The catalogue of positions of optically bright extragalactic radio sources OBRS-2

0.35 MB·English

by  Leonid Petrov

ASTRONOMICALJOURNAL
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THECATALOGUEOFPOSITIONSOFOPTICALLYBRIGHTEXTRAGALACTICRADIOSOURCESOBRS–2
L.PETROV
AstrogeoCenter,FallsChurch,VA22043,USA
(Received2013,January23;Accepted;Published)
3 ABSTRACT
1
Itis anticipatedthatfuturespace-bornmissions, suchas Gaia, willbe abletodeterminein opticaldomain
0
positions of more than 100, 000 bright quasars with sub-mas accuracies that are comparable to very long
2
baselineinterferometry(VLBI)accuracies. Comparisonsofcoordinatesystemsfromspace-bornmissionsand

n fromVLBIwillbeveryimportant,firstforinvestigationofpossiblesystematicerrors,secondforinvestigation
a of possible shift between centroidsof radio and optical emissions in active galaxy nuclea. In orderto make
J
such a comparisonmore robust, a programof densification of the grid of radio sourcesdetectable with both

3 VLBI and Gaia was launched in 2006. In the second observingcampaign a set of 290 objects from the list
2 of 398compactextragalacticradio sourceswith declinations>- 10◦ was observedwith the VLBA+EVN in

2010–2011 with the primary goal of producing their images with milliarcsecond resolution. These sources
]
M are brighterthan 18 magnitude at V band. In this paper coordinatesof observed sources have been derived
with milliarcsecond accuracies from analysis of these VLBI observations following the method of absolute
I astrometryandtheirimageswereproduced. Thecatalogueof positionsof295targetsourcesandestimatesof
.
h theircorrelatedfluxdensitiesat 2.2and8.4GHz is presented. Theaccuraciesof sourcecoordinatesare in a
p rangeof2to200mas,withthemedian3.2mas.
- Subjectheadings:astrometry—catalogues—surveys
o
r
t
s 1. INTRODUCTION systematic differenceswill be very importantforthe assess-
a
mentofthe overallqualityof Gaiaresultsand, possibly,the
[ ThemethodofVeryLongBaselineInterferometry(VLBI)
errorsintheVLBIpositioncatalogue.
first proposed by Matveenkoetal. (1965) allows us to de-
1 This comparisonwill producevaluable results if 1) it will
rive source positions with nanoradian precision (1 nrad
v ≈ 0.2 mas). Since 1971 when the first catalogue of be limited to those common sources which VLBI positions
7 are known with errors smaller than several tenths of a mil-
source coordinates determined with VLBI was published
0 liarcsecond; 2) the number of sources will be large enough
(Cohen&Shaffer 1971), the number of extragalactic com-
4 to derive meaningful statistics; and 3) the sources will be
pactradiosourceswhichpositionswerederivedusingVLBI
5 uniformlydistributed overthe sky. However, the numberof
underabsoluteastrometryobservingprogramsgrewfrom35
.
1 objectsto 7215in20121. For95%thesesources,accuracies quasars that have a compact core and are bright in both op-
0 oftheirpositionsareinarangeof0.05to6.5maswiththeme- tical and radio wavelengths, and therefore, can be detected
3 with both techniques, currently is rather limited. The ob-
dian0.5mas. Thesesourcesformadensegridontheskythat
1 serving program for densification of the list of such objects
canbeusedformanyapplications,suchasdifferentialastrom-
: was launched in 2006 (Bourdaetal. 2008) with the even-
v etry, phase-referencing VLBI observations of weak objects,
tual goal of deriving highly accurate position of sufficiently
i space navigation,Earthorientationparameterdetermination,
X radio-loud quasars from VLBI observations in the absolute
andspacegeodesy.
astrometry mode. The original observing sample consisted
r However, high accuracy of positions of these objects can
a of 447 optically bright, relatively weak extragalactic radio
be exploited directly only by applications that utilize the
sources with declinations above - 10◦. The detailed observ-
VLBItechnique.Applicationsthatusedifferentobservational
ingschemeofthisprojectispresentedinBourdaetal.(2008).
techniques can benefit from the high accuracy of VLBI po-
ThefirstVLBIobservingcampaignin2007resultedindetec-
sitions only indirectly by observing common objects from
tionof398targetswiththeEuropeanVLBINetwork(EVN)
the VLBI catalogue with instruments at other wavelengths.
(Bourdaetal.2010),althoughnoattempttoderivetheirposi-
The European Space Agency space-born astrometry mis-
tions or produce images was made. During the second ob-
sion Gaia, scheduled to be launched in 2013, according to
serving campaign a subset of 105 sources detected in the
Lindegrenetal.(2008)promisestoreachsub-masaccuracies
previouscampaign was observed with the global VLBI net-
ofdeterminingpositionsofquasarsof16–20magnitudethat
work thatcomprisesthe VLBA and EVN observingstations
willrivalaccuraciesofabsoluteastrometryVLBI. Sincepo-
withthegoalofrevealingtheirmorphologyonmilliarcsecond
sitioncataloguesproducedwithGaiaandVLBIwillbecom-
scales from VLBI images (Bourdaetal. 2011) for consecu-
pletelyindependent,theirmutualrotations,zonaldifferences
tivescreeningtheobjectswith structurethatpotentially may
andpossiblyothersystematiceffectscanbeinterpretedaser-
cause non-negligible systematic position errors. Their posi-
rors of one of the techniques after resolving the differences
tionswerederivedbyPetrov(2011)andformedtheOBRS–1
duetoamisalignmentofcentersofopticandradioimagesof
catalogue.
quasarsand a frequency-dependentcore-shift(Kovalevetal.
In2010–2011remaining290sourceshavebeenobservedin
2008;Porcas2009;Sokolovskyetal.2011). Investigationof
thethirdcampaign,hereaftercalledOBRS-2,withtheglobal
[email protected] network that comprises the VLBA and EVN observing sta-
1seehttp://astrogeo.org/rfc. tionsinamodesimilartothesecondcampaign.Ipresenthere
2
fortyingpositionestimates to the existingcatalogueandfor
Table1
evaluationofpositionaccuracy.
Summaryofobservingsessions
3. DATAANALYSIS
2010.03.23 gc034a 48 97
2011.11.08 gc034bcd 58 118 The data were correlated with DiFX software correlator
2011.03.15 gc034ef 40 77 (Delleretal.2011)attheNationalRadioAstronomyobserva-
tory. Thecorrelatorcomputedthespectrumofcrosscorrela-
Note. —Thecolumnsare: 1)epochofanobservingsession,2)session tionandautocorrelationfunctionswithafrequencyresolution
ID,3)totalsessiondurationinhoursexcludingmaintenancegaps,and4)the
of0.25MHzataccumulationintervalsof1slong.
numberofprogramsources.
3.1. Preliminaryanalysis
resultsofdataanalysisofthisobservations.Observationsand
theiranalysisaredescribedinsections 2and3. Theposition Theprocedureoffurtheranalysisisdescribedinfulldetail
catalogueispresentedinsection4anddiscussedinsection5. in Petrovetal. (2011a). I present here only a brief outline.
Concludingremarksaregiveninsection6. First,logfilesareparsed,systemtemperatureandphasecali-
brationareanalyzed,andpointswithoutliersareremoved. No
2. OBSERVATIONS phasecalibrationsignalfornon-VLBAstationswasrecovered
due to a limitation of the DiFX correlator (which was lifted
DuringOBRS–2campaigntherewerethreeobservingses-
laterin2011). Next,thefringeamplitudeswerecorrectedfor
sions with 10 VLBA stations and 5–6 EVN stations from
adistortioninthesamplerduetodigitization. Then,theam-
this list: EFLSBERG, MEDICINA, ONSALA60, YEBES40M,
plitudeswere calibrated by multiplyingthem by the a priori
DSS63, HARTRAO, NOTO. First four EVN stations partici-
pated in every experiment, three remaining stations partici- SystemEquivalentFluxDensity(SEFD) g1g2 ,where
pated in some experiments. Each program source was ob- rTsys1Tsys2
servedinonesession,in3–4scans,each5minuteslong.Two g and T are the gain and system temperature of the ith
i sysi
sources,1148+387and1203+109,wereobservedintwoses- antenna respectively. Then a group delay, phase delay rate,
sionsin5scans. Inadditionto290programsources,8strong groupdelayrate,andfringephaseweredeterminedforevery
calibratorswereobserved. observations for each baseline at X and S bands separately
ObservationsweremadeatXandSbandssimultaneously. usingthewide-bandfringefittingprocedure.Theseestimates
Data were sampled with 2 bits per sample at the aggregate maximizetheamplitudeofthecross-correlationspectrumco-
rate of 512 Mbit/s. The intermediate frequencies were se- herentlyaveragedoverallaccumulationperiodsofascanand
lected to provide continuous bandwidth [8.37699, 8.44099] over all frequency channels in all intermediate frequency of
and[2.22699,2.29099]GHz.Suchasetupisnotfavorablefor aband. Afterthefirstrunoffringefitting,12observationsat
astrometry. Spanningfrequenciesover500 MHz, as it done eachbaselinewithareferencestationwiththestrongestsignal
inallotherdual-bandabsoluteastrometryVLBAsurveys,im- to noise ratios (SNRs) were used to adjust the station-based
provesprecisionofgroupdelaydetermination,andtherefore, complex bandpass corrections, and the fringe fitting proce-
source position precision by one order of magnitude. The durewith the bandpassesappliedwas repeated. Thispartof
positionaccuracyof sourcesobservedwith a wide-bandfre- analysisisdonewithPIMA software2.
quencysetupanddetectedatbaselines3,000–8,000kmlong Furtheranalysiswassplitintotworoutes: astrometricand
is limited by systematic errors. The position accuracy of imaging. Followingtheastrometricroute,totalgroupdelays
sourcesobservedin a frequencybandspannedover64MHz andphasedelayrateswerecomputedonacommonfringeref-
islimitedbythethermalnoise. erencetimeepochwithinascanusingresultsoffringefitting.
SecondlimitationoftheOBRS–2schedulefortheastrom- Theseobservables,alongwithauxiliaryinformationdescrib-
etryuseisarelativelyrareobservationofsourcesatlowand ingobservations,wereexportedtotheVTD/post-SolveVLBI
high elevations for better estimation of the tropospherepath analysissoftware3 forinteractiveanalysis. Initially,onlyob-
delay in zenith direction, which increases systematic errors. servables with the high SNR to ensure that the probability
VCS1 survey (Beasleyetal. 2002) suffered a similar defi- of false detectionis less than 0.001were chosen. ThisSNR
ciency in design, and analysis revealed systematic errors at thresholdis 5.8 forOBRS–2 experiments. Detailed descrip-
a level of 0.4–0.5mas. We can assume systematic errors of tion of the method for evaluation of the probability of false
OBRS–2wereatasimilarlevel. Butsincethecontributionof detectioncanbefoundin(Petrovetal.2011a).
thethermalnoiseintheerrorbudgetofOBRS–2experiments Then, theoreticalpath delayswere computedaccordingto
is one order of magnitude higher than in VCS experiments, the state-of-theart modelas well as their partialderivatives.
these increased systematic errors did not cause a noticeable Smalldifferencesbetweengroupdelaysand theoreticalpath
furtheraccuracydegradation. delaywereusedforestimationofcorrectionstoaparametric
Of 290 program objects, 58 sources were observed modelthatdescribestheperturbationofthetheoreticalmodel
and detected in other absolute astrometry programs, such usingleastsquares(LSQ).Coordinatesoftargetsources,po-
as Very Long Baseline Array (VLBA) Calibrator Sur- sitions of all stations, except the reference one, parameters
vey (Beasleyetal. 2002; Fomalontetal. 2003; Petrovetal. of the spline with the time span of 1 hour that models cor-
2005, 2006; Kovalevetal. 2007; Petrovetal. 2007), reg- rections to the a priori path delay in the neutral atmosphere
ular VLBA geodetic observations (Petrovetal. 2009; in the zenith directionforall stations, and parametersof an-
Pushkarev&Kovalev2012),andongoingVLBIobservations othersplinewiththesametimespanthatdescribestheclock
of2MASSgalaxies(Condonetal.2011). Positionsofallbut functionforallstationsbutthereferenceoneweresolvedfor
foursources0106+315,0809+483,1213+097,and2247+140,
areknownwith accuracybetterthan0.4mas. These54pro- 2Availableathttp://astrogeo.org/pima.
gram sources and 8 calibrators provided a sufficient overlap 3Availableathttp://astrogeo.org/vtd.
ThepositioncatalogueOBRS–2 3
inseparateleastsquaresolutionsthatusedgroupdelaysat X thesesources,i.e. torestorea pointthatwassuppressedand
andSbandsrespectively. suppressthepointthatwasusedinthesolution,andtore-run
Thedatasetwascleanedforoutliers,i.e. observationswith theprocedureofoutliereliminationthenwecangetaposition
residualgroupdelaysexceeding5σ. Themostcommonrea- thatisconsistenttoapositionattheoppositeband.Thesecan
sons foran observationto have a large residualare a failure be explained if to suggest that a sources has multiple com-
of fringe fitting procedure to find the main maximum, the ponentsseparatedatanarcsecondlevel,strongenoughtobe
presenceofradiointerference,orfalsedetection. Simultane- detected.Iusedthefollowingtechniqueforcomponentsepa-
oussolvingforsourcepositionswithonlyfewdetectionsina rationinorderto investigatethese sources. Iclonedvisibili-
presenceofoutliersposesacertainrisk. Onebadobservation tiesofthesesourcesandtreatedthemasdifferentobjectswith
maycorruptthesolution,andasaresult,remaininggoodob- differentpositions.Usingtheirpreliminarypositionsasapri-
servationsofthatsourcemaygethighresidualsandtheoutlier ori, a shifted phases of visibilities to the new positions and
eliminationproceduremaydiscardthem.Identifyingthatbad performed the fringe search in a narrow window, the same
observation(s)oftenrequiredseveraltrials. Whenthedataset wayasItreatedoutliers. Thisapproachyieldedasignificant
was cleaned, I gradually lowered the SNR limit from 5.8 to numberofnewdetectionsforbothcomponentsatoneorboth
5.0 with a step of 0.1. The status of all observations with bands.Newdetectionsconfirmedthehypothesisthatasource
SNR<5.8wasinitiallysetto“suppressed”.Thatmeansthese ismultiple.
observationsdidnotcontributetoestimationoftheparamet-
ricmodel,buttheparametricmodelevaluatedattheprevious 3.3. Globalastrometricsolution
step was applied for computing their residuals. Suppressed
The result of the interactive solution provided a clean
observationswithresidualsbymodulolessthan5σatSband
datasetof X andS-bandgroupdelayswith updatedweights.
and less than 4σ at X band were restored one by one start-
The dataset that was used for the final parameter estima-
ingwith the smallestnormalizedresidual. Afterflippingthe
tionutilizedalldual-bandS/X dataacquiredundertheabso-
status“suppressed”,theLSQsolutionwasupdatedforinclud-
luteastrometryandspacegeodesyprogramsfromApril1980
ingthisobservationintotheparameterestimationmodel,and
throughDecember2012,including76079observationsfrom
theprocesswasrepeated. Therewasasignificantfractionof
OBRS–2experiments,intotal8.89millionobservations.AsI
falsedetectionswithSNR<5.8. Butsuchobservationshave
mentioned,amongprogramsources,58commonobjectswere
randomgroupdelayestimatesuniformlydistributedover4µs
observed in other absolute astrometry VLBI experiments. I
wide search window. The probability that the group delay
madefoursolutions. Thefirstthreesolutionsusedtheglobal
from a false detection will have a residual within 2–5 ns of
dataset, except observationsof 58 common objects, and ob-
theexpectedvalue,andthereforemaybeidentifiedasagood
servablesfromtheOBRS–2experiments:1)thefirstsolution
observation,isanorderof1–2·10- 3. Suppressionofobserva- used the X-band group delays, 2) the second solution used
tionswithhugeresidualdelayrates(severalsuchpointswere the S-band group delays, and 3) the third solution used the
found)allowstoreducethisprobabilityevenfurther. ionosphere free linear combinations of X and S-band group
Then the fringefitting procedurewas repeatedwith a nar- delays. Thefourthreferencesolutionusedallexperimentsin
rowfringesearchwindowforthosepointsthatweremarked theglobaldatasetwiththeonlyexceptionofOBRS–2data.
as outliers. The center of fringe search window over delay OBRS–2experimentswereanalyzedexactlythesameway
anddelayratewassettotheexpectedvalueofthedelayand as5497otherVLBIobservingsessions,usingthesameanal-
delay rate computed as a sum of the a priori delay or delay ysis strategy that was used forprocessing priorobservations
rate and the contributionfrom the parametric modelderived forICRF(Maetal.1998),VCS, VGaPS,LongBaselineAr-
duringtheLSQadjustment. Thewidthofthesearchwindow ray Calibrator Survey (LCS) (Petrovetal. 2011b), and K/Q
was set to 5 ns for S band and 3 ns forX band. In addition survey(Lanyietal.2010)catalogues. Theestimatedparame-
to that, I re-ranthe fringefitting procedurewith the updated tersarerightascensionsanddeclinationofallsources,coor-
a priori model for all the sources that have position adjust- dinatesand velocitiesofall stations, coefficientsofB-spline
mentsexceeding1′′. Forinstance,0744+092hadtheapriori expansionofnon-linearmotionfor26stations,coefficientsof
position 4.′′8 off the VLBI position. When a source has a harmonicsite positionvariationsof48stationsat4 frequen-
large position error, a non-linearfringe phase change over a cies:annual,semi-annual,diurnal,semi-diurnal,andaxisoff-
scanof300slongbecomessignificantandcausesnoticeable sets for69 stations. Estimated variablesalso includedEarth
de-correlation. orientationparametersforeachobservingsession,parameters
The interactive analysis procedure was repeated with up- ofclockfunctionandresidualatmospherepathdelaysinthe
dated results of fringe fitting. The SNR threshold was low- zenithdirectionmodeledwiththelinearB-splinewithinterval
ered to 4.8 because the probability of false detection is less 60and20minutesrespectively.Allparameterswereadjusted
foranarrowfringesearchwindow. Theprocedureofoutliers inasingleLSQrun.
eliminationwasrepeated.Baselinedependentadditiveweight Thesystem of LSQ equationshas an incompleterankand
correctionswere computed in such a way as the ratio of the definesa familyof solutions. Inorderto picka specificele-
weighted sum of post-fit residuals to their mathematical ex- mentfromthisfamily,Iappliedtheno-netrotationconstraints
pectation was close to unity. This computationprocedureis on the positions of 212 sources marked as “defining” in the
describedinfulldetailbyPetrovetal.(2009). ICRF catalogue that required the positions of these sources
inthenewcataloguetohavenorotationwithrespecttotheir
3.2. Multiplesources
positionsintheICRFcatalogue. No-netrotationandno-net-
Careful analysis revealed five sources that had more than
translation constraints on site positions and linear velocities
15% points with SNR > 5.8 marked as outliers that were
were also applied. The specific choice of identifying con-
notdiagnosedasradiointerferenceorerrorsinfringefitting.
straintswasmadetopreservethecontinuityofthenewcata-
ThesesourceshaddifferentpositionsderivedfromXandS-
loguewithotherVLBIsolutionsmadeduringlast15years.
bandgroupdelays.Iftoinvertsuppressionstatusofpointsfor
4
3.4. Imageanalysis longerthan5000km,similartowhatIdidinimageprocess-
ing. Comparisonofestimatesofmediancorrelatedfluxden-
Thesame datasetof visibilitieswas usedforsourceimag-
sitiesofstrongsourcesderivedbythismethodwithestimates
ing. Atfirst,Idiscardedallvisibilitiesfromobservationsthat
ofcorrelatedfluxdensitiesderivedfromimagesproducedby
were marked as outliers in the final step of the interactive
a rigorousself-calibration procedureshowed they agree at a
analysis procedure. Next, the data should be averaged over
levelof15%.
time and frequency after phase rotation for the contribution
Detailed analysis of produced images goes beyond the
ofgroupdelay,groupdelayrate, andphasedelayratefound
scopeofthepresentpaperandwillbegiveninthefutureina
bythefringesearchprocedure.Sincethefringefittingproce-
separatepublication(G. Bourdaetal., 2013,paperin prepa-
dure was baseline-based, the baseline-dependentparameters
ration).
offringefittingshouldbetransformedtostation-basedparam-
eters in orderto preservephase closuresof visibilities. This
transformationwasdoneforthevisibilitiesofeachscan,each 4. RESULTS
subarray,andeachbandseparately.Althoughtheexperiments I have detected at least at one band all but one source
werescheduledforallthestationsofthenetworktoobserve 0843- 025(J0845- 0241).Inpresentpaperobjectswithmulti-
the same source during scan time, it may happen that some plecomponentsaretreatedasdifferentsources,althoughmost
sourcesweredetectedonlyatasubsetofbaselinesthatdonot likelythesearepartsofthesameobjects.
havecommonstations.ForeachscanIfoundascan-reference Since58of295detectedsourceshavebeenobservedwith
time as a weighted mean epoch of used observations. I se- VLBAindifferentprogramsatXbandinawide-bandmode
lected a reference station for each subarray and solved with with frequency spanned over 494 and 992 MHz, their posi-
leastsquaresforstation-dependentgroupdelays,groupdelay tion uncertainties are a factor of 10–20 better than the posi-
rates,andphasedelaysusingbaseline-dependentestimatesof tion uncertainties from OBRS–2 experiments with the same
these quantities found by the fringe search procedure as the SNRandthesamenumberofobservables. Therefore,forthe
right-hand-side.Thesestation-dependentquantitiesrelatedto purposeofcomparisonwithOBRS–2catalogue,positionsof
a commonepochwithin a scan were used forphase rotating these sources can be considered as precisely known. I ex-
thevisibilities. Afterphaserotation,thevisibilitieswereaver- cludedfromcomparison4 sourcesthatare resolvedandhad
agedover32spectralchannelsineachintermediatefrequency uncertaintiesexceeding0.4masinthereferencesolution.
andover4sintervals. Comparisonshowedthatthesolutionthatusedionosphere
The averaged visibilities and accompanyingweights were freelinearcombinationsof X/Sgroupdelayobservablesdid
split into sources and written in separate files. Fur- notimprovethe agreementbetweenpositionestimatesof 54
ther processing was done using DIFMAP software package commonsourceswithrespecttothesolutionthatusedX-band
(Shepherd 1997). I used automatic imaging procedure mu- onlyobservables. ThepositionuncertaintiesofOBRS–2ob-
petdevelopedbyMartinShepherdandGregTaylor. Itstarted servationsare toolargeforthe residualionospherecontribu-
froma pointsourcemodelasan initialguessandperformed tionstoaffectpositionsatasignificantlevel. Icomputedthe
a sequence of image cleaning, phase and amplitude self- variancethat,beingaddedinquadraturetosourcepositionun-
calibrationswithandwithouttaper. certainties,makestheratioofthesumofweightedsquaresof
I developed a web application that allowed me to inspect positiondifferencesto theirmathematicalexpectationsclose
images visually and flag those that showed visible artifacts. to unity. These variancesare 1.7 masin rightascension and
Of564imagesmadeautomatically,Iselected63imagesthat 2.1 mas in declination for the X-band solution, and 2.1 mas
I processed interactively. These were the sources that either in right ascension and 2.4 mas in declination forthe S-band
had points with amplitude outliers that an automatic proce- solution.
dure was unable to flag out or sources with too few obser-
vations, in a range of 15–35, when an automatic procedure 4.1. OBRS–2catalogue
of hybrid imaging becomes unstable. In total, images were
The majority of sources were detected at both bands, and
produced for 279 sources at X band and for 285 sources at
S band4. As a measure of a source strength, I derived two there are position estimates from two solutions that used X-
bandandS-bandobservablesfromtheOBRS–2experiments.
quantities from source brightness distributions: I computed
The preference was given to the solution that used X-band
themediancorrelatedfluxdensitiesatbaselineswithprojec-
observables because first, the detection limit at X band was
tion lengths shorter than 900 km and the median correlated
lowerandsecond,thecontributionoftheresidualionosphere
fluxdensitiesforbaselineswithprojectionlengthslongerthan
after applying reduction for the total electron contents from
5000km.
GPS observations at X band is one order of magnitude less
Some weak sources have too few points for successful
than at S band. If the reweighted position uncertainty from
imaging. In order to provide a measure of source strength
the S-band solution was at least a factor of 1.5 smaller than
at long and short baselines for these objects, I made a sim-
the uncertainty from the X-band solution, the position from
plified amplitude analysis similar to what was done in pro-
theS-bandsolutionwasusedinthefinalcatalogue.Although
cessing OBRS–1 observations. I used averaged corrections
thereareadditionalmorepreciseobservationsof58common
to gains evaluated during self-calibration of strong sources
sourcesinotherabsoluteastrometryexperiments,theseobser-
and applied these corrections to the a priori SEFDs. From
vationswereexcludedin solutions1 and2 usedforderiving
fringeamplitudescalibratedthiswayIcomputedthemedian
theOBRS–2catalogue.
correlated flux densities at baselines with projection lengths
The positions of 295 sources observed in OBRS–2 exper-
shorterthan900kmandforbaselineswithprojectionlengths
iment are listed in Table 2. The 1st and 2nd columns pro-
vide the IVS source name (B1950 notation) and IAU name
4 ImagesinFITSformataswellascalibratedvisibilities areavailableat (J2000notation). The3rdand4thcolumnsgivesourcecoor-
http://astrogeo.org/obrs. dinatesattheequinoxontheJ2000epoch. Columns5and6
ThepositioncatalogueOBRS–2 5
Table2
First8rowsoftheOBRS–2sourcepositioncatalogue.
IAUname Sourcecoordinates Positionerrors #pnt FcorrSband FcorrXband Flags
B1950 J2000 α δ σα σδ corr band S X short unres short unres
hrmnsec ◦ ′ ′′ mas mas Jy Jy Jy Jy S X
J0001+0632 2358+062 000123.694601 +063230.93754 3.52 15.24 - 0.811 X 38 31 0.018 0.022 0.018 0.008 m m
J0005+1609 0003+158 000559.237650 +160949.02157 1.86 2.53 - 0.342 X 168 168 0.143 0.070 0.196 0.057 m m
J0015+3052 0012+305 001536.022281 +305229.79522 2.98 7.35 - 0.437 X 12 32 0.026 0.015 0.017 0.006 m
J0017+1451 0015+145 001736.903866 +145101.88067 1.92 3.10 - 0.557 X 164 166 0.065 0.039 0.053 0.030 m m
J0027+4514 0025+449 002742.262713 +451457.07879 2.72 2.88 - 0.477 X 222 129 0.048 0.042 0.024 0.015 m m
J0035+1553 0033+156 003555.537977 +155316.45642 23.81 23.11 - 0.267 S 29 3 0.074 <0.017 -1.00 -1.00 m
J0037+3938 0034+393 003736.725767 +393811.79014 2.44 2.86 - 0.354 X 210 121 0.142 0.044 0.039 0.012 m m
J0041- 0143 0038- 019 004126.008767 - 014315.67847 2.03 5.16 - 0.625 X 143 125 0.056 0.049 0.037 0.032 m m
Note. — Units of right ascension are hours, minutes and seconds. Units of declination are degrees, minutes and seconds.
(Thistable isavailable initsentirety inmachine-readable andVirtualObservatory (VO)formsintheonline journal. A portionisshownhereforguidance
regardingitsformandcontent.)
give reweighted source position uncertaintiesin right ascen-
sionanddeclinationinmas(withoutcosδfactor),andcolumn
7givesthecorrelationcoefficientbetweentheerrorsinright
ascensionanddeclination.Column8showsbandIDoftheso-
lutionthatwasusedtoderivepositionofagivensource. The
numberofgroupdelaysusedinanalysisislistedincolumns
9 and 10. Columns 11 and 12 provide the median value of
thecorrelatedfluxdensityinJanskyatSbandatbaselinepro-
jection lengths shorter than 900 km and at baseline projec-
tionlengthslongerthan5000km. Thelatterestimate serves
as a measure of the correlated flux density of an unresolved
sourcecomponent.Columns13and14providethemedianof
thecorrelatedfluxdensityatXbandatbaselinesshorterthan
900 km and longer than 5000 km. If no information about
correlatedfluxdensityisavailable,- 1.000isusedasaplace-
holder. Thelasttwocolumnshaveflagswhetheranimageis
availableforSandXbands:“m”ifavailable,blankifnot.
The semi-major axes of error ellipses range from 2.1 to
200mas, with the median 3.2mas, andfor80%sourcesthe
positionerrorsareunder5.2mas.Themajorfactorthatresults
inpositionuncertaintyexceeding5masisalackofdetections
at long baselines. Sources with large position uncertainties
are either highlyresolvedor veryweak moderatelyresolved Figure1. Triple source 0809+483/0809+48B/0809+48C VLA image at
4.86 GHz on epoch 1995.10.15 with beam size 1′′.27, project AS561, is
objects.
shownasacontourmap.VLBIpositionsareshownwithfilledcircles.Com-
ponentAisthemostcompactandhasfluxdensity5mJyatintercontinental
4.2. Analysisofmultiplesources baselines. ComponentsBandCareresolvedandvisibleonlyatshortbase-
0154+31A/0154+31B has two components 2′′.592 apart. lines.
Component A is compact and was detected at 5 mJy level
atXbandatintercontinentalbaselineswithEffelsberg.Itwas tion 3′′.157. Although component B is 5 times stronger at
not detected at intercontinental baselines at S band. Since
X band at the VLA image (Figure 2) than component A
the sensitivityof baselineswith Effelsbergis a factorof 4–5
— 21 mJy versus 4 mJy, it is not detected at VLBA scale
worse at S band than at X band, this can be explained if a
sourcehasaspectralindexsteeperthan- 1.1(S∼ fα). Com- a4tmXJybaatnindt.ercoCnotimnepnotnaelnbtasAelinweisthliecsowrriethlaitned0′′.fl2u0xofdWenIsSitEy
ponentBisstrongeratSband,buthasonlythreedetectionsat
J132529.70+651513.3whichhasmagnitude14.6at3.6µ.
5mJylevelatXbandatshortbaselinesonly.Cross-matching 1335- 06A/1335- 06B has two components 4′′.896 apart.
against Wide-field Infrared Survey Explorer (WISE) infra-
Component B, associated with a radiolobe (Figure 3) was
red catalogue of point sources (Wrightetal. 2010) revealed
W3.6ISµEwJi0th1i5n701′5′..3362+o3f105145149+.231oAb.ject with magnitude 13.9 at ddeetnescitteyd7atmSJybanadt Xonlyb.anCdoimspwacitthcinomopfon0e′′n.t18AowfithWflISuEx
0809+483/0809+48B/0809+48C has three components J133807.98- 062711.0,whichhasmagnitude13.9at3.6µ.
within 6′′. Components B and C coincide with extended 3′′1.031440.+60TAh/e13s4o0u+rc6e0Bloohkass tewloongcaotemdpoantetnhtes sVeLpaAratiemdagaet
radio-lobes visible at the VLA image (Figure 1). Compact
at 1.4 GHz (Figure 4). Component B was detected at
component A that has X-band flux density 5 mJy at inter- S band only. Component A is within 0′′.23 of WISE
continental baselines is located between radio lobes. WISE
J081336.05+481302.7 with magnitude 13.5 at 3.6 µ was J134213.26+602142.9whichhasmagnitude14.2at3.6µ.
foundwithin0′′.11ofcomponentA.
1323+65A/1323+65B has two components with separa- 5. DISCUSSION
6
Figure4. Doublesource1340+60A/1340+60B VLAimageat1.4GHzon
epoch2002.02.07withbeamsize5.′′4,projectAB950(FIRST),isshownas
acontourmap.ComponentBwasdetectedonlyatSband.
Figure2. Doublesource1323+65A/1323+65B VLAimageat8.4GHzon
epoch1999.08.08withbeamFWHM0.′′27,projectAR415,isshownasa
contour map. Although component B is stronger at the VLAimage, it is
resolvedoutatXbandatVLBAresolution. ComponentAhasfluxdensity
27mJyatbothbands.
Figure5. Dependence of the median correlated flux density at 8 GHz at
baselineslongerthan5000kmversusVmagnitudeofthoseactivegalaxynu-
cleathathavebeendetectedwithVLBIunderabsoluteastrometryprograms.
Solid blue filled circles show sources detected at X band at OBRS–1 and
OBRS–2campaigns.Greenhollowcirclesshowassociatedsourcesobserved
underotherprograms.
found that 1676 objects, or 23%, have a counterpart within
a 4′′ searchradius. Of them, 825objectsarebrighterV 18m
mag. Of them, 377, or 46%, were observed in OBRS–1
or OBRS–2 programs, and 293 of them were observedonly
in these two programs. Five OBRS–2 sources, 0012+305,
0232- 042, 0744+092, 1146+249, 1632+198 have position
offsets with respect to the optical catalogue of quasars of
Figure3. Doublesource1323+65A/1323+65BattheVLAimageat1.4GHz Veron-Cetty&Veron(2010)exceeding4′′.Sincetheiroffsets
on epoch 2001.04.29 with beam size 6.′′45, project AB950 (FIRST), is withrespecttoWISEcatalogueareinarangeof0.08′′–0.43′′,
shownasacontour map. Component Bwas detected inOBRS–2 onlyat Iconsiderthattheirpositionsintheopticalcataloguehadan
Sband.
error.
Figure5 showsthe dependenceofthecorrelatedfluxden-
Position accuracy, 2–5 mas for 80% OBRS–2 sources, is sity versus V magnitudeforthe entire sample of radio-optic
too coarse to make a meaningfulcomparison with Gaia be- associationsandforthesub-sampleobservedintheOBRS–1
causethefrequencysetupwasnotfavorableforpreciseabso- andOBRS–2programs. Thereisnosignofobviouscorrela-
luteastrometry. tionbetweenopticalbrightnessandradiobrightness. Wesee
Cross-referencing the cumulative catalogue of radio thattheOBRS–1/OBRS–2sourcesaresystematicallyweaker
sourcesdetectedwithVLBIintheabsoluteastrometrymode than those observed in other programs. Of 400 sources de-
at 8 GHz against the optical catalogue of active galaxy nu- tectedinOBRS–1andOBRS–2,only148haveanunresolved
clea, including quasars, of Veron-Cetty&Veron (2010), I component at X band stronger than 30 mJy, while the to-
ThepositioncatalogueOBRS–2 7
tal numberof radio sources associated with quasarsbrighter ful conclusions from comparison of Gaia and VLBI posi-
V 18m mag and with unresolved component stronger than tions. Approximately 1/3 of observed sources have strong
30mJyis683. unresolved core and their positions can be determined with
If a source is too weak at long baselines, the position un- accuraciesbetterthan0.1masinfutureVLBIobservingpro-
certainty 0.1 maswill notbe achievedfora reasonableinte- gramswithappropriatefrequencysetup.
grationtimebecauseofthethermalnoise. Themediansemi-
majoraxisof thepositionerrorellipse ofOBRS–1/OBRS–2
TheNationalRadioAstronomyObservatoryisafacilityof
sources without reweighting is 2.2 mas. The frequency se-
the NationalScience Foundationoperatedundercooperative
quence used in these observing programs results in a group
agreementby Associated Universities, Inc. This publication
delay uncertainty at a given SNR that is a factor of 11.2
makesuseofdataproductsfromtheWide-fieldInfraredSur-
greater than the group delay uncertainty of regular geodetic
veyExplorer,whichisajointprojectoftheUniversityofCali-
observationsunderRDV program. Even if OBRS programs
fornia,andtheJPL/CaliforniaInstituteofTechnology,funded
wereobservedwiththesamefrequencysequenceasRDVex-
bytheNASA.
periments,themedianpositionuncertaintyduetothethermal
Facilities:VLBA(projectcodeGC034).
noise would have been 0.2 mas. Therefore, future observa-
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