Multi-step VLBI observations of weak extragalactic radio sources to align the ICRF and the future Gaia frame

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Multi-step VLBI observations of weak extragalactic

radio sources to align the ICRF and the future Gaia

frame

G. Bourda, P. Charlot, R. Porcas, S. Garrington

To cite this version:

G. Bourda, P. Charlot, R. Porcas, S. Garrington. Multi-step VLBI observations of weak extragalactic

radio sources to align the ICRF and the future Gaia frame. SF2A-2008: Semaine de l’Astrophysique

Française, 2008, Paris, France. p. 7. �hal-00375921�

C.Charbonnel,F.CombesandR.Samadi(eds)

MULTI-STEP VLBI OBSERVATIONS OF WEAK EXTRAGALACTIC RADIO

SOURCES TO ALIGN THE ICRF AND THE FUTURE GAIA FRAME

Bourda,G. 1 ,Charlot, P. 1 ,Por as,R. 2 and Garrington,S. 3

Abstra t. Thespa eastrometrymissionGaiawill onstru tadenseopti alQSO-based elestialreferen e frame. For onsisten ybetween opti aland radiopositions, it will be important to align the Gaia frame and theInternational CelestialReferen e Frame(ICRF)with thehighesta ura y. Currently, itis found thatonly10%of theICRFsour es(70 sour es)are suitabletoestablish thislink, eitherbe ause theyare not bright enoughat opti al wavelengthsor be ausethey have signi ant extended radio emissionwhi h pre ludesrea hingthehighestastrometri a ura y. Inordertoimprovethesituation,wehaveinitiateda VLBIsurveydedi atedtondingadditionalsuitableradiosour esforaligningthetwoframes. Thesample onsists ofabout 450sour es,typi ally 20timesweakerthanthe urrent ICRFsour es, whi hhavebeen sele ted by ross- orrelating opti al and radio atalogues. This paper presents the observing strategy to dete t,image, and measure a uratepositions for these sour es. It alsoprovides results aboutthe VLBI dete tabilityofthesour es,asderivedfrominitialobservationswiththeEuropeanVLBINetworkinJune andO tober2007. Basedontheseobservations,anex ellentdete tion rateof89%isfound,whi hisvery promisingforthe ontinuationofthisproje t.

1 Context

TheInternationalCelestialReferen eFrame(ICRF)istherealizationatradiowavelengthsoftheInternational Celestial Referen e System (ICRS; Arias et al. 1995), through Very Long Baseline Interferometry (VLBI) measurements of extragala ti radio sour e positions (Ma et al. 1998; Fey et al. 2004). It was adopted by theInternationalAstronomi alUnion (IAU)asthefundamental elestialreferen eframeduringtheIAU 23

rd

GeneralAssemblyatKyoto,in1997. TheICRF urrently onsistsofa ataloguewiththeVLBI oordinatesof 717extragala ti radio sour es(fromwhi h212aredeningsour es),withsub-milliar se onda ura y.

TheEuropeanspa eastrometrymissionGaia,tobelaun hedby2011,willsurveyabout(i)onebillionstars in our Galaxy and throughout the Lo al Group, and (ii) 500000Quasi Stellar Obje ts (QSOs), down to an apparentopti al magnitudeV of 20(Perrymanet al. 2001). Opti al positions with Gaiawill bedetermined withanunpre edenteda ura y,rangingfromafewtensofmi roar se onds(as)atmagnitude15{18toabout 200asatmagnitude20. UnlikeHippar os,Gaiawillpermittherealizationoftheextragala ti referen eframe dire tlyat opti al bands, based onthe QSOsthat havethemost a urate positions (i.e. those with V 18 (Mignard 2003); it is expe ted to dete t at least 10000of su h QSOs (Mignard 2002)). A preliminary Gaia atalogueisexpe tedto beavailableby2015withthenalversionreleasedby2020.

Inthe future, aligningthe ICRFand the Gaiaframe will be ru ial for ensuring onsisten ybetween the measuredradio and opti al positions. This alignment, to be determined with the highest a ura y, requires severalhundredsof ommonsour es,withauniformsky overageandverya urateradioandopti alpositions. Obtainingsu h a urate positions impliesthat thelink sour es must have(i)an apparent opti almagnitude V brighter than18(forthehighestGaiaastrometri a ura y),and (ii)noextended VLBIstru tures(forthe highestVLBIastrometri a ura y). Inapreviousstudy,weinvestigatedthe urrentstatusofthis alignment basedonthe present listof ICRF sour es(Bourda et al. 2008). We showedthat although about 30%of the ICRFsour eshaveanopti al ounterpartwithV 18, onlyone thirdof theseare ompa tenoughonVLBI

1

Laboratoired'AstrophysiquedeBordeaux,UniversitedeBordeaux,CNRSUMR5804,BP89,33271Floira Cedex,Fran e 2

Max-Plan k-InstituteforRadioAstronomy,P.O.Box2024,53010Bonn,Germany 3

s ales for the highest astrometri a ura y. Overall only 10% of the urrent ICRF sour es (70 sour es) are available todayfor thealignmentwith thefuture Gaia frame. This highlightstheneed to identify additional suitableradio sour es,whi histhepurposeoftheproje tdes ribedhere.

2 Strategy toidentifynew VLBI radiosour esforthe ICRF{Gaiaalignment

Sear hingforadditionalradiosour essuitableforaligninga uratelytheICRFandtheGaiaframe ouldrely ontheVLBACalibratorSurvey(VCS;Petrovetal. 2008andreferen estherein),a atalogueofmorethan3000 extragala ti radiosour esobservedwiththeVLBA(VeryLongBaselineArray). Thisinvestigationis urrently underway. Anotherpossibilityisto sear hfor newVLBIsour es, whi h impliesgoingtoweakerradio sour es thathavea ux densitytypi allybelow100mJy. This annowbeenvisionedowingto there entin reasein theVLBInetworksensitivity(i.e. re ordingnowpossibleat 1Gb/s)andbyusinganetworkwithbigantennas like theEVN (European VLBI Network). A sample of about450radio sour esthat mostly haveneverbeen observedwithVLBI(i.e. notpartoftheICRForVCS)hasbeensele tedforthis purposeby ross-identifying theNRAOVLASkySurvey(NVSS;Condonetal. 1998),adeepradiosurvey( ompleteto the2.5mJylevel) that overstheentireskynorthof 40

Æ

,withtheVeron-Cetty&Veron(2006)opti al atalogueofQSOs. This sampleisbasedonthefollowing riteria: V 18(forana uratepositionwithGaia), Æ 10

Æ

(forpossible observingwith northernVLBIarrays),and NVSS ux density 20mJy (forpossibleVLBIdete tion). The observingstrategytoidentify theappropriatelink sour esinthesamplein ludesthree su essivesteps: (1)to determinethe VLBIdete tability of these weak radio sour es, mostlynot observed before with VLBI;(2) to imagethesour esdete tedinthepreviousstep,in orderto revealtheirVLBIstru ture;and (3)to determine ana urateastrometri positionforthemostpoint{likesour esofthesample.

3 VLBI results

Initial VLBI observations for this proje t were arried out in June and O tober 2007 (during two 48{hours experiments),withanetwork of4or5VLBIantennasfrom theEVN. Thepurpose ofthese twoexperiments was to determine the VLBI dete tability of the 447 weak radio sour es in our sample based on snapshot observations. Ourresultsindi ateex ellentdete tionratesof97%atXbandand89%atSband. Overall,398 sour esweredete tedatbothfrequen ies. Theoverallmean orrelated ux densitiesweredeterminedforea h sour eandband bythemeanoveralls ansandbaselinesdete ted. AtXband,432sour esweredete tedand themean orrelated uxesrangefrom1mJyto190mJy,withamedianvalueof26mJy. AtSband,399sour es were dete tedand themean orrelated uxes rangefrom 8 mJyto 481mJy, with amedian valueof 46mJy. A omparisonbetweentheX-band ux densitydistributionforoursour es,thosefromtheVCSandtheICRF showsthatthesour esofoursampleareindeedmu hweaker. Onaverage,theyare27timesweakerthanthe ICRFsour es and 8times weakerthan theVCS sour es. The spe tral index (S /

, S beingthe sour e ux density and  thefrequen y) wasdetermined forthe 398radio sour esdete ted at both frequen ies; the sour eswitha ompa t oreareexpe tedtohave> 0:5. Themedianvalueofinoursampleis 0:34and about70%ofthesour eshave> 0:5,hen eindi atingthattheymusthaveadominating ore omponent, whi hisverypromisingforthefuturestagesofthisproje t. Thenextstepwillbetargetedatimagingthe398 sour esthat wehavedete tedat bothfrequen ies,byusingtheglobalVLBInetwork(EVN+VLBA), inorder toidentifythemostpoint{likesour esandthereforethemostsuitableonesfortheICRF{Gaialink.

Referen es

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