-thermal X-ray source 1000 year old, Distance ~ 1 kpc, dense environment? First TeV gamma-ray SNR (and first image, Nature 432, 75 ) Closely correlated keV/TeV morphology… Moon For Scale ASCA contours TeV Shells e.g. RX J

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 material IC 443 and W 28, Old (>10 4 yr) SNRs near mol. Clouds Both have associated GeV sources pp → π 0 →  ?

Relativistic e - /e + plasma wind driven by pulsar - confined by SNR of pulsar progenitor Efficient conversion of rotation power into relativistic particles Associated with young pulsars - high ‘spin-down power’ Expansion in non-uniform medium may lead to complex morphol. High ISM density Low ISM density Reverse shock crushes PWN Blondin et al. ApJ 563 (2001) 806 G 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 emission Many 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/d 2 > erg/s/kpc 2 are TeV sources Search for TeV PWN HESS

PSR J  erg/s spin-down power, ~2  10 4 years old 5’ X-ray PWN G (Gaensler et al 2002) 1° TeV  -ray source HESS J (Aharonian et al 2005) Energy dependent morphology A first at TeV energies Cooling of electrons away from pulsar? (t cool  1/E) [ 2 keV synchrotron emission comes from 200 TeV electrons (if B  10  G)…,  -rays come from lower energy electrons ] HESS HESS J

Gamma-ray binaries Three (4) systems, two basic scenarios PSR B / SS 2883, LSI , LS (Cyg X-1) Mirabel 2007

Gamma-ray binaries High mass companions O and B stars PSR B = NS LS 5039/LS I Nature of compact object not clear Both 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 B = NS LS 5039/LS I Nature of compact object not clear Both 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 J  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 difficult Need 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 10m Confirmed by CANGAROO, HESS + MAGIC Gravitational centre of our galaxy – dark matter annihilation? Deep HESS observations Precise (10”) localisation of source Spectrum measured over two decades in energy Discovery of diffuse emission in the central 200 pc HESS SNR/PWN G Sgr A

Supernova remnant Sgr A East Pulsar wind nebula G Supermassive Black Hole Sgr A* Dark matter cusp? H.E.S.S Contours - VLA radio Sgr A East 100'' Sgr A* Pulsar? - G Sagittarius A

H.E.S.S preliminary Contours - VLA radio Sgr A East 100'' Sgr A* Pulsar? - G Supernova remnant Sgr A East Pulsar wind nebula G Supermassive Black Hole Sgr A* Dark matter cusp? 10'' Sag A* G Chandra – X-ray HESS 2007 stat. +sys. 10'' G359.95

1 degree CS Line Emission (dense clouds) smoothed to match H.E.S.S. PSF HESS pp → π 0 →  ? 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 nuclei All but one (M 87) are blazars Particle acceleration in relativistic jets Beaming 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 objects The 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-sight Highly 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 HESS 28 th July 2006 MAGIC 30 th June 2005 Crab Nebula Flux × Quiescent Flux Mrk 501 (MAGIC), PKS (HESS) e.g. Begelmann, Fabian, Rees 2007 e.g. Albert et al 2007 TeV Blazar Flares

New HESS and MAGIC AGN x x x  VHE  EBL  e + e - EBL Approx. `Gamma-ray horizon’ 3C 279 (z=0.54) RGB J known TeV AGN 1ES 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 galaxy 16 Mpc, Jet angle ~30° HESS 2 day variability Emission region < 5  R S 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

HESS MAGIC VERITAS CTA GLAST AGILE Phase 1 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 years Lots of activity on new instruments Great scientific potential The UK and Ireland have been involved since the beginning – and could play substantial roles in the future Need new people / institutes Funding?