Compton Efficiencies; Fermi and
the sequence
Can we see statistical evidence for the
influence of external (AGN) photons in
blazar SEDs?
(Jenny Gupta, Mike Peel)
Blazar SEDs
• Have Characteristic twin peaks
– Synchrotron dominating radio through optical
– Inverse Compton dominating hard X-rays and gamma-rays
• Major question:
– Are the photons that get Compton scattered those
internally generated (synchrotron self-Compton) or
external ones from the AGN?
• Secondary question:
– Is there a progressive sequence of blazar SED properties
that depends on the availability of external photons?
(Blazar sequence)
Compton Efficiencies
• Blazars dominate the Fermi sky – i.e. objects
selected by their Compton scattered emission
• Consider the Fermi blazar catalogue
• Start with the premise that radio core emission is
a good proxy for the energy available in ultrarelativistic electrons that can scatter photons to
produce gamma rays.
• This implies that those Fermi sources with the
weakest radio cores have the highest “Compton
efficiencies”
External photons and Compton
efficiencies
• Expect those blazars with a higher energy density
in low frequency photons to have higher
Compton efficiencies
• FSRQs have lots of AGN photons (directly from
the accretion disk and reprocessed by dust) while
BL Lacs only have their synchrotron photons
• Predict:
– FSRQs would have higher Compton efficiencies than
BL Lacs
– Within the FSRQ population itself one might expect a
trend with AGN luminosity
A quick look
1. Plot of Gamma ray
flux (1 – 100GeV)
against 8.4 GHz flux
density
2. Objects from Fermi
First year catalogue
(Abdo et al ???) and
identified with
CRATES radio sources
3. BL Lacs red +
4. FSRQs green X
5. BL Lacs have lower
radio flux densities
CRATES objects with OCRA
measurements
The sample: Fermi Blazars with CRATES
identifications
• We want a gamma ray-selected sample with
radio identifications and with information on
the compact radio emission.
• CRATES is a suitable sample and has the
advantage that Abdo et al. list CRATES sources
which match detections in their first year
sample.
– Gamma ray fluxes plus photon indices
– Radio flux densities at 8.4GHz of cores <~100mas
Measuring Compton efficiencies
• Don’t take previous plot too seriously!
• A proper definition of efficiency would be in
terms of energies or luminosities, not flux
densities.
• As a measure of the gamma ray luminosity we
take the peak νS(ν) in the SED. This takes care of
the fact that BL Lac and FSTQ SEDs peak at
different energies
• For a measure of the jet power we take νS(ν) at
8.4GHz (no k-correction done)
Specrtal energy plot. BLLacs + and FSRQs X
Compton efficiency
distributions
•The Peak Compton
efficiencies for BL Lacs
and FSRQs are not
significantly different
but, anything the
FSRQs are less
efficient.
•The FSRQ distribution
is broader at the 2.5%
significance level
Nasty details!
• Finding the peak νS(ν). Not all the objects in the sample have the
peak in the Fermi band.
–
Abdo et al. give a recipe to find the peak frequency in the SED using
the photon index
– Fit a parabola to the Fermi points using the recipe to fix the peak.
(Additional assumptions required in a few cases.)
• Less than 50% of the gamma ray sources have identified radio
counterparts.
–
Hope that resulting selection effects are the same for all the sample
so should not change the conclusions based on comparisons within
the sample
• Variability blurs any effect (8.4GHz flux densities measures in 1990s)
• Would really like to select sample in νS(ν) not S(ν).
What does is all mean?
• The availability of external photons does not
affect the Compton efficiency.
– FSRQs have lots
– BL Lacs have very few
• This is another reason to be sceptical about
any “blazar sequence” idea that attempts to
unify BL Lacs and FSRQs.
Evidence for the effects of external
photons within FSRQs?
• Detailed SED modelling (e.g. 3C454.3) requires
external Compton to get a fit.
• Can we see statistical evidence using Compton
efficiencies?
– Look for a correlation within the FSRQ population
– Do more optically luminous quasars have higher
efficiencies? No!
Compton efficiencies for objects with
Veron&Veron absolute magnitudes
Compton efficiencies in FSRQs
• There is no dependence on optical luminosity
• Why?
– It’s really energy density in photons within the jet
that’s relevant
– Some of the optical luminosity is beamed.
(Emission line luminosity is better parameter)
• Conclusion; it would have been nice to see a
correlation but we don’t.
The Sequence is dead, long live the
Sequence!
• External photons are a fact of life; we need them
to explain detailed SEDs
• The mistake is to believe that they have an
energetically dominant effect. Compton efficiency
argument say that they do not
• The SED fitting indicate that the integrated
gamma luminosities in the external Compton
component is sub-dominant.
• Importantly it usually contributes a lower energy
gamma component
Today’s new Sequence
• The SSC component is the same in BL Lacs and
FSRQs. (Jet production cares little about the
optical AGN.)
• Scattering of external photons produces a subdominant gamma component at lower energies
than the SSC component
– This component drags the SED peak to lower energies
in FSRQs than in BL Lacs
– It accounts for the broader range of Compton
efficiencies for FSRQs
Compton efficiency
distributions
•The Peak Compton
efficiencies for BL Lacs
and FSRQs are not
significantly different
but, anything the
FSRQs are less
efficient.
•The FSRQ distribution
is broader at the 2.5%
significance level