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Gamma-ray absorption by cosmic radiation fields
Susumu Inoue (Kyoto University)
with (more than) a little help from my friends
Fermi
z~60-6
blazar
GRB
CTA
outline
1. probe of high-z UV background
SI, R. Salvaterra, T. R. Choudhury, A. Ferrara,
B. Ciardi, R. Schneider, MNRAS in press (arXiv:0906.2495)
Y. Inoue, M. Kobayashi, T. Totani, SI et al., work in prep.
2. probe of Lorentz invariance violation
effect in Galactic sources?
diffuse extragalactic background radiation at z=0
EBL=
extragalactic
background
light
1. gamma-ray absorption:
probe of diffuse radiation fields
g + g → e+ + eE e
threshold condition: E e (1-cos q )>2 me2c4
s peak
,,
=4 me2c4
e.g. TeV + 1eV (IR)
100 GeV + 10 eV (UV)
pair production
cross section
CM energy
Costamante et al 03
probe of local IRB through
gamma absorption in TeV blazars
probing local IR background with gamma-ray absorption
HESS observations of TeV blazars @z=0.165, 0.186
Aharonian+ 06 Nat.
• strongly disfavors NIR peak
no strong Pop III
• close to lower limits from galaxy counts
EBL constraints at z=0.536
Albert+ 08 Sci.
Eg(=1)
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MAGIC observation
80-500 GeV
3C279 @z=0.536
close to lower limits from galaxy counts (little missing light)
if “normal” blazar spectra >1.5
EBL constraints at z=4.35
Abdo+ 09, Sci. 323, 1688
Fermi/LAT detection of GRB 080916C up to 13 GeV without cutoff
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slides from T. R. Choudhury
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cosmic reionization epoch
z>6 current observational frontier
When? early? late? two-epoch?
How?
topology?
What? Pop III? Pop II?
mini-QSOs?
dark matter decay?
So what?
suppression of
dwarf galaxy
formation
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Madau 07
slides from T. R. Choudhury
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high-z UV radiation field (“background”)
crucial for understanding early universe
• reionizes the IGM (feedback on later galaxy formation)
• dissociates H2 molecules (feedback on later star formation)
• Ly  pumping (determines HI hyperfine 21cm level population)
but direct detection impossible!
model of cosmic reionization
Choudhury & Ferrara 05, 06, Choudhury 09
parameters e*II, e*III, hesc, l0IGM
semi-analytical model with Pop III+II stars+QSOs, radiative+chemical feedback
consistent with large set of high-z observations: WMAP, xHI, HUDF NIR counts, etc.
xHI
WMAP3
(WMAP5 OK)
SFR
Ly
LLS
reionization
begins z~15
90% z~8
100% z~6
Gphotoion
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HUDF
Lyb
TIGM
intergalactic radiation field (volume average)
caveat: model only for 4<z<22, no Pop I/dust
SI+ arXiv:0906.2945
spectra
“local’’ optical depth
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• realistic UV IRF -> opaque for Erest~10’s-100’s GeV at z<~10
• sharp cutoff at Erest~18 GeV from HI absorption above Ly edge
(reionizing radiation cannot be probed directly)
gg absorption optical depth
(high UV)
SI+ arXiv:0906.2495
low z model
Kneiske+ 04
(high stellar UV)
see also e.g.
Stecker+ 06
Razzaque+ 09
Gilmore+ 09
gg~0.4
@z=4.35, E=13.2GeV
(GRB 080916C)
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significant optical depth >12 GeV at z~5, down to 6-8 GeV at z~8-10
but not much effect z>~8 due to declining star formation, path length
gg absorption attenuation factor
low z model
Kneiske+ 04
(high stellar UV)
(high UV)
Fermi
CTA, AGIS
5@5
appreciable differences in attenuation between z~5-8 at several GeV
→ unique, important info on evolution of UV IRF below Ly edge
during cosmic reionization/first star formation
alternative models
e.g. late reionization model
with no Pop III stars
differences not large, attenuation not sensitive to reionization history
→ predictions reasonably robust?
detectability
GRBs
most luminous GRBs (LGeV~1054 erg/s, e.g. GRB 080916C)
detectable @10GeV by Fermi out to z~7
out to higher z by future large arrays! CTA, AGIS, 5@5
blazars
most luminous blazars (LGeV~1049 erg/s, e.g. 3C454.3)
detectable @10GeV by Fermi out to z~7
few such objects plausible Y. Inoue, SI, Totani+, in prep.
distinction between intrinsic cutoffs
spectral variability: IRF cutoff t-indep. <-> intrinsic cutoff t-dep.
statistical: similar cutoffs for similar z, decreasing trend with z
cosmic star formation rate
Y. Inoue, Totani, SI et al.
in prep.
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CAUTION:
data at z>6
mostly lower limits
to cosmic SFR
large uncertainties in SFR at z>6
models with low Pop II but high Pop III also possible?
cosmic star formation rate
estimated from GRB rate Kistler+ 09
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gg optical depth: comparison
Y. Inoue et al., in prep.
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large differences in
absorption!
-> distinguishable
through CTA obs.
2. gg absorption: probe of Lorentz invariance violation?
Kifune 99, Aloisio+ 00, Protheroe & Meyer 00…
following Jacob & Piran 08, PRD 78, 124010
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n=1

0.1
1
10
100
E*(TeV)
11
23
50
110
c.f. n=2
E*(=1)=0.9x1017 TeV
modified threshold
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Jacob & Piran 08
modified absorption feature
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Jacob & Piran 08
caveat: assume LIV affects only eth
effect on blazar spectra
CTA
100 TeV observations!
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Jacob & Piran 08
BUT
- depends on EBL uncertainties
- blazar spectra to >10 TeV unlikely?
effect on Galactic sources?
20,0,90
R,z,
0,0,0
20,0,180
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Moskalenko+ 06
- Galactic 100 TeV sources likely (e.g. SNRs)
- appreciable >10 TeV absorption in Galactic Center region
- probe of >20 TeV LIV recovery
summary
g-ray absorption by cosmic radiation fields
probe of UV intergalactic radiation fields at high z
based on a model of cosmic reionization
significant >12 GeV at z~5, down to 6-8 GeV at z>~8
-> valuable info on evolution of UV IRF below Ly edge
(constraints on Ly/H2 dissociating radiation)
but not much effect above z~8
detectable in high-L GRBs/blazars to z~<7 by Fermi
possibly to higher z by CTA, AGIS, 5@5
BUT alternative models with different Pop II/III SFRs possible?
-> distinguishable through GeV observations?
crucial contribution to understanding early star/galaxy formation
probe of Lorentz invariance violation?
recovery of absorption feature above ~20 TeV
effect in Galactic center sources promising?
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