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 ultra-relativistic 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 X5. 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 sub-dominant 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