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The Origin of the Extragalactic Background Light:

The Origin of the Extragalactic Background Light: Constraints from High Energy Gamma-Ray Observations. P. Coppi Yale University. ?. Energetic Particles!. The Diffuse Extragalactic Background. Henry 1999. Why do “low energy” astrophysicists care about VHE astronomy?.

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The Origin of the Extragalactic Background Light:

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

  2. Energetic Particles! The Diffuse Extragalactic Background Henry 1999

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

  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! ? EBL Spectrum Matters! Pair Production Threshold

  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 Don’t forget cascades! Including IR/O absorption Coppi & Aharonian 1997

  11. The cascade spectrum from a cosmological population of VHE sources – independent of primary source spectrum for E>10 TeV! Coppi & Aharonian 1997

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  28. 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.)

  29. 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.

  30. 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…

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

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

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

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

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

  36. 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…

  37. Krawczynski, Coppi, & Aharonian 2002

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

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