A low EBL level revealed by the H.E.S.S. spectra of 1ES and H

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1 A low EBL level revealed by the H.E.S.S. spectra of 1ES and H Luigi Costamante (MPI K Heidelberg), H.E.S.S. Collaboration EBL absorption: interplay between TeV and EBL spectra Breakthrough of H.E.S.S. data: derivation of the upper limit (within the assumptions) Observational tests

2 Extragalactic Background Light: the SED Lower limits (source counts) Gardner et al HST Madau & Pozzetti 2000 HST Fazio et al Spitzer Elbaz et al ISO Dole et al Spitzer STARLIGHT UV OPT NIR FIR DUST CMB

3 Pop III stars? Santos et al. 02 Salvaterra & Ferrara 03, Kashlinsky et al z = 7 15 z = 9 30 spectrum of zodiacal light (interplanetary dust emission) Dwek et al BUT huge budget problem: where so many baryons went, to avoid conflict with present time Universe data?? (metallicity, soft X bkg, BH mass density; see Madau & Silk 2005)

4 TeV gamma rays can probe the EBL in the intergalactic space: e + e F obs E =F intr E e E Nikishov 62 Gould & Schreder 66 Jelley 66 Need sources of TeV beams: Blazars! OK: large apparent luminosities > can be detected at large distances > provide good photon statistics Problem: they are NOT standard candles (variable, wide range of spectra)

5 How absorption deforms original TeV spectra? TeV: steepening 2 5 TeV : flattening (partial recover of original spectrum: n( )~ 1 > (E) ~ E 0 ~constant ) >6 TeV: cut off

6 Zoom in: Cherenkov Telescopes in TeV band generally measure an absorbed spectrum still as power law (if intrinsic spectrum is a power law), but of steeper index

7 Dependencies: for a fixed EBL spectral shape For the same intrinsic spectrum: Steepening increases with redshift Steepening increases with EBL normalization The two combines: redshift gives leverage! (same EBL change > gives larger spectral changes at larger z) TeV fits, power law

8 Dependencies: EBL SED changes

9 Dependencies: EBL SED changes

10 Problem: interpretation of TeV blazars spectra With a high EBL: IC peak > 10 TeV Lc >> Ls Bolometric luminosity is strongly under estimated 1ES one of the most problematic Г obs = 3.5 ± 0.3

11 H.E.S.S. spectrum of PKS didn't solve the issue (it allowed only reasonable bets) Aharonian et al Г obs = 3.37 ± 0.07

12 Breaktrough: H.E.S.S. spectra of 1ES & H Г =2.88 ±0.17 z = Г =3.06 ±0.21 z = Aharonian et al. 2006, Nature 440, 1018

13 1ES same epoch SED Such hard spectra (Г 0 ) are at odds with the current blazar physics and phenomenology. If we believe the standard picture for blazar emission (broad band SED due to broad band electron distribution, particle acceleration in shocks), then: Wolter et al. 2000, Aharonian et al. 2006, in prep. since particle acceleration s 1.5 (non linear theory; Malkov & Drury 2001) Г 1.5 under most circumstances. Assumption: true average TeV blazar spectrum not harder than 1.5 Г 1.5 does not need to change the known blazar phyics scenario to agree with observations

14 1ES H

15 Upper limit: robustness Data uncertainties and evolution P = 0.34 ( for ) Primack et al.2001 Comoving EBL density EBL Evolution: Г <0.2 > P <0.1 stat + sys > P<0.1 Energy scale ( 15%) > P<0.05

16 Upper limit: robustness on EBL SED changes

17 Alternative scenarios? High UV background > too high Origin of redshift is not cosmological Breaking of Lorentz Invariance (different interaction) More viable: possible intrinsic blazar feature

18 Possible mechanisms to produce very hard spectra Bulk motion Comptonization in deep KN regime of a narrow band photon distribution (e.g. BB) > sharp pile ups (Aharonian 2001) Pile up /maxwellian e distributions seem natural outcome for turbulent acceleration (Henri & Sauge` 2004, Schlickeiser 98, Henri & Pellettier 91, Petrosian et al 94 04) Sharp low energy cut off ( min > 10 5 ): ~ monoenergetic electrons, 1/3 > Г=0.66 (Katarzynski et al. 2005) But such mechanisms should not know the level of the EBL... >

19 TeV observed spectra HESS MAGIC past CTs Galaxy counts, +evol A high EBL would require strong evolution of blazar properties between z=0.2 and 0.1

20 Bottom line: Though not (yet) the smoking gun, the H.E.S.S. spectra provide strong circumstantial evidence that the EBL density is very low, near the galaxy counts limits. > The EBL is mainly determined by the starlight from normal galaxies (no strong Pop III contribution > solve the energy budget problem). > The intergalactic space is more transparent to TeV gamma rays > we can look further > Strongly reduced uncertainty on blazar spectra reconstruction :)

21 How can we test this conclusion? Detecting and monitoring low redshift TeV blazars (look for Г < 1) Observing high redshift objects (z= ): below gal. counts? > test blazar physics AND Lorentz invariance Primack et al. 2001, 0.8x

22 Possible high z targets for MAGIC and VERITAS: Need: observations at zenith (lowest threshold) + high X ray flux objects 1ES z=0.460 RXJ z= ES z=0.444 Costamante & Ghisellini 2002

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24 Conclusions A low EBL level, near the galaxy counts, in agreement with standard galaxy evolution models, provide the simplest explanation to all observational results (HESS data, galaxy formation and evolution, metallicity, soft X ray bkg etc..)