Probing the diffuse Extragalactic Background Light (EBL) with gamma ray blazars

Transcription

1 Probing the diffuse Extragalactic Background Light (EBL) with gamma ray blazars Luigi Costamante (HEPL/KIPAC, Stanford University) Diagnostic: the interplay between GeV TeV and EBL spectra Progresses in the 3 bands: Opt NIR, MIR, FIR Next steps & Caveats

2 EBL: the Spectral Energy Distribution See talk by R. Gilmore 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 DUST FIR 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 to hide so many baryons in order to avoid conflicts with data of the Universe at present time?? (metallicity, soft X bkg, BH mass density; see Madau & Silk 2005)

4 GeV TeV gamma rays can probe the EBL in the intergalactic space: GeV TeV photons collide with Opt NIR photons of the diffuse EBL: e+ e F obs E =F intr E e E Nikishov 62 Gould & Schreder 66 Jelley 66

5 Diagnostic: how EBL absorption deforms TeV spectra? TeV: steepening 2 5 TeV : flattening (partial recover of original spectrum: 1) EBL SHAPE: n( )~ 1 > (E) ~ E0 ~constant ) >6 TeV : cut off Aharonian ICRC 2001

6 Opt NIR: net effect = steepening TeV powerlaw > powerlaw with steeper index

7 NIR MIR rollover steepening or flattening? Primack et al 2005, Stecker et al 2006

8 Diagnostic 2: Normalization & Redshift Steepening increases with z Steepening increases with EBL level The two combine: redshift gives leverage (same EBL change > gives larger spectral changes at larger z)

9 Further Dependencies: high flux NIR

10 Further Dependencies: high flux UV

11 Blazars as gamma ray beamers: PRO: powerful gamma ray engines, relativistically beamed: large apparent luminosities detectable at large distances provide good photon statistics only extragalactic sources detected so far at VHE... (GRBs??) CONTRA: they are NOT standard candles: highly variable, wide range of properties among objects, not yet understood, emission mechanism not yet established; wide range of spectra (both curved and not, Г = 1.5 4) We are not able (yet) to reliably predict the emission spectrum from mwl data.

12 Different objects probe different EBL wavebands (needed optimal combinations ray spectrum redshift ) 22 Z: > <0.05 Costamante et al ; Dwek & Krennrich 2005 ; Mazin & Raue

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14 Opt NIR constraints 22 Z: > 0.1 Most detections at VHE

15 Problem of blazar interpretation: observed gamma ray spectrum obs = 3.37 ± 0.07 is: int High EBL int Low EBL Aharonian et al Hard TeV sources needed!

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

17 Г = 1.5

18 Г = 1.5 What is NOT: it's not the hardest possible theoretical spectrum it's not the hardest imaginable spectrum in blazars it's not a sharp, hard limit Examples: bulk motion Comptonization (Aharonian et al 2001, 2006) high energy low energy cutoff in particle spectrum (Katarzynski et al 2007) internal absorption on narrow banded target field (Aharonian et al 2008) uncooled particle acceleration spectrum > Г~1.2 (Aharonian et al 2006) pile up particle distributions or fine tuned shock acceleration conditions (e.g. Stecker et al 2007, but dibated, anyway with Г >1.2)

19 Г = 1.5 What it is: It is the borderline between reality and speculation Г 1.5 we are sure: observationally confirmed and can be obtained theoretically in many circumstances (no special tuning); Г <1.5 is progressively more and more unlikely: it requires either parameters pushed to the limits, or ad hoc scenarios not supported by data Synchrotron emission traces directly the particle spectra: never observed in blazars spectra from high energy particles ( >103) with photon index Г<1.5 +/ 0.2 (Г~1.2 1 only as low energy cutoff for low energy electrons) Never observed a naked source: hard TeV features always seen in connection with the EBL effect (cosmic conspiracy??): it requires dramatic evolution of blazar properties with z ( )

20 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 > 105): ~ monoenergetic electrons, 1/3 = 0.66 (Katarzynski et al. 2005) But such mechanisms should not know the level of the EBL... >

21 TeV BLLacs observed vs simulated spectra Past CTs HESS MAGIC VERITAS A high EBL would require strong evolution of properties between z=0.2 and 0.1

22 Simultaneous mwl observations confirm absence of such features 1ES same epoch SED Such hard spectra ( 0 ) are difficult to explain (harder than even monochromatic particle distr.) EBL level is lower Wolter et al. 2000, Aharonian et al. 2006, 2008

23 Bottom line: Though not (yet) the smoking gun, the TeV blazar spectra (now many of them) 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

24 NIR MIR constraints: Need objects with spectrum >1 TeV, at z~ : only 2 detected so far up to 10 TeV

25 1ES (z=0.129) HEGRA (Aharonian et al 2003) int 0.7 int 1.6 1ES (z=0.140) HESS (Aharonian et al 2007)

26 Costamante et al 2008, in prep

27 Costamante et al 2008, in prep

28 Far IR constraints: >10 TeV 22 Z: > <0.05

29 Mkn 501 and Mkn 421 HEGRA spectra (>10 TeV) 1999 Problem Finkbeiner 1996, MKN 501 HEGRA (Aharonian et al 1999) Famous IR bkg TeV gamma ray crysis! (Aharonian ICRC 2001)

30 Latest EBL model: Franceschini et al. 2008; almost OK Franceschini et al 2008

31 Franceschini et al EBL model: compatible with both lower limits (source counts) and upper limits from blazars but see talk by R. Gilmore

32 How can we test/falsify these conclusions? Find direct evidence of TeV spectra < 1 (monitor low z objects) Observe high redshift objects (z=0.5 1) at TeV: below gal. counts? test both blazar physics + Lorentz invariance/exotic physics Populate this zone!

33 Photon wise, NO need (yet) of new physics At present, detections and spectra are all consistent/explainable with low EBL level and standard blazar physics. Even OK objects at z=1 See also S. Wagner's talk on recent time travel limits on c(e)

34 Two Warnings / Caveats:

35 1) At present, data on this type of graph is meaningless! Cut off is NOT measured in spectra with passband TeV (E)=1 cannot be derived from spectra measured in that energy range: GeV FERMI LAT data needed!

36 2) FERMI LAT will sample the UV Opt part of the EBL (see L. Reyes talk); BUT results will be ambiguous! Many factors / selection effects give same evolution with z: 1) most Fermi LAT blazars will be FSRQ (intense BLR emission) BLR spectrum peaks at UV rest frame: intrinsic cutoff evolving with resdhift as the EBL evolution of BH accretion, same as EBL effects. (Reimer ) selection effects on flux limited sample: blazar sequence! distant objects more luminous > redder SED > steeper (more curved) spectra > false cutoff!

37 Evolution of Accretion rates Fixed BLR intensity EBL different evolutions A. Reimer 2007

38 Flux limited sample => luminosity bias Blazar sequence (Fossati et al 98 Donato et al 2001) The higher the redshift the more luminous the sources the redder the SED the steeper the Spectra mimic EBL evolution effects!

39 Conclusions Great progress in last 5 years: (low EBL, no large PopIII flux ) but further refinements require better blazar knowledge Next steps: most critical & promising, multi TeV range, sampling MIR FIR (but have to wait CTA/AGIS, or TenTen) Fermi LAT will give tremendous progress on blazar physics, but no easy recipy for EBL studies: high degeneracy! Fermi LAT + (HESS, MAGIC) phase II nice sinergy: sample the UV part of EBL and leverage for cutoff. At present no compelling need for new physics (best tests will be given by obs. at large TeV energies and high z!)

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41 back up slides

42 Dependencies: for a fixed EBL spectral shape TeV fits, power law 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)

43 absorption on narrow banded target field => selective absorption selective absorption => hardening of the spectrum Attenuation factor Bbody Spectral changes w.r.t. incident spectrum Aharonian et al 2008

44 Possible high z HBL targets for HESS (south) and MAGIC+VERITAS (north): Needs: culmination near the telescope's zenith (lowest possible threshold) + high X ray flux objects (large number TeV electrons) 1ES ES z=0.487 z= ES z=0.460 RXJ z= ES z=0.444 Costamante & Ghisellini 2002 Costamante 2007

45 Upper limit: robustness on EBL SED changes

46 From reverberation mapping technique on AGNs over wide range of luminosity: Relation R Ldisk1/2 Energy density

47 Simultaneous mwl observations confirm absence of such features 1ES same epoch SED Wolter et al. 2000, Aharonian et al. 2006, 2008 Such hard spectra ( 0 ) are difficult to explain (harder than even monochromatic particle distr.) EBL level is lower If we believe the standard picture for blazar emission (broad band SED due to broad band electron distribution, particle acceleration in shocks), then: particle acceleration s 1.5 (non linear theory; Malkov & Drury 2001) 1.5 under most circumstances. Assumption: true average spectrum of these sources was not harder than 1.5