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9 0 VLBI Polarisation with the Yebes (EVN) and Hobart 0 2 (LBA) antennae n a J 2 1 ] M ∗ RichardDodson I ObservatorioAstronomícoNacional,España . h andUniversityofWesternAustralia p - E-mail: [email protected] o r t s Workhasbeenon-goingforthedevelopmentoftherequiredcodeforfullpolarisationprocessing a [ of VLBI data using some new antennaemounts. The extensionsof AIPSallows the supportof two new mount types; the left-handed and right-handed Nasmyth antennae (Pico Veleta in the 1 v GMVAandYebes-40mintheEVN)andtheEW-mount(HobartintheLBA).Thedatahandling 4 processis seamless, oncethe correctmounttype hasbeen selected. All subsequentcalls to the 9 4 parallactic angle subroutinePARANG will returnthe feed anglesfor Leftor Right Nasmythor 1 EW-mount.Thesearerequired,respectively,forPicoVeleta,Yebes-40m(lowfrequencybranch) . 1 andHobart. 0 9 0 : v i X r a The9thEuropeanVLBINetworkSymposiumonTheroleofVLBIintheGoldenAgeforRadioAstronomy andEVNUsersMeeting September23-26,2008 Bologna,Italy ∗ Speaker. (cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/ VLBIPolarisationwiththeYebesandHobartantennae RichardDodson Figure1: RR/LLPhasebetweenYebesandEffelsbergafterfeedrotationcorrection. Introduction To solve for the VLBI polarisation calibration terms one needs to remove the rotation of the feed. Until now the only feed terms supported in AIPS, and there could be solved for, were the Cassegrain and Equatorial types. The Giant mm-VLBI array (GMVA), the European VLBI network (EVN) and the Australian Long Baseline Array (LBA) included feed types which were not supported in AIPS; that is the Nasmyth (Pico Veleta and Yebes) and EW-mount (Hobart) types. New additions to the AIPS code, now included in the general distribution, will allow the full polarisation calibration oftheseantennae, andtheirrespective arrays. Theseroutines havebeen used tocalibrate theLBAandtoproduce thefirstpolarised VLBIimagesofMethanolMasers(Dodson 2008). Followup EVNand LBAobservations are being made for this project. Preliminary results from EVN network experiment N08K2 show successful feed angle calibration. Here we present recent results from test-time LBA experiment VX014, showing the comparison of the polarised fluxseenbytheseobservations andthoseoftheMOJAVEsurvey(withtheVLBA). YebesfirstVLBIfringeswiththeEVN AspartofthefirstVLBIlightofthenew40mantennaatYebes,Spain,observationswereper- formed at22-GHzin thenetwork monitoring experiment N08K2. Fringes were obtained between Yebes and the other antennae. In Figure 1 we present the phase difference between the RCP and LCPdata after the feed angle phases have been corrected for, showing that this wassuccessful. It was not possible to take this analysis further, as the amplitude calibration was poor, which makes theseperation ofthecontributions fromsourcepolarisation andinstrument polarisation difficult. ComparisonsoftheLBAwithATCA We have used the new code to successfully map the polarisation direction of the magnetic fields of theMethanol Masers inG339.88-1.26 (experiment V148). Methanol Masers, it has been claimed, lend important support to the model of the formation of Massive Stars from in-falling disks (Norrisetal.,1993). Themagnetic fieldsfound viaVLBIarenotconsistent withthemasers being in the disks, and therefore remove this strong support for the disk model. As part of this analysis wecompared theintegrated spectral linefluxesfrom ATCAandLBAdata whichshowed agreement between the ATCA detection of the polarised flux by velocity channel and that found withtheLBA.SeeDodson(2008)forthefiguresanddetailsoftheanalysis. ComparisonsoftheLBAwithVLBA FoursourceswereobservedinVX014,ofwhichtwoareintheMOJAVEprogram(Listeretal., 2005); the Monitoring Of Jets and AGNs with VLBA Experiments. We compared the images of 2 VLBIPolarisationwiththeYebesandHobartantennae RichardDodson a) b) Figure2: a)Thepolarisedfluxfrom3C273,asobservedbytheLBAandtheVLBA in2008. Thecolour scale is the LBA total intensity, the black contoursare the VLBA polarised intensity, the red those of the LBA with the LBA polarisation vectors overlaid. b) The polarised flux from 3C279, as observed by the LBA andtheVLBAin2008. ThecolourscaleistheLBA totalintensity,thebluecontoursaretheVLBA polarisedintensity,theredthoseoftheLBAwiththeLBApolarisationvectorsoverlaid. these twosources, 3C273and 3C279, despite thedifferences inresolution andfrequency between the two observations. The LBAat 8.4-GHz has a resolution of ∼3mas and the VLBAat 15-GHz hasaresolution of∼0.5mas. NeverthelesswesmoothedtheVLBAimagestotheresolution ofthe LBA, and amplitude scaled the LBA data to match the fluxes; assuming a simple spectral index between the two frequencies. With these simple corrections the total power images from the two instruments areinverygoodagreement, givingustheconfidence tocomparethepolarisation. The images in Figure 2 show the total power, with a log scaled colour index. Overlaid are the contours from the linear polarised flux for LBA and VLBA data. The vectors are the field directions for the LBA data, these are in good agreement with those of the VLBAdata. However this is not total independent, as the VLBA image was used to provide the calibration in LPCAL. The D-term solutions for the two sources (both assuming a polarised source and an unpolarised source) wereinagreement toafewpercent, dominated bytheerrors inMopra. Itisnotable tothe first order that there is a good match between the results from the LBA and the VLBA,but at the second order there are notable differences. These will need further investigation, as it is possible that they are due tothe second order effects from the very high polarisation feed correction in the LBAcalibration terms. Work isundergoing to improve these, particularly for Mopra, which were knowntobeanomalous. References [1] R.Dodson,2007,IT-OAN-2006-16,InforméTechnico,“Onthesolutionofthepolarisationgainterms forVLBIdatacollectedwithantennaehavingNasmythorE-Wmounts.”. [2] R.Dodson,2008,AA,480,767,“FirstVLBIobservationsofmethanolmaserpolarisation,in G339.88-1.26” [3] M.Lister,D.Homan,2005,AJ,130,138,“MOJAVE,1.VLBAFirst-Epoch15GHzimages” [4] R.Norris,etal.,1993,ApJ,412,222,“Synthesisimagesof6.7GHzmethanolmasers” 3