Initial deep LOFAR observations of Epoch of Reionization windows: I. The North Celestial Pole

2.8 MB·English

by  S. Yatawatta

Astronomy&Astrophysicsmanuscriptno.ncp˙eor c ESO2013
(cid:13)
January14,2013
Initial deep LOFAR observations of Epoch of Reionization
windows: I. The North Celestial Pole
S.Yatawatta2,A.G.deBruyn1,2,M.A.Brentjens2,P.Labropoulos2,V.N.Pandey2,S.Kazemi1,S.Zaroubi1,
L.V.E.Koopmans1,A.R.Offringa1,33,V.Jelic´1,O.MartinezRubi1,V.Veligatla1,S.J.Wijnholds2,W.N.Brouw2,
G.Bernardi12,1,B.Ciardi3,S.Daiboo1,G.Harker4,G.Mellema5,J.Schaye6,R.Thomas1,H.Vedantham1,
E.Chapman27,F.B.Abdalla27,A.Alexov28,J.Anderson9,I.M.Avruch10,1,F.Batejat29,M.E.Bell11,16,M.R.Bell3,
M.Bentum2,P.Best13,A.Bonafede14,J.Bregman2,F.Breitling15,R.H.vandeBrink2,J.W.Broderick16,
M.Bru¨ggen17,14,J.Conway29,F.deGasperin17,E.deGeus2,S.Duscha2,H.Falcke20,R.A.Fallows2,C.Ferrari30,
3 W.Frieswijk2,M.A.Garrett2,6,J.M.Griessmeier21,2,A.W.Gunst2,T.E.Hassall16,22,J.W.T.Hessels2,8,M.Hoeft19,
1 M.Iacobelli6,E.Juette18,A.Karastergiou23,V.I.Kondratiev2,31,M.Kramer9,22,M.Kuniyoshi9,G.Kuper2,J.van
0
Leeuwen2,8,P.Maat2,G.Mann15,J.P.McKean2,M.Mevius2,1,J.D.Mol2,H.Munk2,R.Nijboer2,J.E.Noordam2,
2
M.J.Norden2,E.Orru2,20,H.Paas32,M.Pandey-Pommier24,6,R.Pizzo2,A.G.Polatidis2,W.Reich9,
n
H.J.A.Ro¨ttgering6,J.Sluman2,O.Smirnov25,B.Stappers22,M.Steinmetz15,M.Tagger21,Y.Tang2,C.Tasse7,S.ter
a
J Veen20,R.Vermeulen2,R.J.vanWeeren6,2,12,M.Wise2,O.Wucknitz26,9,andP.Zarka7

1
(Affiliationscanbefoundafterthereferences)
1
Received
]
M
I ABSTRACT
.
h
Aims. TheaimoftheLOFAREpochofReionization(EoR)projectistodetectthespectralfluctuationsoftheredshiftedHI21cmsignal.This
p
signalisweakerbyseveralordersofmagnitudethantheastrophysicalforegroundsignalsandhence,inordertoachievethis,verylongintegrations,
-
o accuratecalibrationforstationsandionosphereandreliableforegroundremovalareessential.
r Methods. OneoftheprospectiveobservingwindowsfortheLOFAREoRprojectwillbecenteredattheNorthCelestialPole(NCP).Wepresent
t resultsfromobservationsoftheNCPwindowusingtheLOFARhighband antenna(HBA)arrayinthefrequencyrange115MHzto163MHz.
s
a ThedatawereobtainedinApril2011duringthecommissioningphaseofLOFAR.Weusedbaselinesuptoabout30km.Thedatawasprocessed
[ usingadedicatedprocessingpipelinewhichisanenhancedversionofthestandardLOFARprocessingpipeline.
Results. Withabout3nights,of6hourseach,effectiveintegrationwehaveachievedanoiselevelofabout100µJy/PSFintheNCPwindow.
2 ClosetotheNCP,thenoiselevelincreasestoabout180µJy/PSF,mainlyduetoadditionalcontaminationfromunsubtractednearbysources.We
v
estimatethatinourbestnight,wehavereachedanoiselevelonlyafactorof1.4abovethethermallimitsetbythenoisefromourGalaxyandthe
0
receivers.OurcontinuumimagesareseveraltimesdeeperthanhavebeenachievedpreviouslyusingtheWSRTandGMRTarrays.Wederivean
3
analyticalexplanationfortheexcessnoisethatwebelievetobemainlyduetosourcesatlargeangularseparationfromtheNCP.Wepresentsome
6
detailsofthedataprocessingchallengesandhowwesolvedthem.
1
Conclusions. AlthoughmanyLOFARstationswere,atthetimeoftheobservations,inastillpoorlycalibratedstatewehaveseennoartefactsin
.
1 ourimageswhichwouldpreventusfromproducingdeeperimagesinmuchlongerintegrationsontheNCPwindowwhichareabouttocommence.
0 Thelimitationspresentinourcurrentresultsaremainlyduetosidelobenoisefromthelargenumberofdistantsources,aswellaserrorsrelatedto
3 stationbeamvariationsandrapidionosphericphasefluctuationsactingonbrightsources.Weareconfidentthatwecanimproveourresultswith
1 refinedprocessing.
:
v Keywords.Instrumentation:interferometers–Techniques:interferometric–Cosmology:observations,diffuseradiation,reionization
i
X
r
1. Introduction (Giant Metrewave Radio Telescope). While MWA (Murchison
a
WidefieldArray)andPAPER(PrecisionArraytoProbetheEoR)
AmajorepochinthehistoryoftheUniverseyettobeunderstood
are notin fullhardwaredeploymentyet, there are still relevant
indetailisitsDarkAgesandtheEpochofReionization(EoR).
results being produced.In Ordetal. (2010) and Williamsetal.
Observational evidence for this era can be gathered with high
(2012), initial widefield images of the southern sky using 32
probabilitybystudyingthefluctuationsoftheredshiftedneutral
MWA Tiles are presented. In Jacobsetal. (2011), full sky im-
hydrogenat redshiftscorrespondingto 6 < z < 12. Therefore,
agesandsourcecatalogsusingPAPERarepresented.
there are numerous experiments becoming operational and al-
readycollectingdata,especiallyinthefrequencyrangefrom115 InpreparingfortheLOFAREoRprojectwehaveconducted
MHzto240MHztoreachthisgoal. severalpilotexperimentswiththeLowFrequencyFrontendson
At the forefrontof such experimentsis the Low Frequency the WSRT in a relevant frequency range: 138-157 MHz. The
Array (LOFAR) (vanHaarlemetal. 2013). Similar EoR exper- results of these observations, and a discussion of their limita-
iments using other radio telescopes are already underway. For tions, have been described by Bernardietal. (2009, 2010). For
instance,Pacigaetal.(2011)provideanewlowerboundforthe LOFAR in its commissioning phase we have adopted a multi-
statisticaldetectionthresholdofHIfluctuationsusingtheGMRT faceted observing strategy, building on the experience gained
1
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
from the WSRT data. The rationale behind this is described in scribesthedataprocessingpipeline.Insection4,wepresentini-
moredetailindeBruynetal.(2013).Abriefsummaryfollows: tial results with deep very widefield images, new sources, and
UsingLOFARinitscommissioningphasewehaveobserved the noise behaviour. We give an analytical explanation for the
andprocessedthreeverydiversewindows.Onewindowcontains noise behaviorin section 5 that considersthe excess noise due
averybrightcompactsource,3C196,whichallowsexquisiteab- tosourcesfarawayfromthephasecenter.Finally,wedrawour
solute calibration as well as a study of the systematics at very conclusionsinsection6.
highspectralandimagedynamicrange.Atadeclinationofonly Notation(mostlyinsection5):Weuseboldlowercaseletters
48 degreesit will allow a study of elevation dependenteffects. for vectorsand bold uppercase letters for matrices. The matrix
Themanybrightcompactfieldsourcesaround3C196alsoallow Frobeniusnormisgivenby . .Thematrixtranspose,Hermitian
a study of ionospheric non-isoplanaticity. The results of these transposeandpseudoinversekkaregivenby(.)T,(.)H and(.) ,re-
†
observations,withemphasisonallthosetopics,willbedescribed spectively. The trace of a matrix is given by trace(.). The real
in a separate paper(Labropoulosetal. 2013). The second win- part of a complex number is denoted by Re(.). The statistical
dowwas chosento ascertainpossible damagingeffectsof faint expectationoperatorisdenotedbyE . .
{}
signalsduetoinstrumentalleakageofbrightpolarizedGalactic
foregroundsignals.Theseresults,ontheElaisN1window,will
bedescribedbyJelicetal.(2013)inthesecondpaperinthisse- 2. ObservationalSetup
ries.
In this section we provide details of the LOFAR stations used
Thethirdwindow,withoutbrightsourcesandwithrelatively
intheNCPobservations.Wealsoprovidethemotivationbehind
faintdiffuselinearlypolarizedemissionfromourGalaxy,isac-
observingtheNCP.
cessible throughout the year. This window is centered on the
NorthCelestialPole(NCP).Ithasbeenusedtoexperimentwith
various calibration approaches as well as conduct a thorough 2.1.LOFARstations
analysis of the noise levels attainable with the current, still in-
complete,LOFARarray.Theseresultsarethesubjectofthefirst We give a brief overview of LOFAR hardware and a com-
paperintheseries. plete overview can be found in vanHaarlemetal. (2013) and
The three windows described abovehave thus far been ob- deBruynetal. (2013). Each LOFAR HBA station consists of
served using a single digital beam with about 48 MHz band- multipleelements(dipoles)withdual,linearpolarizedreceivers.
width.Betweenthesethreewindowsweexpecttoaddressmost Foracorestation(CS),thereare384dipoles,arrangedin24tiles
of the issues that will affect much longer observations with thathavedipolesona4 4grid.Foraremotestation(RS),there
×
LOFAR, which should go one order of magnitude deeper in are768dipoles,arrangedin48tiles.Thesignalsofeachdipole
noise level. The analysis of the results obtained in these three inatilearecoherentlyadded(orbeamformed)toformanarrow
windows,astheypertaintoEoRsignallevels,willbediscussed fieldofview(FOV)alongagivendirectioninthesky.Theeffec-
inmoredetailinsubsequentpublications. tive beam shape is the productof the array (beamformer)gain
In this paper, we present results of LOFAR observations with the dipole beam shape. It should be noted that the dipole
pointed at the NCP in the frequency range 115 MHz to 163 beamshapeisstronglypolarized,andalongsomedirections in
MHz. TheNCP was previouslyobservedusing the Westerbork the sky the polarization could be as much as 20% of the total
SynthesisRadioTelescope(WSRT)inasimilarfrequencyrange, intensity.
albeitwith limited integrationtime and resolution.As reported The nominalLOFAR FOV at around150 MHz at the NCP
by Bernardietal. (2010), the WSRT observations are mainly (fromnulltonull)isabout11degreesindiameterforaCSand
limited by broadband (and low level) radio frequency interfer- about 8 degrees in diameter for a RS. Therefore, the effective
ence(RFI),andclassicalconfusion(duetohavinglimited<3km FOV is about 10 degrees in diameter. There is also a compli-
longest baseline) that prevents reaching the theoretical noise cated low level sidelobe pattern surrounding the FOV, and the
level. sidelobes change with time and frequency, as the beamformer
LOFAR provides significant challenges as well as advan- weights change. In order to minimize the cumulative effect of
tagesoverconventionallowfrequencyradiotelescopesinterms the sidelobes, each LOFAR station is givena differentrotation
of calibration. Unlike the WSRT, antennas on the ground are initsdipolelayout(butkeepingthedipolesparallel).Duetothis
muchlesssusceptibletobroadbandRFI.Ontheotherhand,cal- reason,eachLOFARstationwillhaveauniquebeamshape,that
ibration of a LOFAR observation is challenging due to many varies in time, frequency as well as according to the direction
reasonsincludingspatiallyandtemporallyvaryingbeamshapes being pointed at. For a widefield image, this naturally leads to
with wide fields of view as well as mild to severe ionospheric beam variations that depend on time, frequency as well as the
distortions. Therefore,it is paramountthat we test and demon- directioninthesky.
strate the feasibility of LOFAR for EoR observations before
startingthelong,dedicatedobservingcampaign.
2.2.TheNCPanditssurroundings
The results reported in this paperare based on integrations
oftheNCPwhichconsistedof3nightswith6hourseachnight- ItisofsignificantimportancethattheNCP FOVliesonarela-
timeobserving.Weprovidedetailsofthecalibrationandimag- tivelycold(i.e.,havinglowskytemperature)spotintheGalactic
ing that lead us to almost reach the expected theoretical noise haloinordertominimizetheeffectsofGalacticforegrounds.In
level(withinafactorof1.4).Wealsoprovidedetailsofcurrent Fig. 1, we show the NCP window (orFOV), which is overlaid
limitationsandwhatweexpectwiththecurrent,stillincomplete onafullskyimageobservedat50MHz.Thefullskyimagewas
LOFAR. Based on this result, we see no show stoppers forthe made using an array of 16 LOFAR lowband dipoles, with the
launchof the dedicatedLOFAR EoR observationsonthiswin- longestbaselineof450m.
dowwhichwilllastseveralhundredhours. The Galactic plane lies along an arc joining Cassiopeia A
This paper is organized as follows: In section 2, we give (CasA)andCygnusA(CygA)inFig.1butitisresolved.CasAis
anextensiveoverviewoftheobservationalsetup.Section3 de- 32degreesawayfromthepolewhileCygAisabout40degrees
2
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
tothebeamgainalongthedirectionoftheNCP(whichisunity)
andconstantintime.However,theelement(dipole)gainvaries
andthisistakenintoaccountduringcalibration.
Fig.1.NCP FOV overlaidona fullsky imageat50 MHz.The
fullskyimageismadeusingtheLOFARlowbanddipoles.
away. The closest 3C source to the pole is 3C61.1 which is 4
degreesaway,withatotalfluxabout35Jy(peakabout7Jy)at
150MHz.Itisfullyresolvedandanimagemadefromaprevious
LOFAR observationis showninFig.2.Thereareseveralother Fig.3. Radio spectrum of NVSS J011732+892848 compiled
bright3Csourcesinthevicinityincluding3C390.3(11degrees
from data in the literature (Rengelinketal. 1997; Cohenetal.
away). 2007).
2.3.MotivationforobservingtheNCP
The NCP is one of several observationalwindows for LOFAR
EoR observations (deBruynetal. 2013). The reasons behind
choosingtheNCParenumerousalthoughthisdoesnotimplyit
tobetheoptimalchoice.Welistsomeofthepositiveandsome
ofthepotentiallynegativeaspectsofthischoice:
+ The geographicallocation of LOFAR makesthe NCP win-
dow observable at night time, throughout the year, at high
elevation(53degrees).
+ Dueto minimumprojectioneffectsof theuv tracks, we get
Fig.2. 3C61.1 model image at 150 MHz. The two hot
almostcircularuvcoverageandtherefore,wegetanalmost
spots in this source, a giant double radio galaxy at z=0.188
circularpointspreadfunction(PSF).
(Lawrenceetal. 1996), have peak flux densities of about 7 Jy
The strongest source in the FOV, 3C61.1 is attenuated by
inan8 PSF.ThecolourbarunitsareinJy/PSF. ±
′′ about 70% and the strongest (apparent) source is about 5
Jy in peakflux.Nothavinga strongsourcein theFOV has
bothadvantagesanddisadvantages.First,nothavingastrong
Duetothestationbeams,manyofthestrongsourcesoutside source means a not so high signal to noise ratio (SNR) in
themainbeamaresuppressedsignificantlyandthebrightest(ap- calibration.However,there are less artefactsresulting from
parent)sourceintheFOVisNVSSJ011732+892848,about30 deconvolutionresidualsofstrongsources.
′
away from the pole. This source is unresolved in our observa- GeostationaryRFIthatmanagestoescapeflaggingroutines,
±
tionsandhasapeakfluxof5.4Jyat352MHz(Rengelinketal. which work on high-noise samples, may end up near the
1997) and a peakflux of 5.3Jy at 74MHz (Cohenetal. 2007) North Celestial Pole. Nonetheless, this providesa sensitive
as reported in the WENSS and VLSS surveys, respectively. diagnosticofthepresenceofanyfaint,stationary,undetected
Therefore,weassumethissourcetohaveaflatspectrumwithin RFI.
theobservingbandasinFig.3,whichmakesitasuitablecandi- - Finally,theNCP islocatedataGalacticlatitudeofonly38
dateforabsolutefluxcalibrationandnoiseestimation.Because degrees.Theoverallsystemnoiseisthereforehigherthanthe
itisveryclosetothepole,thenominalstationbeamgainisequal coldestregionsneartheNorthGalacticPole.
3
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
2.4.Observationalparameters 256channelsat2sintegration)isflaggedusingtheaoflagger
(Offringaetal. 2010; Offringaetal. 2012) and averaged to 15
We used40 corestationsand7 remotestationsin ourobserva-
channels(afterremovingthe8channelsatbothsubbandedges,
tions. The shortest baseline is 60 m and the longest baseline is
mainly to remove edge effects from the polyphase filter). This
about30km.Theobservingfrequencyrangeisfrom115MHz
significantlyreducesthesizeofthedatathathastobeprocessed
to 163 MHz. There are about 240 subbands in each observa-
in the following stages. However, with improvements in soft-
tion, within this observing frequency range. Each subband has
ware, we intendto processdata ata finerfrequencyresolution,
256channels,coveringabandwidthof195kHz.Themonochro-
onceregularEoRobservinghasbegun.
maticuvcoverageforatypical6hourNCPobservationisshown
inFig.4.
3.2.Skymodel
Because there is no single bright source in the FOV, the sky
modelusedforinitialcalibrationoftheNCPdatacontainsabout
threehundreddiscretesources,spreadacrosstheFOVofabout
10 10squaredegrees.The mostcomplexsourcein this region
×
is 3C61.1as shown in Fig. 2. In orderto efficiently modelthis
source,weuseamodelincludingshapelets(Yatawatta2011)and
pointsources. Therest of the skymodelis modeledas a set of
discretesources,havingmultiplepointsourcecomponents.The
brightestsource(NVSSJ011732+892848)ismodeledasasin-
glepointsourcewithaflatspectrumandapeakfluxof5.3Jy.We
liketoemphasizeherethatwedivergefromthetraditional’clean
component’basedskymodelconstructioninordertominimize
the number of components used without the loss of accuracy
(Yatawatta2010).Inordertoautomatethisprocess,wehavede-
veloped custom software (buildsky)that creates a sky model
withtheminimumnumberofsourcecomponentsrequired.The
principlebehindbuildskyistoselectthesimplestmodelfora
givensourcebychoosingthecorrectnumberofdegreesoffree-
dom(Yatawatta2011).Whileapointsourcehasonlyonedegree
of freedom (for its shape), a double source has two and so on.
Thereare additionaldegreesoffreedomduetoitspositionand
Fig.4.6hourmonochromaticuvcoverageattheNCP,using40
flux.WeuseinformationtheoreticcriteriaasgiveninYatawatta
corestationsand7remotestations.Thelongestbaselineisabout
(2011)toselecttheoptimumnumberofdegreesoffreedomfor
30 km.The two (redand blue)coloursshow the symmetricuv
any given source. All the sources in the sky model are unpo-
pointsobtainedbyconjugationofthedata.
larized. The sky model was updated using two calibration and
imagingcycles.
With this uv coverage, we get a resolution of about 12 at
′′ 3.3.Calibration
150MHz.Forcomparison,inpreviousNCPobservationsusing
theWSRT(Bernardietal.2010),thelongestbaselineusedwas TheaimofcalibrationofLOFAREoRobservationsistwofold:
only 2.7km yielding an angular resolution of only 120′′. The (i)correctionforinstrumentalandionosphericerrorsinthedata
correlatorintegrationtimeissetat2s.Weuseddatatakenon3 (ii)removalofstrongforegroundsourcesfromthedatasuchthat
differentnightsfortheresultspresentedinthispaper.InTable1, specialized foreground removal algorithms (e.g. Harkeret al.
wesummarizetheobservationalparametersforthese3different (2009))canbeapplied.ThebasicdescriptionoftheLOFAREoR
nights. datamodelusedincalibrationis givenbyLabropoulos(2010).
WeuseanenhancedversionoftheLOFARcalibrationpipeline
(Pandeyetal.2009)fortheEoRdatacalibration.
3. DataReduction
Inthissection,wedescribethemajorstepstakentocalibratethe
3.3.1. Datacorrection
NCP observations. Apart from the initial processing, the data
wascompletelyprocessedinaCPU/GPU1 clusterdedicatedfor Majorstepsinourcalibrationpipelineareasfollows:
LOFAR EoR computingneeds. The processing of these obser-
vationsalsoenabledustofinetunethesoftwareusedinvarious 1. We first calibrate for clock errors as well as small time
processingsteps. scale ionospheric errors along the center of the FOV. This
isthesocalleduvplaneordirectionindependentcalibration
(Labropoulos2010)andisperformedusingtheBlackBoard
3.1.Initialprocessing Selfcal (BBS) package (Pandeyetal. 2009). At this stage,
eachsubbandhas15channelsat2sintegrationtime.Wede-
The LOFAR correlator (Romeinetal. 2010) outputs data at a
terminethecalibrationsolutionsforevery10s,andonesolu-
veryfineresolution(2sand0.78kHz),mainlytofacilitateRFI
tionpersubband.Sincewedonothaveadominantsourceat
mitigation.However,thedatavolumemakesitcumbersomefor
thecenteroftheFOV,thesolutionsthusobtainedcorrespond
furtherprocessing.Therefore,thedataofeachsubband(having
to small time scale ionosphericphase fluctuationscommon
1 CPU:CentralProcessingUnit,GPU:GraphicsProcessingUnit tothefullFOVplustheclockerrors.
4
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
ObservationNo. StartTime(UTC) No.Stations No.Subbands Noise(µJyPSF 1)
−
(duration6hours) (deliveringgooddata) (processed)
L24560 27-March-201120:00:05 45 229 125
L25085 10-April-201120:00:05 43 185 255
L26773 19-May-201119:30:00 41 187 224
Table1.Summaryofobservationalparameters(andnoiselevelachievedwithuniformlyweightedimages,attheedgeoftheFOV)
forthe3nightsofdatataken.Thevariabilityofthenoiseisduetovariabilityinthesensitivityofthestations,someofwhichwere
lessfocusedduetobeamformingerrorsatthetimeoftheobservations.
2. Once uv plane calibration is done, the data is corrected for
theseerrors.Thedataisalsocorrectedfortheelementbeam
gain along the center of the FOV. As discussed previously,
thedipolebeamofLOFARisstronglypolarizedandweuse
anelementbeammodelbasedonnumericalsimulations.For
anareaabout10degreesindiameterinthesky,thevariation
ofthedipolebeamshapeisassumedtobesmallandcorrec-
tionforthecenteroftheFOVisconsideredaccurateenough
forthefullFOV.
3. Thecorrecteddataisaveragedto183kHz(onechannelper
subband)and10sintegrationtime.Thedataisalsoflagged
byclippinganyspikespresentinthedataaftercorrection.
3.3.2. Sourcesubtraction
Fig.5.Clusteringoftwo sources5 apart.Theleftpanelshows
LOFAR has a very wide field of view and along each direc- ′
tion, the errors present in the data are different due to vary- the two (point) sources with identical error patterns. The right
ing beam shape and ionospheric effects (Koopmans 2010). panelshowstheimagemadeafterdeterminingacommonerror
(at an interval of 20 minutes) for both sources and subtracting
Therefore, source subtraction is not a simple deconvolution
theircontributionfromthedata.Theskymodelhasbeenrestored
problem for LOFAR observations.Even for a simple deconvo-
intherightpanel.Thecolourbar(bottom)unitsareinJy/PSF.
lution,itisbettertosubtractthe sourcesdirectlyfromthe visi-
bilities(Yatawatta2010).InthecaseofLOFAR,thissubtraction
hastobedonewiththeappropriategaincorrectionsalongeach
direction.
3.4.Imaging
In orderto efficientlyandaccuratelysolvethe multi-source
We make images at differentstages during calibration. All im-
calibration problem, we have developed algorithms and soft-
agesaremadeusingCASA2.Inordertoupdatetheskymodel,
warebasedonExpectationMaximization(Yatawattaetal.2009;
we make images of the calibrated and source subtracted data.
Kazemietal. 2011b; Yatawattaetal. 2012). We have imple-
We keep the highest available resolution in order to create ac-
mented these algorithms (SAGECal) with accelerated process-
curatesourcemodels.Fortheresultspresentedinthispaper,we
ing using graphics processing units (GPUs). In the NCP win-
havearesolutionofabout12 andwechooseapixelsizeof4
dow, there are about 500 bright discrete sources (note that we ′′ ′′
withuniformweighting.EventhoughthenominalFOVisabout
subtract more sources than what we use for the uv plane cali-
10degrees,we makeimagesthathaveanFOV ofabout13de-
bration, for which we only use about 300) that are subtracted
grees,todetectsourcesattheedgeofthebeam.We restorethe
fromcalibratedvisibilitieswiththecorrectdirectionalgains.We
subtractedsourcesontotheseimages,afterconvolvingwiththe
have ’clustered’(Kazemietal. 2011a) these sources into about
nominal(Gaussian)PSF.Afterwards,weuseDuchamp(Whiting
150differentdirections.Thusweeffectivelydeterminetheerrors
2012)toselectareaswithpositivefluxandupdatetheskymodel
along150directionsduringthesourcesubtraction.Anexample
usingbuildsky.
of clustering is shown in Fig. 5. In the left panel of Fig. 5, we
MorerelevantforEoR signaldetection are imagesmadeat
showtwosourcesthatareabout5 apartandapparentlyhaving
′ low resolution using the short baselines of LOFAR. Therefore,
identical error patterns. Therefore, instead of calibrating along
wealsomakeimagesusingbaselineslessthan1200wavelengths
eachsourceindividually,wecanclusterthemintoonecomplex
at35 pixelsize.Theimagesizeischosentobeabout65degrees
sourceanddeterminethecommonerrors.TherightpanelinFig. ′′
so that we can see any contributions from the Galactic plane,
5showstheresultaftersubtractingtheclusterandrestoringthe
whichisabout30degreesawayfromtheNCP.
model.
There are still some errors remaining in the right panel of
4. Results
Fig. 5 mainly because of errors in the sky model and due to
theeffectofsurroundingsources(thatwerenotincludedinthe
Inthissection,wepresentresultsmainlytohighlightimportant
sky model) and also due to short time scale ionospheric er-
stages in the calibration and finally, to present the noise limits
rors.Itshouldalsobementionedthatwhilemostofthesources
that we have reached using LOFAR. The results are based on
subtracted lie within the FOV, we have also subtracted strong
all threedatasets givenin Table 1, unlessstated otherwise, and
sourcesfarawayfromtheNCPasshowninFig.1,forexample
CasA. 2 CommonAstronomySoftwareApplications,http://casa.nrao.edu
5
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
continuumimagesaremadeusinginversevarianceweightedav- ously,therearethreemajorreasonsfortheseartefacts:(i)vary-
eragingofthe240subbandimages. ing LOFAR beam shapes which are different for each station,
(ii) ionospheric phase errors, and (iii) classical deconvolution
errorsduetohavingpartiallyresolvedsources.Forinstancefor
4.1.TheperformanceofSAGECalindirectionalcalibration
thecaseof3C61.1,allthreeoftheaforementionedcausescreate
artefacts,whichareclearlyvisibleclosetothebottomlefthand
The effects due to beam shapes and the ionosphere are major
corner.
causes of errors in LOFAR images. Therefore, directional cal-
ibration is essential. We present a few images to highlight the The only way to improve the image in Fig. 8 is multi-
performanceofSAGECal. directional calibration as described in section 3.3.2. We have
showntheimageobtainedafterrunningSAGECalinFig.9.The
In Fig. 6, we present the area around the brightest source
circle indicatesan area of diameter10 degrees.Comparison of
NVSSJ011732+892848.Theimageontheleftisbeforemulti-
Figs.8and9showsthatmostsignificantartefactsinFig.8have
directionalcalibrationandsourcesubtraction,with only a deep
been eliminated in Fig. 9. The prominentartefacts that still re-
CLEAN based deconvolution.The imageon the rightin Fig. 6
mainareduetothefactthatCSandRSbeamshapeshavediffer-
isafterrunningSAGECalandafterrestoringtheskymodelonto
entFOVsandalsoduetofrequencysmearing.We nowreacha
theresidualimage.Itisclearthattheerrorsduetobeamvaria-
noiselevelofabout100µJy/PSFattheoutskirtsofFig.9,while
tionsandionosphericvariationshavelargelybeeneliminatedin
thepeakvalueintheimageisabout5.3Jy.Thiscorrespondsto
the right panel of Fig. 6. Some errors are still present, mainly
aformaldynamicrangeof50,000:1.
caused by inaccurate source models used in multi-directional
calibration. We emphasize that longer baselines are needed to InFig.10,wegivetheimagemadeonlywiththeshortbase-
constructaccuratemodelsforsuchcomplexsources. lines (< 1200 wavelengths) using the same data of Fig. 9. In
this image, the circle shows an area with 10 degreesin diame-
InFig.7,wegiveanotherexamplefortheeffectofthetime
ter and the density of the sources close to this circle is clearly
intervalchoseninSAGECalformulti-directionalcalibration.The
less than in other areas of the image. We also see a significant
imageontheleftiswithoutanymulti-directionalcalibrationand
number of sources away from the FOV that are seen through
significanterrorsduetobeamshapeandionospherearevisible.
sidelobesofthe beam.Mostofthese sourceshavenotbeenin-
TheimageinthemiddleofFig.7showstheimageobtainedafter
cludedinourmulti-directionalcalibrationandhence,theyhave
running SAGECal with directional calibration performed every
significantartefacts. The noise level in this image is about 300
20 minutes. The beam variations, which are slower, are com-
µJy/PSFwiththepeakfluxofabout5.3Jy.Note,however,that
pletely eliminated by this procedure.However,the ionospheric
thenoiselevelisastrongfunctionofthedistancefromthefield
variations,thatcouldhavetimescalesmuchlessthan20minutes
centre.
arestillpresent.
The right panel of Fig. 7 shows the result after running
SAGECalwitha’hybrid’solutionscheme.Inthiscase,wesolve 4.3.Newsources
forbrightsourcesonceevery5 minutesand forfaintersources
onceevery20minutes.Mostoftheionosphericerrorspresentin Sincewereachanoiselimitofabout100µJy/PSF,wedetecta
themiddlepanelhavebeenremovedinthisfigure.Therearestill largenumberofsourcesthathavenotbeendetectedinprevious
errorsduetoinaccuratesourcemodels(thesourcewasassumed observations,evenathigherfrequencies.InFig.11,wepresent
tobeaperfectpointsource,butthisisnotaccurateenough)and asmallarea(0.6 1.0degrees)ofFig.9tocomparetoanimage
×
alsoduetoionosphericphasevariationswithatimescalesmaller fromWENSS.
than5minutes. WepresentanareaclosetotheNCPinFig.12.Theleftpanel
Thetimeintervalof20minuteschosenforobtainingtheso- inFig.12showsanimagemadewithallbaselineswhichgives
lutions gives us 120 time samples (each sample is of 10 s du- aPSFof12 .Therightpanelin Fig.12showsanimagemade
′′
ration). For each time sample, we have 990 baselines with 45 usingonlythecorebaselinesandhasaPSFofabout150 . An
′′
stations. Therefore, we have about 8 120 990=1 million real importantlessonthatcanbedrawnfromFig.12isthetotalab-
× ×
constraintstoobtaina solution.The numberof realparameters senceofartefactsclosetothepole.Iftherewouldbeanyresidual
inasolutionis45 150 8=54000for150directionsinthesky. geostationaryRFI,theireffectswouldaccumulatenearthepole.
× ×
Therefore,the ratio between the numberof constraintsand the However,weseenounexplainedartefacts.
numberofparametersisabout18whichismorethansufficient
toobtainareliablesolution.Thiscanbefurtherimprovedbyus-
ingdatapointsatdifferentfrequenciesasproposedbyBregman 4.4.Effectsofbrightsourcesatlargeangulardistances
(2012).
AsshowninFig.1,thereareafewbrightsourcesintheneigh-
borhoodoftheNCP.Wehavealreadymentioned3C61.1which
is still well inside the FOV. The other source that has a signif-
4.2.Widefieldimages
icant effect is CasA, which is about 30 degrees away from the
We presentwidefieldimagesobtainedforthefulldatasetgiven NCP.InFig.13,wepresenttheimagesaroundCasA,madewith
in Table 1. First, in Fig. 8, we present the image obtained af- only the core station baselines. The images with baselines us-
ter calibration as described in section 3.3.1, but before run- ing core stations only are more affected by CasA than images
ning SAGECal. Therefore, no source subtraction is performed thatincluderemotestations.Thereareatleastfourreasonsthat
andonly traditionalCLEAN based deconvolutionhasbeenap- contributeto this. First, core stations have wider station beams
plied. The circle indicates an area of diameter10 degrees.The (comparedto a remote stations) and therefore,CasA is less at-
peak flux of this image is about 5.3 Jy and the noise level is tenuatedoncore-corebaselines.Secondly,thecorestationshave
about 400µJy/PSF. The complex source 3C61.1 is at the bot- more shortbaselinesthan remote stations, hencesee more flux
tomlefthandcorner.Thestrikingfeaturesinthisimageare the from CasA, which is heavily resolved at baselines longer than
artefacts surroundingalmost every source. As described previ- 1000λ. Thirdly, time and frequency smearing lead to a signif-
6
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
Fig.6. An area (about 0.5 deg 0.5 deg) close to the NCP before and after multi-directional calibration using SAGECal. The
×
imageontheleftisbeforerunningSAGECalandafteradeepCLEANdeconvolutionanderrorsduetobeamvariationand(some)
ionosphericvariationsareclearlyvisible.Ontherighthandimage,thesourcesaresubtracteddirectlyfromthevisibilitydataand
the sky modelis restored onto the residual image. Most of these errors visible in the left hand image are eliminated in the right
handimageasCLEANbaseddeconvolutionfailstoconsiderthedirectionalerrorsintoaccount.Thepeakfluxis5Jy/PSFandthe
colourbarunitsareinJy/PSF.
Fig.7.TheperformanceofSAGECalwithdifferentsolutionintervals.Theimageontheleftiswithoutmulti-directionalcalibration.
Theimagein themiddleisafterrunningSAGECalwitha solutionintervalof20minutes.Theimageontherightisafterrunning
SAGECalwithahybridsolutioninterval,wheresolutionsareobtainedalongbrightsourceclustersatevery5minutesandforfainter
sourceclusters,every20minutes.Itisclearthatthesmallscaleionosphericerrorspresentinthemiddlefigurearemostlyeliminated
intherightpanel.However,ionosphericvariationsduetodecorrelationeffectswithinthe5minuteintervalarestillpresentonthe
rightpanel.Thecolourbar(bottom)unitsareinJy/PSF.
7
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
Fig.8.The NCP image aftercalibration,butbeforerunning SAGECal,whichhasalso been deconvolvedusingCASA. Thecircle
indicatesanareaofdiameter10degrees.Thesource3C61.1isatthebottomlefthandcorner.Theimagehas7200 7200pixelsof
×
size4 withaPSFof12 andthenoiselevelatthisstageisstillabout400µJy/PSF.ThecolourbarunitsareinJy/PSF.
′′ ′′
icant attenuation of the visibilities of distant sources. Fourthly, IntherightpanelofFig.13,wehaveincludedCasA inour
ionospheric effects such as non-isoplanaticity rapidly increase multi-directionalcalibrationusingSAGECal.Mostoftheripples
withthelengthoftheinterferometerbaseline. in the left panel are eliminated in the right panel of Fig. 13.
Furthermore,thenoiselevelisreducedbyafewpercentafterin-
InFig.13leftpanel,weshowanimagewherewehaverun cludingCasAinthecalibration.Therearestillsomeerrorsclose
SAGECalwhileignoringtheeffectofCasA.Inotherwords,we tothelocationofCasA.ThisismainlyduetoerrorsintheCasA
didnotincludeCasAinourskymodelandneitherdidwesolve sourcemodelusedinthesubtractionandweexpecttogetbetter
along the direction of CasA. CasA is at the bottom of this im- resultswithanupdatedsourcemodel(Yatawattaetal.2012).
ageandisheavilydistortedduetobeamandionosphericerrors
aswellastimeandfrequencysmearing.Theringlikestructures
centeredatCasAandspreadingthroughouttheimageisclearly
visible. However,thereis an area shapedlike a conethatis di-
rectedtowardstheNCP wherethereare noripples.Thisisdue
to the fact that multi-directional calibration that ignores CasA Because CasA is a strong source, we clearly see its effect
(mainly close to the NCP) has absorbed the effect of CasA di- aswehavejustdescribed.Conversely,evenfaintsourceswould
rectedtowardstheNCP.Similareffectscanbeseeninsequential have a similar effect, albeit at a low magnitude. We perform a
sourcesubtractionschemessuchas’peeling’. statisticalanalysisofthiseffectduetofaintsourcesinsection5.
8
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
Fig.9. The NCP image after multi-directionalcalibration and source subtraction using SAGECal. After a shallow deconvolution
using CASA (mainly to estimate the PSF), the skymodelis restored onto the image. The circle indicates an area of diameter 10
degrees.Theimagehas12000 12000pixelsofsize4 withaPSFof12 andthenoiselevelisabout100µJy/PSF.Duetothefact
′′ ′′
×
thatRSandCSbeamshapeshavedifferentFOVsthesourcesattheedgeoftheimagearedistorted. Inaddition,duetofrequency
smearing,thesourcesattheedgeoftheimageappear’attracted’towardsthecenter.ThecolourbarunitsareinJy/PSF.
9
S.Yatawattaetal.:InitialdeepLOFARobservationsofEoRwindows
Fig.10.TheNCP imageaftermulti-directionalcalibrationandsourcesubtractionusing SAGECal,usingonlythe shortbaselines.
After a shallow deconvolution using CASA (mainly to estimate the PSF), the sky model is restored onto the image. The circle
indicatesanareaofdiameter10degrees.Theimagehas2000 2000pixelsofsize35 withaPSFof150 andthenoiseisabout
′′ ′′
×
300µJy/PSF.ThecolourbarunitsareinJy/PSF.
10