Theoretical aspects of VHE ?-ray astronomy: Exploring Nature

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Theoretical aspects of VHE ?-ray astronomy
Exploring Natures Extreme Accelerators
APP UK 2008 meeting, Oxford, June 20, 2008

• Felix Aharonian
• Dublin Institute for Advanced Studies, Dublin
• Max-Planck-Institut f. Kernphysik, Heidelberg

2
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 Natures
  perfectly designed machines
• Extreme Particle Accelerators
  

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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
• gt 100 efficiency because of
  nonlinear effects
• acceleration of particles creates better
  conditions for their further acceleration

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Cosmic Rays from 109 to 1020 eV

• 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

SNRs ?

• Extragalactic?

T. Gaisser
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Particles in CRs with energy 1020 eV

• difficult to understand unless we assume
  extreme accelerators
• the Hillas condition - l gt 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
• gt109 Mo BH magnetospheres, small and
  large-scale AGN jets, GRBs

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

• signatures of extreme accelerators?
• synchrotron self-regulated cutoff

energy losses
FA 2000, New astronomy, 5, 377

• neutrinos (through converter mechanism)
• production of neutrons (through p? interactions)
  which travel without losses and at large distan-
  ces convert again to protons gt ?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

confinement
energy losses
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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
  ?elds, 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

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VHE gamma-ray astronomy - a success story

• 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...
• 
  

• 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)
• 
  

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good performance gt high quality data gt solid
basis for theoretical studies
RXJ 1713.7-3946
PSR 1826-1334
PKS 2155-309
28th July 2006
TeV image and energy spectrum of a SNR

• 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

enrgy dependent image of a pulsar wind nebula
variability of TeV flux of a blazar on minute
timescales
huge detection areaeffective rejection of
different backgrounds good angular (a few
arcminutes) and energy (15 ) resolutions broad
energy interval - from 100 (10) GeV to 100
(1000) TeV nice sensitivity (minimum detectable
flux) 10-13 (10-14) erg/cm2 s
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VHE gamma-ray observations Universe is full
of extreme accelerators on all astronomical
scales
TeV gamma-ray source populations

• 
  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

VHE gamma-ray source populations
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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)
• ..

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Potential Gamma Ray Sources
Extragalactic Sources
Galactic Sources
GeV
GeV
GeV
GeV
GeV
GeV
GRBs
AGN
GLX
CLUST
IGM
ISM
SNRs
SFRs
Pulsars
Binaries
Blazars
Radiogalaxies
Normal
Starburst
GMCs
Microquasars
Magnetosphere
Cold Wind
Pulsar Nebula
Binary Pulsars
EBL
G-CRs
Compact Objects
Relativistic Outflows
Cosmology
EXG-CRs

• Major Scientific Topics

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unique carriers of astrophysical and
cosmological information about non-thermal
phenomena in many galactic and extragalactic
sources
TeV neutrinos -- a complementary channel
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

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detection rate of neutrinos with KM3NeT
R.White
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sensitivity of km3 volume neutrino detectors

• 1 Crab after several years of observations
• 
  effective energy range around 10 TeV
• so far only four galactic gamma-ray sources
  are detected with a
• TeV gamma-ray flux
  at the level of 1 Crab
• 10 Crab for less than 1 month (background
  free)
• 
  effective energy range 1-10 TeV
• blazars? quite possible if TeV gamma-rays
  are of hadronic origin
• burst-like events fluence ?t x FE gt 10-5
  erg/cm2
• GRBs, SGRs/Magnetars, SN events,
  ets.

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some remarks concerning the neutrino/gamma
ratio typically gt 1, but

• 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 gt 1018
  cm-3 and/or Bgt106 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

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what should we do if hadronic gamma-rays and
neutrinos appear at wrong energies ?
detect radiation of secondary electrons !
Bethe-Heitler electrons
photomeson electrons
synchrotron radiation of secondary electrons from
Bethe-Heitler and photomeson production at
interaction of CRs with 2.7K MBR in a medium with
B1 ?G (e.g. Galaxy Clusters)
E 3x1020 eV
Kelner and FA, 2008, Phys Rev D
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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
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• exploring Natures Extreme Particle
  Accelerators
• with neutrinos, gamma-rays, and hard
  X-rays

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Galactic TeVatrons and PeVatrons - particle
accelerators responsible for cosmic rays up to
the knee around 1 PeV

SNRs ?
Pulsars/Plerions ?
OB, W-R Stars ?
Microquasars ?
Galactic Center ?
. . .
Gaisser 2001
the source population responsible for the bulk
of GCRs are PeVatrons ?
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Visibility of SNRs in high energy gamma-rays
for CR spectrum with ?2
Fg(gtE)10-11 A (E/1TeV)-1 ph/cm2s A(Wcr/1050er
g)(n/1cm-3 )(d/1kpc) -2
p0 decay (A1)
1000 yr old SNRs (in Sedov phase)
Inverse Compton
Detectability ? compromise between angle q
(r/d) and flux Fg (1/d2) typically A
0.1-0.01 q 0.1o - 1o
TeV g-rays detectable if A gt 0.1
if electron spectrum gtgt 10 TeV
synchrotron X-rays and IC TeV gs
main target photon field 2.7 K
Fg,IC/Fx,sinch0.1 (B/10mG)-2
po component dominates if A gt 0.1
(Sx/10 mJ)(B/10 mG ) -2

nucleonic component of CRs - visible through
TeV (and GeV) gamma-rays !
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RXJ1713.7-4639

• TeV ?-rays and shell type morphology
• acceleration of p or e in the shell to
• energies exceeding 100TeV

can be explained by ?-rays from pp -gt?o -gt2??

and with just right energetics Wp1050
(n/1cm-3)-1 erg/cm3
2003-2005 data
but IC canot be immediately excluded
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leptonic versus
hadronic

• 
  
• 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
• gt 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

IC origin ? very small B-field, B lt 10 mG,
and very large E, Emax gt
100 TeV two assumptions hardly can co-exists
within standard DSA models, bad fit of gamma-ray
spectrum below a few TeV, nevertheless
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Suzaku measurements gt electron spectrum 10 to
100 TeV
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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, 100?G outside
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)
Uchiyama, FA, Tanaka, Maeda, Takahashi, Nature
2007
) explanation by variation of B-field doest
work as demonstrated for Cas A (UciyamaFA, 2008)
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acceleration in Bohm diffusion regime
energy spectrum of synchrotron radiation of
electrons in the framework of DSA
(ZirakashviliFA 2007)
(Tanaka et al. 2008)
with h??0.67 /- 0.02keV
Strong support for Bohm diffusion - from the
synchrotron cutoff given the upper limit on the
shock speed of order of 4000 km/s !
B100 ?G Bohm diffusion - acceleration of
particles to 1 PeV
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RXJ 1713.7-3946

• protons
• dN/dEK E-a exp-(E/Ecut)b
• ?-rays
• dN/dE v E-G exp-(E/E0)bg
• ada, da 0.1, bgb/2, E0 Ecut/20

Wp(gt1 TeV) 0.5x1050 (n/1cm-3)-1 (d/1kpc)2
neutrinos marginally detectable by KM3NeT
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Probing PeV protons with X-rays

• SNRs shocks can accelerate CRs to lt100 TeV
• unless magnetic field
  significantly exceeds 10 mG
• recent theoretical developments amplification of
  the B-field up
• to gt100 mG is possible through plasma waves
  generated by CRs
• gt1015 eV protons result in gt1014 eV
  gamma-rays and electrons
• 
  prompt synchrotron X-rays
• t(e) 1.5 (e/1keV)
  -1/2 (B/1mG) -3/2 yr ltlt tSNR
• typically in the range between 1 and 100 keV
  with the ratio Lx/Lg
• larger
  than 20 (for E-2 type spectra)

hadronic hard X-rays and (multi)TeV
g-rays similar morphologies !
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three channels of information about cosmic
PeVatrons10-1000 TeV gamma-rays 10-1000 TeV
neutrinos 10 -100 keV hard X-rays
10-100 TeV m-neutrinos

• g-rays difficult, but possible with future
  10km2 area multi-TeV IACT arrays
• neutrinos marginally detectable by IceCube,
  Km3NeT - dont expect
• spectrometry, morphology
  uniqueness - unambiguous signatute!
• prompt synchrotron X-rays smooth spectrum
  
• a very promising channel - quality!
  (NexT, NuSTAR, SIMBOL-X)

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broad-band emision initiated by pp interactiosn
Wp1050 erg, n1cm-3
protons
broad-band
GeV-TeV-PeV gs
synch. hard X-rays
no competing X-ray radiation mechanisms above 30
keV
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Searching for Galactic PeVatrons
the existence of a powerful accelerator is not
yet sufficenrt for ?-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
32
older source steeper g-ray spectrum
tesc4x105(E/1 TeV) -1 k-1 yr (R1pc) k1
Bohm Difussion
Qp k E-2.1 exp(-E/1PeV)
Lp1038(1t/1kyr) -1 erg/s
33
Gamma-rays and neutrinos inside and outside of
SNRs
1 - 400yr, 2 - 2000yr, 3 - 8000yr, 4 - 32,000
yr

neutrinos
gamma-rays
SNR W51n1u91
GMC M104 Mo d100pc
d1 kpc
ISM D(E)3x1028(E/10TeV)1/2 cm2/s
S. Gabici, FA 2007
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MGRO J190806 - a PeVatron?
HESS preliminary
Milagro
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gamma-ray emitting clouds in GC region

• diffuse emission along the plane!

HESS J1745-303

1. indirect discovery of the site of particle
   acceleration
2. measurements of the CR diffusion coefficient

36
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
TeV gamma-rays from GC
many of them are potential g-ray emitters -
Shell Type SNRs Plerions, Giant Molecular
Clouds Sgr A itself, Dark Matter
HESS FoV5o
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 ltlt 1 arcmin
37

• Pulsar Winds and Pulsar Wind Nebulae (Plerions)

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Crab Nebula a perfect PeVatron of
electrons (and protons ?)
1-10MeV
Standard MHD theory cold ultrarelativistc pulsar
wind terminates by a reverse shock resulting in
acceleration with an unprecedented rate
tacchrL/c, h lt 100 ) synchrotron radiation
gt nonthermal optical/X-ray nebula Inverse
Compton gt high energy gamma-ray nebula
.
MAGIC (?)
100TeV
HEGRA

• Crab Nebula a very powerful WLrot5x1038
  erg/s
• and extreme accelerator
  Ee gt 1000 TeV
• Emax60 (B/1G) -1/2 h-1/2 TeV and
  hncut(0.7-2) af-1mc2 h-1 50-150 h-1 MeV
• 
  
  
• h1 minimum value allowed by classical
  electrodynamics
• Crab hncut 10MeV acceleration at 10 of
  the maximum rate ( h???10)
• maximum energy of electrons Eg100 TeV gt Ee
  gt 100 (1000) TeV B0.1-1 mG
• very close the value independently derived from
  the MHD treatment of the wind

for comparison, in shell type SNRs DSA theory
gives h10(c/v)2104-105
39

• 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 ltlt 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, ...

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HESS J1825 (PSR J1826-1334)
energy-dependent image - electrons!
red below 0.8 TeV yellow 0.8TeV -2.5
TeV blue above 2.5 TeV
Luminosities spin-down Lrot 3 x
1036 erg/s X 1-10 keV Lx3 x 1033 erg/s
(lt 5 arcmin) g 0.2-40TeV Lg3 x 1035
erg/s (lt 1 degree)
Pulsars period 110 ms, age 21.4 kyr,
distance 3.9 /- 0.4 kpc
the g-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 10mG. but even for LeLrot this
condition alone is not sufficient to achieve 10
g-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.
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Gamma-ray Binaries
Mirabel 2006
42
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 plerions
• but 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
• gt 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
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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
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
44
Probing the wind Lorentz factor with comptonizied
radiation
Khangulyan et al. 2008
HESS
GLAST
Loretz factors exceeding 106 are excluded
the effect is not negligible, but not sufficient
to explain the lightcurve
45
TeV Gamma Rays From microquasars?
HESS, 2005
MAGIC, 2006
microqusars or binary pulsars?
independent of the answer particle
acceleration is linked to (sub)
relativistic outflows
46
LS5039 and LS I 61 303 as TeV gamma-ray emitters
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
47
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!)
48

• can electrons be accelerated to gt 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 ? lt 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)

49
future key observations

• 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

50

• Blazars and EBL

51
Blazars - sub-class of AGN dominated by
nonthermal/variable broad band
(from R to g) adiation produced in relativistic
jets close to the line of
sight, with massive Black Holes as central
engines
UrryPadovani 1995
Sikora 1994
g-rays from gt100 Mpc sources - detectable
because of the Doppler boosting
52
TeV emission from Blazars

• 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 g-rays (tgg
  dj6)
• 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 gt
  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...

53
important results

• before 2004
• detection of 6 TeV Blazars, extraordinary
  outbursts of Mkn 501 in 1999,
• Mkn 421 in 2001, and 1ES 1959650 in 2002
  with overall average flux
• at gt 1 Crab level variations on lt1h
  timescales good spectrometry
• first simultaneous X/TeV observations
• gt initiated huge interest - especially
  in AGN and EBL communities
• today
• detection of gt20 TeV blazars, most importantly
  ?-rays from distant blazars
• remarkable flares of PKS2155-305 -
  detection of variability on min timescales
• gt strong impact on both blazar
  physics and on the
• Diffuse Extragalactic
  Background (EBL) models

54
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
  g-rays
• proton synchrotron
  no neutrinos
• very large magnetic field B100 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
55
PKS 2155-304
leptonic and hadronic
2003-2005 HESS observations
PKS 2155-304
PKS 2155-304
G 3.32 /- 0.06 /- 0.1
a standard phrase in Whipple, HESS, MAGIC
papers SED can be explained within both
electronic and hadronic models ...
56
cooling and
acceleration times of protons
Synchrotron radiation of an extreme proton
accelerator
FA 2004
synchrotron radiation of protons a viable
radiation mechanism Emax 300 ?-1 ?j GeV
requires extreme accelerators ? 1
Ecut90 (B/100G)(Ep/1019 eV)2 GeV tsynch4.5x104
(B/100G) -2 (E/1019 eV)-1 s (relatively)
comfortable numbers tacc1.1x104 (E/1019)
(B/100G) -1 s
57
several min (200s) variabiliry timescale gt Rc
?tvar ?j1014?10 cm for a 109Mo BH with 3Rg
1015 cm gt ?j gt 100, i.e. close to the
accretion disk (the base of the jet), the bulk
motion ? gt 100
HESS 28th July 2006
Crab Flux

• risetime 173 28 s

rise time lt200s
58
gamma-rays of IC origin?

• synchrotron peak of PKS2155-409 is located at
  lt100 eV comparison
• with h?cut100 ?-1 ?j MeV gt ? gt 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 lt 1 G one cannot explain the TeV
  variability (rise time)
• in the frame of the
  jet tvar200 ?j sec
• conclusion hadronic origin of TeV
  gamma-rays?

59

integalactic absorption of gamma-rays
60
new blazars detected at large z HESS/MAGIC at
zgt 0.15 !
HESS
1 ES 1101 G 2.90.2 !
condition corrected for IG absorption g-ray
spectrum not harder than E-G (G1.5) ? upper
limit on EBL
H 2356 (x 0.1) G 3.10.2
61
HESS upper limits on EBL at O/NIR
EBL (almost) resolved at NIR ?
direct measurements
upper limits
G1.5
lower limits from galaxy counts


62
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 ? nFn v
  Eg1.33
• TeV emission from blazars due to
  comptonization of
• cold relativistic winds with bulk
  Lorentz factor G gt 106
• internal gamma-ray absorption

63
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 gt 30
64
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 gt neutrino flux gt10 Crab
should be detected by cubic-kilometer
scale neutrino detectors
65
Gamma Rays from a cold ultrarelativistic wind ?
in fact not a very exotic scenario ...
66
M 87 evidence for production of TeV gamma-rays
close to BH

• Distance 16 Mpc
• central BH 3?109 MO
• Jet angle 30? not a blazar!
• discovery (gt4s) of TeV
• g-rays by HEGRA (1998)
• confirmed by HESS (2003)

67
M87 light curve and variabiliy
one needs a factor of few better sensitivity at
TeV energies to probe fluctuations of the TeV
signal on lt1 day timescales

• X-ray emission
• knot HST-1Harris et al. (2005),ApJ, 640, 211
• nucleus(D.Harris private communication)

Igt730 GeV cm-2 s-1
short-term variability within 2005 (gt4s) ?
emission region R 5x1015 dj cm gt
production of gamma-rays very close to the event
horizon of BH?
68
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 gt 10-11 G,
• the cascade ee- pairs are
  promptly isotropised
• formation of extended structures
  Pair Halos

69
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 two
• variables

70
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 g-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

71
EBL at different z and corresponding mean
freepaths
1. z0.034 2. z0.129 3. z1 4. z2
1. z0.034 2. z0.129 3. z1 4. z2
72
SEDs for different z within 0.1o and 1o
EBL model Primack et al. 2000
Lo1045 erg/s
73
Brightness distributions of Pair
Halos
z0.129

• z0.129

E10 GeV
A. Eungwanichayapant, PhD thesis, Heidelberg, 2003
74
Extragalactic sources of UHECR
synchrotron ?-ray halo around a UHECR
accelerator in strongly magneized region of IGM
(e.g. an AGN within a galaxy cluster)
gamma-radiation of secondary electrons !
Kelner and FA, PRD, 2008
synchrotron radiation of secondary electrons from
Bethe-Heitler and photomeson production at
interaction of CRs with 2.7K MBR in a medium with
B1 ?G (e.g. Galaxy Clusters)
E 3x1020 eV
75
non-variable but point-like gamma-rays source !
secondary synchrotron gamma-rays produced wthin
gt 10 Mpc region of IGM around a UHECR
accelerator

• 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) B0.5 nG, (2) 5 nG , (3) 50 nG
• bottom Ecut 5 x 1020 , 1021, 5 x 1021 eV
• and B1nG
• dotted lines - intrinsic spectra,
• solid lines - absorption in EBL

Gabici and FA, PRL 2005
76
Future

• aim? sensitivity FE gt
  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?) lt10 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)
  
  

77
two possible designs of IACT
arrays gt 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
1500-2000m asl
gt3500m asl
78
5_at_5 - a GeV timing explorer

• 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_at_5 is complementary to FERMI,
• in fact due to
  small FoV needs very much FERMI
• and ... FERMI
  certainly needs a 5_at_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 gt10 GeV gamma-rays from
  pulsars, AGN, GRBs
• a sub-10GeV threshold IACT array can be
  realized during the liftime of FERMI