Theoretical aspects of VHE -ray astronomy: Exploring Nature’s Extreme Accelerators

Felix AharonianDublin Institute for Advanced Studies, DublinMax-Planck-Institut f. Kernphysik, Heidelberg

APP UK 2008 meeting, Oxford, June 20, 2008

Astroparticle Physics

a modern interdisciplinary research field at the interface of

astronomy, physics and cosmology

one of the major objectives: study of nonthermal phenomena in their most

energetic and extreme forms in the Universe (the “High Energy Astrophysics” branch of Astroparticle Physics)

all topics of this research area are related, in one way or another,

to exploration of Nature’s perfectly designed machines

Extreme Particle Accelerators

Extreme Accelerators [TeVatrons, PeVatrons, EeVatrons…]

machines where acceleration proceeds with efficiency close to 100%

efficiency ? (i) fraction of available energy converted to nonthermal particles in PWNe and perhaps also in SNRs, can be as large as 50 %

(ii) maximum (theoretically) possible energy achieved by individual particles

acceleration rate close to the maximum (theoretically) possible rate * sometimes efficiency can “exceed” 100% (!) e.g. at CR acceleration in SNRs in Bohm diffusion regime with amplification of B-field by CRs (Emax= ~ B (v/c)2 ) this effect provides the extension of the spectrum of Galactic CRs to at least 1 PeV

“> 100% efficiency” because of nonlinear effects:

acceleration of particles creates better conditions for their further acceleration

Knee

AnkleAnkle

Extragalactic?Extragalactic?

T. Gaisser

SNRs ?

up to 1015-16 (knee) - Galactic

SNRs: Emax ~ vshock Z x B x Rshock for a “standard” SNR: Ep,max ~ 100 TeV solution? amplification of B-field by CRs 1016 eV to 1018 eV:

a few special sources? Reacceleration?

above 1018 eV (ankle) - Extragalactic

1020 eV particles? : two options “top-down” (non- acceleration) origin or Extreme Accelerators

Cosmic Rays from 109 to 1020 eV

Particles in CRs with energy 1020 eV

difficult to understand unless we assume extreme accelerators…

the “Hillas condition” - l > RL - an obvious but not sufficient condition…

(i) maximum acceleration rate allowed by classical electrodynamics

t-1=qBc ( x (v/c)2 in shock acceleration scenarios) with ~ 1 (ii) B-field cannot be arbitrarily increased - the synchrotron and curvature

radiation losses become a serious limiting factor, unless we assume perfect linear accelerators …

only a few options survive from the original Hillas (“l-B”) plot:

>109 Mo BH magnetospheres, small and large-scale AGN jets, GRBs

acceleration sites of 1020 eV CRs ? FA, Belyanin et al. 2002, Phys Rev D, 66, id. 023005

confinement

confinement

energy losses

energy losses

signatures of extreme accelerators?

synchrotron self-regulated cutoff:

neutrinos (through “converter” mechanism)production of neutrons (through p interactions) which travel without losses and at large distan-

ces convert again to protons => 2 energy gain ! Derishev, FA et al. 2003, Phys Rev D 68 043003 observable off-axis radiation radiation pattern can be much broader than 1/ Derishev, FA et al. 2007, ApJ, 655, 980

FA 2000, New astronomy, 5, 377

VHE gamma-ray and neutrino astronomies: two key research areas of

High Energy Astrophysics/Astroparticle Physics

VHE- and - astronomies address diversity of topics related to the nonthermal Universe:

acceleration, propagation and radiation of ultrarelativistic protons/nuclei and electrons

generally under extreme physical conditions in environments characterized with

huge gravitational, magnetic and electric fields, highly excited media, shock waves

and very often associated with relativistic bulk motions linked, in particular, to

jets in black holes (AGN, Microquasars, GRBs) and cold ultrarelativistic pulsar winds

over last several years HESS has revolutionized the field before – “astronomy“ with several sources and advanced branch of Particle Astrophysics

now – a new astronomical discipline with all characteristic astronomical key words: energy spectra, images, lightcurves, surveys...

VHE gamma-ray astronomy - a success story

major factors which make possible this success ?

effective acceleration of Tev/PeV particles almost everywhere in Universe the potental of the detection technique (stereoscopic IACT arrays)

good performance => high quality data => solid basis for theoretical studies

28th July 2006

TeV image and energy spectrum of a SNR

enrgy dependent image of a pulsar wind nebula

variability of TeV flux of a blazar on minute timescales

huge detection area+effective rejection of different backgrounds good angular (a few arcminutes) and energy (15 %) resolutionsbroad energy interval - from 100 (10) GeV to 100 (1000) TeV nice sensitivity (minimum detectable flux): 10-13 (10-14) erg/cm2 s

multi-functional tools: spectrometry temporal studies morphology

extended sources: from SNRs to Clusters of Galaxies

transient phenomena QSOs, AGN, GRBs, ...

Galactic Astronomy | Extragalactic Astronomy | Observational Cosmology

RXJ 1713.7-3946 PSR 1826-1334 PKS 2155-309

VHE gamma-ray observations:

“Universe is full of extreme accelerators on all astronomical scales”

Extended Galactic Objects Shell Type SNRs Giant Molecular Clouds Star formation regions Pulsar Wind Nebulae

Compact Galactic Sources Binary pulsar PRB 1259-63 LS5039, LSI 61 303 – microquasars? Cyg X-1 ! - a BH candidate Galactic Center Extragalactic objects M87 - a radiogalaxy TeV Blazars – with redshift from 0.03 to 0.18

and a large number of yet unidentified TeV sources …

VH

E gam

ma-ray source populations

TeV gamma-ray source populations

highlight topics

particle acceleration by strong shocks in SNR

physics and astrophysics of relativistic outflows (jets and

winds)

probing processes close to the event horizon of black holes

cosmological issues - Dark Matter, Extragalactic Background

Light (EBL)

…..

Potential Gamma Ray Sources

Major Scientific Topics

G-CRs Relativistic Outflows

Compact Objects Cosmology

ISM SNRsSFRs Pulsars Binaries

Galactic SourcesExtragalactic Sources

GRBs AGN GLX CLUST

IGM

GMCsM

ag

neto

sph

ere M

icro

qu

asa

rs

C

old

Win

d

Pu

lsar

Neb

ula

Bin

ary

P

uls

ars

Rad

iog

ala

xie

s

B

laza

rs

N

orm

al

Sta

rbu

rst

EXG-CRs

EB

L

GeV GeVGeV GeV GeV GeV

unique carriers of astrophysical and cosmological information about non-thermal phenomena in many galactic and extragalactic

sources

why TeV neutrinos ?

like gamma-rays, are effectively produced, but only in hadronic interactions (important - provides unambiguous unformation)

unlike gamma-rays do not interact with matter, radiation and B-fields

(1) energy spectra and fluxes without internal/external absorption (2) “hidden accelerators” ! but… unlike gamma-rays, cannot be effectively detected even “1km3 volume” class detectors have limited performance: minimum detectable flux approximately equivalent to 1 Crab gamma-ray flux

TeV neutrinos -- a complementary channel

synchrotron radiation of protons - pure electromagnetic process

interaction of hadrons without production of neutrinos

generally in hadronic neutrinos and gamma-rays are produced with same rates, but

in high density environments (n > 1018 cm-3 and/or B>106 G) production of TeV

neutrinos is suppressed because charged mesons are cooled before they decay

on the other hand, in compact objects muons and charged pions can be accelerated and thus significantly increase the energy and the flux of neutrinos, e.g. from GRBs

some remarks concerning the neutrino/gamma ratio: typically > 1, but…

synchrotron radiation of secondary electrons from Bethe-Heitler and photomeson production at interaction of CRs with 2.7K MBR in a medium with

B=1 G (e.g. Galaxy Clusters)

what should we do if hadronic gamma-rays and neutrinos appear at “wrong” energies ?

photomesonelectrons

Bet

he-H

eitl

erel

ectr

ons

Kelner and FA, 2008, Phys Rev D

detect radiation of secondary electrons !

E* = 3x1020 eV

probing hadrons with secondary

X-rays with sub-arcmin resolution! Simbol-X

new technology focusing telescopes NuSTAR (USA), Simbol-X (France-Italy), NeXT (Japan) will provide X-ray imaging and spectroscopy in the 0.5-100 keV band with angular resolution 10-20 arcsec

and sensitivity as good as 10-14 erg/cm2s!

complementary to gamma-ray and neutrino telescopes

advantage - (a) better performance, deeper probes (b) compensates lack of neutrinos and gamma-rays at “right energies”

disadvantage - ambiguity of origin of X-rays

exploring Nature’s Extreme Particle Accelerators with neutrinos, gamma-rays, and hard X-rays

Microquasars ?

Pulsars/Plerions ?

SNRs ?

Galactic Center ?

. . .

Gaisser 2001

OB, W-R Stars ?

* the source population responsible for the bulk of GCRs are PeVatrons ?

Galactic TeVatrons and PeVatrons - particle acceleratorsresponsible for cosmic rays up to the “knee” around 1 PeV

Visibility of SNRs in high energy gamma-rays

F(>E)=10-11 A (E/1TeV)-1 ph/cm2s

A=(Wcr/1050erg)(n/1cm-3

)(d/1kpc) -2

for CR spectrum with =2

if electron spectrum >> 10 TeV synchrotron X-rays and IC TeV ’s

main target photon field 2.7 K: F,IC/Fx,sinch=0.1 (B/10G)-2

Detectability ? compromise between angle (r/d) and flux F (1/d2) typically A: 0.1-0.01 :

0.1o - 1o

1000 yr old SNRs (in Sedov phase)

o component dominates if A > 0.1 (Sx/10 J)(B/10 G ) -2

TeV -rays – detectable if A > 0.1

nucleonic component of CRs - “visible” through TeV (and GeV) gamma-rays !

Inverse Compton

0 –decay (A=1)

TeV -rays and shell type morphology: acceleration of p or e in the shell toenergies exceeding 100TeV

2003-2005 data

can be explained by -rays from pp ->o ->2

but IC canot be immediately excluded…

RXJ1713.7-4639

and with just ”right” energetics

Wp=1050 (n/1cm-3)-1 erg/cm3

leptonic versus hadronic

IC origin ? – very small B-field, B < 10 G, and very large E, Emax > 100 TeV

two assumptions hardly can co-exists within standard DSA models, bad fit of gamma-ray spectrum below a few TeV, nevertheless …

arguments against hadronic models:

nice X-TeV correlaton well, in fact this is more natural for

hadronic rather than leptonic models

relatively weak radio emission problems are exaggerated

lack of thermal X-ray emission => very low density plasma or low Te ? we do not (yet) know the mechanism(s) of electron heating in supernova remnants so comparison with other SNRs is not justified at all

Suzaku measurements => electron spectrum 10 to 100 TeV

Variability of X-rays on year timescales - witnessing particle acceleration in real time

flux increase - particle acceleration

flux decrease - synchrotron cooling *)

both require B-field of order 1 mG in hot spots and, most likely, 100G outside

Uchiyama, FA, Tanaka, Maeda, Takahashi, Nature 2007

*) explanation by variation of B-field does’t work as demonstrated for Cas A (Uciyama&FA, 2008)

strong support of the idea of amplification of B-field by in strong nonlinear shocks through non-resonant

streaming instability of charged energetic particles (T. Bell; see also recent detailed theoretical treatment of the problem by Zirakashvili et al. 2007)

acceleration in Bohm diffusion regime

Strong support for Bohm diffusion - from the synchrotron cutoffgiven the upper limit on the shock speed of order of 4000 km/s !

with h=0.67 +/- 0.02keV

energy spectrum of synchrotron radiation of electrons in the

framework of DSA (Zirakashvili&FA 2007)

B=100 G + Bohm diffusion - acceleration of particles to 1 PeV

(Tanaka et al. 2008)

protons:

dN/dE=K E- exp[-(E/Ecut)]

-rays:

dN/dE v E- exp[-(E/E0)]

=+, 0.1, =/2, E0 = Ecut/20

Wp(>1 TeV) ~ 0.5x1050 (n/1cm-3)-1 (d/1kpc)2

RXJ 1713.7-3946

neutrinos: marginally detectable by KM3NeT

Probing PeV protons with X-rays

SNRs shocks can accelerate CRs to <100 TeV unless magnetic field significantly exceeds 10 G

recent theoretical developments: amplification of the B-field up to >100 mG is possible through plasma waves generated by CRs

>1015 eV protons result in >1014 eV gamma-rays and electrons “prompt“ synchrotron X-rays

t() = 1.5 (/1keV) -1/2 (B/1mG) -3/2 yr << tSNR

typically in the range between 1 and 100 keV with the ratio Lx/L larger than 20% (for E-2 type spectra)

“hadronic“ hard X-rays and (multi)TeV -rays – similar morphologies !

three channels of information

about cosmic PeVatrons:

10-1000 TeV gamma-rays

10-1000 TeV neutrinos

10 -100 keV hard X-rays

-rays: difficult, but possible with future “10km2“ area multi-TeV IACT arrays

neutrinos: marginally detectable by IceCube, Km3NeT - don’t expect spectrometry, morphology; uniqueness - unambiguous signatute! “prompt“ synchrotron X-rays: smooth spectrum a very promising channel - quality! (NexT, NuSTAR, SIMBOL-X)

protons

broad-band

GeV-TeV-PeV s

synch. hard X-rays

broad-band emision initiated by pp interactiosn : Wp=1050 erg, n=1cm-3

no competing X-ray radiation mechanisms above 30 keV

Searching for Galactic PeVatrons

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

the existence of a powerful accelerator is not

yet sufficenrt for -radiation; an additional component – a dense gas target - is required

older source – steeper -ray spectrum

tesc=4x105(E/1 TeV) -1 -1 yr (R=1pc); =1 – Bohm Difussion

Qp = k E-2.1 exp(-E/1PeV) Lp=1038(1+t/1kyr) -1 erg/s

Gamma-rays and neutrinos inside and outside of SNRs

neutrinosgamma-rays

SNR: W51=n1=u9=1

ISM: D(E)=3x1028(E/10TeV)1/2 cm2/s

GMC: M=104 Mo d=100pcd=1 kpc

1 - 400yr, 2 - 2000yr, 3 - 8000yr, 4 - 32,000 yr

[S. Gabici, FA 2007]

MGRO J1908+06 - a PeVatron?

HESSpreliminary

Milagro

gamma-ray emitting clouds in GC region

HESS J1745-303

diffuse emission along the plane!

(1) indirect discovery of the site of particle acceleration

(2) measurements of the CR diffusion coefficient

HESS: FoV=5o

GC – a unique site that harbors many interesting sources packed with un- usually high density around the most remarkable object

3x106 Mo SBH – Sgr A*

many of them are potential -rayemitters - Shell Type SNRsPlerions, Giant Molecular CloudsSgr A * itself, Dark Matter …

all of them are in the FoV an IACT,and can be simultaneously probed down to a flux level 10-13 erg/cm2s and localized within << 1 arcmin

TeV gamma-rays from GC

Pulsar Winds and Pulsar Wind Nebulae (Plerions)

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

Crab Nebula – a very powerful W=Lrot=5x1038 erg/s

and extreme accelerator: Ee > 1000 TeV

Emax=60 (B/1G) -1/2 -1/2 TeV and hcut=(0.7-2) f-1mc2 -1 = 50-150 -1 MeV

=1 – minimum value allowed by classical electrodynamics Crab: hcut= 10MeV: acceleration at ~10 % of the maximum rate ( 10)

maximum energy of electrons: E=100 TeV => Ee > 100 (1000) TeV B=0.1-1 mG

– very close the value independently derived from the MHD treatment of the wind

1-10MeV

100TeV

* for comparison, in shell type SNRs DSA theory gives =10(c/v)2=104-105

Standard MHD theory

cold ultrarelativistc pulsar wind terminates by a reverse shock resulting in acceleration

with an unprecedented rate: tacc=rL/c, < 100

*)

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

.MAGIC (?)

HEGRA

TeV gamm-rays from other Plerions (Pulsar Wind Nebulae)

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 large spin-down flux

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 ! – many famous PWNe are already detected in TeV gamma-rays - MSH 15-52, PSR 1825, Vela X, ...

HESS J1825 (PSR J1826-1334)

Luminosities: spin-down: Lrot= 3 x 1036 erg/s

X: 1-10 keV Lx=3 x 1033 erg/s (< 5 arcmin)

: 0.2-40TeV L=3 x 1035 erg/s (< 1

degree)the -ray luminosity is comparable to the TeV luminosity of the Crab Nebula, while the spindown luminosity is two orders of magnitude less ! Implications ? (a) magnetic field should be significantly less than 10G.

but even for Le=Lrot this condition alone is not sufficient to achieve 10 % -ray production

efficiency (Comton cooling time of electrons on 2.7K CMBR exceeds the age of the source) (b) the spin-down luminosity in the past was much higher.

red – below 0.8 TeVyellow – 0.8TeV -2.5 TeVblue – above 2.5 TeVPulsar‘s period: 110 ms, age: 21.4 kyr,

distance: 3.9 +/- 0.4 kpc

energy-dependent image - electrons!

Gamma-ray Binaries

Mirabel 2006

PSR1259-63 - a unique high energy laboratory

binary pulsars - a special case with strong effects associated with the optical star on both the dynamics of the pulsar wind and the radiation before and after its termination

the same 3 components - Pulsar/Pulsar Wind/Synch.Nebula - as in plerionsbut with characteristics radiation and dynamical timescales less than hours

both the cold ultrarelativistic wind and shocke-accelerated electrons are illuminated by optical radiation from the companion star => detectable IC gamma-ray emission

on-line watch of creation/termination of the pulsar wind accompanied with formation of a shock and effective acceleration of electrons

time evolution of fluxes and energy spectra of X- and gamm-rays contain unique information about the shock dynamics, electron acceleration, B(r), plus … a unique probe of the Lorentz factor of the cold pulsar wind

HESS: detection of TeV gamma-rays from PSR1259-63 several days before the periastron and 3 weeks after the peristron

the target photon field is function of time, thus the only unknown parameter is B-field? Easily/robustly predictable X and gamma-ray fluxes ?

unfortunately more unknown parameters - adiabatic losses, Doppler boosting, etc. One needs deep theoretical (especially MHD) studies to understand this source

Probing the wind Lorentz factor with comptonizied radiation

Loretz factors exceeding 106 are excluded

the effect is not negligible, but notsufficient to explain the lightcurve

GLAST

HE

SS

Khangulyan et al. 2008

TeV Gamma Rays From microquasars?

HESS, 2005

MAGIC, 2006microqusars or binary pulsars?

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

scenarios? -ray production region within and outside the

binary system cannot be excluded periodicity expected? yes – because of periodic variation of the geometry

(interaction angle) and density of optical photons – as target photons for IC scattering

and absorption, as a regulator of the electron cut-off energy; also because of

variation of the B-field, density of the ambient plasma (stellar wind), ...

periodicity detected ! is everything OK ?

may be OK, but a lot of problems and puzzles with interpretation of the data …

LS5039 and LS I +61 303 as TeV gamma-ray emitters

LS 5039 as a perfect TeV clock

and an extreme TeVatron

close to inferior conjuction - maximum

close to superior conjuction – minimum one needs a factor of 3 or better sensitivity compared to HESS to detect signals within different phase of width 0.1 and measure energy spectra (phase dependent!)

can electrons be accelerated to > 20 TeV in presence of radiation? yes, but accelerator should not be located deep inside the binary system, and even at the edge of the system < 10

does this excludes the model of “binary pulsar” yes, unless the interaction of the pulsar and stellar winds create a relativistic bulk motion of the shocked material (it is quite possible)

can we explain the energy dependent modulation by absorption ? yes, taking into account the anysotropic character of IC scattering ? can the gamma-ray producton region be located very deep inside the system

no, unless magnetic field is less than 10(R/R*)-1 G (or perhaps not at all)

TeV observations with a sensitivity a factor of 3 (or so) better than HESS, to measure, in particular, the fluxes and spectra within narrow phases , very import are both 10 TeV (maximum electron energy and no absorption)

and 0.1 TeV regions (maximum absorption, maximum anysotropy effect, etc.)

GeV observation (GLAST) to measure the cascade component

X-ray observations - synchrotron radiation of primary and secondary electrons

neutrinos - if -ray are of hadronic origin, and less than several percent of the original flux escapes the source, one may expect neutrino flux marginally detectable by km3 volume detectors (current limit from X-ray observations), could be higher If GLAST detects high (cascade) fluxes

future key observations

Blazars and EBL

Blazars - sub-class of AGN dominated by nonthermal/variable broad band (from R to ) adiation produced in relativistic jets close to the

line of sight, with massive Black Holes as central engines

Urry&Padovani 1995

Sikora 1994

-rays from >100 Mpc sources - detectable because of the Doppler boosting

Large Doppler factors: make more comfortable the interpretation of variability timescales (larger source size, and longer acceleration and radiation times), reduces (by orders of magnitude) the energy requirements, allow escape of GeV and TeV -rays ( ~ j

6)

Uniqueness: Only TeV radiation tells us unambigiously that particles are accelerated to high energies (one needs at least a TeV electron to produce a TeV photon) in the jets with Doppler factors > 10 otherwise gamma-rays Cannot escape the source due to severe internal photon-photon pair production

Combined with X-rays: derivation of several basic parameters like B-field, total energy budget in accelerated particles, thus to develope a quanititative theory of MHD, particle acceleration and radiation in rela-

tivistic jets, although yet with many conditions, assumptions, caveats...

TeV emission from Blazars

important results before 2004

detection of 6 TeV Blazars, extraordinary outbursts of Mkn 501 in 1999, Mkn 421 in 2001, and 1ES 1959+650 in 2002 with overall average flux at > 1 Crab level; variations on <1h timescales; good spectrometry; first simultaneous X/TeV observations => initiated huge interest - especially in AGN and EBL communities

today

detection of >20 TeV blazars, most importantly -rays from distant blazars; remarkable flares of PKS2155-305 - detection of variability on min timescales

=> strong impact on both blazar physics and on the Diffuse Extragalactic Background (EBL) models

Hadronic vs. Electronic models of TeV Blazars

SSC or external Compton – currently most favoured models: easy to accelerate electrons to TeV energies easy to produce synchrotron and IC gamma-rays recent results require more sophisticated leptonic

models

Hadronic Models: protons interacting with ambient plasma neutrinos very slow process:

protons interacting with photon fields neutrinos low efficiency + severe absorption of TeV -rays

proton synchrotron no neutrinos very large magnetic field B=100 G + accelaration rate c/rg

“extreme accelerator“ (of EHE CRs) Poynting flux dominated flow

variability can be explained by nonradiative losses in expense of increase of total energetics,but as long as Doppler factors can be very large (up to 100), this is not a dramatic issue :

PKS 2155-304

= 3.32 +/- 0.06 +/- 0.1

PKS 2155-304

a standard phrase in Whipple, HESS, MAGIC papers “SED can be explained within both electronic and hadronic models“ ...

PKS 2155-304

2003-2005 HESS observations:

leptonic and hadronic

cooling and acceleration times of protons

Ecut=90 (B/100G)(Ep/1019 eV)2 GeV

tsynch=4.5x104(B/100G) -2 (E/1019 eV)-1 s (relatively) comfortable numbers

tacc=1.1x104 (E/1019) (B/100G) -1 s

synchrotron radiation of protons: a viable radiation mechanism

Emax =300 -1 j GeV

requires extreme accelerators: ~ 1

FA 2004

Synchrotron radiation of an extreme proton accelerator

risetime: 173 ± 28 s

Crab Flux

HESS28th July 2006

several min (200s) variabiliry timescale => R=c tvar j=101410 cm

for a 109Mo BH with 3Rg = 1015 cm => j > 100, i.e. close to the

accretion disk (the base of the jet), the bulk motion > 100

rise time <200s

gamma-rays of IC origin?

synchrotron peak of PKS2155-409 is located at <100 eV; comparison

with hcut=100 -1 j MeV => > 106 j - quite a large number, i.e. very low efficieny of acceleration ...

acceleration rate of TeV electrons (needed to produce the IC peak in the SED at energies 10GeV or so (for large Doppler factors, 10-100):

tacc= RL/c = 105 j (B/1G)-1 sec

Since B < 1 G one cannot explain the TeV variability (rise time)

in the frame of the jet tvar=200 j sec

conclusion: hadronic origin of TeV gamma-rays?

integalactic absorption of gamma-rays

new blazars detected at large z: HESS/MAGIC at z> 0.15 !

1 ES 1101 = 2.9±0.2 !

H 2356 (x 0.1) = 3.1±0.2

HESS

condition: corrected for IG absorption -ray spectrumnot harder than E- (=1.5) upper limit on EBL

HESS – upper limits on EBL at O/NIR:

“direct measurements” upperlimits

favored EBL – before HESS

HESS upper limits

lower limits from

galaxy counts

=1.5

EBL (almost) resolved at NIR ?

two options:

claim that EBL is “detected“ between O/NIR and MIR

or

propose extreme hypotheses, e.g. violation of Lorentz invariance, non-cosmological origin of z ...

or propose less dramatic (more reasonable) ideas, e.g.

very specific spectrum of electrons F v E1.33

TeV emission from blazars due to comptonization of cold relativistic winds with bulk Lorentz factor > 106

internal gamma-ray absorption

internal gamma-gamma absorption

can make the intrinsic spectrum arbitrary hard without any real problem from the point

of view of energetics, given that it can be compensated by large Doppler factor, j > 30

TeV gamma-rays and neutrinos (?) and secondary X-rays

2-3 orders of magnitude suppression of TeV gamma-rays ! if gamma-rays are of hadronic origin => neutrino flux >10 Crab should be detected by cubic-kilometer scale neutrino detectors

Gamma Rays from a cold ultrarelativistic wind ?

in fact not a very exotic scenario ...

M 87 – evidence for production of TeV gamma-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)

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) emission region R ~ 5x1015 j cm

=> production of gamma-rays very close to the ‘event horizon’ of BH?

one needs a factor of few better sensitivity at TeV energies to probe fluctuations of the TeV signal on <1 day timescales

Pair Halos (Aharonian, Coppi, Voelk, 2004)

when a gamma-ray is absorbed its energy is not lost !absorption in EBL leads to E-M cascades suppoorted by Inverse Compton scattering on 2.7 K CMBR photons photon-photon pair production on EBL photons

if the intergalactic field is sufficiently strong, B > 10-11 G, the cascade e+e- pairs are promptly isotropised

formation of extended structures – Pair Halos

how it works ?

energy of primary gamma-ray

mean free path of parent photons

information about EBL flux at

gamma-radiation of pair halos can be recognized by its distinct variation in spectrum and intensity with angle ,and depends rather weakly (!) on the features of the central VHE source

two observables – angular and energy distributions allow to disentangle twovariables

Pair Halos as Cosmological Candles

informationabout EBL density at fixed cosmological epochs given by the redshift of the central source

unique !

estimate of the total energy release of AGN during the active phase

relic sources objects with jets at large angles - many more -ray emitting AGN

but the “large Lorents factor advantage“ of blazars disapeares: beam isotropic source therefore very powerful central objects needed

QSOs and Radiogalaxies (sources of EHE CRS ?) as better candidates for Pair Halos this requires low-energy threshold detectors

EBL at different z and corresponding mean freepaths

1. z=0.034

2. z=0.129

3. z=14. z=2

1. z=0.034

2. z=0.129

3. z=14. z=2

SEDs for different z within 0.1o and 1o

EBL model – Primack et al. 2000 Lo=1045 erg/s

Brightness distributions of Pair Halos

z=0.129

z=0.129

E=10 GeV

A. Eungwanichayapant, PhD thesis, Heidelberg, 2003

synchrotron radiation of secondary electrons from Bethe-Heitler and photomeson production at interaction of CRs with 2.7K MBR in a medium with

B=1 G (e.g. Galaxy Clusters)

synchrotron -ray “halo” around a UHECR accelerator in strongly magneized region of

IGM (e.g. an AGN within a galaxy cluster)

Kelner and FA, PRD, 2008

gamma-radiation of secondary electrons !

E* = 3x1020 eV

Extragalactic sources of UHECR

non-variable but point-like gamma-rays source !

photon spectra for a source at a distance of 1 Gpc in a 20 Mpc region of the IGM: power of UHECR source is 1046 erg/s(proton spectral index = 2)

top: Ecut = 1021 eV, (1) B=0.5 nG, (2) 5 nG , (3) 50 nG

bottom: Ecut = 5 x 1020 , 1021, 5 x 1021 eV and B=1nG dotted lines - intrinsic spectra, solid lines - absorption in EBL

secondary synchrotron gamma-rays produced wthin

> 10 Mpc region of IGM around a UHECR accelerator

Gabici and FA, PRL 2005

Future

aim? sensitivity: FE => 10-14 erg/cm2 s (around 1TeV)

realization ? 1 to 10 km2 scale 10m+ aperture IACT arrays

timescales short (years) - no technological challenges

price no cheap anymore: 100+ MEuro

expectations guaranteed success - great results/discoveries first priority? ”classical” 100 (30) GeV - 30 (100 ) TeV IACT arrays next step (or parallel?): <10 GeV threshold IACT array

0.1-1 TeV threshold all sky monitor: “HAWK” (an analog of Fermi in VHE band with comparable angular and energy flux sensitivity)

two possible designs of IACT arrays =>

the slide shown first time in Padova in 1995 at the 4th “Towards a major …” Workshop but published 2years later, in: Aharonian 1997, LP97 (Hamburg)

HESS Phase 1

>3500m asl

1500-2000m asl

Detector : several 20 to 30m diameter IACTs to study the sky at energies from several GeV to several 100 GeV with unprecedented photon and source statistics

Potential: can detect “standard“ EGRET sources with spectra extending beyond 5 GeV for exposure time from 1 sec to 10 minutes Targets: Gamma Ray Timing Explorer for study of nonthermal

phenomena AGN jets, Microquasars, GeV counterparts of GRBs,

Pulsars ...

5@5 is complementary to FERMI, in fact due to small FoV needs very much FERMI and ... FERMI certainly needs a 5@5 type instrument

(1) The Magic detection of 25 GeV gamma-rays from the Crab pulsar demonstrated that a sub-10GeV threshold IACT array can be realized with advanced Cherenkov detectors

(2) GLAST detects >10 GeV gamma-rays from pulsars, AGN, GRBs

a sub-10GeV threshold IACT array can be realized during the liftime of FERMI

5@5 - a GeV timing explorer