VLBI observations of four radio quasars at $z>4$: blazars or not?

0.36 MB·

by  H. -M. Cao

MNRAS000,1–11(2017) Preprint12February2017 CompiledusingMNRASLATEXstylefilev3.0
VLBI observations of four radio quasars at z > 4: blazars or not?
H.-M. Cao,1,2⋆ S. Frey,3 K. É. Gabányi,3 Z. Paragi,4 J. Yang,5,6 D. Cseh,7
X.-Y. Hong,6 and T. An6,8
1SchoolofPhysicsandElectricalInformation,ShangqiuNormalUniversity,298WenhuaRoad,Shangqiu,Henan476000,China
2XinjiangAstronomicalObservatory,ChineseAcademyofSciences,150Science1-Street,Urumqi,Xinjiang830011,China
3KonkolyObservatory,MTAResearchCentreforAstronomyandEarthSciences,POBox67,H-1525Budapest,Hungary
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4JointInstituteforVLBIERIC,Postbus2,7990AADwingeloo,theNetherlands
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5DepartmentofEarthandSpaceSciences,ChalmersUniversityofTechnology,OnsalaSpaceObservatory,SE-43992Onsala,Sweden
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6ShanghaiAstronomicalObservatory,ChineseAcademyofSciences,80NandanRoad,Shanghai200030,China
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7DepartmentofAstrophysics/IMAPP,RadboudUniversityNijmegen,POBox9010,6500GLNijmegen,theNetherlands

n 8KeyLaboratoryofRadioAstronomy,ChineseAcademyofSciences,Nanjing210008,China
a
J

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AcceptedXXX.ReceivedYYY;inoriginalform2016December9
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A ABSTRACT
Blazarsareactivegalacticnuclei(AGN)whoserelativisticjetspointnearlytothelineofsight.
G
Theircompactradiostructurecanbeimagedwithverylongbaselineinterferometry(VLBI)
. onparsecscales. Blazars atextremelyhighredshiftsprovidea uniqueinsightintothe AGN
h
p phenomenaintheearlyUniverse.We observedfourradiosourcesatredshiftz > 4withthe
- EuropeanVLBINetwork(EVN)at1.7and5GHz.Theseobjectswerepreviouslyclassifiedas
o
blazarcandidatesbasedonX-rayobservations.Oneofthem,J2134–0419isfirmlyconfirmed
r
t asablazarwithourVLBIobservations,duetoitsrelativisticallybeamedradioemission.Its
s radiojetextendedto∼10milli-arcsecscalemakesthissourceapromisingtargetforfollow-up
a
[ VLBIobservationstorevealanyapparentpropermotion.Anothertarget,J0839+5112shows
a compact radio structure typical of quasars. There is evidence for flux density variability

1 andits radio“core”hasa flatspectrum.However,the EVNdatasuggestthatitsemissionis
v not Doppler-boosted.The remaining two blazar candidates (J1420+1205and J2220+0025)
0
show radio properties totally unexpected from radio AGN with small-inclination jet. Their
6
emissionextendstoarcsecscalesandtheDopplerfactorsofthecentralcomponentsarewell
7
below1.Theirstructuresresemblethatofdouble-lobedradioAGNwithlargeinclinationto
4
0 thelineofsight.Thisisincontrastwiththeblazar-typemodelingoftheirmulti-bandspectral
. energydistributions.Ourworkunderlinestheimportanceofhigh-resolutionVLBIimagingin
1
confirmingtheblazarnatureofhigh-redshiftradiosources.
0
7 Keywords: radiocontinuum:galaxies–galaxies:active–galaxies:high-redshift
1
:
v
i
X
1 INTRODUCTION SMBHs,i.e.masseslargerthan∼108M (e.g.Sikoraetal.2007).
r ⊙
a Theradioemissionofradio-loudAGNoriginatesfromincoherent
Activegalacticnuclei(AGN)areenergeticcelestialobjectspromi-
synchrotron processes in the jets (e.g. Blandford&Königl 1979)
nent inawiderangeofelectromagnetic bands, fromtheradioup
andtheextendedlobes(e.g.Burbidge1956).Iftheanglebetween
tothehighestenergies(X-raysandγ-rays).Therichobservational
theapproachingjetandthelineofsight(i.e.theviewingangle)is
phenomena of AGN can be understood in the framework of the
small,theradiationfromtheapproachingjetisstronglyenhanced
orientation-basedunifiedscheme(e.g.Urry&Padovani1995).Ap-
byrelativisticbeaming.TheseAGNarecalledblazars.Blazarscan
proximately 10 per cent of the optically-selected AGN areradio-
bedividedintotwomainclasses:flat-spectrumradioquasars(FS-
loud(Mushotzkyetal.2004).Theradio-loudfractionofAGNmay
RQs)andBLLacobjects.Theformerhaveprominentopticaland
evolve withredshift (Volonterietal.2011), but the causes of this
ultraviolet(UV)emissionlines, whileBLLacsexhibit weak(the
evolution are not well understood yet. Radio-loud AGN launch
restframeequivalentwidthEW < 5Å)orevennoemissionlines
powerfulpairsofrelativisticjetsintheoppositedirectionsperpen-
intheirspectra(e.g.Fossatietal.1999;Massaroetal.2015).
diculartotheaccretiondiskwhichsuppliesmaterialforthecentral
engine,asupermassiveblackhole(SMBH,∼106−1010M ).The
⊙ FSRQs and BL Lacs are thought to have different accretion
powerfulrelativisticjetsareusuallyfoundinAGNwithhigh-mass
mechanisms, i.e. different types of accretion disk (Sbarratoetal.
2014).Theformerarequiteluminousinoptical,thereforecanbe
⋆ E-mail:[email protected](HMC) seen from large cosmological distances. The most distant FSRQ
(cid:13)c 2017TheAuthors
2 H.-M. Cao et al.
currently known is Q0906+69301 at z = 5.48 (Romani 2006; refine the cosmological model (e.g. Vermeulen&Cohen 1994;
Zhangetal.2016),whileBLLacsarerarelyfoundatredshiftz>2 Kellermannetal. 1999; Britzenetal. 2008). Core–jet structures
(Plotkinetal.2008).Themulti-bandlog(νS )−logνspectralen- have also been proposed as “standard ruler” to measure cosmo-
ν
ergy distribution (SED) of blazars shows a typical double-hump logicalmodelparametersusingtheapparentangularsize–redshift
feature. In the one-zone leptonic model (Ghisellini&Tavecchio relation(e.g.Kellermann1993;Gurvits1994;Gurvitsetal.1999).
2009) which is widely employed to explain the blazar SED, the CompilinglargersamplesandincorporatingmoreVLBIobserva-
low-energy hump (peaked at the infrared/X-ray bands) is due to tionsofhigh-zblazarsareessentialforsuccessfullyrevisitingthese
the synchrotron radiation of the relativistic electrons in the jet, cosmological tests because model differences are the most pro-
while the high-energy one (peaked at hard X-ray/γ-ray bands) is nouncedatthehighestredshifts.
producedbytheinverseComptonradiation(IC)ofthesameelec- High-resolution VLBIimaging isapowerful tool todirectly
tronpopulationthatcausesthesynchrontronhump.Dependingon confirmtheblazarclassificationofthecandidates.Ifasourceisin-
whether the seed photons are internal or external to the jet, IC deedablazar,itshouldshowDoppler-boostedbrightnesstemper-
can be further classified as Synchrotron-Self-Compton (SSC) or ature, flat-spectrum radio core, and possibly rapid and prominent
External-Compton (EC) emission model. The latter is often used variation in flux density. With this method, Gabányietal. (2015)
tofit theFSRQSEDs,so astoobtain the physical parameters of recently confirmed the blazar nature of a newly discovered radio
the jets, e.g. the jet bulk Lorentz factor Γ and the viewing an- quasar, SDSS J0131−0321 with an extremely high luminosity at
gleϑ(Ghisellini&Tavecchio2009).Highradioloudness(R>100, z=5.18foundbyYietal.(2014).
∼
whereRistheratioofrest-frame5-GHzand2500-Åfluxdensities, Inthispaper,wereportonourdual-frequencyEuropeanVLBI
e.g.Sbarratoetal.2013),highX-rayluminosity(νL >1039W;the Network (EVN) observations of four high-redshift blazar candi-
ν∼
inverseComptonhumpmovesintotheX-rayband)andhardX-ray dates. Our aims were to confirm their blazar nature and to probe
spectrum are typical characteristics of high-redshift blazars (e.g. theirjetstructureson∼1−10masangularscales.Iftheyhavejets
Sbarratoetal.2013;Ghisellini2015). on these scales, multi-epoch observations could be carried out to
Systematic search for high-redshift (here and henceforth, measure proper motions. Section 2 describes our target selection
“high-redshift” means z > 4) blazars have been carried out method.ThedetailsoftheVLBIexperimentandthedatareduction
by Sbarratoetal. (2012, 2013) and Ghisellinietal. (2014). Their aregiveninSect.3.WepresentourresultsinSect.4,andprovide
methodwastofirstselecthighlyradio-loudsources(radioloudness thediscussionandconclusionsinSect.5andSect.6,respectively.
parameter R > 100) from the Sloan Digital Sky Survey (SDSS) A flat ΛCDM cosmological model with H0 = 71 kms−1Mpc−1,
DataRelease7(DR7)quasarcatalogue(Schneideretal.2010)and Ωm=0.27andΩΛ =0.73(Spergeletal.2007)isadoptedthrough-
thentouseX-raydatatofurtherconfirmtheblazaridentityofthe out this paper. In this model, 1 mas angular size corresponds to
candidates.Blazarscouldbeusedtoinfertheirparentpopulation: 7.08pcprojectedlinearsizeatz=4.
statistically, for one blazar with a viewing angle ϑ ≤ 1/Γ, there
should be∼ 2Γ2 radiosourceswiththeirjetspointing elsewhere.
Besides,oncethemassoftheSMBHpoweringtheblazarisknown,
the number density of SMBHshosted by radio-loud quasars asa 2 TARGETSELECTION
functionofredshiftcouldthenbeexplored(Sbarratoetal.2015).
We searched in the recent literature (e.g. Volonterietal. 2011;
Blazars are compact and bright in the radio, and thus ideal
Sbarratoetal. 2015; Ghisellinietal. 2015) and found four z > 4
targetsforverylongbaselineinterferometry(VLBI).Inparticular,
objectswhichwereidentifiedasblazars,havedeclinations reach-
high-z blazars allow us to probe the radio emission of the AGN
able for the EVN but had no published VLBI observations (Ta-
intheearlyUniverse.TheVLBIimagesofblazarstypicallyshow
ble 1). We call these objects blazar candidates. All of them have
milli-arcsec (mas) scale compact “core” emission or a one-sided
high radio loudness values (R > 100), and they were claimed as
core–jet structure. However, the high-z ones rarely have promi-
blazars because of their high X-ray luminosities and hard X-ray
nent jets. This could be explained by taking into account the re-
spectra,whichcouldnotbeexplainedbytheAGNcoronamodel.
lation between the emitted (rest-frame) and observed frequencies
Oneofourtargets(J2220+0025)appearedparticularlyinteresting
intheexpanding Universe: ν = (1+z)ν .Whilethecorehas
em obs from the point of view of its extended radio emission on arcsec
typically flat spectrum (with a spectral index α ≥ −0.5, where
S ∝ να and S is the flux density), the spectrum of the jet is scale,evenbeforetheacquisitionofthenewhigh-resolutiondata.It
ν issomewhatresolvedinthe1.4-GHzimagetakenintheVeryLarge
steep (α < −0.5). Therefore, if we observe at a fixed frequency,
Array(VLA)FaintImagesoftheRadioSkyatTwenty-Centimeters
thehighredshiftimpliesahigheremittedfrequencyandtherefore
(FIRST)2 survey (Whiteetal. 1997), with a resolution of about
theextendedsteep-spectrumemissionappearsfainterrelativetothe
5arcsec.Observedatthesamefrequencybutwithhigherresolution
flat-spectrumcore(Gurvits1999).Todate,onlyonehigh-redshift
(1.8arcsec)withtheVLA,thesourceshowsanextended(∼10arc-
blazar (J1026+2542, z = 5.27) is known to have a pronounced
sec)linearstructureintheimagefoundintheVLA–SDSSStripe
jetstructureextendedto∼ 10-masscalewhichallowedFreyetal.
82survey3 (Hodgeetal.2011).However,thereisnoindicationof
(2015) to estimate the apparent proper motion of the jet compo-
multiplecomponentsinitsoptical(SDSS)image4.Thereforeitis
nentsusingtwo-epochVLBIobservationsseparatedbymorethan
unlikelythattheextendedradiostructureiscausedbygravitational
7yr.
lensing.Theremaining3sourcesareunresolvedintheFIRSTsur-
Blazar jets often show apparent superluminal motion, and
veyimages.
the apparent proper motion–redshift relation can be used to
1 Veryrecently,thissourceisclassifiedasasteep-spectrum radioquasar 2 http://sundog.stsci.edu/
based on VLBI observations, although at high rest-frame frequencies 3 http://sundog.stsci.edu/cgi-bin/searchstripe82
(Coppejansetal.2016). 4 http://www.sdss.org/dr13/
MNRAS000,1–11(2017)
Fourradioquasarsat z > 4:blazarsor not? 3
Table1.Fourz>4radioquasarsclaimedasblazarsobservedinourEVNexperiment.
Name Rightascension Declination S1.4 z
hms ◦ ′ ′′ mJy
J0839+5112 083946.204 +511202.88 41.6±4.0 4.390†
J1420+1205 142048.011 +120546.40 87.3±6.6 4.034†
J2134−0419 213412.012 −041909.67 311.3±18.3 4.346
J2220+0025 222032.608 +002536.03 92.7±6.4 4.205†
Notes.Theequatorialcoordinates(J2000)and1.4-GHzfluxdensities(S)arefromtheFIRSTsurvey(Whiteetal.1997).Theredshifts(z)markedwith†are
foundinSchneideretal.(2010),theotheroneisfromHooketal.(2002).
3 VLBIOBSERVATIONSANDDATAREDUCTION lowedtheEVNDataAnalysisGuide7.Aprioriamplitudecalibra-
tionwasdoneusingtheknownantennagaincurvesandthesystem
3.1 Observations
temperatures measured at the VLBI stations during the observa-
OurEVNobservationswereconductedine-VLBImode(Szomoru tions or nominal system equivalent flux density values. Parallac-
2008). The data were taken at 1024 Mbps rate at each partic- ticanglecorrectionforthealtitude-azimuthmountedantennas,and
ipating radio telescope, with two circular polarizations, 8 base- ionosphericcorrectionwerethenapplied.Manualphasecalibration
bands(orintermediatefrequencychannels,IF)perpolarizationand andglobalfringe-fittingwereperformed.Bandpasscorrectionwas
16MHzbandwidthperbaseband,andtransferredtotheJointInsti- carried out using the corresponding fringe-finder source. For the
tute for VLBI ERIC (JIVE, Dwingeloo, the Netherlands) via op- calibrationofJ2134−0419,manualphasecalibrationwasnotper-
tical fiber cables. The data streams were correlated in real time formed, and the bandpass was corrected using 1156+295 at both
at the EVN software correlator (SFXC, Keimpemaetal. 2015) 1.7and5GHz.
with 2 s integration time and 32 spectral channels per IF. The Thecalibratedvisibilitydataforphasecalibratorsourceswere
experiment was separated into 4 segments with project codes first transferred to and imaged in the Difmap program (Shepherd
EC054A, EC054B, EC054C, and EC054D. Three of our targets 1997).Antenna-basedcorrectionfactorsweredeterminedforeach
(J0839+5112,J1420+1205,andJ2134−0419)wereobservedwith IF in the first step of the amplitude self-calibration. These were
theEVNon2015September15–16at1.7GHz,andon2015Oc- fed back into AIPS and applied to the visibilities using the task
tober6–7at5GHz.Theremainingone(J2220+0025)wasfirstob- clcor. In this step, only the amplitude corrections exceeding 5
servedon2016January13at1.7GHz,andthenon2016February percentwereconsidered.Globalfringe-fittingwasthenrepeated,
3at5GHz. buttheimageofeachphasecalibratorproducedearlierinDifmap
Theobservationswerecarriedoutinphase-referencingmode wastakenintoaccountthistime,tocorrectfortheresidualphases
(Beasley&Conway 1995). The radio telescopes were pointed to resulting from the calibrator’s structure. Finally, the phase, delay
the a priori positions taken from the FIRST catalogue (Table 1). and delay-rate solutions obtained for the phase-reference calibra-
Thenearestsuitablephasecalibrators,withinabout2◦angularsep- torswereinterpolatedandappliedtotherespectivetargetsources.
aration from the respective targets (see Table 2), were selected In addition to phase-referencing described above, global
from the Astrogeo database5. The observing parameters, includ- fringe-fittingwasalsodirectlyandsuccessfullyperformedfortwo
ingthenamesofradiotelescopesparticipatingintheprojectseg- targetsources,J2134−0419andJ0839+5112.Duetothelackofthe
ments,arelistedinTable2.Thebrightfringe-finderradiosources most sensitiveEVNantenna(Ef)intheproject segment EC054B
used in this experiment were 0528+134 (EC054A), 1156+295 at 5 GHz (see Table 2), a longer solution time (10 min) was ap-
(EC054A and B), DA193 (EC054B), 3C454.3 (EC054C), and plied for the direct fringe-fitting on J0839+5112, rather than the
CTA102(EC054D). 1.5minusedinothercases,inordertoproperlycalibratethevisi-
Thetotalobservingtimewasabout3hforeachcombination bilityphasesonthelongestbaselinestoSh.Atlast,thecalibrated
oftargetsourceandphasecalibratorateachfrequency.Thephase- visibilitydataofeachtargetsourcewereexportedfromAIPSand
referencingdutycirclewas5minlong,with3.5minspentonthe loadedintoDifmapforimaging andfittingbrightnessdistribution
targetsource.Therefore,approximately2hwerespentoneachtar- models.
get,exceptforJ2134−0419.Inthiscaseweadoptedadifferentob- The traditional hybrid mapping method (cycles of modeling
serving strategy, because of the high flux density of J2134−0419 thebrightnessdistributionand, ifallowedbythesufficientlyhigh
(seeTable1).Toincreasetheon-targettimeto2.5h,onlyacou- signal-to-noiseratio,self-calibrationofphasesandpossiblyampli-
pleofphase-referencingscansweredistributedoverthewholeob- tudes) wasemployed to produce theimages of thetarget sources
servingperiod,toallowfortheprecisedeterminationofthesource presented in Sect. 4. The details of the procedure varied from
position.TherestofthetimewasspentonobservingJ2134−0419 source to source, depending on their properties. For J0839+5112
only. andJ2134−0419,weusedseveralcyclesofcleancomponentmod-
eling and phase (later also amplitude) self-calibration. In these
caseswecouldworkwithtwotypesofdatasets:theoneobtained
3.2 Datareduction fromphase-referencing,andanotherfromdirectfringe-fitting.We
also fitted Gaussian brightness distribution models to the self-
ThedatacalibrationwasconductedintheNRAOAstronomicalIm-
ageProcessingSystem6 (AIPS,Greisen2003),andgenerallyfol- calibrated data in the visibility domain in Difmap, to allow for a
5 http://astrogeo.org/calib/search.html
6 http://www.aips.nrao.edu/index.shtml 7 http://www.evlbi.org/user_guide/guide/userguide.html
MNRAS000,1–11(2017)
4 H.-M. Cao et al.
Table2.ObservationinformationoftheEVNexperiment.
Projectsegment Targetsource(s) νobs Participatingtelescopes Phasecalibrator Separation
GHz ◦
J0839+5112 J0849+5108 1.60
EfHhJbMcO8
EC054A J1420+1205 1.7 J1415+1320 1.71
TrWbSh
J2134−0419 J2142−0437 2.12
J0839+5112 J0849+5108 1.60
HhJbMcNtO8
EC054B J1420+1205 5 J1415+1320 1.71
TrWbSh
J2134−0419 J2142−0437 2.12
EfHhJbMcO8
EC054C J2220+0025 1.7 J2226+0052 1.62
TrWb
EfMcNtO8Tr
EC054D J2220+0025 5 J2226+0052 1.62
YsWbHh
Notes.Telescopecodes:Ef–Effelsberg(Germany),Hh–Hartebeesthoek(SouthAfrica),Jb–JodrellBankMk2(UnitedKingdom),Mc–Medicina(Italy),
Nt–Noto(Italy),O8–Onsala(Sweden),Tr–Torun´(Poland),Wb–Westerbork(theNetherlands),Ys–Yebes(Spain),Sh–Sheshan(China).Hhdidnot
participateintheobservationsofJ0839+5112becauseofthehighdeclinationofthesource.
4.2 Imagesandbrightnessdistributionmodels
Table3.Theaccurateastrometricpositionsofthefourtargetsourcesdeter-
minedfromthephase-referencedEVNobservationsat5GHz. The VLBI image and model parameters obtained in Difmap are
listedinTables4and5,respectively.Theerrorsofthemodelpa-
Name Rightascension Declination rametersareestimatedaccordingtoLeeetal.(2008),assumingthat
hms ◦ ′ ′′ the errors are stochastic and independent for each estimated pa-
J0839+5112 083946.21606 +511202.8257 rameterofthecomponent(Fomalont1999).Forthefluxdensities,
J1420+1205 142048.00993 +120545.9807 weassume an additional 5 per cent error added inquadrature, to
J2134−0419 213412.01074 −041909.8610 account for the uncertainties of the VLBI amplitude calibration
J2220+0025 222032.50276 +002537.5107 which is based on antenna system temperatures and gain curves.
Alsoshown inTable5arethe source parametersand their errors
derivedfromourmeasurements:brightnesstemperature,two-point
quantitativedescriptionofthesourcestructuresbyobtainingesti- spectralindexbetweenourobservingfrequencies,andmonochro-
matesforcomponentsizes,positionsandfluxdensities. maticluminosity of thecorecomponent inthesource restframe.
For thetwoweaker sources(J1420+1205 andJ2220+0025), Note that the angular resolution at the two frequencies is differ-
onlyphase-referenceddatasetswereavailable.Forproducingthe entwhichmaycausesomefluxdensitycontributionfromextended
images,weusedGaussianmodelinginsteadofcleancomponents, structureat1.7GHz,leadingtoanartificialsteepeningofthede-
whichgavebetterresultsduetotheextendedemissionregionsof rivedspectrum.
theseobjects.Nophaseandaplitudeself-calibrationwasattempted Aninterferometercanprobesourcestructuressmallerthanthe
fortheseweaksources,exceptforJ1420+1205at1.7GHz,where synthesized beam, and the minimum resolvable component size
itappearedsufficientlystrongforphase-onlyself-calibrationover couldbeestimatedaccording toe.g. Kovalevetal.(2005).Inour
solutionintervalsof10sec,andexcludingthelongbaselinestoHh experiment,allthefittedcomponent sizesreportedinTable5ex-
andSh. ceed the minimum resolvable angular size. The images, modelfit
resultsandderivedparametersaredescribed below foreachindi-
vidualtargetsource.
4 RESULTS
4.1 Phase-referencedpositions 4.2.1 J0839+5112
Thephase-referenceddatasetswereusedtomeasuretheastromet- Theimagesmadefromthephase-referencedandfringe-fittedvisi-
ricpositionsofthefourtargetsources.Itispossiblebecausetheco- bilitydatasetsarepracticallythesameintermsofsourcestructure,
ordinatesofthereferencesourcesareaccuratelyknownintheInter- peak brightness and noise level for this source at both observing
nationalCelestialReferenceFrame(Feyetal.2015).Thesources frequencies.InFig.1,weshowthe1.7-GHzimageofJ0839+5112
were first imaged in Difmap, and then the AIPS verb maxfit was producedfromthefringe-fitteddataset,whileat5GHz,thephase-
usedtoderivethecoordinatesofthebrightnesspeakintheimages. referenced image is displayed. The latter is somewhat less noisy
Thepositionsmeasuredat1.7and5GHzwereconsistentwitheach thanitsfringe-fittedcounterpart,duetothebettercalibratedphases
otherwithintheuncertainties.Themoreaccurateresultsobtained onthelongbaselinesfromtheEuropeanantennastoSheshanmade
at5GHzareshowninTable3.Theerrorsarisefromthepositional possiblebythestrongphase-referencecalibrator.Therelativeco-
uncertaintyofthephasecalibrator,thethermalnoiseoftheinterfer- ordinatesintheimagesaremeasuredwithrespecttothelocationof
ometerphases,andsystematicerrorsscaledbythetarget–calibrator thebrightnesspeak.
separation(seee.g.Pradeletal.2006).Weestimatethat theright Asinglecomponent isseeninthe1.7-GHzimage, therefore
ascensionanddeclinationcoordinatesgiveninTable3areaccurate onecircularGaussianmodelwasusedtofitthesourcebrightness
within0.5mas. distribution. At5 GHz, thereisa weakjet-likecomponent tothe
MNRAS000,1–11(2017)
Fourradioquasarsat z > 4:blazarsor not? 5
Table4.VLBIimageparametersofthefourtargetsources.
Name νobs Beamsize P.A. Peak RMS Fig.
GHz mas×mas ◦ mJybeam−1 µJybeam−1
J0839+5112 1.7 15.4×3.2 15 37.35 41 1a
5 3.4×1.3 10 36.90 58 1b
J2134−0419 1.7 5.9×3.3 76 163.22 363 2a
5 2.5×1.2 81 138.01 127 2b
J1420+1205 1.7 3.8×3.2 61 10.41 60 3a
5 1.6×1.2 80 4.76 88 3b
J2220+0025 5 8.3×4.4 141 1.92 37 4
Notes.Weonlypresent5-GHzVLBIimageforJ2220+0025(seethetextfordetails).Col.3–synthesizedbeamsize(FWHM),Col.4–positionangleofthe
beammajoraxis,measuredfromnorththrougheast,Col.5–peakbrightness,Col.6–RMSbrightness,Col.7–figurenumber.
Table5.ParametersofthefourtargetsourcesfromtheEVNmeasurements.
Name νobs Comp. Sν ∆α ∆δ θ Tb α Lν
GHz mJy mas mas mas 109K 1027WHz−1
J0839+5112 1.7 C 51.3±3.3 0 0 2.99±0.10 13.9±1.8 −0.2±0.1 2.61±0.71
5 C 41.6±3.0 0 0 0.60±0.02 30.0±4.6 2.11±0.60
E 1.4±0.4 6.1±0.1 –0.8±0.1 0.67±0.16
J2134−0419 1.7 C 192.6±15.3 0 0 1.42±0.07 230.2±20.5 −0.2±0.1 10.18±3.03
E 44.0±6.5 21.0±0.3 2.4±0.3 3.25±0.39
5 C 150.4±9.7 0 0 0.37±0.01 288.4±36.2 7.95±2.25
E 27.8±3.7 12.7±0.3 0.9±0.3 5.19±0.62
J1420+1205 1.7 C 20.9±2.1 0 0 3.45±0.26 4.0±1.0 −0.9±0.2 2.75±1.34
NW 27.0±7.7 –275.2±4.0 1299.3±4.0 22.7±6.4
5 C 8.0±1.3 0 0 1.12±0.15 1.6±0.7 1.05±0.58
J2220+0025 1.7 C 3.7±1.3 0 0 11.6±3.5 0.06±0.05 −0.4±0.5 0.23+0.48
−0.16
SE 4.5±1.7 2447.7±8.0 –1928.6±8.0 24.0±8.1
5 C 2.5±0.5 0 0 2.9±0.4 0.07±0.03 0.15+0.26
−0.10
Notes.Col.2–observingfrequency,Col.3–componentdesignation,Col.4–fluxdensity,Cols.5–6–relativerightascensionanddeclinationwithrespect
tothecore,Col.7–fittedGaussiancomponentdiameter(FWHM),Cols.8–10–rest-framebrightnesstemperature,spectralindexandmonochromatic
luminosityofthemain(core)component.ForJ2220+0025,thefluxdensityerrorsarelikelyoverestimated,andthecoreluminosityupperandlowerbounds
arederivedbyassumingthespectralindexupperandlowerbounds.
eastofthebrightcoreatabout6masseparation(Fig.1b).Inthis measured with much higher angular resolution, is 23 per cent
case, we modeled the source with two circular Gaussian compo- higher than VLA value at 1.4 GHz in the FIRST survey, 41.6±
nents(Table5). 4.0mJy(Table1).Giventheverycloseobservingfrequencies,this
The dual-frequency observations allowed us to calculate the canonlybeexplainedbyfluxdensityvariabilitybetweenthetwo
two-pointspectralindexofthedominant(core)componentpresent measurementepochs.Thecompactcore–jetradiostructure(Fig.1),
inbothimages,usingthemodel-fittedfluxdensities(Table5).The theflatcorespectrumandtheimpliedvariabilityareallcharacter-
valueα = −0.18indicatesaflatradiospectrum. Wealsoderived isticofblazars.Ontheotherhand,themeasuredbrightnesstemper-
thecorebrightnesstemperatureinthesourcerestframe(Table5) ature,T =(3.0±0.5)×1010K(Table5)doesnotindicateDoppler-
b
usingtheformula boostedemission.Ifweadopttheequipartitionbrightnesstemper-
Tb=1.22×1012(1+z)θ2Sνν2 [K] (1) atetumrpee(rTaetqur≈e(5T×int1)0,1th0eKD,Ropepaldehrefaadct1o9r9δ4)=aTsbth/Teiinnttbriencsoimcbersigsmhtanlelsesr
obs than unity. If we assume a somewhat smaller T = 3×1010 K
int
(e.g.Condonetal.1982;Leeetal.2008),wherezistheredshift,Sν proposed byHomanetal.(2006),theDoppler factorisδ ≈ 1.In
thefluxdensityinJy,νobs theobservingfrequencyinGHz,andθ any case, theradio emission in J0839+5112 does not seem to be
thefullwidthathalf-maximum(FWHM)sizeofthecircularGaus- highlyenhancedbyrelativisticbeaming.Itispossiblethatweun-
sianmodelcomponent inmas.Themonochromatic luminosityof derestimate the Doppler boosting by a factor of ∼ 2 because the
thedominantcomponentinthesourcerestframe(Table5)wascal- brightnesstemperatureismeasuredatahighrest-framefrequency
culatedusing (∼ 27 GHz) where the intrinsic brightness temperature may be
S lower(Lee2014).
L =4πD2 ν (2)
ν L(1+z)1+α
(Hoggetal.2002),whereD istheluminositydistanceandαthe
L
spectralindex.
The1.7-GHzVLBIfluxdensityof51.3±3.3mJy,although
MNRAS000,1–11(2017)
6 H.-M. Cao et al.
4.3GHz.This,anda7.6-GHzimageshowingthecorecomponent
withamarginallyresolvedeasternextension(within∼2 mas)are
1.7 GHz
availabeintheAstrogeodatabase8.Theseshort1-minVLBAsnap-
shotsweretakenaftertheacceptance ofour EVNobserving pro-
posal,on2015September1(projectcode:BP192),about2and6
weeksbeforeourobservationsat1.7and5GHz,respectively.The
C
overallcore–jetstructuretowardstheeastisthesameintheVLBA
imagesandourmoresensitiveEVNimages(Fig.2).
InourEVNdataat1.7GHz,thehighervisibilityamplitudes
on the shortest Ef–Wb baseline indicate the presence of a large-
scalestructurewhichisresolvedoutwiththeEVN.Also,theim-
agenoiseissignificantlyhigherthanthetheoreticalthermalnoise
level (21 µJybeam−1), suggesting some extended emission in the
field.Indeed,the1.4-GHzFIRSTfluxdensity(311.3mJy,Table1)
(a)
is ∼32 per cent higher than the integrated flux density recovered
inour VLBIcomponents (236.6 mJy, Table 5) at aclose observ-
ing frequency of 1.7 GHz. This cannot be explained by spectral
changes alone, and may be due to a radio structure extended to
∼0.1–1arcsecscales,or/andfluxdensityvariability.
ThedominantcomponentofJ2134−0419hasaflatspectrum
(α=−0.2).ItsbrightnesstemperatureisT =(28.8±3.6)×1010K,
5 GHz b
wellexceedingT .TheimpliedDopplerfactorisδ≈6−10.For
eq
thehigherestimate,T = 3×1010 Kwasassumed(Homanetal.
int
2006).TheflatcorespectrumandthelargeDopplerfactorleaveno
C doubtabouttheblazaridentificationofJ2134−0419.
E
4.2.3 J1420+1205
Awidedoublestructureextendedto∼1.33′′isalreadyapparentin
the1.7-GHzdirtyimageofthissource.Thissizeissmallerthanthe
restoringbeamoftheVLAintheFIRSTsurveyandthustheobject
appearsunresolvedinFIRSTat1.4GHz.Torestorethe1.7-GHz
(b) EVNimageofJ1420+1205seeninFig.3a,wefittedtwocircular
Gaussianbrightnessdistributionmodelcomponentstothevisibil-
ity data (Table 5). These represent the extendend emission better
thancleancomponentmodelstraditionallyusedinVLBIimaging.
Thebrighterandmorecompactsouth-easterncomponentwasalso
Figure 1. Naturally weighted VLBI images of J0839+5112 at 1.7 and detectedat5GHz(Fig.3b).OnecircularGaussianmodelwasfit-
5GHz.Thefirstcontoursaredrawnat±3σimagenoiselevel.Theposi- ted in this case, itsbrightness temperature is low, in the order of
tivecontoursincreasebyafactorof2.Thesynthesizedbeam(FWHM)is 109K(Table5).Theweaker,moreextendednorth-westerncompo-
shownat thelower-left corner ofeach image. Theimage parameters are nentseeninthe1.7-GHzimageremainedundetectedat5GHzat
listedinTable4. the6σimagenoiselevelof∼0.5mJybeam−1.
The VLBI-recovered integrated flux density at 1.7 GHz
4.2.2 J2134−0419 (nearly 47.9 mJy, Table 5) is more than 45 per cent lower than
theFIRSTvalueat1.4GHz(Table1).Thisindicatesthatmostof
Theimages produced fromthephase-referenced and fringe-fitted theradioemissionoriginatesfromarcsec-scaleextendedstructures
visibility data show consistent structures at both frequencies. We in J1420+1205. The optical position of the object (right ascen-
present in Fig. 2 the images of J2134−0419 made after direct sion14h20m48s.01,declination+12◦05′45′.′96)fromSDSSDR13
fringe-fitting which are based on more data and have higher dy- (Albaretietal.2016)coincideswiththelocationofthemorecom-
namicrange.Thejetstuctureisalignedtotheeast–westdirection. pactsouth-easternVLBIcomponent(Table3)withintheuncertain-
Twocomponentsareseeninboththe1.7-and5-GHzimages,the ties.Wecanthereforeidentifythiscomponentwiththegalacticnu-
westernmost one is the brighter and more compact. We identify cleus.TheheavilyresolvedVLBIcomponenttothenorth-westof
thiswiththecore.Accordingtotheaccurateastrometryprovided thenucleusismostlikelyalobeinaradioquasar,ontheapproach-
bythephase-referencedimages,thecorecomponentspositionally ingside.Theprojectedlineardistancebetweenthetwocomponents
coincideatbothfrequencies.TwocircularGaussianmodelcompo- ofJ1420+1205inthe1.7-GHzimageis∼9.5kpc.Inthispicture,
nentswerefittedtothebrightnessdistributionofthissourceat1.7 theapparentasymmetryofthestructuremaybecausedbyaview-
and5GHz,respectively(Table5).Thepositionoftheeasternjet ingeffect:theradioemissionofthelobeontherecedingsideofthe
component(E)isslightlydifferentat5GHz,consistentwithitsre- nucleusisbelowthedetectionthreshold.Boththeextendedstruc-
solvednatureapparentinFig.2b,wherethelowsurfacebrightness tureandthelowbrightnesstemperatureofthecentralcomponent
emissionalsotracesoutthejetstructuretowardstheeast.
Thetwo-component structure of J2134−0419 isalsoseenin
an image made with the Very Long Baseline Array (VLBA) at 8 http://astrogeo.org/cgi-bin/imdb_get_source.csh?source=J2134-0419
MNRAS000,1–11(2017)
Fourradioquasarsat z > 4:blazarsor not? 7
1.7 GHz 5 GHz
C
C
E E
(a) (b)
Figure2.NaturallyweightedVLBIimagesofJ2134−0419at1.7and5GHz.Thefirstcontoursaredrawnat±3σimagenoiselevel.Thepositivecontours
increasebyafactorof2.Thesynthesizedbeam(FWHM)isshownatthelower-leftcornerofeachimage.TheimageparametersarelistedinTable4.
indicatealargeinclinationangleofthestructurewithrespecttothe sityat1.4GHz9.Thelatterisalobeblendedwiththeweakcorethat
lineofsight.Thiscanhardlybereconciledwiththeblazarscenario. wedetectedwiththeEVN.Thesetworadiolobesadduptheentire
Thelobe-dominatednatureofJ1420+1205isconsistentwith FIRSTfluxdensity(92.7±6.4mJy,Table1)withinthemeasure-
itssteepoverall spectrumwithα ≈ −0.6.Thetotalfluxdensities ment errors. The 1.4-GHz flux density given in the NRAO VLA
ofthesourcemeasuredinabroadrangeof frequenciesareavail- SkySurvey(NVSS)is88.0±2.7(Condonetal.1998),whichalso
ablefrom theliterature,at 74 MHz(750±130 mJy, Cohenetal. indicatestheabsenceofradiostructureonscaleslargerthan∼10′′.
2007), 365 MHz (248 ± 29 mJy, Douglasetal. 1996), 1.4 GHz If we assume that the structure of the double-lobed radio source
(92.1±2.8mJy,Condonetal.1998),and4.85GHz(55±10mJy, isintrinsicallysymmetric,thefluxdensitydifferencebetweenthe
Gregory&Condon1991). approaching jetside(S ) andthereceding counterjet side(S )is
j cj
causedbyDopplerbeaming.Thisallowsustoestimatetheview-
ing angle of the structure (ϑ) with respect to the line of sight or
4.2.4 J2220+0025 the jet speed. According to e.g. Arshakian&Longair (2004), the
viewingangleofradiosourcewithacontinuoustwinjetcouldbe
ThisobjectfallsintheskyareacoveredbytheVLA–SDSSStripe
estimatedas
82 survey (Hodgeetal. 2011). This survey is about three times
deeper than FIRST and mostly used the A configuration of the S 1+β cosϑ 2−α
VLAthatprovidesthelongest baselinesandthusthehighestres- Sj = 1−βjcosϑ! (3)
cj j
olution. The 1.4-GHz VLA image of J2220+0025 reproduced in
Fig.4showsanelongatedstructureextendedto∼10′′.Imagingthis where βj is the jet speed expressed in the unit of light speed
extendedsourcewithVLBIproveddifficult.At1.7GHz,thereis c. From a simple orientation-based unified model of radio AGN
aclearindicationoftwoemissionregionsinthedirtyimage.Two (Barthel 1989), the viewing angle of quasar jets is expected at
circular Gaussianbrightness distributionmodel components were or below 45◦. If we assume a typical value for spectral index
fittedtothe visibilitydata (Table5). One of them coincides with α = −0.6(Arshakian&Longair2004),thenwegetβj ≈ 0.15for
thecentreof thelarge-scaleradioemission, theother oneisvery J2220+0025 at ϑ ≈ 45◦. Smaller inclination angles would infer
closetothebrightness peak, locatedtothesouth-east of thecen- even slower jet speed. Note that for low-redshift sources usually
tre. The sum of their flux densities (8.2 mJy) is much below the βj = 0.4isassumed (Arshakian&Longair 2004).Inturn, higher
1.4-GHz FIRST value (92.7 mJy, Table 1), consistently with the βj would result in inclination angles exceeding 45◦, being incon-
resolvednatureofthesource. sistentwiththequasarclassification(Schneideretal.2010),unless
Only the central component was clearly detected at 5 GHz thereissubstantialchangeintheorientationoftheinnerandouter
with the high resolution of the EVN. We fitted a circular Gaus- jets.Inanycase,forreasonableβj andαvalues,arobustresultis
sianmodeltocharacteriseitsproperties(Table5).The5-GHzim- that the inclination angle of the arcsec-scale structure is large in
age of this faint but relatively compact feature is shown in the J2220+0025.
inset in Fig. 4. This component is located in the SDSS DR 13 Wecanconcludethat,asinthecaseofJ1420+1205,thecom-
(Albaretietal.2016)opticalpositionofthequasar(rightascension pact radio emission decected with the EVN in this high-redshift
22h20m32s.49, declination +00◦25′37′.′49) within the astrometric source originates from an AGN as it shows brightness tempera-
uncertainties. It is therefore right in the centre of a double-lobed ture well above the ∼ 105 K limit for normal galaxies (Condon
radioAGNreminiscentofanFR-IItypesource(Fanaroff&Riley 1992). Also, the monochromatic radio luminosity exceeds by or-
1974)withatotalprojectedlinearextentupto∼70kpc.Inthissce- dersofmagnitudethelowerlimitof∼2×1021WHz−1typicalfor
nario,thesouth-easterncomponentdetectedwiththeEVNonlyat radio AGN (Kewleyetal. 2010; Middelbergetal. 2011). On the
1.7GHzisahotspotinthelobeontheapproachingside. otherhand,TbismuchlowerthanexpectedfromaDoppler-boosted
TheVLA–SDSSStripe82survey image(Fig.4)shows two
majorcomponents. Thesouth-eastern oneisthebrightest(56.2±
9.5mJy),thenorth-westerncomponenthas33.3±7.7mJyfluxden- 9 http://sundog.stsci.edu/cgi-bin/searchstripe82
MNRAS000,1–11(2017)
8 H.-M. Cao et al.
1.7 GHz
5 GHz
NW
C
20 mas
C
(b)
(a) 20 mas
Figure3.Naturally weightedVLBIimagesofJ1420+1205at1.7and5GHz.At5GHz,onlythesouth-easterncomponentwasdetected, whichappears
brighterandmorecompactinthe1.7-GHzimage.Thefirstcontoursaredrawnat±5σimagenoiselevel.Thepositivecontoursincreasebyafactorof2.The
synthesizedbeam(FWHM)isshownatthelower-leftcornerofeachimage.TheimageparametersarelistedinTable4.
5 DISCUSSION
EVN at 5GHz 1.4 GHz
00 25 44
5.1 ThecompactradiosourcesJ0839+5112andJ2134−0419
42
ThesourceJ0839+5112showspropertiestypicalforblazars:com-
40 pact mas-scale structure (Fig. 1), flat spectrum and flux density
20 mas variability. However, itsmoderate brightness temperatureisclose
n (J2000) 38 C ttohetihreloiwnt-rbinrisgichtnveaslusestcahteara(Hctoemrisatnicettoala. 2la0r0g6e),sainmdpidleationfgAnGoNsigin-
natio 36 nificant Doppler boosting. There is at least another radio quasar
Decli 34 known at high redshift with similar properties. J1146+4037 at
SE
z=5.01hasacompactstructurewithmoderatebrightnesstemper-
32
ature(Freyetal.2010;Coppejansetal.2016)measuredwithVLBI
butitsX-raydataindicateitsbalazarnature(Ghisellinietal.2014).
30
OurEVNobservationsconfirmedthatJ2134−0419isablazar.
28 VLA A-con guration Moreover,thediscoveryofitsprominentjetstructure(Fig.2)will
enablefutureattemptstodetectapparentpropermotionofthecom-
22 20 33.2 33.0 32.8 32.6 32.4 32.2 32.0
Right ascension (J2000) ponents.LikeinthecaseofJ1026+2542,asofaruniqueblazarat
veryhighredshift(z=5.27)withdirectlyestimatedjetcomponent
Figure 4. The background image of J2220+0025 was made at 1.4 GHz
propermotions(Freyetal.2015),thismaybecomefeasibleovera
withtheVLAinitsAconfiguration(Hodgeetal.2011).ThecircularGaus-
sianrestoringbeamsizeis1.8′′ (FWHM),thefirstcontoursaredrawnat timebaselineof5–15yrwithmulti-epochVLBImeasurementsat
±0.3mJybeam−1,thepeakbrightnessis40.05mJybeam−1.Thepositive 5GHz.Ourobservationspresentedherecouldhelpdetectingstruc-
contoursincreasebyafactorof2.Thetwo+symbolsindicatethepositions tural variations which appear slower by a factor of (1+z) in the
ofthetwocomponentsseeninour1.7-GHzEVNdirtymapofJ2220+0025 observer’s frame because of the cosmological time dilation. In a
(notshownhere).The5-GHzEVNimageoftheonlycomponentdetected separatestudy,wewillattempttocomparethecurrentVLBIdata
isdisplayed intheinset. Thedata frombaselines longerthan 50million ofJ2134−0419witharchivaldata.
wavelengths(50Mλ)wereexcluded.Thefirstcontoursaredrawnat±5σ
imagenoiselevel.Thepositivecontoursincreasebyafactorof2.Theimage
parametersarelistedinTable4. 5.2 Radioemissionofthenon-blazarsourcesJ1420+1205
andJ2220+0025
WeobservedfourblazarcandidateswiththeEVNandfoundthat,
contrarytotheexpectations,twoofthemarenotcompactandare
inclinedatlargeanglestothelineofsight.ForJ2220+0025whose
extendedstructureonarcsecscalewasalreadyknownfromVLA
observations (Hodgeetal. 2011) we could identify a weak com-
blazar jet. This, and the two-sided lobe-dominated structure in- pactcoreinthecentreofthelarge-scaleradiostructure,confirmig
clinedatalargeangletothelineofsightseenintheVLAimageare thedouble-lobedmorphology.Inthesetwosources,theradioemis-
strongargumentsagainsttheblazarclassificationofJ2220+0025. sionisdominatedbystructuresextendedto∼0.1−1arcsecscales.
MNRAS000,1–11(2017)
Fourradioquasarsat z > 4:blazarsor not? 9
TheyalsoshowclearevidenceagainstDoppler-boostedinner(mas- X-rayemissioncanalsooriginatefromtherelativisticjet.Infact,
scale) jet emission. Intriguingly, these two objects (J1420+1205 theSEDisdominatedbythejetemissionoverthewidestrangeof
andJ2220+0025) wereclaimedasthebestblazar candidatesina elecromagneticradiation,fromtheradiotoγ-rays.Forblazars,the
complete sample of 19 extremely radio-loud quasars at z > 4 by one-zone leptonic model (Ghisellini&Tavecchio 2009) can suc-
Sbarratoetal.(2015),andX-rayobservationswithSwiftseemedto cessfullyexplainthedouble-humped SED.Herethedominant X-
confirmthisnotion,withestimatedjetviewinganglesofϑ≈3◦. rayemissionalsoresultsfrominverse-Comptonprocess,andcomes
RecentlyCoppejansetal.(2016)collecteddataofallz> 4.5 fromasmall dissipationregion inthejet near thecentral engine,
radiosourcesobservedtodatewithVLBI.Theselectioncriteriafor where a high density of photons and relativistic electrons is ex-
these 30 objects partly taken from the literatureare not homoge- pected.
neousbutmostofthemweretargetedwithVLBIbecausetheirra- Blazars are divided into three types according the position
dioemissionisknowntobecompactinFIRST,i.e.with∼5arcsec of the synchrotron peak frequency: low, intermediate, and high
angular resolution. Verysimilarlytothecaseof J1420+1205 and synchrotron-peak sources, abbreviated as LSPs, ISPs, and HSPs,
J2220+0025 presentedinthispaper,thesourceJ1548+3335 (z = respectively (e.g. Fanetal. 2016). Different types may have dif-
4.68) was found to have two widely separated (∼0.8′′, ∼5.3 kpc) ferentoriginofseedphotonsfortheinverse-Compton hump(e.g.
componentswiththeEVNat1.7GHzbyCoppejansetal.(2016). Kangetal.2016).Thehigh-zblazarsbelongtotheLSPs,because
One of them was undetected at 5 GHz. The most plausible ex- they are very luminous, and luminosity appears to correlate with
planationof theradiopropertiesof J1548+3335 isthatacore(at the frequency peak location (the “blazar sequence”, Fossatietal.
both frequencies) and a hot spot (at 1.7 GHz only) in one of the 1998;Ghisellini2016).Blazarsoftenshowstrongγ-rayemission.
two symmetric lobes in the kpc-scale extended structure of the However,therearenoFermidetectionsofz>4blazarsreportedso
source are seen with the EVN. This is our preferred explanation far.Fortheveryhigh redshiftsources, theinverse-Compton peak
forJ1420+1205 andJ2220+0025 aswell.Heretheprojected lin- movesintotheX-rayband,andalsotheγ-raysarelikelyabsorbed
ear distances between the core and the brighter lobe are 9.5 and in the intergalacticspace along the path from large cosmological
21.5kpc,respectively.Alternatively,thenorth-westerncomponent distances.Onlyacoupleofγ-rayblazarsareidentifiedat3<z<4
ofJ1420+1205couldalsobeinterpretedasaforegroundorback- (Fanetal.2016).
ground radio source physically unrelated to component C. Deep Can extended X-ray structures be responsible for the emis-
follow-upradiointerferometricobservationswith∼ 0.1arcsecan- sion of J1420+1205 and J2220+0025? To date, kpc-scale X-ray
gularresolutioncouldsettlethisquestionbydetectingasymmetric jetshavebeenfoundinmorethan100objects10,themostdistant
lobestructureandpossiblyjetfeaturesconnectingthecorewiththe ofthemistheblazarJ1510+5702atz=4.3(Siemiginowskaetal.
lobe(s). 2003;Cheung2004).TheirX-rayradiationmechanismisnotwell
Another remarkable object in the list of 30 VLBI-imaged understood.Synchrotronemissionofasecondaryhigh-energyelec-
high-redshift radio AGN compiled by Coppejansetal. (2016) is tronpopulationisfavoredatlowredshifts(e.g.Meyeretal.2015),
J0311+0507 (z = 4.51).Adetailedradiointerferometricstudyof but the acceleration mechanism producing the high-energy elec-
thispowerfulFR-IIsourcebyParijskijetal.(2014)foundacom- trons is unclear. The inverse-Compton scattering of cosmic mi-
plex two-sided core–jet–lobe stucture. The distance between the crowave background (CMB) photons by the relativistic electrons
componentidentifiedwiththegalacticnucleusandthebrighterhot inthejet(theIC/CMBscenario) mayalsoplayanimportantrole
spot in the south-western lobe is ∼1.3′′(equals to about 8.7 kpc at high redshifts, where the energy density of CMB is (1 + z)4
projectedlinearseparation),comparabletowhatwasmeasuredin times higher (e.g. Schwartz 2002; Simionescuetal. 2016). If the
J1420+1205 andJ1548+3335, andnearly3timessmallerthanin IC/CMB model works, then luminous X-ray jet components can
J2220+0025.Theinclinationangleofthestructurewithrespectto evenoutshinetheemissionofthenucleusinhigh-zsources.How-
thelineofsightcouldbecloseto45◦inJ0311+0507(Parijskijetal. ever,suchsourcesarenotfoundatz>4sofar(e.g.Wuetal.2013).
2014). Thetypicaljet-to-coreX-rayluminosityratiois∼ 0.01orsmaller
Therefore our discovery of supposedly FR-II type arcsec- forlower-redshiftquasarsobservedwithChandra(Marshalletal.
scale radio structures in the high-redshift AGN J1420+1205 and 2005).
J2220+0025isnotunprecedented.Inthisscenario,theradiostruc- The relativisticelectrons in the radio lobes deposited by the
tures can naturally be interpreted as expanding young symmetric jetscannot produce X-rayemission viasynchrotron radiation be-
sourcesintheearlyUniverse,aswasalsodoneforJ0311+0507by cause they have lower energies. At redshifts above z ∼ 3, the
Parijskijetal.(2014).Simplyassumingaconstantexpansionspeed CMBenergydensitymaydominateoverthemagneticfieldenergy
β =0.15andamoderatejetinclinationϑ=45◦,weobtainarough density.Thisintensifiesinverse-Comptoncoolingoftheelectrons,
j
estimateoftheirage,whichisbelow106yr.Itisconsistentwiththe leading to an enhanced (diffuse) X-ray emission from the lobes
value less than 106 yr claimed for J0311+0507 by Parijskijetal. (Ghisellinietal.2015).X-rayemissionfromradiolobeshasbeen
(2014).WhatispuzzlingisthatbothJ1420+1205andJ2220+0025 observedinseveralsourcesatredshiftz < 1andisinterpretedas
showpropertiestypicalofblazarsinX-rays(Sbarratoetal.2015). inverse-ComptonscatteringoffCMBphotons(e.g.Hardcastleetal.
2002).However,thereisnoevidenceforthediffusenatureofthe
X-rayemissioninJ1420+1205andJ2220+0025.Onthecontrary,
5.3 X-rayemissionofJ1420+1205andJ2220+0025
Wuetal. (2013) found J1420+1205 unresolved with Chandra at
Since our VLBI data do not support the blazar classification of a resolution of ∼ 1′′, which is however not much smaller than
J1420+1205andJ2220+0025,thestrongandhardX-rayemission the overall size of the radio structure in our VLBI image. The
and the overall SED (Sbarratoetal. 2015) call for an alternative X-ray source position measured from the image11 (right ascen-
explanation. In general, AGN are prominent X-ray emitters (e.g.
Rees1981;Schwartz2005).Forradio-quietAGN,theX-raysare
producedbyinverse-ComptonscatteringoftheUV/softX-raydisk 10 https://hea-www.harvard.edu/XJET/
photons by the electrons in the hot corona. For radio-loud AGN, 11 AvailablefromtheChandraarchive,http://cda.harvard.edu/pop/
MNRAS000,1–11(2017)
10 H.-M. Cao et al.
sion14h20m48s.040,declination+12◦05′46′.′22)ismoreconsistent sources requiresthe refinement of the broad-band SEDmodeling
withthatofthecompactnuclearVLBIcomponentinFig.3a. andposesaninterestingquestionaboutthephysicaloriginoftheir
ThereareinprincipletwopossibilitiesfortheoriginofX-ray high-energyemission.Ourresultsdrawtheattentiontotheutmost
emission in the two sources (J1420+1205 and J2220+0025) that importance of VLBI imaging obervations for reliably classifying
are clearly not blazars based on our high-resolution VLBI imag- blazarsathighredshift.
ingobservations.However,noneoftheexplanationsisparticularly
compelling.
(1)TheX-rayemissionmaybeacombinationofthecontribution
fromthe(unbeamed)innerjetandfromthekpc-scalejetsandouter ACKNOWLEDGEMENTS
lobes(hotspots)oftheseexpanding,powerful,presumablyyoung
The EVN is a joint facility of independent European, African,
radiosources.Itistobeseenifanymeaningfulphysicalmodelcan
Asian, and North American radio astronomy institutes. Scientific
possiblyreproducetheobservedX-rayfluxandspectralindex,as
resultsfromdatapresentedinthispublicationarederivedfromthe
wellastheblazar-likeSEDoftheseAGN.
followingEVNprojectcode:EC054.H.-M.C.acknowledgessup-
(2)TheX-raysoriginatefromthejetlaunchnig regionveryclose
portbytheprogramoftheLightinChina’sWesternRegion(Grant
totheSMBH.Theapproachingjetthensuddenlychangesitsdirec-
No.2016-QNXZ-B-21&YBXM-2014-02) andtheNational Sci-
tiononpcscaleandappearsalreadymisalignedwithrespecttothe
enceFoundationofChina(GrantNo.11573016&11503072).S.F.,
lineofsightandthusproducesnoDoppler-boostedradioemission
K.É.G.,andD.C.thanktheHungarianNationalResearch,Develop-
onthescaleprobedbyVLBI.Whilejetdirectionchangesarenot
mentandInnovationOffice(OTKANN110333) forsupport.T.A.
unprecedentedatlargerscalesinradioAGNduetotheinteraction
thanksforthegrantsupportbytheYouthInnovationPromotionAs-
with the dense intestellar medium, it is unlikely to happen here,
sociationoftheChineseAcademy ofSciences(CAS).Thiswork
especiallyintwosourcesfromasmallsampleoffour.
wassupportedbytheChina–HungaryCollaborationandExchange
Future sensitive spectral and high-resolution X-ray imaging
Programme by the International Cooperation Bureau of the CAS
observations of these sources, as well as the other two similar
and the Program for Innovative Research Team (in Science and
arcsec-scaleextended high-zsources, J0311+0507 (Parijskijetal.
Technology)attheUniversityofHenanProvince.Weacknowledge
2014)andJ1548+3335(Coppejansetal.2016),wouldhelpunder-
theuseofAstrogeoCenterdatabaseofbrightnessdistributions,cor-
standhowtheseobjectsworkandwouldprovidethenecessaryin-
related flux densities, and images of compact radio sources pro-
putfordetailedmodelingoftheX-rayemission.Itispossiblethat
ducedwithVLBI.ThisresearchhasmadeuseoftheNASA/IPAC
otherX-rayemitting,highlyradio-loudAGNalsomimicthetypical
ExtragalacticDatabase(NED)whichisoperatedbytheJetPropul-
blazarproperties,yettheybelongtodifferenttypesofsources.This
sionLaboratory,CaliforniaInstituteofTechnology,undercontract
may contribute to the apparently larger fraction of blazars found
withtheNationalAeronauticsandSpaceAdministration.
in the early Universe (cf. Ghisellini&Sbarrato 2016). In the ra-
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