1. The Origin of the Extragalactic Background Light: Constraints from High Energy Gamma-Ray Observations ? P. Coppi Yale University

2. Henry 1999 The Diffuse Extragalactic Background Energetic Particles!

3. Starlight Dust Why do “low energy” astrophysicists care about VHE astronomy? BIG zodiacal light foreground

4. Mean free path for VHE photons Absorption (pair production) and Cascading important for cosmological VHE sources. Coppi & Aharonian 1997

5. Absorption at Specific Energy Probes EBL Intensity In Relatively Narrow Wavelength Range! Pair Production Threshold EBL Spectrum Matters! ?

6. N.B. Shape of background does matter! Optical depth at ~20 GeV actually very sensitive to 60 micron flux… (Finkbeiner et al. “detection” of 60 micron EBL a big problem.)

7. Effects of Absorption: Non-Negligible! (z=0.129) (z=0.048) (z=0.034)

8. Absorption by the EBL – Might not seem too important, but… Don’t Ignore!

9. Mean free path for VHE photons Absorption (pair production) and Cascading important for cosmological VHE sources. Coppi & Aharonian 1997

10. Blazar Background Models, a la Stecker & Salamon 1996 Including IR/O absorption Don’t forget cascades! Coppi & Aharonian 1997

11. The cascade spectrum from a cosmological population of VHE sources – independent of primary source spectrum for E>10 TeV!

12. Response to Change in IR/O Background GeV background measurement = calorimeter for VHE universe! Coppi & Aharonian 1997

13. S.J. Lee, Coppi, & Sigl

15. Can we see cascade radiation from individual sources? Due to likely IGMF, “No” is the standard answer, but … ?

16. Pair Halo Intensity Coppi, Aharonian, & Voelk 1994

17. Most sources can think of, even decaying/annihilating CDM particles, trace large scale structure… look for clustering signal! Bromm et al. 2003

18. Strong et al., 2003, astro-ph/0306345 Would be very useful to know what the real gamma-ray background is! Contamination by galactic halo I.C. component…?

19. Mkn 421 GeV Blazars… TeV Blazars… EGRET Pian et al. 1998 Gaidos et al. 1996 Fossati et al. 2002 Rapid spectral variability!

21. The potential advantage of TeV blazars… they are much simpler? SSC model BeppoSAX CAT/ HEGRA Klein-Nishina effects important! Internal, self-consistently generated photon field… Testable Predictions! Coppi & Aharonian 1999

22. GeV Blazar Models & Complications… Blazejowski et al. 2000 Boettcher et al. 2001 vs. 3C279 Seed photons: IR from dust Beamed from behind, reduced efficiency? Which photon field(s) does jet interact with???

23. Steady X-Ray Component?? N.B. June 1997 data (after main flaring) included! The stability problem… Linear Axes! Key – 3 keV flux tracks TeV flux relatively poorly

24. Oops!! -- 1ES1959 May-Aug 2002 Krawczynski et al. 2004 Multiple Emission Components!

25. April 16, 1997 SSC fits (e- distribution obtained by “inverting” X-rays) to quasi-simultaneous (< 6hr difference) data for Mrk 501 April-May flare. Time Averaged over April/May

26. Simultaneous SSC fit to BeppoSax and CAT for Mrk 501 flare of April 16, 1997 using fully self-consistent model. Synchrotron I.C.

27. Using Mrk 501 April 1997 data can start to constrain DEBRA models – if SSC hypothesis is correct. Key which allows this is simultaneous, broadband X-ray and TeV data. Better data on the way! ? Coppi, Krawczynski, & Aharonian 2002

28. Model used for simulation (t cool ) is slightly different at low energies compared to fit model (high  min ). Both models give excellent fit to current data – but not HESS data! Example of Data Quality Expected for Next Generation Instruments – Simulated 5hr observation of April 16,1997Mrk 501 flare as as seen by HESS.

29. Presentation to Snowmass 2001 Gamma-Ray Working Group What if we put HESS/VERITAS up at 5000 m?

30. Even better! Same as last slide, except simulated for response of the proposed 5@5 (5 GeV threshold) instrument. Blue is curve is best-fit no absorption SSC model – strongly ruled out! (“Salpeter” DEBRA absorption assumed in generating faked data.)

31. A 5@5 instrument would have sensitivity even on 5 minute timescales – although now one cannot rule out no-absorption model. Still, very useful for probing possible SSC model dynamics or signatures, e.g., looking for gamma-ray vs. X- ray lags, etc.

32. How to calculate EBL: Method 1: Start from what see today and and extrapolate to the past … An “empirically based” model by Malkan & Stecker 2001…

33. Arendt & Dwek, ARAA, 2002 Method 2: Do a first principles calculation starting from the past… Discrepancies (!) = different physics (e.g., dust)/evolution assumptions…

34. Why 1-5 micron shape seems relatively secure… [Assuming no Pop. III stars!!]

35. Playing around w/different assumptions… z~1g.f. Z~5 g.f. z~1-3 g.f. Watch out! Coppi & Aharonian 1997

36. The latest and greatest: Primack et al. 2005 Be wary of those bearing SAMs… Looks great, but remember it’s a multi-parameter fit! ??

37. Mrk 501 (1ES 1959+650) Mrk 421 Telescope Sensitivities For TeV Blazars EXIST: Synchrotron Emission from “Blue” TeV Blazars RXTE ASM 2 Years 3 hrs EXISTGLASTVERITAS 1 Month

38. Summary EBL at gamma-ray energies places important constraints on very high energy luminosity/non-thermal activity of the Universe. Effects of cascading => problems for many exotic physics scenarios. If could detect cascade radiation, good probe of EBL (cascade spectrum “standard” – don’t have to work in exponential absorption regime). Shape of EBL at 50 GeV – 1TeV (GLAST) range crucial. Reflects redshift distribution of gamma-ray sources and EBL intensity. If no cutoff seen, we’re not measuring extragalactic background, or something is wrong with our physics. In principle, gamma-ray absorption in bright, distant sources (GRBs? or blazars) powerful probe of UV-IR EBL => important information on cosmology, structure formation, primordial fluctuation spectrum at small scales, etc. (even dark energy!) [Don’t let the SAM people tell you they already know everything ] In practice, measuring absorption is a very messy business. Need to know intrinsic source spectrum and don’t have good “spectral standard” yet. Maybe huge flares in TeV blazars? Please, don’t assume power law spectra!!! GLAST and next generation telescopes Cherenkov should increase source pop. by ~10x, so there’s hope…

39. Krawczynski, Coppi, & Aharonian 2002

40. Gabici & Blasi 2003 Converging flows in merging/accreting clusters => clusters should be gamma-ray sources …