1. Blazars as beamlights to probe the EBL Problem: EBL, emitted SED: both unknown ! Aim: measure n EBL (z) at different redshifts  local normalization, cosmic evolution Tools: homogeneous set @ different redshifts  one same model (parameters) at all z blazars ( e.g., High-Peaked BLs [HBLs] ) Emission physics: simple  one-zone SSC emission  synchrotron + compton, PL electron spectrum Nijil Mankuzhiyil (INFN, Udine U.) Massimo Persic (INAF+INFN, Trieste) Fabrizio Tavecchio (INAF, Milano)

2. One-zone SSC: model parameters : Plasma blob: R, B,  j Electron pop: n 0,  1,  2, E br, E min, E max Kino+ 2002 Tavecchio + 2001 ApJ, 554, 725

3. Simultaneous multi- obs’s:  optical    X-rays    HE  -ray  VHE  -ray Model SED : use SED w/out (EBL-affected) VHE  -ray data :   2 -minimization  SSC model ( check structure of multi-D parameter space ) TTTTTTTTTTTTTTTT simulated data The method ( 1 )

4. Extrapolate model SED into VHE regime  “intrinsic” blazar VHE emission Observed vs “intrinsic” emission    (E,z) Assume (concordance) cosmology  n EBL ( ,z j ) (parametric:  a nj  n ) s i m u l a t e d d a t a …the method ( 2 )

5. EBL de-absorbed Aharonian+ 2009 ApJ, 696, L150 HESS FERMI R XTE ATOM … assumed electron spectrum is triple -PL … … caveat… Swift Checking the method … locallyPKS 2155-304 z = 0.12 SSC param’s  min = 1  br1 = 1.4  10 4  br2 = 2.3  10 5  max = 3  10 6  1 = 1.3  2 = 3.2  3 = 4.3 B = 0.018 G R = 1.5  10 17 cm  = 32

6. data: Aharonian+ 2009 ApJ, 696, L150 … our effort H.E.S.S. Fermi ATOM Swift, RXTE R   t  12 hr  OK observed SSC parameters from  2 minim. n e =150 cm   min = 1  br = 2.9  10 4  max = 8  10 5  1 = 1.8  2 = 3.8 B = 0.056 G R = 3.87  10 16 cm  = 29.2

7. p r e l i m i n a r y

8. Stecker 1999 z=0.12 p r e l i m i n a r y Stecker & de Jager 1998 Malkan & Stecker 1998

9. Franceschini + 2008 z=0.1 z=0.003 z=0.01 z=0.03 z=0.3 z=0.12 p r e l i m i n a r y

10.  (E,  )  max by ( for head-on collision )  2.5 E , TeV  m Heitler 1960  (E ,z) =  d l /d z  x /2  n EBL (  )  ( 2xE  /(1+z) 2 ) d  d x d z  (E, ,  )  [1 – 2(m e c 2 ) 2 / E  (1-cos  )] 1/2 0  z  z s  > 2(m e c 2 ) 2 / [Ex(1+z) 2 ] For EBL photon energies  > 2(m e c 2 ) 2 / E (1-cos  ) x  1-cos  n EBL ( ,z) unknown  parameterize n EBL (  n  z j ) =  a n,j  n (E,,)(E,,) Stecker 1971 cosmology Stecker 1999 Optical depth

11. Conclusion Cons: indirect measurement of EBL method depends on blazar model Pros: unbiased method no assumptions on EBL, blazar SED SSC well tested locally on different emission states Need: simultaneous multi- obs’s of several blazars in shells of z possibly each source seen @ different levels of activity ( to increase statistics )  plan simultaneous obs’s involving IACTs + Fermi + X-rays + optical Aim: to probe EBL out to z  1 with Fermi/LAT + current/upcoming enhanced IACTs ( + x-ray, optical tel’s ) (  long live Fermi to see CTA / AGIS !! ) Check: on local (z=0.12) blazar PKS 2155-304 deduced  ’s within reasonable range

12. Thanks