The Origin of the Extragalactic Background Light: Constraints from High Energy Gamma-Ray Observations

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P. Coppi Yale University

Henry 1999

The Diffuse Extragalactic Background

EnergeticParticles!

Starlight Dust

Why do “low energy” astrophysicists care about VHE astronomy?

BIG zodiacal light foreground

Mean free pathfor VHE photons

Absorption (pair production) andCascadingimportant for cosmologicalVHE sources.

Coppi & Aharonian 1997

Absorption at Specific Energy Probes EBLIntensity In Relatively Narrow Wavelength Range!

Pair Production Threshold 2 2(1 cos ) 2( )t eE E m c

EBL Spectrum Matters!

2 2

( ),

peaks at 3.4( )

largest for

1.3 (E /1 TeV)

t

t e

EBL

f E E

E E m c

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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.)

Effects of Absorption: Non-Negligible!

(z=0.129)

(z=0.048)

(z=0.034)

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

1/ 2peak

true observedpeak peak

E ( / )

EBL Abs: E 3 E

by up to factor 9!

B

B

peak peak

IC

kinetic B e-

More: E larger Higher

More in KN limit lower IC component

BUT... EBL abs. also L (another factor 10)

L ! U / U (very out of equipartition)

R

Mean free pathfor VHE photons

Absorption (pair production) andCascadingimportant for cosmologicalVHE sources.

Coppi & Aharonian 1997

Blazar Background Models, a la Stecker & Salamon 1996

Including IR/O absorption

Don’tforgetcascades!

Coppi & Aharonian 1997

Coppi & Aharonian 1997

6( ) (1 )n z z

3( ) (1 )n z z

0( ) (1 )n z z

The cascadespectrum froma cosmologicalpopulation of VHE sources –independent ofprimary sourcespectrum forE>10 TeV!

Response to Change in IR/O Background

GeV background measurement= calorimeter for VHE universe!

Coppi & Aharonian 1997

S.J. Lee, Coppi, & Sigl

Can we see cascade radiation from individual sources?

Due to likely IGMF, “No” is the standard answer, but …

B

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Pair Halo Intensity

Coppi, Aharonian, & Voelk 1994

Most sources can think of, even decaying/annihilating CDM particles, trace large scale structure… look for clustering signal!

Bromm et al. 2003

Strong et al. , 2003, astro-ph/0306345

-5 -2 -1 -1

Keshet, Loeb, & Waxman (2003)

I ( 100 MeV) [integral flux]

< 0.5 10 ph cm sec sr (3 )!

E

Would be very useful to know what the realgamma-ray background is! Contamination by galactic halo I.C. component…?

Mkn 421

Mkn 421

GeV Blazars…

TeV Blazars…

EGRET

Pian et al. 1998

Gaidos et al. 1996

Fossati et al. 2002

Rapid spectral variability!

The potential advantage of TeV blazars… they are much simpler?SSC model

BeppoSAX

CAT/HEGRA

Klein-Nishina effects important!

Internal, self-consistentlygenerated photon field…

TestablePredictions!

Coppi & Aharonian 1999

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???

2 (naive SSC)TeV xL L

(ERC, SSC,

hadronic model)

TeV xL L

Steady X-RayComponent??

N.B. June 1997 data (after main flaring) included!

The stability problem…

LinearAxes!

Key – 3 keV fluxtracks TeV fluxrelatively poorly

Oops!! -- 1ES1959 May-Aug 2002

Krawczynski et al. 2004

Multiple EmissionComponents!

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

Simultaneous SSCfit to BeppoSax andCAT for Mrk 501flare of April 16, 1997using fully self-consistent model.

SynchrotronI.C.

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!

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Coppi, Krawczynski, & Aharonian 2002

Model used for simulation (tcool) is slightly different at low energies comparedto fit model (high min).

Both models give excellent fit to current data – but not HESSdata!

Example of Data Quality Expected for Next Generation Instruments –

Simulated 5hr observation of April 16,1997Mrk 501 flare as as seen by HESS.

Presentation to Snowmass 2001 Gamma-Ray Working Group

What if we put HESS/VERITAS up at 5000 m?

Even better!

Same as last slide,except simulated forresponse of theproposed 5@5(5 GeV threshold) instrument.

Blue is curve isbest-fit no absorptionSSC model – stronglyruled out! (“Salpeter”DEBRA absorptionassumed in generating fakeddata.)

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.

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…

Arendt & Dwek, ARAA, 2002

Method 2: Do a first principles calculation starting from the past…

Discrepancies (!) = different physics (e.g., dust)/evolution assumptions…

Why 1-5 micron shape seems relatively secure…

[Assuming no Pop. III stars!!]

Playing around w/different assumptions…

z~1g.f.

Z~5 g.f.

z~1-3 g.f.

3

( , )

(1 ) [ /(1 ),0]

n z

z n z

Watch out!

Coppi & Aharonian 1997

The latest and greatest: Primack et al. 2005

Be wary of those bearing SAMs…

Looks great, butremember it’s amulti-parameter fit!

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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

EXIST GLAST VERITAS

1 Month

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…

Krawczynski, Coppi, & Aharonian 2002

Gabici & Blasi 2003

Converging flows in merging/accreting clusters=> clusters should be gamma-ray sources …