Table Of ContentFirst results from 18-22cm VLBA polarisation
observations of the MOJAVE-I AGNs
1
1
0
2
n Colm Coughlan∗
a DepartmentofPhysics,UniversityCollegeCork,Ireland
J
E-mail: [email protected]
1
3
RonanMurphy
] DepartmentofPhysics,UniversityCollegeCork,Ireland
O
E-mail: [email protected]
C
. Kyle Mc Enery
h
p DepartmentofPhysics,UniversityCollegeCork,Ireland
- E-mail: [email protected]
o
r
t HeatherPatrick
s
a DepartmentofPhysics,UniversityCollegeCork,Ireland
[ E-mail: [email protected]
1
RedmondHallahan
v
2 DepartmentofPhysics,UniversityCollegeCork,Ireland
4 E-mail: [email protected]
9
5
DeniseGabuzda
.
1 DepartmentofPhysics,UniversityCollegeCork,Ireland
0
E-mail: [email protected]
1
1
: WeareintheprocessofobtainingVLBApolarisationdataforthe135MOJAVE-IActiveGalactic
v
i Nucleiat fourwavelengthsin the 18-22cmband. These observationswill enable studiesof the
X
evolutionoftheintensityandmagnetic-fieldstructuresoftheseAGNjetsastheypropagatefrom
r
a parsec to kiloparsec scales, as well as studies of the thermal plasma present in the vicinity of
the jetsonthese scales, manifestviaFaradayrotation. A widerangeofothermulti-wavelength
studiescanalsobecarriedoutusingthesedata. Preliminaryresultsforselectedsourcesfromthe
first3of9observingsessionswillbepresented.Weaimtohave18-cmintensityandpolarisation
imagesavailableviatheMOJAVEwebsitewithin18monthsafterthelastobservingsession.
10thEuropeanVLBINetworkSymposiumandEVNUsersMeeting:VLBIandthenewgenerationofradio
arrays
September20-24,2010
Manchester,UK
∗
Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
Firstresultsfrom18-22cmVLBApolarisationobservationsoftheMOJAVE-IAGNs ColmCoughlan
1. Studying the JetsofAGNonscalesof10-100pc
Active Galactic Nuclei (AGN) are compact regions at the centres of galaxies where there is
believed to be a supermassive black hole which attracts mass to it via its gravitational pull. The
massattractedtotheblackholeformsanaccretiondisk,andasmassspiralsfromtheaccretiondisk
totheblackhole,itheatsupandbeginstoemitlargeamountsofhigh-energy radiation. Itisinpart
thisintenseemissionfromtheaccretion diskthatmakestheAGNconsidered tobe‘active’.
Much of the gas from the accretion disk spirals in towards the black hole until it passes the
event horizon, after which no further emission can be detected. However, for reasons not fully
understood, but which are thought to be connected with strong magnetic fields generated by the
relativistic plasma in the accretion disk, in about 10% of AGN some of the gas falling into the
black hole is instead accelerated to relativistic speeds and sent flying away from the AGN along
collimated relativistic ‘jets’, presumably along the rotational axis of the system. These jets can
be detected because of the large amounts of radio synchrotron radiation they emit. If the jet is
orientated towards the Earththen relativistic beaming boosts the electromagnetic fluxfrom thejet
receivedonEarth,makingthejetmoreprominent.
The polarisation of the radiation detected from the jet is of particular interest because it pro-
vides information about the underlying synchrotron magnetic field: in optically thick/thin regions
theemittedpolarisationisparallel/perpendicular tothismagneticfield. Changesinthepolarisation
angle of the radiation with frequency due to Faraday rotation can give information about both the
magnetic field and thermal electron density in the immediate vicinity of the jet. Multiwavelength
VLBA observations at 18-22cm probe the jet on scales from parsecs to 100s of parsecs, making
them a useful tool to study Faraday rotation. Other jet physics that can be studied using such ob-
servations include jet collimation, bending and interaction with the surrounding medium and the
evolution and morphology of the magnetic field along the jet. Specific proposed magnetic field
structures, suchashelicaljetmagneticfields(expectedduetotherotationofcentralaccretiondisk,
jetoutflowandshocks), canalsobeinvestigated.
2. The Experiment : BG 196
Weareobtainingsimultaneous VLBAobservations at1358,1430,1493and1665MHz(22.1,
21.0, 20.1 and 18.0 cm) of the 135 AGN making up the MOJAVE-1 sample [1]. The first obser-
vations were taken in early 2010 and the observations and processing are expected to be finished
in 2011-2012. These observations are designed for intensity and polarisation studies of AGN jets
on scales out to 100s of parsecs. The spectral index and Faraday rotation distributions can also
be studied. Thefirst3epochs (A,Band C)have been calibrated, and some preliminary results are
presented here.
3. CalibrationandImaging
Thepreliminarycalibration andpolarisation (D-term)calibration weredoneintheAstronom-
2
Firstresultsfrom18-22cmVLBApolarisationobservationsoftheMOJAVE-IAGNs ColmCoughlan
Figure1: J0841+7053(EpochC)withEVPAcorrectionsappliedatallfourfrequencies.
J0841+7053 1358 MHZ J0841+7053 1430 MHZ J0841+7053 1493 MHZ J0841+7053 1665 MHZ
20 20 20 20
0 0 0 0
-20 -20 -20 -20
-40 -40 -40 -40
MilliARC SEC --6800 MilliARC SEC --6800 MilliARC SEC --6800 MilliARC SEC --6800
-100 -100 -100 -100
-120 -120 -120 -120
-140 -140 -140 -140
-160 -160 -160 -160
-180 -180 -180 -180
10 0-10-20-30-40-50-60-70 10 0-10-20-30-40-50-60-70 10 0-10-20-30-40-50-60-70 10 0-10-20-30-40-50-60-70
MilliARC SEC MilliARC SEC MilliARC SEC MilliARC SEC
(a) 1358MHz (b) 1430MHz (c) 1493MHz (d) 1665MHz
Frequency Jan2004 Feb2010 Mar2010 May2010
MHz BG139 EpochA EpochB EpochC
D c ◦ D c ◦ D c ◦ D c ◦
1358 128 130 91 132
1430 113 113 112 113
1493 100 147 84 96
1665 82 91 49 37
Table1:EVPAcorrectionsforepochsA,BandCandtheearlierexperimentBG139.
ical Image Processing System (AIPS) using standard techniques. In all cases, Los Alamos was
used as the reference antenna. The polarisation angles for epochs A and B were calibrated using
integrated (VLA)observations of compact polarised sources. However as no electric vector posi-
tion angle (EVPA) calibrator was included in epoch C, this epoch was instead calibrated using a
previous VLBI observation. Table 1 gives the resulting EVPA corrections for each frequency for
eachepoch. Forcomparison wealsopresent theEVPAcorrection forthesamefrequencies forthe
observations of Gabuzda and Hallahan [2], obtained in January 2004, which were reduced using
the same reference antenna. The EVPA corrections are sometimes, but not always, remarkably
stable over as long as 6 years. These EVPA corrections were tested to ensure that they resulted
in self-consistant images for a range of sources. One example is shown in Figure 1, which shows
thecalibratedpolarisation imagesforJ0841+7053observedatEpochC:thepolarisation structures
areclearly consistant across thefourfrequencies. Semi-automated scripts werewritteninAIPSto
speedupthecreationofintensity, polarisation, spectral indexandFaradayrotation maps.
3
Firstresultsfrom18-22cmVLBApolarisationobservationsoftheMOJAVE-IAGNs ColmCoughlan
4. Results
Intensity, polarisation, spectral index
and Faraday rotation maps have been ob-
Figure2:3C120.
tained for the first three epochs. The ex-
pected wavelength dependence for Fara-
3C 120 Faraday Corrected Polarisation Map
40
dayrotation is
20
0
c obs =c o+RMl 2 MilliARC SEC --2400
-60
RM(cid:181) n ~B·d~l -80
Z e
-100
50 0 -50 -100 -150 -200 -250
MilliARC SEC
~
where n is the electron density, B is the
e
magnetic field and the integral is taken
(a) FaradayCorrectedPolarisationMapat1358MHz
along the line of sight. The amount and
direction of rotation are thus determined 3C 120 Rotation Measure Map
20 40 60
by the electron density and the line-of- 40
sight magnetic field. Results for two in- 20
0
dividual sourcesarepresented below.
MilliARC SEC --2400
4.1 3C120 -60
-80
The well known AGN 3C 120 has -100
50 0 -50 -100 -150 -200 -250
MilliARC SEC
anextendedjetwithcomplexpolarisation
structure. Thepolarisationmapof3C120 (b) RotationMeasureMap
at 1358 Mhz presented in Figure 2(a) has
been corrected for Faraday rotation. The
rotation measure distribution is shown in
Figure 2(b). A tendancy for the RM to
be higher along the upper edge of the jet
of 3C 120 is visible; one possible origin
for thistransverse gradient inFaradayro-
tation is a helical magnetic field carried
bythejet: theFaradayrotationgradientis
due to a systematic change in the line-of-
sight magnetic fieldacross thejet. Figure
2(c)showsarotationmeasuresliceacross
thejetabout40masfromthecore,which
(c) RMof3C120acrossthejet.About40masfromthecore.
displays a clear systematic gradient. Pre-
vious evidence that the jet of 3C 120 car-
ries a helical field has been presented by
Gómezetal[3].
4
Firstresultsfrom18-22cmVLBApolarisationobservationsoftheMOJAVE-IAGNs ColmCoughlan
Figure3: UnusualbehaviourinJ0121+1149.Figures3(a)and3(c)showthefaradaycorrectedpolarisation
and RM mapsduringMay 2010,Figures3(b)and3(d)show the same mapsduringJanuary2004. Figure
3(e)showsc vs. l 2plotsforthethreeregionsduringMay2010.Thec errorsarecomparabletothesizeof
thesymbols.
J0121+1149 1358 MHZ J0121+1149 1358 MHZ
100 100
80 80
60 60
40 40
20 20
0 0
-20 -20
120 100 80 60 40 20 0 -20 120 100 80 60 40 20 0 -20
MilliARC SEC MilliARC SEC
(a) (b)
J0121+1149 1358 MHZ
0 50 100 150 200
60
40
20
C
C SE
MilliAR 0
-20
-40
60 40 20 0 -20 -40 -60
MilliARC SEC
(c) (d)
(e)
5
Firstresultsfrom18-22cmVLBApolarisationobservationsoftheMOJAVE-IAGNs ColmCoughlan
4.2 J0121+1149
The Faraday rotation map of this source, shown in Figure 3, displays 3 distinct regions with
differentcharacteristicRMvalues: thecore,innerjetandouterjet. AftercorrectingforthisFaraday
rotation, the magnetic field structure is complex, but appears to become longitudinal in the outer
jet. Figure 3 also shows a comparison of the polarisation images obtained at epoch C and in
January 2004. The three distinct regions of the Faraday rotation are present at both epochs. The
polarisation structure in the core and inner jet changed between the two epochs, while that in the
outer jet remained constant. The c vs. l 2 plots in Figure 3 clearly show the linear dependence
expected forFaradayrotation, withtheRMdecreasing markedlywithdistance fromthecore.
5. Conclusions
The images presented here show just two examples of the suitability of 18-22 cm VLBA
observations for studying AGN jets on scales out to 10s or 100s of parsecs. We have worked
out a scheme for calibrating and imaging the data for BG196 in a timely fashion, including the
development of semi-automated scripts. We plan to provide calibrated data and images to the
communitywithin18monthsafterthelastobservations, viatheMOJAVEwebsite[1].
Acknowlegements
This work was supported by the Irish Research Council for Science, Engineering and Tech-
nology (IRCSET).
References
[1] Lister,M.L.,Aller,H.D.,Aller,M.F.,Cohen,M.H.,Homan,D.C.,Kadler,M.,Kellermann,K.I.,
Kovalev,Y.Y.,Ros,E.,Savolainen,T.,Zensus,J.A.,Vermeulen,R.C.,2009,AJ,137,3718.
Website: http://www.physics.purdue.edu/MOJAVE/
[2] Hallahan,R.,andGabuzda,D.C.,2008,Proc.9th EVNSymposium,PoS(IX EVN
Symposium)029
[3] Gómez,J.L.,Marscher,A.P.,Jorstad,S.G.,Agudo,I.,Roca-Sogorb,M.,2008,ApJ,68,L69.
6
