Astrophysics and Cosmology with TeV gamma-rays

1
Astrophysics and Cosmology with TeV
gamma-rays

• F.A. Aharonian (MPI-K, Heidelberg)

Fermilab Colloquium, July 13, 2005
2
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 ...
• 
  is already (partly) opened

3
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

poorly explored
4
Crab Nebula broad-band SED 20
decades
g-rays 9 decades (detected !)
100 keV 100 TeV
COMPTEL
EGRET
HEGRA
CELESTE (MAGIC)
5
Status of the field

• in 1990s, after several decades of struggles and
  controversial
• developments ground-based
  gamma-ray astronomy
• became an observational
  discipline and entered the
• main stream of modern
  astrophysics and cosmology with
• viable detection technique Imaging Atmospheric
  
• Cherenkov Telescope (IACT)
  Arrays -
• emerged as prime tool for detection of
  VHE g-rays
• more than 25 reported objects representing
  several
• galactic and extragalactic source
  populations
• the principal results obtained at TeV
  energies with IACTs
• (plus some interesting results with a
  water Cherenkov detector)

6
Intensity Shower Energy
Image Shape Background rejection
7
stereoscopic approach
image of source is somewhere on the image axis
need several views to get unambiguous shower
direction
8
Why Cherenkov telescopes ?

• large detection area typically 0.1 km2,
• 
  potentially up to 10km2
• low energy threshold typically 0.1-1 TeV,
• potentially down to a
  few GeV
• first result 3s signal from Crab - 3 years
  observations with
• the Whipple non-imaging
  10m telescope (1969)

but ... cosmic ray detectors
rather than g-ray telescopes...
CT an optical reflector with a PMT in
focus fast (ns) electronics
9
Why Imaging ?

• because it allow reconstruction of shower
  parameters
• (certain) information about arrival direction
• capability to separate g- and proton induced
  showers
• larger FoV (larger collection areas)
• first result 10 sigma signal from Crab with
  the
• Whipple imaging 10 m
  telescope (1989)
• a good gamma-ray detector but ...
• not
  yet a telescope...

10
Why Stereoscopy ?

• better separation of hadronic and E-M showers
• better sensitivity
• angular resolution of about 3 arcmin
• better sensitivity,
  source localization, morphology
• energy resolution 10 to 15 per cent
• better spectrometry
• rejection of local muons, better rejection of
  N.S.B.
• lower energy threshold,
  systematics under control
• quite large (up to 5 degree) FoV
• extended sources,
  surveys, huge collection areas
• first results HEGRA system of small (4m
  diameter)
• IACTs in La
  Palma (1996-2002)

11

• IACT Arrays as perfect g-ray telescopes
  
• in the interval 100 GeV - 10 TeV (TeV
  Astronomy)
• and (hopefully) also in the
  intervals
• several GeV to 100 GeV (multi-GeV
  astronomy)
• 10 TeV to several 100 TeV (multi-TeV
  Astronomy)

12
CANGAROO-III
March 2004
Feb 2000
Woomera, Australia
13
VERITAS
Prototype, May 2004
Artists conception
Kitt Peak, Southern Arizona
14
MAGIC a 17m diameter telescope
La Palma, Canary Islands
15
Stereoscopic Imaging of Air Showers

• (almost) commonly
  accepted approach
• CANGAROO-III, H.E.S.S., VERITAS, MAGIC-2
• for
  TeV astronomy
• 5_at_5, ECO-1000
• 
  for multi-GeV astronomy

16
H.E.S.S. - High Energy Stereoscopic System
13m diameter dish
920 pixel, 5 deg FoV camera
17
Potential of IACT Arrays

• sensitivity down to 10-13 erg/cm2s
  angular resolution a few arcminutes
• energy resolution 10 to 20
  dynamical range 3 GeV to 100TeV

Crab Nebula
Energy Flux, E2J(E), erg/cm2 s

?
HEGRA EGRET
18

• energy range
• 100 GeV - 10 TeV
• energy resolution
• 15 - 20
• angular resolution
• 3 - 6 arcmin
• sensitivity
• 1 Crab 30 sec
• 0.1 Crab 20min
• 0.01 Crab 25 hours
• 10 Crab 1 sec
• Field of View
• 5o

• 1 Crab 3 x 10 -11 erg/cm2 s
• 0.1 Crab - min detection time
• for Whipple 50-100 hour
• 0.003 Crab requires 200 h
• 10-13 erg/cm2 s level
• better than Chandra/XMM for gt0.1 deg objects !
• 10 Crab (i) strong flares of Mkn 421/501
• (ii) energy flux sensitivity
  of EGRET
• (iii) several orders of
  magnitude
• less than typical GRB
  fluxes
• 3 arcmin - angular resolution of ASCA
• 5o FoV plus 0.1 Crab for lt 0.5 h
• sufficient for effective surveys !

19
TeV Sky before 2004

• TeV sources not many, but represented by
  several populations

20
Reported TeV Sources
before 2004

• Blazars Markarian 421 Markarian 501
  1es2344514
• 1es1959650
  1es1426428 PKS 2155-304
• Plerions Crab Nebula PSR 1706-44
  
• SNRs Cas A SN 1006
  RX1713.7-394
• Radiogalaxies M87 Cen A
• X-ray binaries Cygnus X-3, Cen X-3,
  GRS1915105
• Starburst Galaxies NGC 253
• First Uniden.source TeV J20324131

21
First (published) H.E.S.S results

• 
  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 a Microquasar
• 
  Galactic Center
• 
  Extragalactic objects
• M87 - a radiogalaxy
• new TeV Blazars PKS 2155-304,PKS 2005-489,
  
• and a large number of yet unidentified TeV
  sources

22
Expectations - near 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
• Stereoscopic IACT Arrays large photon
  statistics !
• tens (hundreds ?, thausands ?) objects
  based on datasets
• consisting of more than 1,000 TeV g-ray
  photons per source
• also exploration of a few (many ?) objects in
  gt 10TeV domain

23
distinct features of Cherenkov
Astronomy
huge detection areas ( large photon statistics)

nice (a few arcminutes) angular resolution

reasonable (10 to 15 ) energy resolution
high quality spectrometric, temporal, and
morphological studies of nonthermal objects
representing different G- and EXG- source
population and many hot topics of HE
Astrophysics, Astroparticle Physics and Cosmology
24
time average spectra of Mkn 421 and Mkn 501
Spectrometry beyond 3Ecutoff !

• Unprecedented photon statistics
• Mkn 421 60,000 TeV photons
• detected in 2001
• Mkn 501 40,000 TeV photons
• detected in 1997
• spectra canonical power-law
• with exponential cutoff
• Cutoff 6.2 TeV and 3.8 TeV
• for Mkn 501 and Mkn 421

TeV
25
Mkn 421 extraordinary high
state in 2001
time variations on sub-hour timescales !
Whipple
RXTE
26
morphology and
spectrometry
Spatially resolved Energy Spectra (HESS)
RX 1713.7-3946
preliminary
preliminary
27
TeV g-ray Astronomy - a viable discipline in
its own right

• TeV emission as an extension of GeV
  domain ?
• visibility of sources in GeV g-rays
  does not
• authomatically imply visibility in TeV
  g-rays,
• 
  and vice versa
• Reasons ? Efficiency of acceleration process,
  spectral cutoffs due to
• internal and external absorption, diffusion of
  charged parent particles
• ...

TeV astronomy is not an extension of MeV/GeV
astronomy, but a viable discipline in its own
right with several major objectives
28
major objectives of TeV g-ray
astronomy

• Origin of Galactic Cosmic Rays
• SNRs, Molecular clouds, Diffuse radiation
  of the Galactic Disk, ...
• Galactic and Extragalactic Sources with
  relativistic flows
• Pulsar Winds, mQSOs, Small and Large Scale
  jets of AGN, GRBs...
• Observational Gamma Ray Cosmology
• Large Scale Structures (Clusters of
  Galaxies), Dark Matter Halos,
• Diffuse Extragalactic Background radiation,
  Pair Halos
• ........
• ....
  

energy domain E gt 0.1 TeV (VHE astronomy TeV
astronomy)
29
Origin of Cosmic Rays

• 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)
• thanks to the new generation of ground and
  space-based
• gamma-ray detectors , in particular
• HESS and
  GLAST

30
g-rays as tracers of GCRs

• we know a lot about Galactic Cosmic Rays (energy
  spectra,
• composition, propagation) but we do not know
  
• acceleration sites, source populations,
  acceleration mechanisms
• reason ? deflection (diffusion) of CRs in
  interstellar B-fields
• solution ? probing CRs with high energy
  gamma-rays
• discrete g-ray sources
  productions sites oF CRs
• diffuse g-ray emission
  propagation of CRs in ISM
• the major (historical) motivation of gamma-ray
  astronomy
• (P.
  Morrison, V. Ginzburg, S. Hayakawa, ...)

31
Galactic PeVatrons accelerators responsible for
CRs up
to (at least) 1 PeV (1015 eV)

SNRs ?
Pulsars/Plerions ?
O B stars ?
Microquasars ?
Galactic Center ?
. . .
Gaisser 2001
the source population responsible for the bulk
of GCRs are PeVatrons ?
32
SNRs the most probable factories of GCRs ?

• (almost) common belief based two arguments
• necessary amount of available energy 1051 erg
• Diffusive Shock Acceleration 10 efficiency
  and E-2 type spectrum
• 
  up to ? at least
  1015 eV
• Straightforward proof detection of g-rays (and
  neutrinos) from
• pp interactions (as products of decays
  of secondary pions)
• Objective to probe the content of nucleonic
  component of CRs in
• SNRs within 10 kpc
  at the level 1049 -1050 erg
• Realization sensitivity of detectors - down
  to 10-13 erg/cm2 s
• crucial energy domain
  - VHE/UHE (up to 100 TeV)

33
Cosmic Ray Accelerators ?
SNRs in our Galaxy 231(Green et al. 2001
with nonthermal X-ray emission - 10 or so
best candidate - young SNRs with
synchrotron X-rays
SN1006
Diffusive source
Tycho
Kepler
CasA
?
30 arcmin
TeV emission
H.E.S.S. PSF
34
energy spectrum
RXJ1713.7-3946 is a TeV source !
morphology
do we see a shell structure ?
G2.2 -evidence of DSA of protons?
HESS 2004 data preliminary !
35
RX 1713.7-3946 morphology and energy spectrum
obtained with H.E.S.S.
the key issue - identification of g-ray
emission mechanisms p0 or IC ? new! -
energy spectra 150GeV-30 TeV from
different parts - NW, S W, E,C if a
coordinate-independent single power law
from 100 GeV to 10 TeV
hardly can be explained by IC
implications ? if p0 - hadronic component
is detected ! estimate of Wp (with an
uncertainty related to the
uncertainty in n/d2 ) if IC - model
independent estimate of We (multi-TeV
electrons) and also LeLx ! model
independent map of B-field
TeV-keV correlations what this could mean?
36
Origin of radiation ?

• hadronic origin seems preferable given
• the high density environment
  
• Wp (above 10 TeV) 3x1049 (n/1 cm-3) -1 erg
  
• IC origin is not excluded, but this model
• requires B field less than 10-20 mG
• More complex scenario, e.g. g-rays from NWSW are
  contributed by
• protons while gamma-rays from remaining parts are
  due to IC g-rays,
• cannot be
  excluded
• HESS observations with 4 telescope in
  2004 and 2005
• provide higher quality data and
  certain answers ?

FA, Nature 2002
37
New ! Vela Junior (a 2o diameter remnant)
B-fields RXJ 10 mG Vela Jr 4 mG
B-fields RXJ 10 mG Vela Jr 4 mG
CANGAROO , HESS Flux - 1 Crab at 1 TeV
uncertainty in d as large as factor of 3, n
poorly known if no nearby clouds - Wp could be
as large as 1050 erg
IC ? very small magnetic field at
the level of lt 4 mG
38
searching for galactic PeVatrons ...
TeV gammarays 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
39
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
40
older source steeper gamma-ray spectrum
41
Giant Molecular Clouds (GMCs)
as tracers of Galactic Coismic Rays

• GMCs - 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 ...

42
First
Unidentified TeV source TeV J20324130

• Found by HEGRA seredipiously (6 sigma signal
  accumulated 100h from
• the Cygnus region and confirmed in 2002 by
  pointing observations (130 h)
• Basic features hard power-law spectrum (photon
  index 1.9), constant flux
• and slightly
  extended (about 5 arcmin) source
• Origin ? leptonic (IC) origin is
  almost excluded, possibly dense gas cloud(s)
• illuminated by
  protons arriving from a recent nearby Pevatron
  ?
• if this object is a representative of a large
  source population, the planned surve
• of the Galactic Disk by H.E.S.S. will
  reveal (many ?) more new hot spots

detected earlier by the HEGRA array and
Crimean and recently by Whipple groups(?)
43
A new unidentified sources is found
by HESS !
Feb 2004
March 2004
PSR1259-63
44
HESS detected new galactic sources
unidentified HESS sources
45
HESS
Aharonian et al. 2005
TeV and CO data narrow distributions in
the Galactic Plane because of GMCs ? or
Star Formation Regions ? or (most likely)
both ?
NANTEN CO observations
Fukui et al.2005
46
Crab Nebula g-rays up to 100TeV !
1-10MeV
100TeV

• Standard MHD theory cold ultrarelativistc
  pulsar wind terminates by a reverse
• shock resulting in
  (re)acceleration of electrons up to gt 1015 eV
  
• Synchrotron radiation gt
  nonthermal optica and X-ray nebula
• Inverse Compton scat. gt
  high energy gamma-ray nebula
• Crab Nebula a very powerful
• and extreme accelerator
  
  
  (tacchRg/c)
• hncut 10MeV acceleration at 1 to 10
  of the maximum rate ( h10-100)
• maximum energy of electrons
• Eg100 TeV gt
  Ee gt 100 (1000) TeV gt B lt 1 mG (close to 0.1
  mG)

for comparison, in shell type SNRs DSA theory
gives h10(c/v)2104-105
47
Challenges

• measurements of the energy-dependent size of IC
  component
• detection of possible hadronic component
• gt 1 TeV neutrinos (marginally)
  detectable by Ice Cube
• to probe location of creation and the Lorentz
  factor of kinetic energy dominated wind through
  IC scatering of wind electrons
• cold wind can be visible/detectable in
  gamma-rays with energy
• E me c2 x wind Lorentz factor G
  (because of K-N effect)
• unique feature of VHE gamma-ray
  astronomy - discovery of
• ultrarelativistic flows through
  bulk motion Comptonzation

48

• TeV gamm-rays from other Plerions ?
  
• Crab Nebula is a very effective accelerator
• 
  but not an effective IC g-ray emitter
• We see TeV gamma-rays from the Crab Nebula
  because of
• very large spin-down luminosity
  
• but 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 !

Plerions Pulsar Driven Nebulae
49

MSH 15-52
dN/dE ? E-G G 2.27?0.03?0.15 ?2/n
13.3/12 Flux gt 280 GeV 15 Crab Nebula

• the energy spectrum - a perfect hard power-law
  with photon index G2.2-2.3
• over 2
  decades !
• cannot be easily explained by IC
• hadronic (po-decay) origin of g-rays ?

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)

50
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
• both the electrons of the cold wind and
  shocke-accelerated electrons are illuminated
  by
• optical radiation from the companion star
  detectable IC g-ray emission
• the photon field is a strong function of time,
  thus the only unknown parameter is B-field
• TeV electrons are cooled and and radiate in deep
  Klein-Nishina regime with
• very interesting effects on both synchrotron
  X-ray and IC gamma-rays

HESS detection of TeV gamma-rays from
PSR1259-63 at lt 0.1Crab level several days
before the periastron and 3weeks after the
periastron
but with characteristic timescales much shorter
- less than 1 h !
51
energy flux of starlight close to the
periastron around 1 erg/cm3 B-field is
estimated between 0.1 to 1 G
predictable X and gamma-ray fluxes ?
time evolution of fluxes and energy spectra of X-
and g-rays contain unique information about the
shock dynamics, electron acceleration, B(r), ...
52
if the gamma-ray energy spectrum can
be (more or less) explained by IC the
lightcurve is still a puzzle deep
theoretical (in particular MHD) studies
needed to understand the source
53
new ! HESS detects TeV g-rays from a
microquasar !
VLBA-VLA image Paredes et al. 2000
NASA,, ESA, and F. Mirabel

• LS 5039 X-ray binary - BH O7 star
• presence of two basic components for TeV
  gamma-ray production !
• 0.2c jet as accelerator of electrons (protons ?)
• 1039 erg/s luminosity star as source of seed
  photons for IC or pg
• scenario ? both gamma-ray production region
  within (despite tgg gtgt 1) and
• outside binary system (jet
  termination site) cannot be excluded

mQSOs one of the highest priority targets
of the HESS project
54
TeV Blazars and Diffuse Extragalactic

Background Radiation

• two topics relevant to different
  research areas

TeV Blazars ideal laboratories to study particle
acceleration and MH structures in relativistic
jets, and powerful factories of GeV/TeV g-ray
beams DEBRA (also EBL, CIB,) thermal emision
components - between O/UV and FIR - produced
by stars and absorbed/re-emitted by dust, and
accumulated over the entire history of the
Universe

• but tightly coupled through intergalactic g-g
  absorption

only a few remarks about the problem
55
Impact of
the intergalactic absorption
56

• TeV g-rays - carriers of unique cosmological
  information about
• epochs and history of
  evolution of galaxies
• such information can be extracted through
  studies of intergalactic
• absorption features in the energy spectra of
  blazars of given z, if
• one can unambiguously identify the
  intergalactic absorption features
• two (both not perfect) approaches
• measure the intrinsic spectrum based on
  comprehensive time-
• dependent modeling of multiwavelength
  data (broad-band SED)
• but this is a very
  hard (almost impossible) task
• accept a principle the intrinsic spectrum
  Jo(E)Jobs(E) expt(E) should be reasonable
• but what means
  reasonable ?
• or if gamma-rays are of hadronic
  (pp-gtpo-gtgg) origin
• measure the spectrum
  of TeV neutrions
• nice dream and still not sufficient
  (intrinsic absorption of gs)

absorption does not mean spectral cutoff
57
X-TeV flares of 1ES 1959650 in 2002

• Basic conclusions
• correlations
• X-TeV do correlate
• No optical TeV (X) correlations
• Radio essentialy stable
• puzzles
• strong TeV flare on June 4 not
• accompanied by an X-ray activity
• very hard gamma-ray spectrum below 1 TeV (photon
  index could be significantly less than 1.5 (!)
• (after correction for IG absorption)

58
1ES 1426428 a different blazar ?
IC ?

Proton synchrotron?

• 1ES 1426428 does not agree with
• the red-blue phenomenology

59
Gamma-rays and X-rays from DM in GC
B2.5 mG, Eo8, 25, gt 1000 TeV
Eo10 TeV, B1, 3, 10 mG
60
Models

• SSC or external Compton currently
  most favoured models
• easy to accelerate electrons to TeV energies
• easy to produce synchrotron and IC gamma-rays
• recent blazar observations 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

expect neutrinos from EGRET AGN but not from
TeV blazars
61
Cooling and acceleration times in Markarian 501
in TeV blazars synchrotron cooling time always ltlt
photomeson colling time
no neutrinos from TeV
blazars
no VHE gamma-rays from most powerful and distant
AGN and QSOs but (possibly)
detectable fluxes of VHE and UHE neutrinos
62
1ES1426428 - a special case

• many puzzles
• difficult to believe... TeV gamma-rays from
  this source at z0.129 despite
• the intergalactic absorption gtgt 10
• TeV peak significantly higher than the X-ray peak
• violation of the red-blue blazar
  paradigm cannot be easily explained
• by standard SSC or external Compton models
• only a specific class of EBL models allows
  reasonable instrinsic TeV spectrum

63
near future

• observations of 1ES1426428 (Whipple/HEGRA/CAT)
  with
• different instruments and different times - no
  robust conclusions
• further bservations of 1ES1426428 and 2155-304,
  and especially
• discovery of new blazars with similar and larger
  redshifts (z gt 0.11),
• 
  very important and promising ...
• let assume (hope) that new observations will
  confirm the conclusion
• that only a specific class of EBL model allows
  reasonable intrinsic
• TeV spectra of these blazars ... then what ...
  ?

64
two options

• claim that EBL is detected between
  O/NIR and MIR !
• propose extreme hypotheses, e.g.
• violation of Lorentz invariance,
  non-cosmological origin of z ...
• or propose less dramatic ideas, e.g
• TeV emission from blazars due to the
  comptonization of
• cold ultrarelativistic winds with
  Lorentz factor gt 106
• Solution ?
• detect many blazars at different redshifts and
  ... try to detect
• Pair Halos formed (unavoidably) around TeV
  extragalactic sources

65
Gamma Rays from a cold ultrarelativistic wind ?
in fact not a very exotic scenario
66
Pair Halos

• 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

67
how it works ?

• 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

68
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 powerful
  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 now
• disapeares
  beam isotropic source
• therefore more powerful central objects
  needed
• powerful QSOs and Radiogalaxies (sources of EHE
  CRS ?)
• as
  better candidates for Pair Halos
• this requires low-energy threshold
  detectors

69
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
70
SEDs for different z within 0.1o
and 1o
EBL model Primack et al. 2000
Lo1045 erg/s
71
Brightness distributions of Pair
Halos

• z0.129

z0.129
E10 GeV
A. Eungwanichayapant, PhD thesis, Heidelberg, 2003
72
TeV g-rays from GC
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 g-ray emitters -
Shell Type SNRs Plerions, Giant Molecular
Clouds Sgr A itself, Dark Matter
HESS FoV5o
all of them are in the FoV HESS ! and can be
probed down to a flux level 10-13 erg/cm2 s
and localized within ltlt 1 arcmin
73
Position?
systematic and statistical errors on source
location by HESS are comparable
20-30 arcseconds
74
two comments

• typically (often) theorists face problems of
  interpreting g-ray
• observations in the frameworks of
  "standard" models, but in the
• case of TeV observations of GC we face an
  opposite problem
• TeV data can be explained within several
  (essentially different)
• scenarios and by several
  different radiation mechanisms
• the FoV, PSF, and sensitivity of HESS (and
  GLAST) perfectly match
• the performance of other relevant instruments
  at other wavelengths
• (Chandra, XMM, INTEGRAL, VLT, radio and mm
  telescopes, etc.)
• both for compact objects like Sgr A and
  diffuse structures
• HESS and GLAST can provide perfect temporal,
  spectroscopic and
• morphological studies over six (100 MeV
  to 100 GeV) g-ray decades

75
TeV g-rays from central lt10 pc region of GC

• Annihilation of DM ? mass of DM particles gt 12
  TeV ?
• Sgr A 3 106 Mo BH ? yes
• even the inner R lt 10 Rg region is
  transparent for TeV g-rays !
• SNR Sgr A East ? why not ?
• Plerionic (IC) source(s) why not ?
• Interaction of CRs with dense molecular
• gas (clouds) ? easily
  

76

• or Dark Matter ?

annihilation of SUSY or other DM candidate
particles
77
DM Annihilation?

• HESS Spectrum requires a gt 10
  TeV DM particles
• most WIMPs models favour a
• lt 2 TeV mass neutralinos
• other DM particle candidates ?
• GMSB Gauge mediated
• Supersymmetry Breaking
• Kalusa-Klein Dark Matter
• also a rather cuspy profile and a high density of
  DM in the very central part (around SBH/Sgr A)

Wimp annihilation spectra have a cutoff at
(0.20.3) M?
78
Point-like but not variable TeV source an
argument in favor of DM origin of detected TeV
gamma-rays

• angular size of TeV signal can be explaind by
  DM annihilation for
• n(r) profile like r-a with a gt 1.1 i.e.
  Qg(r)C1n2 Qor-2a
• note that the same can be the case of CR
  interactions with gas
• Qg(r)C2ncr(r)
  nH(r)Qor-(a1a2) ,
• e.g. CR density decreases like r-2 and the gas
  density like r-0.2
• but the absolute intensity of the TeV signal
  requires much sharper
• density
  profile n(r) within lt 0.1 pc
• DM TeV signal from GC - point-like with an
  angular size ltlt 1 arcmin

C1 and C2 interaction constants
(cross-sections)
79
CONCLUSION unfortunately TeV g-ray data cannot
provide information about Qg(r) (and therefore
about DM cusp), inside 1 pc (less than
30) what about synchrotron X-radiation of
secondary electrons ? (from decays of p/-
mesons accompanying neutral p mesons) Lx
comparable with Lg , hn few (B/1 mG) (Eg/10
TeV)2 keV provided that the B-field inside
0.1 pc exceeds 1 milliGauss important synchr.
cooling time of secondary electrons is shorter
than other characteristic times
(unlike radio emitting electrons) gt X-rays
are produced simultaneously with po decay
g-rays important Chandra detected 1.4 arcsec
size diffuse X-ray
emission around Sgr A this can be used to
constrain the flux
of the DM TeV signal
80
X-ray and g-ray fluxes associated with DM in GC
B2.5 mG different g-ray spectra
fixed g-ray spectrum B1, 3, 10 mG
production and cooling of electrons in a constant
B-field over 1010 yr