The Interna*onal Celes*al Reference Frame (ICRF) from

VLBI DavidGordon

NVIInc.,Code61AJune22,2017

•  Mul=pleradiotelescopesobservethesamedistantradiosourcessimultaneously.

•  Quasarsignalsarriveatdifferent=mesateachantenna.

•  Signals(+noise)arerecordedondisksindependentlyateachsta=on.

•  DisksaresenttoaVLBIcorrelatorwheretherecordeddatastreamsarecross-correlatedtoextracttheVLBIobservables(delays,delayrates,fringephases,andfringeamplitudes).

VeryLongBaselineInterferometry(VLBI)

VLBIisanastronomicaltechniquethatcanbeusedforastronomy,astrometry(absoluteandrela=ve),precisegeodesy,spacecraTnaviga=on.TheGSFCVLBIgroupusesVLBIforgeodesyandabsoluteastronomy.Geode=c/AstrometricVLBIsessions:•  Sessionsareusually24-hrsinlength.•  MostsessionshavebeendualfrequencyatX(~8.5GHz)andS(~2.3GHz)bandstoallowcorrec=ngfortheionosphere.

•  Typically~50-100differentquasarsobserved.•  Datacorrelated/fringedtogetobserveddelaysanddelayrates.•  Observeddelaysandtheore=caldelaysusedinleastsquaressolu=onstodeterminesiteposi=ons,Earthorienta=onparameters,sourceposi=ons,etc.usingtheGSFCCalc/SolveandμSolvesoTwarepackages.

Whydoweneedapreciseceles;alreferenceframeforgeodesy?VLBIoperatesbetweentworeferenceframes.VLBIantennasare‘fixed’inarota=ngterrestrialreferenceframe(TRF).Quasarsare‘fixed’inaniner=alceles=alreferenceframe(CRF).TheTRFandtheCRFareconnectedbyasetof5Earthorienta=onangles(EOP’s:Xpole,Ypole,UT1,Xnut,Ynut).Averypreciseceles=alreferenceframeisneededbyVLBIinordertodetermineaverypreciseterrestrialreferenceframeandverypreciseEarthorienta=onparameters.

Interna;onalCeles;alReferenceFrames–ICRF1:InJanuary1998,theIAUadoptedtheVLBIX/Sceles=alreferenceframeastheInterna;onalCeles;alReferenceFrame-ICRF1,replacingtheFK5op=calframe.ICRF1wascomposedoftheceles=alcoordinatesof608compactradiosourcesandhadanes=matednoisefloorof250μ-arc-seconds.Althoughaninterna=onaleffort,theICRF1catalogwasgeneratedhereatGSFC.TheaxesofICRF1weredefinedbythecoordinatesofasetof212‘defining’sourcesthoughttobestableandwell-observed.Axisstabilitywases=matedat~20μ-arc-sec.Inlatersolu=ons,thedefiningsourceswereheldtoano-net-rota=onconstraintfromtheirICRF1coordinates.Theothersourcesweretobefreelyadjustedintheleast-squaressolu=ons.TwoextensionstoICRF1weremade,ICRF-ext1andICRF-ext2,whichaddedaddi=onalsources.

ICRF1:AdoptedbytheIAUinJan.1998.•  X/Sfrequencies•  608totalsources•  212definingsources•  250μ-arc-secondnoisefloor

WeaknessesofICRF1:•  Fewdefiningsourcesinthefar-south.•  Severaldefiningsourcesfoundtobeunstable.•  Fewoverallsourcesinthefar-south.(MostVLBIantennasareinthenorthernhemisphere.)

Interna;onalCeles;alReferenceFrames–ICRF2:Asecondrealiza=onoftheInterna=onalCeles=alReferenceFrame,ICRF2,wasgeneratedbyanIAU/IVSWorkingGroupin2009,andadoptedbytheIAUinJanuary2010.ICRF2hadprecisecoordinatesof3414sourcesandanes=matednoisefloorof40μ-arc-sec(equivalenttoaquarterasseenfrom~125,000kmaway).Thesolu=onusedtoconstructICRF2wasalsogeneratedhereatGSFC.TheaxesofICRF2weredefinedby295‘defining’sources.Axisstabilitywases=matedat~10μ-arc-sec.Themainreasonforthelargeincreaseinsourcesisthat~2200addi=onalsourceswereobservedin24VLBACalibratorSurvey(VCS1-VCS7)sessions(1994-2007),whichwereruntodensifythephasereferencingcalibratorcatalogforastronomicalVLBI.

ICRF2:AdoptedbytheIAUJan.2010.•  X/Sbands•  3414totalsources•  295definingsources•  40μ-arc-secnoisefloor

WeaknessesofICRF2:•  ~2/3ofthesourceswereobservedinonlyonesession,theVLBACalibratorSurveys(VCS),andhadposi=onuncertain=es~5=mesgreaterthantheother1/3.

•  Scarcityofsourcessouthof~-30°declina=on.(VLBAlimitis~-40°.)

TheNatureofQuasarRadioSources:Attheradiofrequencies,manyquasarsshowstructures(jets,etc.)whichcanvaryinintensity.ThisproducesshiTsintheirapparentposi=ons.Someareverystable,someveryunstable,mostaresomewherein-between.SomeoftheICRF1definingsourceswerefoundtoshowfairlylargeposi=onvaria=ons.ForICRF2,considerableeffortsweremadetoiden=fyandusethemoststableandleaststructuredsourcesasdefiningsources.

AStableSource AnUnstableSource

UsesoftheICRF:•  VLBIgeodesy.ForimprovedUT1andprecession/nuta=onleadingto

improvedgeophysicalmodeling.•  Phasereferencecalibrators.ForVLBIimagingofcompactsources,

measurementsofparallaxesandpropermo=ons(masers,pulsars,radiostars)improvingtheunderstandingofgalac=cstructure.

•  SpacecraTnaviga=on.•  Alignmentoftheplanetaryephemerides.•  Gaiaop=calalignmentandcomparisons.Gaiacandetectmostofthe

ICRF2(andfutureICRF3)sources.Radioandop=calreferenceframesneedtobealigned.Canleadtoaccuratemeasurementsofcore-shiTs.

•  Studiesofgalac=caberra=on.

ICRF-3:WorktowardsICRF3beganwiththecrea=onofanIAUWorkingGroupin2012.Goalswere:•  ImproveupontheprecisionofICRF2,especiallyforthe‘VCS’sources.•  Increasethenumberofsources.•  Provideforaframe=eandaccuracycomparisonswiththean=cipatedGaiaop=calcatalog.

•  PresentittotheIAUforapprovalattheAugust2018GeneralMee=ng.ThreeGSFCcode61ApersonnelaremembersoftheICRF3WorkingGroup(DavidGordon,ChopoMa,SergeiBolo=n).

CurrentworktowardsICRF3:ICRF3willhavecatalogsatthreedifferentfrequencies.X/S-band:Manyofthesingleepochsourceshavenowbeenobservedin3ormoresessions,andhavemuchsmallerformalerrors.Mosthavebeenre-observedinVLBAsessions.Also,~860addi=onalsourceshavebeenadded,mostlyfromVLBAsessionsandsouthernhemisphereastrometrysessions.Sourceposi=on=me-seriesplotsandVLBImapswillbeexaminedtofindthebestdefiningsources,withemphasisonuniformskydistribu=on.TheX/Scatalogwillprobablybeacombina=onfrom~7differentgroupsusingseveralindependentsoTwarepackages(Calc/Solve,VieVS,OCCAM,QUASAR).

CurrentworktowardsICRF3:(con=nued)K-band:AnearlierK-band(24GHz)astrometryprogramwasrestartedin2014.Now~30K-bandsessions(VLBAandSouthAfrica-Australia)and788sources.FormalerrorsnearlyasgoodasatX/S.UsingGPSionospheremapsforionospherecorrec=ons.Ka/X-band:Observa=onsbeingmadebyChrisJacobs(JPL)usingDSNantennas.Mostlysinglebaselinesessions,~700sources.TheKa(~34GHz)andX(~8.4GHz)frequenciesallowforionospherecorrec=ons.FuturespacecraTnaviga=onworkwillshiTtoKabandsoaKa/Xcatalogwillbeveryimportant.Sourcestructuregenerallydecreaseswithincreasingfrequency.Therefore,thehigherfrequencycatalogs(K-bandandX/Kaband)canbemorestable.TheVLBIposi=oncanalsomoveclosertothequasar’score.Thesourcesgenerallybecomeweakerthough,sofewersourcescanbedetected.

Parameter ICRF1(Jan.1998) ICRF2(Jan.2010) ICRF3(2018/2019)

DataSpan: Aug1979-July1995(16years)

Aug1979–March2009

(29.5years)

X/S:Aug1979-~Dec2017(38.4years)

K:2002–2017(15yrs)Ka/X:2012–2017(~5yrs)

#Observa=ons(datapoints)

1.6millionX/Sband 6.5millionX/Sband ~15millionX/Sband~400,00Kband

~100,000Ka/Xband

#DefiningSources 212 295 ~300

Total#Sources 608 3414 ~4300X/Sband~800Kband

~700Ka/Xband

NoiseFloor ~250μ-arc-sec ~40μ-arc-sec ~30μ-arc-sec

AxisStability ~20μ-arc-sec ~10μ-arc-sec

≤10μ-arc-sec

ICRF1vs.ICRF2vs.ICRF3Comparison

PreliminaryICRF3X/Ssourcedistribu=onJune12,2017solu=on

4274Sources

VLBIwebpage:htps://lupus.gsfc.nasa.gov/LatestICRF2compliantsolu=on:htps://gemini.gsfc.nasa.gov/solu=ons/2016a_astro/2016a_astro.htmlLatestsource=meseries:htps://gemini.gsfc.nasa.gov/solu=ons/2016a_astro/2016a_ts.html