Multi-Frequency VLBI Observations of the Gravitational Lens B2016+112

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by  A. More

Multi-Frequency VLBI Observations of the
Gravitational Lens B2016+112
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J AnupreetaMore∗

1 Max-Planck-InstitutfuerRadioastronomie

E-mail: [email protected]
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RichardPorcas
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2 Max-Planck-InstitutfuerRadioastronomie
0 E-mail: [email protected]
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7 We present Global VLBI and HSA images of the gravitational lens B2016+112 at 18, 6 and
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/ 3.6cm. PreviousVLBIobservationsshowedthatimagesAandB(whichareclearlylensedim-
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agesofasinglebackgroundsource)andtheelongatedregionCareeachdividedintocomponents.
p
- Ournewhigh-resolutionmapsrevealmorecomponentsinimagesAandB, clearlydemonstrat-
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r ing their expected opposite parities. According to the scenario of Koopmanset al. (2002), the
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s arc-likeregionCconsistsoftwomerging,partialimages(“C1-C2”)ofjustasmallregionofthe
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: samebackgroundsource,seenwithhighlensmagnification.Wehavedeterminedthespectraand
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i relativepositionsofthecomponentswithinallfourimagesinordertotestthisscenario. Wefind
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thattheouternorth-westcomponentsinimagesAandBdoindeedhaveradiospectrasimilarto
r
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thecomponentsseeninC1andC2.
The8thEuropeanVLBINetworkSymposiumonNewDevelopmentsinVLBIScienceandTechnologyand
EVNUsersMeeting
September26-292006
Torun,Poland
∗Speaker.
(cid:13)cCopyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
Multi-FrequencyVLBIObservationsoftheGravitationalLensB2016+112 AnupreetaMore
1. Introduction
The gravitational lens system B2016+112, discovered in 1981 in the MIT-Green Bank 6-cm
survey, showsthree radio components A,Band C(see Figure1left). Theoverall integrated spec-
trumofthissystemisthatofaGigahertzPeakedSpectrumsource. ComponentsAandB,separated
by 3.4 arcseconds, are associated with the lensed images of an AGN at redshift z=3.273 ([1] [2]
[3]; see Figure 1, centre). The radio and optical positions of A and B are coincident within the
measurement errors and they also have similar spectra in both radio and optical, as expected for
lensedimages. Koopmansetal([4])describe theopticalspectrum asthatofatype-IIquasar.
Component C, which is the brightest radio source in the field, lies 2 arcseconds away to the
south-eastofBandisextendedintheE-Wdirection. Itisassociatedwithadiffuseextendedobject
intheinfrared. Agiantelliptical galaxy Datz=1.01([2]),lyingclosetothecentroid ofA,Band
C,observedinK andI bands([5]),actsastheprimarylens(seeFigure1,right). Becausetheradio
spectrum of C is flatter than those of A and B, and there is no associated optical nucleus, it is not
clearwhetherthisisalsolensedemissionfromthesamebackground source.
2.0 A
1.5
1.0
0.5 B
ARC SEC0.0
-0.5
-1.0
C
-1.5
-2.0
2 1 ARC0 SEC -1 -2
Figure 1: Left: MERLIN map of B2016+112 at 6 cm showing A, B and C; C is resolved into two sub-
components. Centre: HST V-band image showing A and B lensed images of the optical nucleus. Right:
HSTH-bandimageshowingthelensinggalaxyD(z=1.01)inadditiontoA,BandC.HSTimagestaken
fromtheCASTLESwebsite.
Images A and B are each further resolved into two components as shown in 18 cm-VLBI
observations ([6]) and C is resolved into four components from 6 cm-MERLIN and 18 cm-EVN
observations ([7],[8]). TheseshowthattheoutertwocomponentsinregionChavesteeperspectra
thantheinnertwocomponents.
The lens model of this system by Koopmans et al. ([4]) consists of a single screen which
containstheprimarylensD,twosingularisothermalsphere(SIS)massdistributionsandanexternal
shear. In this model B2016+112 is interpreted as a quadruply imaged lens system, in which the
diamond caustic crosses the radio source such that just part of the “counter-jet”, host galaxy and
narrow line regions are imaged at A, B and C (where C consists of two partial images, C1 and
C2). Thecomplete source, including all radio components and theoptical nucleus, isonly doubly
imaged(AandB).Thismodelpredicts unusually highmagnification (∼300)inregionC.Support
forthisinterpretation isprovided byspectral observations showingthatChasfeatures atthesame
redshift as A and B. In this work, we analyze the spectra of all the radio components in order to
testKoopmans’scenario.
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Multi-FrequencyVLBIObservationsoftheGravitationalLensB2016+112 AnupreetaMore
2. Observations andData Reduction
Multi-frequencyGlobalVLBIandHSAobservationsweremadeofB2016+112(seeTable1).
B2029+121 was used for phase calibration and B2134+004 as a fringe-finder. The data were
processed attheVLBAcorrelator, producing 16frequency samplesperbaseband channel.
Date Wavelength Antennas Obs. time Integ. BBchans. Polarization
(hours) time(sec) perpolsn.
25Feb02 18cm Eb,Jb,Mc,On,Tr 17 2 4x8MHz LCP-only
VLBA,Y,Ro,Go
17Nov01 6cm Eb,Jb,Mc,On,Wb 17 1 2x8MHz LCP+RCP
VLBA,Ar
30Apr06 3.6cm VLBA,Eb,Gb,Ar 7 1 4x8MHz LCP+RCP
Table1: Observationaldetails
Furtherprocessing ofthedatasetswascarriedoutwiththeNRAOpackage. Noaveraging
of the data in either time or frequency was made, in order to reduce the effects of bandwidth
and fringerate smearing when imaging this wide field lens system. First, the a priori amplitude
calibration derived from system temperature measurements and antenna gains was applied to the
data. Then the instrumental phase, delay and rate solutions determined from fringe-fitting the
B2029+121 data were applied to make a map of B2016+112. Three sub-fields centered on A, B
and C were imaged. This phase referenced map was then used as an initial model for phase self-
calibration ofthelensdata. Severalrunsofthetasksand wereperformed torefinethe
phase andamplitude solutions beforeasatisfactory mapwasobtained. Allmapsweremadeusing
aweighting schemebetween uniform andnatural (parameter =0inthetask )togive
a reasonable balance between high resolution and high sensitivity. In order to determine the flux
densities of components in the maps, single Gaussian models were fitted using task ; where
thiswasnotpossibletheintegrated fluxwasestimated usingor.
3. Results andDiscussion
The maps at all 3 wavelengths are shown in Figure 2 (images A and B) and Figure 3 (region
C).Inadditiontothe2componentsnotedbyKoopmansetal(A1,A2,B1,B2)weidentifyinthese
higherresolutionmaps3newcomponents (labelled3,4and5)inbothAandB.Thenon-collinear
lineofthe5components clearlydemonstrates theopposite imageparities ofAandB.
The 4 main components previously noted in region C are clearly seen at all wavelengths.
The labelling of components in region C has been changed here to reflect the obvious symmetry
between the left and right sides, and its interpretation as 2 merging images. Partial image C1 has
components C1-1andC1-2;theircounterparts inC2becomeC2-1andC2-2.
Figure 4 (left) shows the spectra of all 5 components in A (the corresponding components in
B are essentially the same) and (right) the 2 components in both partial images C1 and C2. All
exhibitsteepspectraexceptA2(andB2),C1-2andC2-2,whichshowflattenedspectrabetween18
and 6cm. Followingastandard description ofextragalactic radio sources wewould identify A2
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Multi-FrequencyVLBIObservationsoftheGravitationalLensB2016+112 AnupreetaMore
0 5 10 15 0 5 10 15
25 25
20 20
15 15
10 10
C 5 C 5 B5
C SE C SE
R 0 R 0
A A
Milli -5 A5 Milli -5 B2
B3
-10 A2 -10
A3
-15 -15 B1
A1 B4
-20 A4 -20
-25 -25
20 10 0 -10 -20 20 10 0 -10 -20
MilliARC SEC MilliARC SEC
0 1 2 3 0 1 2 3
25 25
20 20
15 15
B5
10 10
C SEC 5 C SEC 5 B2
MilliAR -50 A2 A5 MilliAR -50 B3
A3 B1
-10 A4 A1 -10 B4
-15 -15
-20 -20
-25 -25
20 10 0 -10 -20 20 10 0 -10 -20
MilliARC SEC MilliARC SEC
0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5
25 25
20 20
15 15
B5
10 10
C SEC 5 A5 C SEC 5 B2
R 0 R 0
MilliA -5 A3 A2 MilliA -5 B3
A1 B1
B4
-10 -10
A4
-15 -15
-20 -20
-25 -25
20 10 0 -10 -20 20 10 0 -10 -20
MilliARC SEC MilliARC SEC
Figure 2: Each row shows (left) image A and (right) image B at (from top to bottom) 18 cm
(σ=0.12mJybeam−1),6cm(σ=0.07mJybeam−1)and3.6cm(σ=0.04mJybeam−1). Thecontoursare
(-3,3,9,18,27,54)×σintherespectivemaps.Therestoringbeamsusedwere(18cm)11.1×2.6mas2and
PA=-10.58◦,(6cm)2.4×1mas2andPA=-8.03◦and(3.6cm)1.8×0.6mas2andPA=-7.02◦.
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Multi-FrequencyVLBIObservationsoftheGravitationalLensB2016+112 AnupreetaMore
0 2 4
40
30
20
C
E 10
S
C
R 0
A
Milli-10 C2−1
-20 C1−1 C1−2 C2−2
-30
-40
100 50 0 -50 -100
MilliARC SEC
0.0 0.5 1.0
40
30
20
C
E 10
S
C
AR 0 C2−1
Milli-10
-20 C1−1 C1−2 C2−2
-30
-40
100 50 0 -50 -100
MilliARC SEC
0 200 400
40
30
20
C
E 10
S
C
R 0
A
Milli-10 C2−1
C2−2
-20 C1−2
C1−1
-30
-40
100 50 0 -50 -100
MilliARC SEC
Figure3: FiguresaboveshowregionC(partialimagesC1andC2)at18cm(σ=0.1mJybeam−1),6cm
(σ=0.05mJybeam−1)and3.6cm(σ=0.03mJybeam−1)fromtoptobottom. Thecontoursare(-3,3,9,
18,27,54)×σintherespectivemaps. TherestoringbeamsareasforFigure2.
andB2asthe“core”oftheradiostructure, andexpectthistocoincide withtheopticalnucleus.
In the scenario of Koopmans et al, C1 and C2 are images of components in the background
source which appear in the NW end of image A. The similar spectra of C2-2 (C1-2) and A2 (B2)
suggests an identification of C2-2 with A2, and hence A5 with C2-1. As noted by Koopmans,
however,modelspredictthatC2-2andC1-2wouldbemuchmoremagnifiedthantheyappear, and
the coincident optical nucleus would appear in C1and C2. However, the data is consistent with a
schemewherebythediamondcausticpassesthroughA2,causingonlyasmallpart(notcontaining
the small optical nucleus) to be quadruply imaged. From the gap between images C1 and C2
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Multi-FrequencyVLBIObservationsoftheGravitationalLensB2016+112 AnupreetaMore
A1 C1-1
10 A2 10 C1-2
A3 C2-2 C2-2
A4 C2-1
A5 C1-2
mJy) A1 mJy)
Flux ( A2 Flux (
1 1
C2-1
A3
C1-1
A4
A5
0.1 0.1
0.1 1 10 0.1 1 10
Frequency (GHz) Frequency (GHz)
Figure4: Spectraofallcomponents.Left: ImageA;Right:RegionC(imagesC1,C2)
we deduce that A2 in fact consists of a line of smaller, flat-spectrum subcomponents, the caustic
passing between2ofthem. Alternatively, C1andC2maycorrespond tofurther, faintcomponents
asyetundetectedtotheNWofA;however,thiswouldrequiretheretobe2flat-spectrum,core-like
components inthebackground source,whichseemsunlikely.
Acknowledgments
Thisworkwassupported bytheEuropeanCommunity’sSixthFrameworkMarieCurie
ResearchTrainingNetworkProgramme,ContractNo. MRTN-CT-2004-505183"ANGLES".
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