1. A diagnostic for the location of the energy dissipation in blazars.
Markos Georganopoulos University of Maryland Baltimore County & NASA Goddard Space Flight Center

2. External Compton scattering off the broad line ~1-10 eV photons
Sikora, Begelman, & Rees ‘94
>Spherical broad line region
with R~ cm
Uo  UBLR Γ2
oBLR Γ  in mc2 units

3. Marscher et al. Nature 2008: The dissipation is far outside the BLR.
See Vlahakis & Konigl (2004)

4. electrons of at least the same energy (γ~104)
Observed Compton dominance up to ~100 => External Compton losses dominate
Kubo et al ‘98
photons of ~ few GeV 
electrons of at least the same energy (γ~104)
For seed photons o~10-4
oγ~1
GeV emission comes from scatterings in the gray area between the Thomson and Klein-Nishina regimes

5. The devil is in the details…
At GeV electron energies the cooling time is
~ energy independent!
Effect on n(γ) ?

6. The devil is in the details…
At GeV electron energies the cooling time is
~ energy independent!
Effect on n(γ) ?

7. The devil is in the details…
GLAST bandwidth
Klein-Nishina and pair-production cross section for Lyman  photons.
No need to worry about pair production absorption below ~10-20 GeV

8. The electron distribution
See also Dermer & Atoyan ’02 for possible application in extended jets

9. The electron distribution

10. How EC dominated blazars should look
Unavoidable marks:
The hump in the synchrotron component
2. The flat/rising SED of the GeV component
3. Achromatic variability for the synchrotron hump and the GeV regime.

11. How can it not be EC off the BLR? Downsize and flatten the BLR?
Kaspi et al. ’00,
Peterson et al. 04:
BLR size from
reverberation
mapping:
R= L460.7 cm
Argument for
flattened geometry:
Anti-correlation between R (core to extended radio emission
->beaming) and line FWHM (e.g. Wills & Browne ’86)
U1/Γ2

12. Regardless of the shape of the BLR, we may be a factor of 10 away from it. Then the dominant high energy emission process may be synchrotron-self Compton (SSC).
234
Q: But can SSC produce
superquadratic variations
like those seen in 3c 279?
(Wehrle et al. ‘98)
A: Yes, it does so naturally, when the SSC power is comparable or higher than the synchrotron power.
Power:1:10:100 16
7.1 1
3.2 1

13. GLAST’s increased sensitivity will provides us with detailed spectra and energy-dependent variability profiles.
The question of the location of the blazar energy dissipation site will be settled soon.