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Why Gamma Ray Astronomy at energies above 10 TeV ?

Adelaide, Dec 6, 2006

F.A. Aharonian DIAS(Dublin)/MPIK(Heidelberg)

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Gamma-Ray Astronomy

branch of high energy astrophysics for study of the

sky in MeV, GeV, TeV (and more energetic) photons provides crucial window in the spectrum of cosmic E-M radiation for exploration of nonthermal phenomena in the Universe in their most extreme and violent forms

“the last window in the spectrum of cosmic E-M radiation

to be oppened...´´ (a popular phrase since 1950s) is already (partly) opened

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the last E-M window ... 15+ decades:

LE or MeV : 0.1 -100 MeV (0.1 -10 + 10 -100) HE or GeV : 0.1 -100 GeV (0.1 -10 + 10 -100 ) VHE or TeV : 0.1 -100 TeV (0.1 -10 + 10 -100) UHE or PeV : 0.1 -100 PeV EHE or EeV : 0.1 -100 EeV (TDs ?)

the window is opened in MeV, GeV, and TeV bands:

LE,HE – domain of space-based astronomy VHE, .... - domain of ground-based astronomy

‘‘Atmospheric Cherenkov Technique“ covers (potentially) 10 GeV to 1 PeV

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Status of the field

1989: first reliable detection of a TeV -ray signal (Whipple)

1990s: several exciting discoveries (HEGRA, Whipple, CAT, CANGAROO),

but not yet a breakthrough; gamma-ray astronomy recognized as

(perhaps) most advanced area of Astroparticle Physics, but not

yet a nominal astronomical discipline

2000s: HESS revolutionized the field – -ray astronomy emerged as a truly observational (astronomical discipline) with viable detection technique – stereoscopic IACT arrays high quality spectrometric, morphological and temporal studies of more than 40 sources representing several galactic and extragalactic source populations

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Expectations from the foreseeable future

GLAST large source statistics !“Era of gamma-ray astronomy with thausand sources“ (0.1-10 GeV)

also: a few objects and G- & EXG- backgrounds in 10-100 GeV range

HESS/VERITAS/MAGIC-2 large photon statistics !

high quality morphological and spectrometric studies in 0.1-10 TeVrange of up to 100 (or so) sources in both hemisphere based onon data sets consisting of more than 1,000 gamma-ray photons

also: exploration of limited number of objects in > 10TeV domain

HAWK (?) first effective all sky survey at TeV energies

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new activity beyond HESS/MAGIC/VERITAS

next generation of multi-telescope arrays - 2010+

main objectives ? – to be formulated yet … although the ultimate goal is clear:

(1) sensitivity – improvement by an order of magnitude

down to 1 mCrab (at 1TeV)

(2) energy range 10 GeV – 100 TeV

(3) angular resolution 1 arcmin or so

it is believed that both the improvement of the flux sensitivity in the “classical”

0.1-10 TeV interval and reduction of the energy threshold down to 10 GeV

should lead to a discovery of hundreds or, hopefully, thousands of sources.

effective ways of realization of this ultimate goal on realistic timescales ?

different options and opinions under discussion …

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three possible developments (my opinion)

30 GeV to 30 TeV range with emphasis on 100 GeV to 10 TeV realization: 15m diameter class multi (two dozens or more) telescope

array located at 3.5-4 km a.s.l. mountain levels (super HESS)

3-300 GeV range with emphasis on E < 30 GeV realization: a telescope system consisting of several 25+ m diameter

telescopes located at 5+ km a.s.l. altitudes and equipped with high quantum efficiency 50%+ multipixel cameras (concept 5@5)

3 TeV to 300 TeV with emphasis on E > 10 TeV realization: a telescope array consisting of 10 to 30 m2 area and large, 6 deg to 8 deg FoV telescopes located at 250m to 500m distances from each other to cover up to 10km2 at 10 TeV

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a nasty question

do we really need > 10 TeV region ?

given that only the nearby Cosmos (< 100 Mpc or so) is visible at these energies, and that realistically one expects only limited number of sources above 10 TeV

my answer: number of sources is not a key issue; physics/astrophysics is a more important issue...

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TeV astronomy - a viable discipline in its own right Generally, TeV emission is considered as extension of the

GeV region...

however visibility of sources in GeV gamma-rays does not yet imply

visibility in TeV gamma-rays, and vice versa

Reasons ? Efficiency of acceleration mechanisms, spectral cutoffs due to internal and external absorption, particle diffusion... ...

TeV astronomy is not merely an extension of MeV/GeV astronomy but a viable discipline in its own right with several major objectives

> 10 TeV astronomy is an extension of TeV astronomy; and many key scientific ssues are contained in this part of the spectrum

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TeV -ray Source Populations

Extended Galactic Objects

Shell Type SNRs Giant Molecular Clouds (star formation regions) Pulsar Wind Nebulae (Plerions)

Compact Galactic Sources Binary pulsar PRB 1259-63 LS5039, LSI +61 303 – microquasars (?)

Galactic Center Extragalactic objects M87 - a radiogalaxy TeV Blazars – with redshift from 0.03 to 0.18 and large number of yet unidentified TeV sources …

HESS detected > 10 TeV gamma-rays from all of these classes

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Origin of Galactic Cosmic Rays and SNRs

a mystery since the discovery in 1912 by V.Hess … but now we are quite close (hopefully) to the solution of the (galactic) component below the energy 1PeV (1015eV)

standard theory of particle acceleration by SNR shocks: Emax = 100 TeV or so (Lagage-Cesarsky (1974) limit) but what about the knee around 1 PeV ? one needs a strong amplification of the B-field; recent recent developments of theory (e.g. Bell and Lucek 2001) it is possible !

what gamma-ray observations tell us ?

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RXJ1713: > 40 TeV -rays and shelltype morphology: first model-independent evidence of acceleration of

particles (e and/or p) to > 100 TeV

hadronic orgin? – most likely, but unfortunately we cannot make rubust (unambiguous)

statements

2003-2005 data

almost constant

photon index !

can be explained by -rays from pp->o ->2for apw 2 and Eo w 100 TeV

cannot be explained by IC because of KN effect

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spectrum of protons ?

one needs flux measurement below 100 GeV and well above 10 TeV

Wp = 1050 (n/1cm-3)-1 erg ; n close to 1 cm-3 ? preferable – can explain the production rate of GCRs by SNRs

Eo significantly smaller than 1PeV ?, yes, although that could be connected

with fast fast escape of protons from accelerator, so RXJ 1713 still could be treated as a PeVatron

IC vs o

IC vs o

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searching for galactic PeVatrons ...

TeV gamma–rays from Cas A and RX1713.7-3946, Vela Jr – a proof that SNRs are responsible for the bulk of GCRs ?– not yet the hunt for galactic PeVatrons continues

unbiased approach – deep survey of the Galactic Plane – not to miss any recent (or currently active) acceleration site: SNRs, Pulsars/Plerions, Microquasars... not only from accelerators, but also from nearby dense regions

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Gamm-rays/X-rays from dense regions surrounding accelerators

the existence of a powerful accelerator by itself is not sufficenrt for gamma radiation; an additional component – a dense gas target - is

required

gamma-rays from surrounding regions add much to our knowledge about highest energy protons which quickly escape the accelerator and therefotr do not signifi-cantly contribute to gamma-ray production inside the proton accelerator-PeVatron

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older source – steeper gamma-ray spectrum

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Giant Molecular Clouds (GMCs) as tracers of Galactic Coismic RaysGMCs - 103 to 105 solar masses clouds physically connected with

star

formation regions - the likely sites of CR accelerators (with or

without SNRs) - perfect objects to play the role of targets !

While travelling from the accelerator to the cloud the spectrum of CRs

is a strong function of time t, distance to the source R, and the (energy-

dependent) Diffusion Coefficient D(E)

depending on t, R, D(E) one may expect any proton, and

therefore gamma-ray spectrum – very hard, very soft,

without TeV tail, without GeV counterpart ...

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Propagation Effects on the spectrum of Gamma Rays

emissivities and fluxes (M5/d2kpc ) of gamma

rays from a cloud at different times and dis- tances from an impulsive accelerater with W=1050 erg [ D(E)=1026 (E/10GeV)0.5 cm2/s ]

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Relativistic outflows as extreme acceleratorsdistinct feature of relativistic outflows: effective particle acceleration at different stages of their development

close to the central engine during propagation on large scales, at the jet

(wind) termination

the theory of relativistic jets – very complex and not (yet) fully understood – all aspects (MHD, electrodynamics, shock waves,particle acceleration) contain many problems and uncertainties

maximim (theoretically possible) acceleration rate: qBc

or minimum acceleration time: tmin=RL/c

RL [cm] =3 x109 ETeV BG-1 cm – Larmor radius

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close to 1 – extreme accelerators

from tacc=tsynch : tacc=RL/c

hmax = (9/4) f-1 mc2 w 150 -1 MeV for

electrons * w 300 -1 GeV for protons small – signature of extreme accelerators ?

_______________________________ * SNRs – sources of GCRs are not extreme accelerators: particle acceleration in young SNRs through DSA w 10(c/v)2 w105 (in the Bohm regime) h w 1 keV

high energy gamma-rays best cariers of information about extreme accelerators

relativistic outflows – high energy gamma-ray emitters (“on“ and “off“ axis)

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Crab Nebula – a perfect PeVatron of electrons (and protons ?)

Crab Nebula – a powerful We=1/5 Lrot w 1038 erg/s

and extreme accelerator: Ee >> 100 TeV

Emax=60 (B/1G) -1/2 -1/2 TeV and hcut w 150 -1 MeV Cutoff at hcut =10-20 MeV ? w10 - acceleration at 10 % of the max. rate -rays: E > 50 TeV (HEGRA, HESS) => Ee > 200 TeV

B-field w 100 G => w10 - independent and more robust estimate

1-10MeV

100TeV

standard MHD theory (Kennel&Coroniti)

cold ultrarelativistc pulsar wind terminates by reverseshock resulting in acceleration of multi-TeV electrons

synchrotron radiation => nonthermal optical/X-ray nebulaInverse Compton => high energy gamma-ray nebula

.MAGIC (?)

HEGRA

EGRET

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Synch. cutoff at 10 MeVand IC - up to 100 TeV ?

Energy-dependent size 0.1 -10 TeV (MAGIC –II,VERITAS,HESS)

Energy spectrum 100 MeV to 100 GeV (GLAST) Detection of a sharp cutoff around 100 TeV (HESS ?)

Detection of -ray line signatures (at E= me c2 x of the unshocked wind (?)

> 1 TeV neutrinos (marginally) detectable (Ice Cube)

HEGRA

EGRET

important tests

contribution of hadronic component of -rays ? not necessary, but cannot be excluded (provided higher acceleration rate in the past),

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TeV gamm-rays from other Plerions ?

Crab Nebula is a very effective accelerator but not an effective IC -ray emitter

We see TeV gamma-rays from the Crab Nebula because of very large spin-down flux but gamma-ray flux << “spin-down flux“ because of large magnetic field

but the strength of B-field also depends on

less powerful pulsar weaker magnetic field higher gamma-ray efficiency detectable gamma-ray fluxes from other plerions

HESS confirms this prediction ! ( ? ) – several famous PWN already detected - MSH 15-52, PSR 1825, Vela X, ...

* Plerions – Pulsar Driven Nebulae

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HESS J1825 (PSR J1826-1334)

TeV image the -ray luminosity is comparable to the TeVluminosity of the Crab Nebula, while the spin-down luminosity is two orders of magnitude

less ! implications ?(a) magnetic field of order of 10G. (b) spin-down luminosity in the past - higher

noticeable softening of -ray spectrum away from position of the pulsar PSRJ 1926-1334: evidence for IC origin of -

rays!

red – below 0.8 TeVyellow – 0.8-2.5 TeVblue – above 2.5 TeV

S.Funk

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since 2.7 K MBR is the main target field, TeV images reflect spatial distributions of electrons Ne(E,x,y); coupled with synchrotron X-rays, TeV

images allow measurements of B(x,y)

MSH 15-52

the energy spectrum - a perfect hard power-law with photon index =2.2-2.3 over 2 decades !

• cannot be easily explained by IC… (unless intense IR sources around)• hadronic (o-decay) origin of -rays ?

dN/dE E-

= 2.270.030.15

2/ = 13.3/12

Flux > 280 GeV

15% Crab Nebula

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questions: • B-field – as weak as 2 G ? • energy in ultrarelativistic electrons only 2x1045 erg ? • integrated energy over 11kyr: >2.5x1048 erg – in which form the “dark energy“ is released?

HESS J0835-456 (Vela X) – do we see the Compton peak ?

the image of TeV electrons ! (?)

photon index 1.45 with exponential cutoff at 13.8 TeV

spectral index e 2 with abreak around 70 TeVtotal energy We=2x1045 erg !!!

adiabatic losses?, ‘inisible‘ low energy electrons? or in ultrarelativistic protons?

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pulsar wind consisting of protons and nuclei ...

dNp/dE=AE2 exp[-(E/80TeV)2]

Wp = 1.3 x 1049 (n/0.6cm-3) erg

total spin down energy released over the last11kyr: 5 x1048 – 5 x1051 erg depending on thebraking index (time-history of Lrot)

B=10 G , We=1045 (B/10G)-2 erg

for B=100 G – half of X-ray flux can be explained by secondary electrons

Horns et al. 2006

fluxes of TeV neutrinos are detectable by KM3NeT !

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TeV Gamma Rays From LS5039 and LSI+61 303

HESS, 2005

MAGIC, 2006microqusars or binary pulsars?

independent of the answer – particle acceleration is linked to (sub) relativistic outflows

See talk by J. Paredes

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Molecular Clouds in the Galactic Center Region

HESS J1745-303

diffuse emission along the plane!

HESS collaboration: Nature Feb 9, 2006

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Diffuse emission from the GC ridge

Photon index 2.3

Harder CR spectrum Compared to local CRs

Higher CR density above 10 TeV

Source of CRs - Sgr A* ?

very important – detection of E > 10 TeV gamma-rays , hard X-rays, neutrinos (?)

no indication of a cutoff below 10 TeV !

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Extragalactic Sources of > 10 TeV gamma-rays?

neraby BL Lac Objects Clusters of Galaxies Nearby radio galaxies nearby starburst galaxies Microblazars within 1 Mpc ...

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Intergalactic Absorption

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time average spectra of Mkn 421 and Mkn 501

Unprecedented photon statistics

Mkn 421 – 60,000 TeV photons detected in 2001Mkn 501 – 40,000 TeV photons detected in 1997spectra: canonical power-law with exponential cutoff Cutoff = 6.2 TeV and 3.8 TeV for Mkn 501 and Mkn 421

Gamma-rays detected beyond 10 TeV !

TeV

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Multi-TeV gamma-rays expected from Clusters of Galaxies

accretion shocks3 deg diameteer

Coma cluster of galaxies

core – 1 deg

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M 87 – evidence for production of TeV -rays close to BH

Distance: ~16 Mpc

central BH: 3109 M

Jet angle: ~30°not a blazar!

discovery (>4) of TeV

-rays by HEGRA (1998)

confirmed by HESS (2003)

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M87: light curve and variabiliy

X-ray emission:knot HST-1

[Harris et al. (2005),ApJ, 640, 211]

nucleus(D.Harris private communication)

X-ray (Chandra)

HST-1

nucleus

knot A

I>73

0 G

eV [

cm-2 s

-1]

short-term variability within 2005 (>4)constrains size of emission region (R ~ 5x1015 j cm)

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energy spectra for 2004 (~5) and 2005

(~10)

Differential spectra well

described by power-laws:

energy spectra – no indication of a cutoff below 10 TeV

2004 vs. 2005:Photon indices compatible, but different flux levels

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= 10-13 cm-2 s-1 TeV-1

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Probing DEBRA at MIR /FIR with E > 10 TeV -rays from nearby extragalactic sources (d < 100 Mpc)

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Conclusions >10 TeV gamma-rays are expected and/or detected from all potential Galactic TeV source populations – SNRs, PWNe, QSOs, GMCs/SFR, ISM,... as well as from nearby (within 100-200 Mpc) Extragalactic source populations: BL Lac objects, Radiogalaxies, Clusters of Galaxies, Starburst Galaxies, etc.

>10 TeV gamma-rays provide key information about physics and astrophysics of Cosmic TeVatrons and PeVatrons

we need dedicated detectors for studies of multi-TeV sources with detection area 10km2 and energy flux sensitivity better than 10-13 erg/cm2s @10 TeV

arrays of relatively modest IACTs optimized for the energy region from 1 TeV to 100 TeV have significant discovery potential and should be treated

as an important and complementary approach to the ongoing efforts for im- provement of the performance at sub-100 GeV energies (HESS phase-2, MAGIC VERITAS,CANGAROO) – before arrival of the (next generation) major ground based detectors – Very Large Arrays of Very Large Cherenkov Telescopes

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Potntial Gamma Ray Sources

Major Scientific Areas

GCRs Relativistic Outflows Compact Objects Cosmology

ISM SNRsSFRs Pulsars Binaries

Galactic Sources Extragalactic Sources

GRBs AGN GLX CLUST

IGM

GMCsM

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Mic

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Cold

Win

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Pu

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Neb

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Bin

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P

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ars

Rad

iog

ala

xie

s

B

laza

rs

N

orm

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Sta

rbu

rst

EXGCRs

EB

L

GeV GeVGeV GeV GeV GeV