Table Of Content2011 Fermi Symposium, Roma., May. 9-12 1
Gamma-ray Clues to the Relativistic Jet Dichotomy
EileenT.Meyer∗ andGiovanniFossati
DepartmentofPhysicsandAstronomy,RiceUniversity,Houston,TX77005
MarkosGeorganopoulos
DepartmentofPhysics,UniversityofMarylandBaltimoreCounty,Baltimore,MD21250and
NASAGoddardSpaceFlightCenter,MailCode663,Greenbelt,MD20771,USA
MatthewL.Lister
DepartmentofPhysics,PurdueUniversity,WestLafayette,IN47907
2 Inexaminingaselectsampleofover200blazarsofknownjetkineticpower(Lkin)andwell-characterizedSEDs,
1 wefound[1]thatIntermediatesynchrotron-peaking(ISP)blazarsmayhavelowergamma-rayoutputthanhigh
0 synchrotron-peaking (HSP) blazars of similar Lkin, consistent with our hypothesis that ISP blazars are less-
2 beamedversionsofHSPblazars,ratherthanadistinctpopulation. Further,byusingtheradiocoredominance
asameasureofrelativebeaming,wefindthatgamma-rayluminositydepends onbeaminginaconsistentway
n forblazarsrangingoveralljetkineticpowers(1042 −1046 ergss−1). Were-examinethegamma-rayproperties
a
ofthiscoresampleofblazarsusingthe1-yearLATcatalog [2]. Wefindthatforweakjets,theratioofinverse
J
Comptontosynchrotronemissionremainsconstantwithincreasedbeaming,consistentwithanSSCmodelfor
6 thejetemission,whilethemostpowerfuljets showastrongincreaseinCompton dominancewithorientation,
2 consistentwithanexternalCompton(EC)emissionmodel.
]
E
1. Introduction or molecular torus [7, 8, 9]). Generally low-power
H
Radio-loudactivegalacticnuclei(RLAGN)arebe- and lineless objects are believed to radiate by SSC at
.
h lieved to contain an actively accreting super-massive high energies, while individually, many high-powered
-p black hole (107 - 1010 M⊙) which generates bi-polar FSRQhavebeenmoresatisfactorilyfitwithECmod-
o jets of relativistic material reaching up to Mpc in els [10, 11]. However a consistent basis for which
r scale and with luminosities up to 1049 ergs s−1, pow- sourcesrequireEChasnotbeendemonstratedforany
st erful enough to heat the intra-cluster medium and particular class of blazars.
a
potentially produce a feedback mechanism in galaxy
[ ThelocationoftheGeVemissionisalsoamatterof
formation [3]. When these relativistic jets are seen
active debate, as it comes from locations close to the
2 with the axis near to the line of sight, they are
centralenginethatremainunresolved,evenwithvery
v called blazars (see [4] for a review). The jet radia-
0 long baseline interferometry (VLBI). If the emission
tion is strongly Doppler-boosted and dominates over
9 region is located within the sub-pc scale BLR, the
the emission from the host galaxy, the dusty torus
4 GeV emission of blazars with strong lines is due to
6 (thought to surround the nuclear black hole), the ac- ECscatteringofBLR photons[12], whichcanleadto
. cretion disk, and, except at very low frequencies, the
1 moreefficientcoolingandthuslowerpeakfrequencies,
isotropic emission from the giant lobes of gas fed by
1 as seen in the blazar sequence [13]. However, others
the jet overtime. Thus blazarsarenaturallyinterest-
1 agree that the high-energy emission may be coming
1 ing objects as the magnified view of the jet emission
several pc downstream as suggested by some multi-
: canallowustoprobethestructureandnatureofthese
v wavelength observations of variability [14, 15]. The
jets and ultimately to understand the physical mech-
i EC versus SSC origin of the high-energy emission is
X anisms behind their origin.
animportant question as it can be used as a diagnos-
r The broad-band, highly luminous and variable
tic for the location of the gamma-ray emission, and
a
spectrum from blazars is usually characterized as a
the inferred structure of the jet.
‘double-peaked’ one, with one broad emission com-
ponent believed to be synchrotron radiation peaking TheFermi gamma-raytelescopeisgivingusanun-
from sub-infrared energies to X-rays, and a second precedented look at the high-energy emission from
peak from inverse Compton emissionat > MeV ener- blazars, with nearly 700 blazar associations in the 1-
gies. This second high-energy peak in blazars can be yearcatalog(1-LAT)[2]. Withsuchalargesample,we
explained with both synchrotron-self Compton (SSC, canbegin to put some orderin the phenomenologyof
in which the particles in the relativistic jet up-scatter blazarsathighenergies. InSection2weintroducethe
synchrotron photons, [5, 6]) and external Compton ‘BlazarEnvelope’asseenbyFermi anddiscusstheim-
(EC) models, in the latter case with photons from portanceofthejetkineticpowerandorientationofthe
either the accretion disk, broad line region (BLR), jet in determining the gamma-rayluminosity. In Sec-
tion 3 we discuss a recent finding suggesting that the
high-energy emission of high-power jets is dominated
by EC rather than SSC. In Section 4, we summarize
∗Electronicaddress: [email protected] our findings.
eConf C110509
2 2011 Fermi Symposium, Roma., May. 9-12
Figure 1: The synchrotron Lpeak - νpeak plane, in which Figure 2: The InverseCompton Envelope. Figure is
theblazar sequencefirst found an anti-correlation adapted from [16]. The Fermi-detected AGN (large, dark
between bolometric luminosity (with Lpeak) and the bluecircles and magenta squares) show that alignment
synchrotron peak frequency. Using only well-sampled playsa strong effect in thegamma-ray output.
SEDsreveals a possible alternative to thecontinuous Interestingly,while theFR II sources appear to drop
sequence, in which two populations show dramatically directly below thepowerful FSRQsources, FR I radio
different behaviors in this plane. Track (A) is the galaxies appear to follow a more horizontal track,similar
de-beaming trail in synchrotron peak for a standard toour findingsin thesynchrotron envelope (Figure 1,
single Lorentz-factor jet (‘strong’ type),track (B) shows left). The presenceof a forbidden zone (emptyregion at
themarkedly horizontal movement typical of a upperright) suggests that there is a sequencein theIC
decelerating jet model characterized byvelocity gradients plane, similar to that found in the synchrotron (Figure
(‘weak’ type). Adapted from [1]. 1).
2. The Inverse Compton Envelope as
This was the first work to consider both the jet ki-
seen by Fermi
netic power (L , as measured from the isotropic ra-
kin
The relativistic jets in RL AGN are generally con- dioemission)andradiocoredominance(R,ameasure
fined to a small opening angle and thus most blazars ofjetorientation),asimportantfactors. Strikingly,we
are seen within a small range of orientation angles foundthatthe blazarsequenceisactuallybrokeninto
from the jet axis. It is often assumed that bright two populations in the L − ν plane (Figure
peak peak
blazarsin flux-limited samples are “well-aligned”and 1). The ‘weak’ jets consist entirely of sources with
therefore the exact viewing angle is of little impor- L <1044.5 ergs s−1, and extend from mis-aligned
kin
tance (e.g., for modeling, often the assumption θ = FR I radio galaxies at low ν up to well-aligned
peak
1/Γ is used). However, as surveys go deeper, it is (as measured by R) HSP BL Lacs. The ‘strong’ jets
expected that more and more relatively misaligned comprised a population of low ν jets which drop
peak
sources will be observed. in peak luminosity rapidly with decreasing R. The
Fossati et al. (1998) demonstrated an anti- critical transition between weak and strong jets ap-
correlation between the synchrotron peak luminosity pears to occur at an (dimensionless) accretionrate of
and the peak frequency, forming the now canonical m˙ ∼10−2 (see also [21, 22]), as estimated from the
cr
‘blazarsequence’of(presumably)well-alignedsources ratio L /L 1, matching the divide suggested by
kin Edd
[17]. Lower-luminositysourcesappearinginthe space [23].
below the blazar sequence are expected due to some
We note that the large number of blazars seen by
sources being less aligned to the line of sight. Indeed,
Fermi, as well as the detection of severalradio galax-
[18]foundthatnewsourcestheyidentifiedmodifythe
ies [16], implies that we should have a large range in
blazar sequence to an envelope, with the area below
orientationsintheFermi samples. AsseeinFigure2,
the blazar sequence populated with sources. Similar
an‘envelope’isindeedseenintheplaneofgamma-ray
envelopes were found by Anton & Browne and Niep-
pola et al. [19, 20]. In recent work, we filled the
synchrotron L −ν plane using a much larger
peak peak
sample of jets over a wide range of orientations (from 1TheEddingtonluminositywascalculatedusingtheaverage
blazarstosourcesmisalignedenoughtobeseenasra-
blackholemassestimatefromtheliterature,aswillbediscussed
dio galaxies), with much-improved SED sampling [1]. inaforthcomingpublication.
eConf C110509
2011 Fermi Symposium, Roma., May. 9-12 3
3. Evidence for External Compton in
High-Power Jets
With a large sample of blazars spanning a range
of orientations as shown in the previous section, it is
possibletolookforsignaturesofdifferenthigh-energy
emission mechanisms in a population of blazars. The
Dopplerbeamingfactor,δ,isafunctionoftheLorentz
factor Γ and the orientationangle, and the effect of δ
on the observed (monochromatic) luminosity is:
L=L δ3+α (1)
0
where L is the rest-frame luminosity (at δ = 1),
0
andαis the energyspectralindex atthe frequency of
interest. The exponent 3+α is the value assumed for
a‘movingblob’inthe jet. Ifthe emissioncomesfrom
a standing shock, the exponent would be 2+α [24].
Figure 3: Total gamma ray output versusradio core
In the case of emission by SSC, the IC peak has a
dominance for the 130+ sources with good SED coverage
detected by Fermi in the1-LAT.Jet power has a very beamingpatternwhichisidenticaltothesynchrotron
strong effect on thegamma-ray luminosity, but when (i.e., it follows Equation 1). However,for EC models,
sources are sorted bytheir jet powers, thedependenceon thebeamingathighenergiesgoesasL =L δ4+2α
IC 0,IC
radio core dominance also becomes apparent. The slopes [25, 26]. The larger exponent indicates that as a
shown are statistically significant at the95% level. source with significant EC emission is aligned, the IC
peakshouldbemoreandmoredominantoverthesyn-
chrotron peak (i.e., the ratio L /L will increase
IC sync
luminosity (> 100 MeV) versus the LAT photon in- with R. We can measure the Compton Dominance
dex(whichapproximatelytracksthepeakfrequency). R = log(L /L ) ∝log (δ(3+α)/(4+2α)), and see
CD IC sync
There is clearly a ‘forbidden zone’ at the upper right, that the dependence on δ goes with an exponent of 2
with the edge of the envelope presumably comprised whencomparingtheICandsynchrotronpeaks(where
of the most aligned objects. The sources are divided α = 1). At radio frequencies, where the spectral in-
into the typical higher-power and broad-lined flat- dex is confined to a fairly narrow range in values of
spectrumradioquasars(FSRQ)andthe lower-power, α =0−0.5,the beaming exponent will be ∼ 3 - 3.5.
r
lineless BL Lac objects (BLL).While this figurelacks In Figure 4 we examine the relationship between
the precise information needed to find the two popu- R and the radio core dominance, R, for two sub-
CD
lations suggested by [1], it is clear the the FR I radio samples of the 1-LAT blazars. The pink triangles are
galaxies(whichareunifiedwithmostBLLacs)appear high-jet-power sources (log L > 45.5), while the
kin
at much lower IC peak frequencies, forcing a similar blue are low-power (log L < 43.5). The flat dis-
kin
horizontal track from the low-power, aligned BLLs, tribution in the latter group is consistent with the
just as was found in the synchrotron plane. It is in- SSC models generally used to model these types of
teresting to note that the FR II radio galaxies,which sources. For the high-power group, however, we find
wouldbegenerallyunifiedwiththeFSRQpopulation, a significant trend of increasing Compton dominance
arebothdirectlybeneaththeminpeakfrequency,but withincreasingradiocoredominance(i.e.,alignment).
are also not particularly mis-aligned, which suggests ThiscanbeinterpretedasasignatureofanECprocess
that these sources might drop out of Fermi detection for a population of jets characterized by high kinetic
more quickly than the weak sources. powers.
It is interesting to consider the effect of jet kinetic However, from the above relations, the expected
power as well as orientation on the total LAT-band slope of the correlation should be close to 2/3, up
luminosity. As shown in Figure 3, as a total popula- to a max of 1 under reasonable assumptions. The
tion the 1FGL blazars appear as a scatter-plot in a slope in Figure 4 is clearly much higher, in fact con-
plot of L versus R. However, when the jet kinetic sistentwith a value of ∼ 2. While a more detailed in-
γ
power is shown (color scale), it becomes clear the L vestigationisinprogress,wenotethatthishighslope
γ
increases with both radio core dominance (i.e., angle mightmeanthattheassumptionthattheLorentzfac-
totheline-of-sight)and,evenmoredramatically,with tor of the plasma emitting in the gamma-rays is the
the jet kinetic power. We note that L is calculated same as that emitting at GHz frequencies is incor-
kin
fromthelow-frequencyisotropicemissionandisthere- rect. If a roughly 2:1 ratio is assumed (i.e., Γ = 15
IC
fore a completely independent measurementfromL , while Γ = 8), the higher slope can be explained.
γ radio
making it unlikely for this trend to be the result of Thisisconsistenttosomeextentwiththedifferencein
the selection effects of the 1-LAT sample. variability timescales for very high energies (minutes
eConf C110509
4 2011 Fermi Symposium, Roma., May. 9-12
France(http://simbad.u-strasbg.fr/simbad/),andthe
NASA/IPAC Extragalactic Database (NED) which
is operated by the Jet Propulsion Laboratory, Cal-
ifornia Institute of Technology, under contract with
the National Aeronautics and Space Administra-
tion (http://nedwww.ipac.caltech.edu/). We utilized
packagenpintheRlanguageandenvironmentforsta-
tistical computing. GF and EM acknowledgesupport
from NASA grants NNG05GJ10G, NNX06AE92G,
and NNX09AR04G, as well as SAO grants GO3-
4147X and G05-6115X. MG acknowledges support
from the NASA ATFP grant NNX08AG77G and
NASA FERMI grant NNH08ZDA001N. The MO-
JAVE project is supported under National Science
Foundation grant 0807860-AST.
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