blazar Gamma-ray absorption by cosmic radiation fields GRB Fermi z~60-6 CTA Susumu Inoue (Kyoto University) with (more than) a little help from my friends

outline 1. probe of high-z UV background 2. probe of Lorentz invariance violation SI, R. Salvaterra, T. R. Choudhury, A. Ferrara, B. Ciardi, R. Schneider, MNRAS in press (arXiv: ) Y. Inoue, M. Kobayashi, T. Totani, SI et al., work in prep. effect in Galactic sources?

diffuse extragalactic background radiation at z=0 EBL= extragalactic background light

gamma-ray absorption: probe of diffuse radiation fields  +  → e + + e - threshold condition: E  (1-cos  )>2 m e 2 c 4 E  e.g. TeV + 1eV (IR) 100 GeV + 10 eV (UV)  peak,, =4 m e 2 c 4 pair production cross section CM energy probe of local IRB through gamma absorption in TeV blazars Costamante et al 03 1.

probing local IR background with gamma-ray absorption Aharonian+ 06 Nat. HESS observations of TeV no strong Pop III strongly disfavors NIR peak close to lower limits from galaxy counts

E  (  =1) EBL constraints at z=0.536 Albert+ 08 Sci. close to lower limits from galaxy counts (little missing light) if “normal” blazar spectra  >1.5 MAGIC observation GeV

EBL constraints at z=4.35 Abdo+ 09, Sci. 323, 1688 Fermi/LAT detection of GRB C up to 13 GeV without cutoff

slides from T. R. Choudhury

cosmic reionization epoch early? late? two-epoch? When? mini-QSOs? What? So what? suppression of dwarf galaxy formation How? topology? Pop III? Pop II? Madau 07 dark matter decay? z>6 current observational frontier

slides from T. R. Choudhury

high-z UV radiation field (“background”) crucial for understanding early universe but direct detection impossible! reionizes the IGM (feedback on later galaxy formation) dissociates H 2 molecules (feedback on later star formation) Ly  pumping ( determines HI hyperfine 21cm level population)

model of cosmic reionization Choudhury & Ferrara 05, 06, Choudhury 09 semi-analytical model with Pop III+II stars+QSOs, radiative+chemical feedback WMAP3 x HI SFR HUDF Ly  Ly  LLS  photoion T IGM parameters  *II,  *III,  esc, 0IGM consistent with large set of high-z observations: WMAP, x HI, HUDF NIR counts, etc. reionization begins z~15 90% z~8 100% z~6 (WMAP5 OK)

intergalactic radiation field (volume average) “local’’ optical depth spectra realistic UV IRF -> opaque for E rest ~10’s-100’s GeV at z<~10 sharp cutoff at E rest ~18 GeV from HI absorption above Ly edge (reionizing radiation cannot be probed directly) caveat: model only for 4<z<22, no Pop I/dustSI+ arXiv:

significant optical depth >12 GeV at z~5, down to 6-8 GeV at z~8-10  absorption optical depth (high UV) low z model Kneiske+ 04 (high stellar UV) see also e.g. Stecker+ 06 Razzaque+ 09 Gilmore+ 09 but not much effect z>~8 due to declining star formation, path length   E=13.2GeV (GRB C) SI+ arXiv:

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 low z model Kneiske+ 04 (high stellar UV)  absorption attenuation factor Fermi CTA, AGIS (high UV)

alternative models differences not large, attenuation not sensitive to reionization history → predictions reasonably robust? e.g. late reionization model with no Pop III stars

detectability most luminous blazars (L GeV ~10 49 erg/s, e.g. 3C454.3) by Fermi out to z~7 blazars GRBs distinction between intrinsic cutoffs spectral variability: IRF cutoff t-indep. intrinsic cutoff t-dep. few such objects plausible Y. Inoue, SI, Totani+, in prep. most luminous GRBs (L GeV ~10 54 erg/s, e.g. GRB C) by Fermi out to z~7 out to higher z by future large arrays! CTA, AGIS, statistical: similar cutoffs for similar z, decreasing trend with z

cosmic star formation rate 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? Y. Inoue, Totani, SI et al. in prep.

cosmic star formation rate estimated from GRB rateKistler+ 09

 optical depth: comparison Y. Inoue et al., in prep. large differences in absorption! -> distinguishable through CTA obs.

 absorption: probe of Lorentz invariance violation? 2. following Jacob & Piran 08, PRD 78, Kifune 99, Aloisio+ 00, Protheroe & Meyer 00… n=1  E*(TeV) c.f. n=2 E*(  =1)=0.9x10 17 TeV

modified threshold Jacob & Piran 08

modified absorption feature Jacob & Piran 08 caveat: assume LIV affects only  th

effect on blazar spectra Jacob & Piran 08 BUT - depends on EBL uncertainties - blazar spectra to >10 TeV unlikely? CTA 100 TeV observations!

effect on Galactic sources? Moskalenko Galactic 100 TeV sources likely (e.g. SNRs) - appreciable >10 TeV absorption in Galactic Center region - probe of >20 TeV LIV recovery R,z,  0,0,0 20,0,90 20,0,180

summary crucial contribution to understanding early star/galaxy formation detectable in high-L GRBs/blazars to z~<7 by Fermi possibly to higher z by CTA, AGIS, 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  -ray absorption by cosmic radiation fields BUT alternative models with different Pop II/III SFRs possible? -> distinguishable through GeV observations? probe of Lorentz invariance violation? recovery of absorption feature above ~20 TeV effect in Galactic center sources promising?