Table Of ContentAstron. Nachr./AN?,No.?,789–792(2015)/DOIpleasesetDOI!
Radio Properties of the γ-ray Emitting CSO Candidate 2234+282
T.An1,2,4,⋆,Y.-Z.Cui1,2,K.É.Gabányi3,5⋆⋆,S.Frey3,W.A.Baan2,andW.Zhao2
1 SchoolofElectricalandElectronicEngineering,ShanghaiInstituteofTechnology,201418,Shanghai,China
2 ShanghaiAstronomicalObservatory,ChineseAcademyofSciences,NandanRoad80,Shanghai200030,China
6 3 FÖMISatelliteGeodeticObservatory,POBox585,H-1592Budapest,Hungary
1 4 KeyLaboratoryofRadioAstronomy,ChineseAcademyofSciences,Nanjing210008,China
0 5 KonkolyObservatory,MTAResearchCentreforAstronomyandEarthSciences,POBox67,1525Budapest,Hungary
2
v Received2015Oct19,accepted2015Oct21
o Publishedonline
N
Keywords galaxies: active –galaxies: individual (2234+282) –gamma rays: galaxies–radiocontinuum: galaxies–
0
1 techniques:interferometric
Mostoftheγ-rayemittingactivegalacticnuclei(AGN)areblazars,althoughthereisstillasmallfractionofnon-blazar
]
E AGNintheFermi/LATcatalog.Amongthesemisalignedγ-ray-emittingAGN,afewcanbeclassifiedasCompactSym-
metricObjects(CSOs).Incontrasttoblazarsinwhichγ-rayemissionisgenerallythoughttooriginatefromhighlybeamed
H
relativisticjets,thesourceofγ-rayemissioninunbeamedCSOsremainsanopenquestion.Therarityoftheγ-rayemitting
. CSOsisamysteryaswell.Herewepresenttheradiopropertiesoftheγ-rayCSOcandidate2234+282.
h
p
(cid:13)c 2015WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
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o
r
st 1 Introduction as young radio galaxies (reviewed by Fanti, 2009; O’Dea,
a 1998). Theoretical models have predicted γ-ray emission
[
One of the intriguingdiscoveriesof the Fermi Large Area fromCSOs(Kinoetal.,2009;Migliorietal.,2014;Stawarzetal.,
2 Telescope (LAT) survey is the overwhelmingmajority (98 2008). However there are only two γ-ray CSO candidates
v %)oftheFermi-detectedAGNclassifiedasblazars known to date, 4C +55.17 (McConvilleetal., 2011) and
9 (Ackermannetal., 2015). In leptonic models, γ-ray radia- PMN J1603−4904 (Mülleretal., 2014). In this paper, we
5
tion fromblazarsis mostly explainedby Comptonscatter- reportanewγ-rayCSOcandidate2234+282.
8
ing of low-energy synchrotron photons by the same rela-
3
0 tivisticelectronsinthejetsproducingthesynchrotronemis-
2 Sampleselection anddata reduction
. sion at lowerfrequencies(e.g.,Bloom&Marscher, 1996);
1
0 inhadronicmodels,thehigh-energyemissionisdominated In order to increase the sample size of γ-ray CSOs, we
6 by relativistic protons (e.g., Böttcher, 2010). Besides the
looked into the archival MOJAVE1 (MonitoringOf Jets in
1 blazars,therearedozensofmisalignedAGNdetectedbythe
ActivegalacticnucleiwithVLBAExperiments,Listeretal.,
:
v Fermi(e.g.,Abdoetal.,2010a;Ackermannetal.,2015).
2009)databasetosearchfornewcandidates.
i Thesamplesizeisstillsmall,thoughtheyconstituteavery
X Thecriteriaofsampleselectionare:
interestingclassofγ-rayemitters.CenA(NGC5128),Per
r 1. thesourcehasbeenidentifiedbyFermiwithhighstatis-
A (NGC 1275) and M87 (3C 274) are three well-studied
a
ticalsignificance;
nearbyradiogalaxiesinwhichγ-rayemissionwasobserved
2. theradiomorphologyisofCSOtype,characterisedwith
inthecore.Asingle-zonesynchrotronself-Compton(SSC)
compactdoublelobeswithorwithoutacentralcore;
jetmodelwassuccessfullyappliedtofittheLATγ-rayspec-
3. thetotalfluxdensitywhichisintegratedoverthewhole
trum of these radio galaxies, although other explanations
source in each VLBI image shows a steep or GHz in-
exist(e.g.Abdoetal.,2009).Thesenon-blazargamma-ray
verted(GPStype)spectrum.
AGN generally have moderate jet beaming, with Doppler
boostingfactorof2–4.Extendedγ-rayemissionwasalso Intotal,344AGNarecross-matchedbetweentheMO-
detected from Cen A lobes (Abdoetal., 2010b), and it is JAVE and Fermi catalogues. Among them, we found only
interpretedastheinverseComptonscatteringofthecosmic one source, 2234+282 matching the above three criteria.
microwavebackgroundphotonsbytherelativisticelectrons Such a low percentage (0.3%) is consistent with the rarity
inthelobes. oftheγ-rayCSOsingeneral.Detailsoftheradioemission
AmongthemisalignedAGN,thereisasub-classnamed propertiesaregiveninthenextSection,includingthespec-
Compact Symmetric Objects (CSOs), which are regarded tralindexα(definedasSν ∝ να),thebrightnesstempera-
tureTbofthecore,andhotspotadvancespeed(βadv).
⋆ Correspondingauthor:e-mail:[email protected]
⋆⋆ speakerattheconference. 1 http://www.physics.purdue.edu/astro/MOJAVE/index.html
(cid:13)c 2015WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 Anetal.:γ-rayCSOs
3 CSO identification of2234+282
2234+282(alsonamedasCTD135)hasaredshiftz =0.79.
Its optical spectrum was described by Shawetal. (2012)
as transitional between flat-spectrum quasars and BL Lac
NE C typeobjects.ItwasdetectedbyFermiwithastatisticalsig-
nificance of 52.1σ, corresponding to 3FGL J2236.3+2829
and 2FGL J2236.4+2828 (Aceroetal., 2015), but not de-
tectedbytheEnergeticGammaRayExperimentTelescope
(EGRET;Hartmanetal.,1999)oftheComptonGammaRay
Observatory,probablyduetoinsufficientsensitivity.
Theradiospectrumofthetotalfluxdensityiscomplex
SW
above1GHzduetothevariabilityofthenon-simultaneous
data points (Figure 2)2. Except for that, there is a signifi-
10 pc
cantdecreaseinthefluxdensityat408MHz,suggestinga
lowfrequencyturnoveraround1GHzinthespectrum.The
VeryLargeArray(VLA)observationperformedat1.5GHz
in 1984 revealed a bright compactcomponentand a weak
featureabout4.9′′tothesouthwest(Murphyetal.,1993).
Fig.1 Overlaid VLBI images of 2234+282 at 8.4 GHz The Very Long Baseline Array (VLBA) images at 2.3
(red contours)and 15 GHz (blackcontours).The8.4-GHz GHz show a single compactcomponentwhich is resolved
image was made from the VLBA data on 1996 May 15 alongthenortheast-southwestdirectionatfrequencieshigher
(projectcode:BB023),therestoringbeam(shownasa red than5GHz.Figure1showsthe15-GHzimage(blackcon-
ellipse in the bottom left corner) is 2.05 mas × 0.87 mas tours) overlaid on the 8.4-GHz one (red contours). At 15
atPA = −12.8◦.Thepeakbrightnessis0.708Jybeam−1. GHz, the emissionstructure can be fitted with three Gaus-
Thecontourlevelsincreasebyafactorof2,andthelowest sian components,NE,C andSW. ComponentC isthe most
contouris1.5mJybeam−1(3σ).The15-GHzdatawereob- compact,andaccountsfor65%ofthetotalfluxdensity.We
tainedon1996May16(projectcode:BK037).Therestor- usedcircularGaussiansinthefittingtogetthepeaklocation
ingbeamis1.17mas×0.49masatPA= −23.4◦ (shown andthesizeofthemodels,inordertosimplifythecompari-
asablackellipseinthebottomleftcorner).Thepeakbright- sonandtodecreasethenumberoffreeparameters.Onefifth
nessis0.421Jybeam−1,andthelowestcontouris1.1mJy ofthebeamsizeistakenastheerroroftheposition.Compo-
beam−1(3σ). nentsNEandCarestillblendedat8.4GHz.Thecombined
NE+C component shows a somewhat steep spectrum with
a spectral index of α = −0.43 ± 0.07 (Figure 3-a). The
details of the VLBI observationsused here to describe the
spectralindexofthesourcearegiveninTable1.Theweaker
SWcomponentshowsaninvertedspectrumwithaturnover
around4GHz.Thespectrumoftheopticallythinsectionis
rathersteep,showingα=−2.50±0.18.
y) Since 2009 the MOJAVE program observations of the
y (J 100 source were performed in full polarization. According to
sit thosethepolarizedemissionisconcentratedincomponents
n
e
d NEandC.ComponentCdoesnotshowevidentvariationin
x
Flu polarization,butNEshowssignificantvariabilityinthepo-
larizedintensityat15GHzonatimescaleofafewmonths.
ComponentsCandNEcanberesolvedonlyat15GHz
andfrequenciesabove.Since therehasbeennosimultane-
10-1
100 101 102 ous observationat 15 and 24 GHz, and the source is vari-
Frequency (GHz)
able (see below),an accurateestimationof thespectralin-
Fig.2 Radio flux density versus frequency. The data
dexofcomponentCisnotpossible.Eventhough,thecen-
points,obtainedfromthesingledishorVLAmeasurements,
tral component C is the brightest and most compact one
arecollectedfromtheNED.Theradiospectrumiscomplex
among the three. The brightness temperature of C ranges
above 1 GHz due to variability. A significant decrease of from 3.4×1010 K to 1.5×1012 K at 15 GHz. Thus C is
fluxdensityat408MHzsuggestsalowfrequencyturnover
mostlikelythecore.
around1GHz.
2 http://ned.ipac.caltech.edu/
(cid:13)c 2015WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2015) 791
a) C+NE b) 2.3 GHz c)
SW 2.0 81.54 GGHHzz s) 1.5
a
Flux density (Jy)1100-10 alpha = −0.43 alpha = −2.50 Flux Density (Jy) 1.0 SW separation (m 1.0
E-
N 0.5
10-2
100 101 102 1995 2000 2005 2010 2015 1995 2000 2005 2010 2015
Frequency (GHz) Epoch (year) Epoch (yr)
Fig.3 Panela)showstheradiospectraoftheVLBIcomponentsin2245+282.Thedashedlineindicatesthepowerlaw
fittothespectrumofthecomponentC+NE.ThedottedlinerepresentsthemodelfittothespectrumofSWbyusingapower
lawtogetherwiththefree-freeabsorption.Wenotethatthespectrumcanalsobefittedwithasynchrotronself-absorption
model.Panelb)displaysthelightcurvesat2.3,8.4and15GHz.Panelc)showstheseparationofcomponentsNE-SWasa
functionoftime.Thedashedlinedenotestheleast-squaresfittotheobserveddata.
Figure3-b showsthe lightcurvesof thetotalfluxden- Table1 Interferometricobservationsusedtodescribethe
sity at 15,8.4and 2.3GHz. 2.3 and8.4 GHz observations spectralindexofthesource(Figure3Panela)).
were performed in the framework of the VLBA calibrator
survey3.Thefluxdensitiesatallthreefrequencieswerecal- Epoch ν Projectid SC+NE SSW
(GHz) (Jy) (Jy)
culated by summing up the CLEANed components. The 1996May15 2.3 BB023 1.49±0.15 0.02±0.002
sourceexhibitsvariabilityatallthreefrequencieswithaper- 1996Jun5 5 BH019 1.04±0.10 0.18±0.02
centagefluctuationindexdefinedas 1996May15 8.4 BB023 0.89±0.09 0.07±0.01
σ 1996May16 15 BK037 0.64±0.06 0.02±0.002
u=100 (1)
hSi
(Heeschenetal., 1987), in whichhSi is the meanandσ is 4 Discussion
the standard deviation. That gives u = 25.1% (15 GHz),
25.9% (8.4 GHz) and 30.6% (2.3 GHz). Since the central
The location and the radiation mechanism of γ-ray emis-
componentCcontributesthemajorityofthetotalfluxden-
sionfromCSOsremainopenquestions.Asingle-zoneSSC
sity, the variability may in fact represent the flux density
emission model well fits the broadband SEDs from radio
variationinC.
up to GeV energy bands in blazars and also in some mis-
We investigated the position change of component NE
alignedAGN(e.g.,M87:Abdoetal.,2009).Theone-zone
with respectto the assumed coreC, andfoundthatNE ap-
SSC model requires relativistic beaming. However, unlike
parently moves toward the core C. This is actually indica-
theblazarjets,theCSOjetsareingeneralmildlyrelativis-
tive that the componentC is a mixture of the core and an
ticandunbeamed(An&Baan,2012).Itisobviousthatthis
innerfast-movingjet(notresolvableat15GHz)whichwas
SSCmodelcannotreproducetheγ-rayemissionfromCSOs.
ejected toward the northeast.We then used SW component
Someothermodelshavebeeninvestigatedtointerprettheγ-
as the referenceto calculate the hot spotseparation speed.
rayradiationfromCSOs.Forexample,Stawarzetal.(2008)
TheangularseparationbetweencomponentsNEandSWas
proposedthattherelativisticelectronsinjectedfromthehot
a function of time is depicted in Figure 3-c. The separa-
spotstotheCSOlobescanup-scattertheultravioletphotons
tion rate is estimated as β = 0.014±0.004 mas yr−1,
sep from the disk to GeV energy range. Migliorietal. (2014)
corresponding to a projected hot spot separation speed of
proposedatwo-zoneSSCjetmodelfortheγ-rayemission
0.6 ± 0.2c. Assuming that the jet and counterjet have an
fromGPSandCSSquasars,inwhichtheseedsynchrotron
equalspeed,theaveragehotspotadvancespeedis∼ 0.3c.
photonswhicharefromaninnerblazar-likejetknotareup-
ThishotspotspeedisatypicalvalueforCSOs(An&Baan,
scatteredbytheelectronsinanouterslowerjetknot.Inad-
2012),butmuchlowerthanthejetspeedinblazars.
dition, bremsstrahlung emission from the shocked plasma
Tosummarize,allpiecesofevidences,includingthe
in the cocoon of CSO may also have a significant contri-
compacttriplestructure,steepspectrum,andlowjetspeed
butiontothehigh-energyemission(Kinoetal.,2009).The
are consistent with each other to imply that 2234+282 is
model predicts γ-ray emission from CSOs that could be
a CSO. The brightnesstemperatureandmoderatevariabil-
detected by Fermi, since the electron density and temper-
ity of the core indicate that the innermost jet is Doppler
ature are much higher in the young radio lobes than those
boosted.
inolderones.Thedifferenceofthesemodelsisthatthefirst
3 thedataarefromtheVLBIarchiveathttp://astrogeo.org two(Migliorietal.,2014;Stawarzetal.,2008)areofnon-
www.an-journal.org (cid:13)c 2015WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 Anetal.:γ-rayCSOs
thermal origin and and the last one (Kinoetal., 2009) is Heeschen, D. S., Krichbaum, T., Schalinski, C. J., & Witzel, A.
thermal. 1987,AJ,94,1493
Theobservationalpropertiesof2234+282makeitaγ- KinoM.,Ito,H.,Kawakatu,N.,&Nagai,H.2009,MNRAS,395,
ray CSO candidate, increasing the number of γ-ray CSO L43
ListerM.,Homan,D.C.,Kadler,M.,etal.2009,ApJ,696,L22
candidates from 2 to 3. The number of CSOs detected in
McConville,W.,Ostorero,L.,Moderski,R.,etal.2011,ApJ,738,
GeV energy range so far is significantly lower than pre-
148
dictedbytheoreticalmodels.D’Ammandoetal.(2015)
MiglioriG.,Siemiginowska,A.,Kelly,B.C.,etal.2014,ApJ,780,
searched 60 CSOs and analyzed 6 years of LAT data but
165
failed to confirm any γ-ray CSO. The theoretical calcula-
MüllerC.,Kadler,M.,Ojha,R.,etal.2014,A&A,559,115
tionsshowthatthemodel-predictedγ-rayfluxisnearorbe- Murphy D.W.,BrowneI.W.A.,PerleyR.A.1993, MNRAS,264,
lowthecurrentFermidetectionlimit(Migliorietal.,2014). 298
ContinuingsearchofacompleteCSOsamplebasedonthe O’DeaC.P.1998,PASP,110,493
Fermidataatalowerenergyrangeof0.1–10GeVisunder- ShawM.S.,Romani,R.W.,Cotter,G.,etal.2012,ApJ,748,49
way (D’Ammandoetal., 2015) to verifywhetherthe CSO Stawarz,Ł.,Ostorero,L.,Begelman,M.C.,etal.2008,ApJ,680,
emissionisintrinsicallydominatedinlowenergyband.On 911
theotherhand,themorphology,spectralpropertiesandkine-
maticsofthesethreesourcessuggestmildlyrelativisticjet.
VLBI data of γ-ray CSO candidatesprovideimportantin-
formationontheradiojetluminosityandjetspeed,placing
stringent constraints on the γ-ray emission models. Com-
plementary wide-band SED fits of these three sources are
crucialforverifyingwhetherthejetplaysanimportantrole
ingeneratingγ-rayemissionfromCSOs(themodelof
Migliorietal., 2014). If however the γ-ray emission from
CSOs is found to be of thermal origin and related to the
lobes,thatwouldidentifyanewγ-rayemissionmechanism
forAGNasproposedbyKinoetal.(2009).
Acknowledgements. The research is supported by the European
CommissionSeventhFrameworkProgramme(FP/2007-2013)un-
dergrantagreementNo283393(RadioNet3).Thisresearchissup-
portedbytheChineseMinistryofScienceandTechnologygrant
(2013CB837900), Shanghai Rising-Star Program, the Hungarian
ScientificResearchFund(OTKANN110333)andtheChina–Hun-
garyCollaborationandExchangeProgrammebytheInternational
CooperationBureauoftheChineseAcademyofSciences.Thisre-
search has made use of data from the MOJAVE database that is
maintainedbytheMOJAVEteam.TheNRAOisafacilityofthe
National Science Foundation operated under cooperative agree-
mentbyAssociatedUniversities,Inc.
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