VLBI - Australia Telescope National Facility...weightedimagesandcubes,to8×5mas(PA2 ) for the full-resolution images. They correspond, attheredshiftof4C12.50z=0.1217,to32×11pc and

VLBI:Theveryhighresolu3oncomponentofASKAP

ASTRONOMYANDSPACESCIENCE

ChrisPhillips7June2016

LBA

• LBAVLBIarrayoperatedbyCSIRO• UniversityofTasmania,AucklandUniversityofAucklandandHartebeesthoekRadioObservatory

• Fivecoretelescopes• Parkes(64m),ATCA(5x22m),Mopra(22m),Hobart(26m),Ceduna(30m)

• Warkworth(14m/30m),Hartebeesthoek(26m/15m),Tidbinbilla(70m/34m),Auscope(12m)• SomeAsian&EVNinvolvement• 1-100GHzfrequencyrange• ~30days/yearobserving

ASKAPVLBI|ChrisPhillips3|

Frequency(GHz) Baseline(km) Resolu3on(mas)

AusLBA FullLBA AusLBA FullLBA

1.6 1702 9847 22.7 3.93

2.4 1702 10480 15.1 2.46

6.7 1702 10480 5.4 0.88

8.4 1702 10480 4.3 0.70

22.2 1702 9847 1.6 0.28

35 566 3.1

85 114 6.4

AusLBA:Parkes,ATCA,Mopra,Hobart&CedunaFullLBA:AlsoWarkworth,Hartebeesthoek

TheHighResolu3onComponentsofASKAP:Mee3ngtheLongBaselineSpecifica3onsfortheSKA(VLBI)

• UseASKAPasanelementoftheLBA• Acceptedas“technologydemonstrator”ratherthanfullonsurvey• MainrequirementASKAPgedarray• ~35mequivalentsingledish• DoublesmaximumAustraliaonlybaseline• 1.5xlongerwhichincludingNewZealand• FillsinuvcoveragetoSouthAfrica• Explore“mulg-view”VLBI

– MulgbeamVLBI





Frequency(GHz) Baseline(km) Resolu3on(mas)

AusLBA FullLBA AusLBA FullLBA

1.6 3273 9847 11.8 3.93

2.4 1702 10480 15.1 2.46

6.7 1702 10480 5.4 0.88

8.4 3273 10480 2.3 0.70

22.2 1702 9847 1.6 0.28

35 566 3.1

85 114 6.4

ProjectUpdate

•  Installedsinglepixelfeedsat1.6and8.4GHzontwoantenna•  SuppliedHydrogenmaserforobservatory•  InstalledownVLBIbackend• FirstASKAPfringes22April2010• FirsteVLBIfringes27June2011• DeterminedabsoluteposigonforAK29– Originalsurveyposigon17moff

• DemonstratedPAFVLBIwithsingledishbetasystem• Nogedarraydevelopedforbeta•  TiedarrayforfullASKAPspunoffasseparateproject•  ExploringASKAPPAF-PARKESPAFVLBI



• RegularinclusioninregularLBAobservagonssince• 69userexperiments• 1000hoursobserving(includingJune2016session)• Firstscience-1934-638-Tzioumiset.al.2010

– Higherresolugonlowfrequencyobservagonsreinterpretedevolugonchangesasopgcaldeptheffect

• Pulsarastrometry(MillerJoneset.al.)• PKSB1424−418

– Possibleradioassociagonwithextraterrestrialvery-high-energyneutrinosdiscoveredbyIceCube

–  Kadleretal.NaturePhysics2016

ASKAPVLBIScience


5

21

0.5

0.2

0.1

0.05

0.02

0.01

0.005

0

2011-11-13 2012-09-16

1 mas

2013-03-14

FluxDen

sity

[Jy/bea

m]

VLBIwithASKAP/VLBIforASKAP

• ASKAPwillenhancealllowfrequencyLBAobservagons– Benerresolugonmatchingforhigherfrequencyobservagons– Benefitsarangeofscience

• ASKAPSSPsneedVLBIforfollowup– LBAisonlySouthernHemisphereLBAarray– ASKAPhasuniquefrequencycoverage

• Callfor1GHzLBAobservagonswillincrease


PulsarAstrometry

• Accurateastrometrygiveparallaxandpropermogonindependentofgming– Goodtobreaksomedegeneraciesingmingmodel

– ImprovedGRtests•  IndependentparallaxmeasurementanchorselectrondensitydistribugonofMilkyWay– Improveddistanceesgmateforotherpulsars,Galacgcformagonmodels

• Calibratepulsar2Dvelocitydistribugon–kickphysics• Pulsargmingarrayshaveunderesgmatederrorsonparallaxandpm(Delleretal,2016,ApJsubmined)


EMU

• EMUisadeep(10μJy/beamrms)radiocongnuumsurveyof75%ofthesky• Willdetect70millionradiogalaxies• NeedVLBItodisgnguishbetweenAGNandStarforminggalaxy– IdeallyVLBIonallsources….

• HighqualityVLBIonasampleofsources,highresolugon• WidefieldVLBIof1000squaredegrees• SinglebaselinesurveyoflargefracgonofEMUsources

– Parkes-ATCA.WouldincludeASKAPifpracgcalbutresolveoutsomesources

• Want1GHzVLBI


ATLAS third data release 4025

Figure 3. The ATLAS CDFS total intensity mosaic with the linear grey-scale set to the range −100 to +300 µJy beam−1. The image projection used is NorthCelestial Pole (NCP; Greisen 1983), a projection on to a plane perpendicular to the NCP, which is a special case of the orthographic (SIN) projection, oftenused for east–west radio interferometers. The solid black contour indicates the component catalogue boundary (3.6 deg2) of the mosaic defined by: (1) localrms noise ≤100 µJy beam−1; (2) sensitivity loss due to bandwidth smearing <20 per cent; and (3) mosaicked primary beam response ≥40 per cent. The patternof pointings on the sky is identical to that shown in Norris et al. (2006).

3 C O R R E C T I O N O F FI NA L M O S A I C S

3.1 CLEAN bias

CLEAN bias (Condon et al. 1998) is an effect in deconvolution whichredistributes flux from point sources to noise peaks in the image,thereby resulting in a systematic underestimation of the flux densi-ties of real sources. As the amount of flux which is taken away froma real source is independent of its flux density, the fractional errorthis causes is largest for faint sources. The magnitude of the effectwill depend on the uv coverage and to what flux density level theimages are CLEANed.

In order to analyse the CLEAN bias on the flux density of sourcesin our ATLAS mosaics, we followed a similar procedure to thatoutlined in Middelberg et al. (2008). We added 132 point sources atrandom positions to the uv data of one ELAIS-S1 pointing, exceptthat a simulated source could not lie closer than 1 arcmin from areal source (>5σ ) and any other simulated source. The numbers ofsources added with different signal-to-noise ratios (SNRs) are listedin Table 2. We then imaged the data in the same way as the finalimage (see Section 2.3), extracted the flux density of each simulatedsource and divided by the injected flux density. This simulation wasrepeated 30 times to improve the statistical sampling.

The mean normalized flux density of the simulated sources asa function of SNR is shown in Fig. 5. The flux densities of the

simulated sources were measured by extracting the map values atthe simulated source positions, interpolated between pixels. Thiswas done by calculating the map values on a successively finer grid(up to 128 times finer), by repeated convolution with a Gaussian-graded sinc function (Rees 1990). At low SNR, the thermal noisewill introduce a slight shift in the position of a point source, resultingin an increased likelihood for the peak to lie on top of a positivenoise fluctuation. This, in turn, causes the peak flux density of apoint source to be biased slightly high; this is known as the peakflux density bias. We chose to measure the flux densities of thesimulated sources at their true positions, rather than measure theirpeak flux densities, to disentangle the peak flux density bias fromthe CLEAN bias. The peak flux density bias is dealt with separatelyby the BLOBCAT source finder in Section 4.2.

The black circles in Fig. 5 show the CLEAN bias when CLEANingthe images to 5σ , which typically corresponds to 5000 iterations(the final images were CLEANed to this level). The simulations in-dicate that a source with an SNR of 5 would have its flux densityunderestimated by less than 1 per cent as a result of the CLEAN bias,well within the calibration error of 5 per cent (see Section 4.6). Wetherefore chose not to correct the sources’ flux densities for thiseffect.

We repeated the simulations CLEANing the images to 2.5σ (thistypically corresponds to 65000 iterations) to see how the CLEAN biasis affected by the flux density level to which the images are CLEANed.

MNRAS 453, 4020–4036 (2015)

at University of Sydney on June 6, 2016

http://mnras.oxfordjournals.org/

Dow

nloaded from

FLASH

• AblindHIabsorpgon-linesurveyusingbackgroundradiocongnuumsourcestoidengfyandcharacterizeforegroundneutralhydrogen• WantVLBItointerpretresultsbasedonshapeofbackgroundradiosource• AlsoVLBIfollowupofactualHIdetecgons

– 700-1000MHz(needASKAP&ParkesPAF)– Esgmatesizeofabsorbingclouds– Figureoutwhereabsorpgoniscomingfrom– E.g.fastjet-drivenouslowsofneutralgasinnearbyradiogalaxies


Array (VLA), and three European VLBI Network(EVN) telescopes (Effelsberg, Westerbork, andOnsala). The central frequency, corresponding to thefrequencyof the redshiftedHI,was set at 1266MHz.Theseobservations expand on the results obtainedpreviously (12), whichwere limited by the width ofthe observing band. We have used a broader ob-serving band (16MHz, covering gaswith kinematicsdeviatingmore than 1500 km/s from the systemicvelocity) and a longer integration time (14 hours).

The angular resolution ranges from 12 × 4 mas[position angle (PA) of 12°] for the naturallyweighted images and cubes, to 8 × 5 mas (PA 2°)for the full-resolution images. They correspond,at the redshift of 4C12.50 z=0.1217, to 32× 11 pcand 21 × 13 pc, respectively. The noise level ob-tained is 0.65 and 0.90 mJy per beam per chan-nel, respectively, for the different weightings, afterHanning smoothing. A continuum image was alsoobtained using the line-free channels (Fig. 2).

We detected the two HI absorption featurespreviously seen in lower-resolution observations(8) and imaged them at parsec-scale resolution(Fig. 1). The outflowing component (~1000 km s−1

blueshifted from the systemic velocity) is seen atthe end of the distorted southern jet. This HIcomponent appears to consist of a compact cloud(unresolved even by our high-resolution images)and a diffuse, tail-like structure. The compact cloudis seen—in projection—to be cospatial with thehot spot observed in radio continuum (11). Thefaint and diffuse component extends at least~50 pc around and in front of the southern lobe(Fig. 1). This can be considered a lower limit tothe actual extent due to the sensitivity limitationsof the observations. Indeed, the profile from thenaturallyweighted image integrated over the south-ern lobe shows that we recover most of the HIabsorption detectedwith theWesterbork SynthesisRadio Telescope (WSRT) (Fig. 3). The deeper HIabsorption feature at the systemic velocity detectedearlier (12) is confirmed by our observations tocorrespond to a cloud north of the nucleus (Fig. 1).

We have estimated the column density and theHI mass associated with the two clouds. In thesouthern part, the unresolved cloud has a columndensity of NHI = 4.6 × 1021 cm−2 [assuming thespin temperature (Tspin) = 100 K]. The derivedmass of the cloud is MHI = 600 solar masses(M⊙); if we include the extended part, it reachesMHI =1.6 × 104 M⊙. The assumed value of Tspinlikely represents a lower limit because the actualspin temperature, under the extreme conditionsclose to the AGN, may be at least a factor of 10larger (6, 13). This implies that the mass of thecloud, as well, could be much higher. A recentstudy (9) suggests that the outflow carries anestimated cold H2 mass of at least 4.2 × 103 M⊙.

The larger HI cloud located in the northernpart of the radio source is known to have an ex-tremely high optical depth (t = 0.6) (12). This is

Fig. 1. The distribution of the HI in two velocity channels showing thelocation of the two clouds of HI detected in absorption (orange-white)superimposed on the continuum of 4C12.50 (contours). The integratedHI absorption profile is shown in the middle superimposed on the CO profile[taken from (9) with the HI from (8) and inverted for comparison]. The northern HI

cloud is at the systemic velocity (12) whereas the southern one is blueshifted~1000 km/s relative to the systemic velocity. The location of the southern un-resolved cloud producing the blueshifted HI absorption is cospatial (in projection) withthe bright radio hot spot. A diffuse HI component is also observed. [Credit: Dasyraand Combes (9); reproduced with permission The European Southern Observatory]

Fig. 2. Radio continuum images of 4C12.50 illustrating themorphology of the radio jet. (A) Thecontinuum image derived from our VLBI data using the line-free channels. The red dot indicates thelocation of the radio core. The contour levels are 0.0003 × (–1, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512,1024) Jy per beam with a peak brightness of 0.417 Jy per beam. (B) Continuum image taken from aprevious, higher–spatial resolution study (11). The bent structure of the jet and the terminal hot spot inthe southern lobe are clearly seen. This hot spot is characterized by an extremely high (~60%) frac-tional polarization (11). The two images are displayed on approximately the same scale. [Reproduced bypermission of the American Astronomical Society]

www.sciencemag.org SCIENCE VOL 341 6 SEPTEMBER 2013 1083

REPORTS

GASKAP

• HighspectralresolugonsurveyofHIandOHlinesintheMilkyWayandMagellanicSystems• VLBIfollowupofOHmasers:

– Structureanddynamicsofindividualsources– Astrometry–distanceandGalacgcdynamics– Zeemanpairpolarisagon– Needhighresolugontomatchpairsandovercomespagaldepolarisagon

• RequiredVLBIat1.6GHz


Transients

•  VASTandCRAFTinvesggatetheradioskylookingfortransientsatarangeofgmescales• Fasttransientsmustbesmall• NeedVLBItoresolve• VLBIcrucialforinterpregngmanyofthedetecgons• ESE(ExtremeScaneringEvents)– Imagewhileonprogress–infergeometryofthelens

• FRBs(FastRadioBursts)– Locagonofaterglowassociatedwithblackholecoreorelsewhere

• Gamma-rayburstsandsupernova– Measureexpansionrateanddiscriminatebetweendifferencesourcesfortheradioemission


POSSUM

• MeasuretheFaradayrotagonofthreemillionextragalacgcradiosources• Significantpopulagonofde-(orun-)polarizedcompactsteepspectrumsources,aswellasasmallerpopulagonofhigherpolarizagoncompactsources• VLBItoseeiftheyarestructurallydifferent,areembeddedindifferenttypesofmedia,etc– Includingpolarimetry– 1GHzLBAplusatahigherfrequency(with~matchedresolugon)togetroughspectralindicesfortheresolvedcomponents(tohelpdisgnguishbetweenCSOsandblazars).


SETI

• SETIwithASKAPwouldrequiregedarray– Untargetedsearchwouldrequireprohibigvecompute

• MulgplegedarraybeamswithdedicatedSETIbackendperbeam– Presumablywouldwantmanygedarraybeams


18.5kHzNyquistsampled

RFoF188elements

1MHzchannelsOversampled

DigitalReceiver

BeamForm

er

DigitalReceiver

BeamForm

er

Correlator

TiedArrayStrawman

• Developoversampledfinefilterbank– BeamformerèCorrelatorlinkatcapacity

• Addgedarrayfuncgonalityincorrelator– Maybelinearècircularconversion

• AddEthernetpackegsor• Purchase“large”switchandaddlotsofEthernetcables• DevelopGPUbasedsynthesis(inverse)filterbank

– GPUserverwith4GPUscanprobablyhandle4beams– Purchase1or2GPUserversinigally

• Feedbackforarraycalibragon


Conclusion

• ASKAPgedarrayfuncgonalitywouldbeamajorbenefitforarangeofscienceusingtheLBA• VLBIiscrigcalforthefollowupofalotofASKAPscience

– MuchofthisneedstobeatASKAPfrequencies• Thegedarrayprojectissgllbeingplanned


Acknowledgements

• Thanksforthecontribugonsfrom– AdamDeller– KeithBanister– TaraMurphy– ElaineSadler– JimiGreen– LawrenceRudnick– ShaneO'Sullivan– BryanGaensler– RayNorris– RonEkers


ChrisPhillipsLBALeadSciengstt +61291234567e [email protected] www.atnf.csiro.au

CSIROASTRONOMYANDSPACESCIENCE

Thankyou

Documents

VLBI - Australia Telescope National Facility...weightedimagesandcubes,to8×5mas(PA2 ) for the full-resolution images. They correspond, attheredshiftof4C12.50z=0.1217,to32×11pc and