why gamma ray astronomy at energies above 10 tev ?
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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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Cold
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P
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Rad
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EXGCRs
EB
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GeV GeVGeV GeV GeV GeV
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