Table Of ContentAstronomy&Astrophysicsmanuscriptno.chiinstrumentfinal (cid:13)c ESO2012
January26,2012
High Resolution Rapid Response observations of compact radio
sources with the Ceduna Hobart Interferometer (CHI)
JayM.Blanchard1,2,JamesE.J.Lovell1,RoopeshOjha3,4,MatthiasKadler5,6,7,8,JohnM.Dickey1,andPhilipG.
2
Edwards
1 SchoolofMathematics&Physics,PrivateBag37,UniversityofTasmania,HobartTAS7001,Australia
2 AustraliaTelescopeNationalFacility,CSIROAstronomyandSpaceScience,POBox76,Epping,NSW1710,Australia
3 NASA,GoddardSpaceFlightCenter,Greenbelt,MD20771,USA
2 4 InstituteforAstrophysics&ComputationalSciences(IACS),Dept.ofPhysics,TheCatholicUniversityofAmerica,620Michigan
1 Ave.,N.E.,Washington,DC20064,USA
0 5 Institutfu¨rTheoretischePhysikundAstrophysik,Universita¨tWu¨rzburg,AmHubland,97074Wu¨rzburg,Germany
2 6 Dr.RemeisSternwarte&ECAP,Universita¨tErlangen-Nu¨rnberg,Sternwartstrasse7,96049Bamberg,Germany
7 CRESST/NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA
n
8 UniversitiesSpaceResearchAssociation,10211WincopinCircle,Suite500Columbia,MD21044,USA
a
J
Preprintonlineversion:January26,2012
5
2 ABSTRACT
] Context.Frequent,simultaneousobservationsacrosstheelectromagneticspectrumareessentialtothestudyofarangeofastrophys-
M
ical phenomena including Active GalacticNuclei. A key tool of such studies isthe abilityto observe an object when it flaresi.e.
exhibitsarapidandsignificantincreaseinitsfluxdensity.
I
. Aims.Wedescribethespecificobservationalproceduresandthecalibrationtechniquesthathavebeendevelopedandtestedtocreate
h
asinglebaselineradiointerferometerthatcanrapidlyobserveaflaringobject.Thisistheonlyfacilitythatisdedicatedtorapidhigh
p
resolutionradioobservationsofanobjectsouthof−30degreesdeclination.Animmediateapplicationistoproviderapidcontempo-
-
o raneousradiocoverageofAGNflaringatγ-rayfrequenciesdetectedbytheFermiGamma-raySpaceTelescope.
r Methods.AsinglebaselineinterferometerwasformedwithradiotelescopesinHobart,TasmaniaandCeduna, SouthAustralia.A
t softwarecorrelatorwassetupattheUniversityofTasmaniatocorrelatethesedata.
s
a Results.Measurementsofthefluxdensitiesofflaringobjectscanbemadeusingourobservingstrategywithinhalfanhourofatrig-
[ geringevent.Theseobservationscanbecalibratedwithamplitudeerrorsbetterthan15%.Lowerlimitstothebrightnesstemperatures
ofthesourcescanalsobecalculatedusingCHI.
1
v Key words. instrumentation:interferometers – galaxies:active – galaxies:jets – galaxies:nuclei – gamma rays:galaxies–
9 quasars:general–
5
1
5 1. Introduction (Cohenetal.,2007).Theyalsoplayacrucialroleintheidenti-
.
1 ficationofthenatureandlocationofregionswhereγ-rayemis-
0 ThelaunchoftheFermiGamma-raySpaceTelescope(formerly sionoriginatesinAGN(Jorstadetal.,2001;Agudoetal.,2011).
2 GLAST; Atwoodetal., 2009) on June 11th, 2008, has ushered TANAMIVLBIobservationsaresupportedbyradiomonitoring
1 in an era when it is possible to observe astronomical objects, programsatarc-secondresolutionusingtheAustraliaTelescope
v: including AGN, simultaneously across the entire electromag- Compact Array (ATCA) and single-dish resolution using the
i neticspectrum.Suchmultiwavelengthobservationsareessential Ceduna radio telescope (McCullochetal., 2005). With its as-
X to understand the behaviour of astronomicalobjects in general sociated optical/UV and X-ray programs and its unique VLBI
r and AGN in particular. This motivated the TANAMI program dual-frequencycharacteristics,TANAMIhasbecomeoneofthe
a (Tracking Active Galactic Nuclei with Austral Milliarcsecond majormultiwavelengthresourcesfortheFermimissionandthe
Interferometry;Ojhaetal.,2010)whichprovidesradiomonitor- onlyonecoveringsourcessouthof−30degrees.TANAMIVLBI
ing of γ-ray loud sources (and a control sample) south of −30 observationsareconstrained,however,bytheavailabilityofthe
degreesdeclination. LBAwhichonlyobservesinapproximatelyoneweekblocksev-
TANAMIobservationsaremadeattworadiofrequenciesus- ery2∼3months.Thismakesitimpossibletoquicklyobservea
ingthetelescopesoftheAustralianLongBaselineArray(LBA; southernsourcethatexhibitsinterestingbehavioursuchasrapid
e.g.Ojhaetal.,2004a)augmentedbytelescopesinSouthAfrica, changesinfluxdensityatoneormorewavebands.
Chile,AntarcticaandNewZealand.Thisarrayallowstheimag-
ing of the southern sky at milliarcsecond scale or better res- The Ceduna Hobart Interferometer (CHI) was developed
olution on a regular basis (e.g., Mu¨lleretal., 2011). This ob- to provide rapid, high resolution observations of sources in
servationaltechnique,calledVeryLongBaselineInterferometry the southern sky. A fundamental characteristic of AGN is
(VLBI),isessentiallytheonlywaytomeasureintrinsicparame- their variability at all wavelengths in which they are detected
tersofthejetsseeninAGN,asmulti-epochVLBIobservations (Ulrichetal., 1997; Abdoetal., 2010). Thus a proper study
providethesoledirectmeasurementsoftheirrelativisticmotion of the physics of AGN requires simultaneous multiwavelength
1
Blanchardetal.:CedunaHobartInterferometer(CHI)
observations, particularly when the flux density of an object slewing at 40 degrees per minute on both axes to an elevation
changes significantly over a short period of time. Here we de- limitof7degrees(apartfroma 17degreelimitin thekeyhole)
scribethespecificobservationalproceduresandcalibrationtech- andcanobserveatallmajorastronomicalfrequenciesfrom1.4
niques that have been developed to create this uniquely useful through22GHz.The6.7GHzreceiveratHobartiscooledto20
single-baselineinterferometer.Itis importantto notethat,even Kusingheliumcryogenics,givingasystemequivalentfluxden-
thoughthedevelopmentofCHIwasmotivatedbytheFermiand sity ofapproximately820Jy. Thebackendsystematbothtele-
TANAMIprograms,itisbynomeansrestrictedtoobservations scopes producestwo channels (right and left hand circular po-
of γ-ray loud AGN but can be used for multiwavelengthstudy larisation)of32MHzbandwidth.Thebaselinesensitivityofa2
ofarangeofcompactsourcese.g.opticallyflaringAGN,X-ray elementinterferometerisgivenbyequation1(Thompsonetal.,
sources being studied by the INTEGRAL and Swift satellites, 2001).
TeV sources detected by H.E.S.S., radio supernovae and some
microquasars.
1 SEFD ×SEFD
In this paper, we describe the CHI instrument in (Sect. 2) ∆S = i j (1)
and describe our observing strategy (Sect. 3). We then explain ij ηs r 2∆t∆ν
ourcalibrationprocedures(Sect.4)andpresentsomeresultsin
Sect.5.Weendwithasummaryanddescription(Sect.6)offur- whereηsisthesystemefficiency(roughly0.88for2bitsam-
therdevelopmentplannedforCHI.Throughoutthepaperweuse pling),SEFDisthesystemequivalentfluxdensityinJyofeach
thecosmologyH =73kms−1Mpc−1,Ω =0.27,Ω =0.73where antenna,∆tistheintegrationtimeinsecondsand∆νistheband-
0 m Λ
thesymbolshavetheirtraditionalmeanings. widthinMHz.
Thus,injust60secondsofintegrationtime,CHI canreach
a baselinesensitivityofabout15mJywhichissufficientto ob-
2. CHI:TheCedunaHobartInterferometer
serve all the sources in the TANAMI program. The coherence
time at 6.7 GHz is approximately 4.6 minutes (calculated us-
TheCedunaHobartInterferometerisasinglebaselineinterfer-
ing the methods outlined in Briggs 1983 and checked against
ometerformedbya26meterantennalocatedatMountPleasant,
thegivenvaluesinWalker1995).Thisgivesa maximumbase-
Hobart and a 30 meter antenna located at Ceduna in South
linesensitivityofapproximately7mJywithoutphasereferenc-
Australia. They are both re-purposed antennas that are now
ing, more than enough to detect the above mentioned sources.
operated by the Radio Astronomy group at the University of
Table 1 lists the brightnesstemperature sensitivity of CHI to a
Tasmania.CHIhasaphysicalbaselinelengthof1704kmcorre-
hypothetical1Janskysourceforarangeofparameters.
spondingtoaresolutionof6.6milliarcsecondsatitscurrentop-
eratingfrequencyof6.7GHz.TheHobartantennaisatalatitude CHIdataarerecordedtodiskat256Mbpsat2bitsampling
−42.8 degrees while the Ceduna antenna is at latitude −31.87. with the LBA Data Recorder system(Phillipsetal., 2009) and
ThusCHIcan,inprinciple,observesourcesuptomid-northern theCedunadisksareshippedtoHobartforcorrelation.Thedata
latitudesandcaneasilyobservetheentiresouthernhemisphere. arecorrelatedusinganinstallationoftheDiFXsoftwarecorrela-
The 30m telescope at Ceduna is a former Telstra Satellite tor(Delleretal.,2007).Thecorrelatoritselfconsistsofacluster
Earth Station, that was converted to astronomical use after its of 6 PCs each containing dual quad-coreXeon processors and
acquisitionbytheUniversityofTasmaniain1995.Cedunauses 3GBofmemory.Thecorrelatorcanmanagedataratesofupto
a standard alt-azimuth mount, with a slew rate of 40 degrees 512Mbpsonasinglebaseline,givinga2timesrealtimecorrela-
per minute on each axis and an elevation limit of 10 degrees. tionspeedforatypicalCHIobservation.Thebiggestconstraint
Itis a criticalcomponentof the Australian LBA, providingthe on the speed of analysis of data from CHI howeveris the time
major east-west baselines. However, Ceduna is primarily used taken for disks from Ceduna to be shipped to the correlator in
for single dish programsmonitoringγ-ray brightAGN in sup- Hobart.Thismeansatypicalturnaroundtimeofaround5days
port of the TANAMI program as well as Intra Day Variable fordatatobefullyreduced.Transferovernetworkisnotpossible
(IDV;Wagner&Witzel, 1995) AGN in supportof the MASIV due to the limited network capacity to Ceduna and the volume
(MicroarcsecondScintillation-InducedVariability;Lovelletal., ofdatarecorded.
2003,2008)program.Cedunaiscapableofobservationsat2.2,
4.8,6.7,8.4,12.2and22GHzusingroomtemperaturereceivers.
Since the peak variability in amplitude of scintillating AGN is 3. ObservingStrategyforflaringAGN
expectedtobeclosetoafrequencyof6.7GHz,IDVobservations
are carriedoutin this band.As receiverchangesat Cedunaare Duetothepeculiarcalibrationneedsofasinglebaselineinterfer-
not automated, the standard observing frequency for CHI was ometer(seesection4)andthedesiretoobtaingoodabsoluteam-
selectedtobe6.7GHz,allowingforarapidchangebetweenob- plitudecalibration,wehavedevelopedanon-standardobserving
servingprograms.Despitebeinguncooled,the6.7GHzreceiver strategy.AtypicalCHIobservationrunsfortwelvehours.Each
has a System Equivalent Flux Density (SEFD) of roughly 820 hour consists of a roughly 50 minute long scan on the target
Jy,equivalenttothatatHobart.Thisismostlyduetothelarger source and a ten minute scan of PKS B193−638, the primary
collectingareaofthe30mdish,aswellasthefocustype.Hobart amplitudecalibratoroftheATCA.PKSB1934−638isobserved
isaprimefocusinstrumentwithmorespilloverthanthetertiary astraditionalamplitudecalibrationisnotpossiblewithasingle
systematCeduna. baseline(seesection4foradiscussion).
The 26m telescope at Mount Pleasant was opened on 13 Observationsare usually triggeredfollowingan increase in
May 1986after having been movedfrom OrroralValley in the gamma-rayactivityasfoundbytheFermiLATinstrument.The
Australian Capital Territory where it had been a NASA track- flaring source is also added to the single dish monitoring pro-
ingstationforabouttwodecades.Italsoisacomponentofthe gram,providingahighcadencelightcurveattheobservingfre-
Australian LBA and a vital geodetic antenna regularly partici- quency.Ifthesourceisobservedtobeflaringatradiofrequen-
patinginInternationalVLBIService(IVS;Schlu¨ter&Behrend, cies multiple CHI observations are then undertaken to provide
2007) observations. Hobart uses a non traditional X-Y mount informationonthestateandevolutionoftheflare.
2
Blanchardetal.:CedunaHobartInterferometer(CHI)
Fig.2 Calibrated PKS B1934−638amplitude vs (u,v) distance
plotwiththeoverlaidmodel.
FinallytheabsolutefluxscaleofthePKSB1934−638model
at 6.7 GHz was set using simultaneous ATCA and LBA data
of severalpointsources (0506−612,0808+019,1022−665and
1420−679). This was not possible to achieve using the PKS
B1934−638dataitselfasPKSB1934−638isresolvedonVLBI
Fig.1 The brightness distribution of PKS B1934−638 derived
baselines. For a point source however the VLBI and ATCA
fromthemodelcreatedusingAIPSandDifmap.
fluxes should be the same and so the difference in VLBI and
ATCAfluxdensitywascalculatedforeachofthepointsources
mentionedabove,givinganaveragecorrectionfactorof1.45±
4. Calibration 0.2 used to scale the modelof PKS B1934−638.This gavethe
final clean component model as seen in Figures 1 and 2. The
Calibrationofaninterferometerwithjusttwoelementspresents
errorinthiscorrectionfactor(oforder15percent)isbyfarthe
several interesting challenges. Since a minimum of 3 anten-
dominantformoferrorin fluxdensitymeasurementsmadeus-
nas is required for phase closure and 4 antennas for ampli-
ingCHIandisthususedasanestimateofthetotalerrorinsuch
tude closure, it is not possible to use the standard VLBI tools
measurements.
of hybrid mapping (Readheadetal., 1980) or self-calibration
(Pearson&Readhead,1984).Thusnon-standardamplitudecali-
brationmustbeperformedonCHIdata.Calibrationrequirestwo 4.2.CHICalibration
stages,aonceoffinitialcalibrationandacalibrationappliedto
eachindividualCHIobservation. Asdescribedinsection3,PKSB1934−638isobservedineach
CHIobservationtoperformadditionalamplitudecalibration,as
requiredbythesinglebaseline.TheCHIdata(includingboththe
4.1.InitialCalibration targetsource(s)andPKSB1934−638)areinitiallycalibratedin
AIPS (AstronomicalImageProcessing System; Greisen, 1998)
The need for amplitude calibration using a calibrator source is
in the standard manner. Fringe-fitting is carried out to correct
wellmetbyPKSB1934−638,whichhasafluxdensityof3.983
forresidualdelayandrateerrors.Thisisfollowedbyamplitude
Jyat6.7GHzanddoesnotvarysignificantly(Reynolds,1994).
calibrationusingtheknowngain-curvesandsystemtemperature
Unfortunately, at VLBI resolutions, PKS B1934−638 is not a
measurementsmade at each antennaoverthe course of the ex-
pointsourceandsoamodelisrequiredinordertomakeampli-
periment.
tudecalibrationpossible.
The AIPS calibrated PKS B1934−638 data are loaded
To create the model of PKS B1934−638, data from the
into the Caltech Difference Mapping program ’Difmap’
V255 series of Australian LBA observations were used. The
(Shepherdetal., 1995) and the model created as described in
V255 project focuses on methanol maser studies at 6.7 GHz,
Section 4.1 is then overlaid on an amplitude vs (u,v) distance
making use of VLBI to measure accurate positions, distances
plot.Asimplescriptisthenusedtomanipulatethemodeltofit
and propermotionsof these masers. PKS B1934−638was ob-
boththe amplitudevs(u,v)distance plot(see Figure2) andan
served in the V255 observations to act as a polarisation cali-
amplitudevstimeplot,givingacorrectionfactor.
brator. Three epochsof V255 were used to make the modelof
Thiscorrectionfactoristhenusedtoscalethevisibilitiesof
PKSB1934−638usedforCHI.Aspartofthecalibrationtheef-
thebaselineproducingthefinalcalibratedsourceobservation.
fect of elevation on the gain of the receiver must be corrected
for. These gain-curves are not available for all LBA antennas
at 6.7 GHz. Gain-curves from existing 8.4 GHz data were ad-
5. Results
justedforthenewfrequencyfortelescopesthatdidnothave6.7
GHzcurvesavailable.ForCedunathegain-curvewastakenfrom WepresentatypicalCHIobservation.PKS2052−474isablazar
McCullochetal.(2005).ForHobartitisapproximatedasafirst ataredshiftof1.489.Itisestablishedtobeavariablegamma-ray
orderpolynomialandthismayintroduceasmallelevationeffect source. Abdoetal. (2010) show a light curve with two flaring
intoCHIdata(seeFigure3). events, occurring around February and June 2009 respectively.
3
Blanchardetal.:CedunaHobartInterferometer(CHI)
of the single dish data (as expected for an unresolved source)
givingusconfidenceintheamplitudecalibrationtechniqueem-
ployedbyCHI.The(u,v)coverageofatypicalCHIobservation
is shown in Figure 4. Applyingthe radiometerequation(equa-
tion1)tothis7hourobservationofPKS2052−474givesanex-
pectednoise of 0.73mJy rms, agreeingwellwith the observed
noiseinthevisibilitiesof0.75mJyrms.
Fig.3 Amplitude vs time plot for the calibrated 2052−474 ob-
servation. Note the downturn in amplitude at the beginning of
theexperiment.Thiscorrespondstolowelevationofthesource
andisthuslikelyaresidualelevationeffect.Thisisperhapsdue
to the simple nature of the gain-elevationcurve for the Hobart
Antenna
Fig.5SingledishandATCAmonitoringdataofPKS2052−474
overlaidwiththeCHIobservation(thelargefilleddiamond).The
CedunadataarethecirclesandtheATCAdatathetriangles.The
VLBIdataisconsistentwithboththesingledishandtheATCA
monitoring flux density. Note that the ATCA data are interpo-
latedbetweensimultaneous5.5and9GHzdatatomatchthe6.7
GHzCedunasingledishdata.
Aconstraintontheangularsizeofthesourcecanbecalcu-
lated by applying a Gaussian modelto the visibilities. For this
exampleobservationof2052−474amodelwithanangularsize
of0.8masischosen,asthevisibilitiesarewellencompassedby
themodel(seeFigure6).Notethatwearenottryingtofittothe
datadirectly,ratherwearechoosingagaussianwhichliesbelow
the majorityofthe visibilities, thusprovidingan upperlimit to
theangularsize.
Thebrightnesstemperatureofasourceisgivenby
Fig.4The(u,v)coverageobtainedfroma12hourCHIobserva- T = 2ln2Sνλ2(1+z) (2)
tionof2052-474.ThisistypicalofaCHIexperiment. b πk θ θ
maj min
wherekisBoltzmann’sconstant,S isthefluxdensityofthe
ν
source, z is its redshiftand θ the angularsize (see for example
PreviousVLBIobservationshavefoundthesourcetobestrongly
Kovalevetal.2005).
core dominatedon the milli-arcsecondscale (Ojhaetal., 2010,
Using the upper limit to the angular size of the source and
2004b; Pineretal., 2007). On the kilo-parsec scale, ATCA
equation2, a lower limit of 2×1011 K for the brightnesstem-
imaging shows some extended structure (Marshalletal., 2005;
perature is calculated. This is well below the inverse compton
Burgess&Hunstead,2006)extendingover 4arc-seconds.The
limit.
observationpresentedherewasmadeinsupportofaFermi/LAT
multiwavelengthcampaignonthisobjectin2009(Changetal.,
2010)followingtheflaringseeninthegamma-ray.
6. SummaryandFutureDevelopments
Calibrationwasperformedasexplainedinsection4,result-
ing in the amplitude vs time plot seen in Figure 3 and the am- TheCHIinterferometeristheonlyfacilitycapableofrapid,high
plitudevs(u,v)distanceplotseeninFigure6.Thisisconsistent resolutionobservationsofcompactradiosourcessouthofabout
withapointsourcewithafluxdensityof2.6Jy.Figure5shows −30degreesdeclination.Thepeculiarcalibrationchallengesof
single dish data from the Ceduna monitoring program, as well such a single baseline interferometer have been met and ini-
as ATCA data from the C1730 gamma-raysources monitoring tial results are presented. Currently CHI is a critical compo-
program,aroundthetimeoftheCHIobservation.Thefluxden- nentinthemultiwavelengthstudyofAGNintheageofFermi.
sity obtainedfromthe CHI observationsis consistentwith that However,itsroleissettobebroaderasitiscapableofobserving
4
Blanchardetal.:CedunaHobartInterferometer(CHI)
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Second, while CHI observationscan be initiated almostin-
stantly,theremotenatureoftheCedunaobservingsitemakesthe
shippingofdatatothecorrelatorinHobartthebiggestbottleneck
torapidanalysis.Thepossibilityoftakingpreliminarymeasure-
ments using a ‘sniffing’strategy is being explored.This would
involve taking short (about one second) slices of the twelve
hoursof data and transferringthem back via network, thusde-
creasingthetransfertimewithoutsacrificing(u,v)coveragetoo
much. This would allow a quick analysis showing the flaring
stateofatarget(forexample)wheresuchinformationisneeded
totriggerobservationsatotherwavelengths.
Finally, we are investigating the possibility of includ-
ing the new 12 meter antenna at Warkworth, New Zealand
(Gulyaevetal.,2011)intheCHIarray.Thiswouldincreasethe
maximum baseline to 3724 kilometers boosting the resolution
2.5timesatanobservingfrequencyof8.4GHz,aswellaspro-
vidingphaseclosure.
Acknowledgements. We are extremely grateful to Simon Ellingsen of the
University of Tasmania for providing the V255 PKS B1934−638 data essen-
tial to the proper calibration of CHI. This research was funded in part by
NASA through Fermi Guest Investigator grant NNH09ZDA001N (proposal
number31263).ThisresearchwassupportedbyanappointmenttotheNASA
PostdoctoralProgramattheGoddardSpaceFlightCenter,administeredbyOak
RidgeAssociatedUniversitiesthroughacontractwithNASA.
This research has made use of data from the NASA/IPAC Extragalactic
Database(NED,operatedbytheJetPropulsionLaboratory,CaliforniaInstitute
of Technology, under contract with the National Aeronautics and Space
Administration); and the SIMBAD database (operated at CDS, Strasbourg,
France).ThisresearchhasmadeuseofNASA’sAstrophysicsDataSystem.This
researchhasmadeuseoftheUnitedStatesNavalObservatory(USNO)Radio
ReferenceFrameImageDatabase(RRFID).
5
Blanchardetal.:CedunaHobartInterferometer(CHI)
Table1. BrightnessTemperatureSensitivity
Redshift Shape BrightnessTemperaturea
K
0 Gaussian 1×109
0 Sphere 0.5×109
0 Disc 0.4×109
1 Gaussian 2.2×109
1 Sphere 1×109
1 Disc 0.8×109
aFora1janskysource.
6
