First Results of the 74 MHz VLA-Pie Town Link. Hercules A at Low Frequencies

0.62 MB·English

by  Nectaria A. B. Gizani

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed2February2008 (MNLATEXstylefilev2.2)
First Results of the 74 MHz VLA-Pie Town Link.
Hercules A at Low Frequencies
Nectaria A. B. Gizani,1,2 ⋆, A. Cohen,3 and N. E. Kassim,3
1 Grupo de Astrof´isica da Universidade de Coimbra e Observat´orio Astron´omico da Universidade de Coimbra,
5 Santa Clara, 3040 Coimbra, Portugal
0 2Institute of Astronomy and Astrophysics, National Observatory of Athens, I. Metaxa& B. Pavlou, Lofos Koufou, Palaia Penteli,
0 15236 Athens, Greece.
2 3 Naval Research Laboratory, Code 7213, Washington, DC20375, USA

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1 2February2008
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1 ABSTRACT
v
0 We present the results of the first successful observations of the Pie Town link
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with the Very Large Array (VLA) at 74 MHz on Hercules A. The improvement in
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resolution from 25 arcsec to 10 arcsec resolves the helical- and ring-like features seen
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athigherfrequencies.Wealsopresentnewhighdynamicrangeimagesofthispowerful
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radio galaxy at 325 MHz. Our low frequency observations confirm the multiple out-
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burst interpretation of the spectral index differences at high frequencies. Comparison
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/ between our radio and ROSAT X-ray data does not reveal any association between
h
the X-rayemissionfromthe cluster andthe radiolobes.Thereare noextra regionsof
p
radio emission at 74 MHz.
-
o
Keywords: galaxies:active;radiocontinuum:galaxies;galaxies:individual(Hercules
r
t A); clusters: individual (the Hercules A cluster); methods: data analysis; techniques:
s
image processing
a
:
v
i
X
r 1 INTRODUCTION lobes and no compact hotspots. The ring-like/helical fea-
a
turesonbothsidesoftheradioemissionformanalmostsym-
1.1 Hercules A
metrical sequence which suggests successive ejections from
Hercules A (Her A) is a complex extended radio galaxy at theactive nucleus(Gizani & Leahy 2003).
a low redshift of z =0.154 (see Gizani & Leahy 2004, 2003
Her A has also been studied in the X-rays with the
for a review on this source and results from new observa-
ROSAT PSPC (Position Sensitive Proportional Counter)
tions).Itisoneofthemostluminousradiosourcesinterms
and HRI(High Resolution Imager) (Gizani & Leahy 2004).
of apparent and intrinsic brightness. Itstotal power output
The cluster is luminous in X-rays with a bolometric lumi-
is nearly as great as Cygnus A. At low frequency, it is one nosity Lbol = 4.8×1037 W. Her A itself lies at the center
of the brightest radio sources in the sky at 170 and 800
of the cluster, which should also have a cooling flow at the
Jy at 325 (VLA P-band) and 74 MHz (VLA 4-band) re-
center, when the powerful jets are absent, Currently, how-
spectively(Kassim et al. 1993).Itstotalradioluminosity is
ever, it is critically disturbed by the expansion of the radio
∼3.8×1037 Wintheband10MHzto100GHz1.Hercules
lobes. ThePSPCspectrum revealsacool component of the
AisclassifiedasanFR1.5(Dreher& Feigelson1984).With intraluster medium (ICM) with kT = 2.52 keV. The total
a linear size of 540 kpc and a width of ≃ 250 kpc (angular mass of thecluster is estimated to be1.5×1014 M⊙.
size=194×70arcsec),thisradiogalaxyposessesanunusual
jet-dominated morphology, almost symmetrically extended Gizani & Leahy (2003), have made an extensive study
of Her A with the VLA 1.4 GHz band (L-band, A-, B-,
C- configuration) at 1295, 1364.9, 1435.1, and 1664.9 MHz;
8.4 GHz band (X-band,B-, C-, D- configuration) at 8414.9
1 We use H◦=65 kms−1 Mpc−1 and q◦=0 throughout for con-
and 8464.9 MHz and reprocessed the 5 GHz band data
sistencywiththeseriesofpapersonthisobject.Wearenotvery
(C-band, B-, C- and D- configuration) at 4872.6 MHz by
wrong in doing this, since the recent WMAP (Wilkinson Mi-
crowaveAnisortopyProbe,Bennettetal.2003)datagiveH◦=71 Dreher& Feigelson (1984).Gizani &Leahy2003, havepro-
kms−1 Mpc−1 and q◦=-0.595, which do not change our results ducedaspectralindexmapwith1.4arcsecresolutionwhich
significantly. provided a detailed picture of the plasma throughout the
2 Nectaria A. B. Gizani, A. Cohen and N. E. Kassim
complex morphology, andgaveafairly good idea about the array to successfully break the “ionospheric barrier” below
temporal history of the plasma. Clearly the spectral index 100 MHz by achieving ∼ 20 arcsec resolution at 74 MHz.
mapseparatesthesourceindistinctregionsofbrighter,flat- In 2001, NRL and NRAO collaborated again to develop a
ter(youngermaterial)andfainter,steeper(oldermaterial), 74 MHz observing system on the Pie Town VLBA antenna
which is some evidence for spectral curvature, suggesting (hearafter PT) located 50 km from the center of the VLA
thatwemaybewitnessing arenewedoutburstfrom theac- andlinkedtoitviaopticalfiber.In2002 thefirstsuccessful
tive nucleus (Gizani & Leahy 2003). Generally spatial and fringes wereobtained atboth 74and330 MHzontheradio
spectral variations in a source imply temporal variations as galaxy 3C123, thereby marking another important step
well (Leahy 1991). forward in the development of high angular resolution, low
However the multifrequency measurements of frequency imaging.
Gizani & Leahy (2003) contained data from only a Today, low frequency systems on the VLA and the
relativelysmallfrequencyrange(1.4to8.4GHz),witheach Giant Metrewave Radio Telescope (GMRT) are creating
one of the four frequencies of the 1.4 GHz band, not very a quiet renaissance in long wavelength radio astronomy,
far apart from eachother. More quantitative conclusions, producing high angular resolution, high sensitivity images
suchasestimating theaging ofHerArequireamuchwider (Kassim et al.2004)andstimulatingthedevelopmentofin-
frequency range which extends to low enough frequencies strumentswith evengreater capability (e.g.Low Frequency
to map the break frequency throughout this radio galaxy. Array (LOFAR),see Kassim et al. (2002)).
Low frequencies are more likely to trace the shape of the In this paper we present the first successful scientific
injected spectrum, before the aging effects have introduced observations of a cosmic radio source, thepowerful FR I/II
spectral breaks. With this goal in mind, we have observed radiogalaxyHerA,usingthenewVLA+PTsystem,thereby
HerA at both 325 and 74 MHz. achievingthehighestangularresolutionimageeverobtained
ThearchivedVLAdata,atthesefrequenciesareoflow byaconnectedelementinterferometeratafrequencybelow
resolution and especially the 74 MHz data are poor. Fig. 1 100 MHz.
shows contours of the old maps of Her A at 74 (top) and
325 MHz(bottom). Gizani & Leahy(2003) havealso found
thefaintextendedemission ’cocoon’, at1.4GHz,1.4arcsec 1.3 Outline of Paper
resolution, surroundingthecoreandjetsofHerAshownin
We present the total intensity map of Her A at 74 MHz
Fig. 1 (see also Kassim et al. 1993).
employing the VLA-PT link, the corresponding map at
It is clear that even in the A-configuration, the VLA
325MHzaswellasthespectralindexmapbetween74 and
doesnothavetheresolutionrequiredat74MHztoprovidea
325 MHz. The spectral imaging analysis in the frequency
usefulspectralindexmap.Therefore,weturntothemostre-
range (0.074 to 8.4 GHz) and the resulting spectral aging
centadvanceinhigh-resolution,low-frequencyobservations,
studywillbepresentedinasubsequentpaper.Section2de-
the VLA-Pie Town link, which has just been upgraded to
tails the observations we made and Section 3 describes the
operate at 74 MHz.
data reduction and analysis. Results are discussed in Sec-
tion 4 and in Section 5 we summarise our conclusions.
1.2 The VLA-Pie Town Link at 74 MHz
In spite of the importance that low frequencies
2 OBSERVATIONS
(ν ≤100 MHz) have played in the discovery and de-
velopment of radio astronomy (Jansky 1933, Reber 1940, WehaveusedtheNRAO’sVLAtoobtaintotalintensityim-
Ryle&Smith1948,Hewish1952),theregionoftheelectro- ages of Her A at both 325 and 74 MHz, in order to make a
magneticspectrumbelow100MHzremainsamongthemost spectralagingstudy.Asthelimitingfactorincreatinganac-
poorly explored,despite its great scientific potential. While curatespectralindexmapofthissourceisthelowresolution
interferometers such as MERLIN (Multi Element Radio obtainable at 74 MHz, we employed the new VLA-PT link
Linked Interferometer) have brought higher resolution in- to increase the resolution at that frequency. At 325 MHz,
terferometery to frequencies as low as 151 MHz (resolution weobserved bothin theAand B-configurations in orderto
3 arc-second), sub-arcminute imaging below 100 MHz has maintain both high resolution and sensitivity to the large
been limited because ionospheric phase fluctuations have scale source structure. At 74 MHz, we are restricted to 1.5
restrictedinterferometerbaselinesto∼5km(seeforexam- MHz bandwidth, because that is the only part of the fre-
ple Erickson et al. (1982) and Rees (1990)). The resulting quency spectrum in that region that is protected for as-
poor angular resolution and consequently confusion limited tronomy. At 325 MHz we chose the 6.25 MHz bandwidth,
sensitivityhaveleftfrequenciesbelow100MHzinaseeming becausethisisthemaximumthatthecorrelatorcanhandle
darkagesrelativetocentimeterwavelengthradioastronomy while still having channels narrow enough to avoid band-
thatcanexploitthelongbaselines(≥35km)ofinstruments width smearing.
such as the VLA, MERLIN, and the VLBA. An important We observed in the multichannel continuum mode in
breakthrough was the demonstration that the technique of order to reduce the effects of bandwidth smearing and to
self-calibration could mitigate the ionospheric limitations more efficiently remove radio frequency interference (RFI).
and thereby achieve sub-arcminute resolution imaging at In the A-configuration, we observed both frequencies, 325
the longer wavelengths (Kassim et al. 1993). During the and 74 MHz, simultaneously, each in a separate IF. In the
1990s,theNationalRadioAstronomyObservatory(NRAO) B-configuration,weonlyobservedinthe325MHzband,and
and Naval Research Laboratory (NRL) developed 74 MHz we observed in two IF settings (328.5 and 321.5625 MHz).
systemattheVLAwasthefirstconnectedelementimaging The observations contained nearly full tracks, necessary to
First Results of the 74 MHz VLA-Pie Town Link.Hercules A at Low Frequencies 3
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Figure1.Top:Snapshotimageat74MHzA-arraywithonly8antennas.Thepeakbrightnessis119Jybeam−1,thermsnoiseis0.5Jy
beam−1 andtheresolutionis25 25arcsec.Contoursarelogarithmicseparatedbyfactors√2.Thefirstcontour isat1.5Jybeam−1.
Bottom:Snapshotimageat325M×Hz.Thepeakbrightnessis6.3Jybeam−1,thermsnoiseis3.7mJybeam−1 andthebeamis6.47
5.87arcsecwithpositionanglePA=67.5deg.Asbefore,contoursarelogarithmicseparatedbyfactors√2startingat15mJybeam−1×.
InbothfigurescoordinatesaregivenintheJulianJ(2000) coordinatesystemtomatchournewmaps.
maximisetheuv-coveragesothatwecouldmapthecomplex ing to inner channels. For 3C286, which is unresolved at
structureofHerA.Fulldetailsoftheobservationsaregiven this frequency and resolution, we did not require a model
in Table 1. for calibration. No phase calibrator was observed, because
Cygnus A (Cyg A) was used as a bandpass calibrator withthehighamountoffluxdensityinHerAwedetermined
at both frequencies. As Cyg A is resolved at both frequen- thatself-calibrationwouldbesufficientforthispurpose.Ap-
cies, accurate source models were required for calibration proximately90percentofthetimewasspentonthetarget
at each frequency, which are also available online2. For the source, the remainder on calibrators and slewing.
74 MHz model of Cyg A, we have used a high resolution The observations went almost as planned. Some of the
(∼ 9 arcsec) image produced with the PT link only about 74MHzdatawereaffectedbyinterferenceandunstablerack
2 weeks before our own observations (Kassim, Carilli, Har- temperatures, and data of the 325 MHz were occasionally
ris and Perley, private communication). For the 325 MHz lost for other reasons including antenna motor faults. The
model of this source we have made use of the model avail- total downtime per antennawas 10.3 minutes.
ableintheon-linelow-frequencydatareductiontutorial(see
Section 3).
Forfluxcalibrators, weused3C286 andCygA for325
3 DATA REDUCTION & ANALYSIS
and 74 MHz respectively. For Cyg A again we used the
model, because of the resolved source structure, restrict- Thegreatest difficultyinthedatareductionatlowfrequen-
cies comes from the non-coplanar array geometry and the
largefield ofview(f.o.v.) of theVLA.Forthereducingand
2 http://lofar.nrl.navy.mil/pubs/tutorial/node49.html imagingofthedatawehaveusedroutinesintheAstronom-
4 Nectaria A. B. Gizani, A. Cohen and N. E. Kassim
Table 1.Observations ofHerA.
Config Frequency Bandwidth Channels IFs Time Dates
MHz MHz # # hrs
A+PT 74 1.56 64 1 8 April2002
A 325 6.25 16 1 8 April2002
B 325 6.25 16 2 4 August2002
icalImageProcessingSoftware(AIPS).Wehavelargelyfol- Fig. 2 presents the new 325 MHz image. The angular
lowed the methodology described by T. J. W. Lazio, N. E. resolutionofthismapis5.68×4.77arcsecatapositionangle
Kassim,&R.A.Perleyin“Low-FrequencyDataReduction 45.17◦.Thedynamicrangeintheimageis830:1andtherms
attheVLA:ATutorialforNewUsers”,alsofoundon-line3. noiseis≃8mJy/beam.Fig.5showsthe74MHzmapwith
After performing bandpass (using Cyg A) and flux PT overlayed on top of the 325 MHz map. The resolution
calibration (using Cyg A at 74 MHz and 3C286 at ofthemap is25.12×9.75 arcsec ataposition angle30.95◦.
325 MHz),excision of RFI was performed by first clipping The dynamic range in the image is ∼ 385:1 with rms noise
all data pointswell abovetheactual fluxin theimage. Our 0.2 Jy/beam. The quality of the 74 MHz map in particular
clip levels were 300 and 5,000 Jy/beam for 325 MHz and is limited by the fact that the uv coverage for the PT link
74MHzrespectively.OncetheworstRFIwasremovedfrom is sparse and not very good near the equator. One can see
thechanneldata,weaveragedthechannelsasfaraspossible artifacts near the ends of the lobes in Fig. 5, probably due
withoutintroducingbandwidthsmearing(aipstasksplat). to residual calibration problems. Fig. 3 shows that the PT
At 325 MHz, we averaged the entire bandwidth to a single baselineshardlytouchtheVLAones.Self-calibrationcannot
channelperIF.At74MHz,wetookthecentral56channels get ridof smallerrors ifthebaselines from alltelescopes do
out of the original 64 and averaged this down to 7 chan- notcrossovereachotherverydensely,andthismayhaveled
nels. After channel averaging, additional RFI flagging was to the artifacts in the map. Though the synthesised beam
performed by hand (aips task tvflg). is highly elongated, the axis of maximum resolution fortu-
nately is aligned nicely with the axis of the jets in Her A,
allowing many features to be resolved which were not pre-
3.1 Mapping
viously seen at 74 MHz (c.f. Fig. 1, top). Fig. 4 shows the
The full f.o.v. , defined roughly by the primary beam area, 74 MHz map without the PT link for comparison.
has a diameter of 2.5◦ and 11.5◦ for 325 and 74 MHz re-
spectively. However, because Her A is one of the brightest
objectsinthesky,itcompletelydominatesallothersources
in the field, making it unnecessary to clean other sources 4 DISCUSSION
in the primary beam area. Therefore, only a small region
4.1 The Radio Characteristics
surrounding Her A (size ∼3 arcmin) needed to be imaged,
removingtheneedfortheusual“fly’seye”approximation4 The total radio power of Her A at 325 and 74 MHz from
tothethree-dimensionalFourierinversionsnecessarytomap our observations is P325 MHz =1.15×1027WHz−1sr−1 and
theentire primary beam area. P74 MHz =4.65×1027WHz−1sr−1respectivelyusingamean
Forthe325MHzdata,wecombinedtheA-configuration spectral index α≃−1.01.
and B-configuration data in the uv plane before imaging Itisclear from comparingFigures 4and 5that thead-
(aips task dbcon). For the 74 MHz data, we have included dition of the PT-link data has resolved many features in
PT link data, which increases the maximum baseline to the jets that had only previously been identified at higher
73 km. However, the coverage of the outer regions of the frequencies. The extended diffuse emission which forms the
uv-plane, provided by the VLA-PT baselines, is sparse and ‘cocoon’ is more clearly defined in the PT-link map. At
elongated nearly east-west (Fig. 3) This is due to the near 325 MHz, almost all emission was seen in the old maps
equatorial location of Her A (δ ≃ 5◦), and results in a withintheerrors(angularsize199×68arcsec). Intheorig-
highly elongated synthesised beam. Imaging was performed inalobservationsat74MHz(Kassim et al.1993)thesource
for each frequency with a nearly uniform weighting (robust hadappearedlarger(227×95arcsec)butwiththenewPT-
factor of −2) and with 1.5 arcsec pixels (aips task imagr). linkresolutionthesizeis185×71arcsec,whichagreesmuch
Severalroundsofphase-onlyself-calibrationwereperformed betterwiththe325MHzresult.Thefluxdensitiesmeasured
oneachdatasetuntilthemapnoisestoppeddecreasing(aips fromournewmapsareS325 MHz=206JyandS74 MHz=835
taskcalib).Noamplitudeself-calibration wasdone.Thefi- Jy. Direct comparison between Figs. 5, 2 and our 1.4 GHz
nal images produced this way are presented in Figs. 2, 4 data shows that all radio emission, faint and bright, young
and 5. and old, is detected (see also Paper II).
The325and74MHzimages(Figs.2and 5)showmany
3 http://lofar.nrl.navy.mil/pubs/tutorial/ of the same bright features on both jets as are seen in the
4 A “3–D” wide-field imaging technique, which does an inter- higher resolution 1.4 GHz image. These are identified with
polation between a set of small overlapping images of multiple thehelical-andring-likestructuresintheeasternandwest-
pointings,calledfacets,producingasingleimagecenteredonthe ern side of the radio emission respectively. These features
facetcentered onthesource. alsohaveflatterspectrumthanthesurroundingmediumand
First Results of the 74 MHz VLA-Pie Town Link.Hercules A at Low Frequencies 5
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Figure 2. The 325 MHz map produced by combining both the A and B arraydata. The beam size is 5.68 4.77 arcsec at a position
angle45.17deganditisshowninthelowerleft-handcorner.Thedynamicrangeintheimageis800:1andthe×rmsnoiseis 8mJybm−1.
Contoursareseparatedbyfactors√2,startingat25mJybeam−1.CoordinatesareasinFigure1. ≃
sotheybelongtotheyoungermaterialejectedfromthenu- spectrum. However the spectral index of the jets and rings
cleus(Gizani & Leahy2003).Neithermap resolvesthecore is contaminated by the lobe material superposed along the
which isfound tobeveryfaint and,surprisingly enough,to lineofsight,andsotheapparentspectralindexofthehigh-
haveasteepspectrum(Gizani & Leahy2003).Ourspectral brightness features is steeper than thetruevalue.
aging study will be given in a follow-up paper (Gizani, Co-
Gizani & Leahy (2003) interpreted the spectral index
hen,Kassim & Leahy,in prep.) where we expect to present
differencesat high frequency in terms of multiple outbursts
adetailed characterization ofthespectralcurvature.Atthe
which assume spectral curvature. This outburst interpreta-
currentobservingfrequenciesweexpectthespectrum tobe
tionisconfirmedbyourlowfrequencyobservations,because
flatterthanathigherfrequenciesbecauseoftheagingofthe
thespectraldifferencesbetweendifferentregionsoftheradio
relativistic particle energy spectrum.
emission are smaller at the α(74,325) map. The spectrum
oftheinnerregioncurves,asotherwisetherewouldbealot
of emission with α<−2.0 (c.f. Gizani & Leahy (2003)).
4.2 Spectral Index
The steep spectrum core found by Gizani & Leahy
We present here the resulting spectral index map from our
(2003) is hinted in the 325 MHz map (see Fig. 2) and it
observations at low frequencies, leaving however the more
is basically absent in Fig. 5 at 74 MHz. This absence is not
sophisticated, detailed analysis for a subsequent paper by
really a resolution problem, since convolving our high reso-
Gizani et al. in preparation as stated above. Fig. 6 shows
lution stacked map at 1.4 GHz (see Gizani & Leahy 2003)
the spectral index map between 74 and 325 MHz, i.e. the
withthe325MHzbeam,stillshowsthepresenceofthecore.
image of α37245. We adopt the convention for spectral index Sincethecompact core is self-absorbed, we suggest that its
α, flux density Sν ∝να, where α< 0. subsequent dissapearance at lower frequencies is because of
Fromthismap,theaveragespectralindexofthesource
spectral curvatureoccuring at these frequencies.
is α ≈ −1.1. The image hints at the distinction between
the bright and diffuse structures noted by Gizani & Leahy The relation between theturnoverfrequency νbreak (in
(2003). The former have flatter, younger, spectra and the GHz)versustheprojected linear size θ (inmas) inahomo-
latter, as well as the overall ’cocoon’, have older, steeper geneous,self-absorbed,incoherentsynchrotronradiosource,
6 Nectaria A. B. Gizani, A. Cohen and N. E. Kassim
Figure 3. Theuv-coverage forthe 74MHzobservations of HerA utilizingthe PTlink.The near equatorial locationof Her Acauses
thelongVLA-PTbaselinestobemostlyintheeast-westdirection.Asaresult,thesynthesisedbeamfortheVLA-PTlinkimageshown
inFigure5 is elongated, whilein the case of all other images we present it ismorenearly circular because they didnot utilizethe PT
link. Because the outer regions of the uv-plane are relativley sparsely covered compared to the inner regions, it was necessary to use
nearlyuniformweightingtoachievethefullresolvingpoweroftheVLA-PTlink.
when the turnover in the spectrum is due to synchrotron mass which should be equal to the heat loss rate over the
self-absorption, is given by O’Dea & Baum (1997): heat per unit mass Ikebeet al. 1997).
We expect the low frequency radio emission to origi-
θ≃13.45(S2B(1+z))14 nate, most likely, from theoldest electron populations. Our
ν5
break 74MHzmapcouldclarifyifthereisanyassociationbetween
the X-ray and radio emission shown as holes (areas empty
where B is the magnetic field in Gauss, S is the flux
from X-ray gas) or bright patches (cooler X-ray emitting
densityatthepeakinJy,andz istheredshift.FortheHer-
clouds of gas swept by the radio lobes), or other features
cules A cluster the central magnetic field is of the order of
µG(3<B◦(µG)<9,Gizani & Leahy(1999)andinprepara- pointing to the fossil radio lobes, currently devoid of ener-
tion).A∼ssuming∼thattheturnoverinthespectrumoccursat getic electrons. However there are no extra regions of radio
74 MHz where the flux of the compact core is ≃ 1090 mJy, emissionat74MHz,sothereseemstobenosuchinteraction.
then its size θ is estimated to be ≥ 10 mas. Gizani, Gar- Gizani & Leahy, 2004 have shown that the low signal-to-
noiseratio HRIdataand thelow resolution PSPCdata are
rett&Leahy,2002 andinpreparation,usingnewEVNand
insufficienttorevealasignofinteraction.Ourhighandlow
MERLINobservationsofthecoreregion at18cm,revealed
emission coming from 10× 20 mas scales (20 × 50 pc). frequency radio observations on Hercules A together with
the analysis of the higher resolution and deeper observa-
tionsbyChandrashouldhelpclarifyingthismatter(Wilson
et al., 2005, in prep.).
4.3 The Radio/X-ray interaction
The analysis of the ROSAT PSPC data suggested that
the X-ray emission extends well beyond the radio emission
5 CONCLUSIONS
(Gizani & Leahy 2004). The total mass of the cluster at
500 arcsec radius is 1.5×1014M⊙, and the accretion rate We reported on the high resolution VLA observations of
is M˙ ≃ 15M⊙ yr−1 (adopting the relation of the cooled Her A employing the newly established VLA-PT link at
First Results of the 74 MHz VLA-Pie Town Link.Hercules A at Low Frequencies 7
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Figure 4. The 74 MHz map without the PT link. The beam size is (25.4 21.1 arcsec with PA = 55.0 arcsec) shown in the lower
left-handcorner.Contoursareseparatedbyfactors√2,andstartingat0.5m×Jybeam−1.CoordinatesareasinFigure1.
74 MHz. We have also presented the new A+B configura- Our low frequency data do not find any extra areas
tion map of the source at 325 MHz. The Her A image at of radio emission so the situation between the radio–X-ray
74 MHz with PT is dramatically different than the one ex- correlation remains as reported in Gizani & Leahy (2004).
cludingthelink.Thebright ring-likeand helical features in Ourlongtrackobservationsforthisexceptionallybright
thewesternandeasternlobeareresolvedatthehighresolu- sourcewillprovidecriticalinputtotheongoingeffortstoim-
tionof≃10arcseccurrentlyprovidedbythePTlink.These prove the sophistication of VLA calibration for all low fre-
structureswerenotevenhintedintheearlierpublishedmap quencyobservations, and toextend theirapplication to the
at thesamefrequency.Inaddition,thenewimages support PT link. Our observations will enlarge the currently short
thepresenceof acomplicated extendedstructureandadif- list of long, uninterupted track data on bright objects at
fuse well-defined emission. However the ends of the eastern highresolutioninordertoexplorerefractive,differentialre-
and western lobes suffer from thepresence of ripples, prob- fractive, and related ionospheric effects, and to gauge the
ably due to residual self-calibration problems, because the algorithms’abilitytocorrectthemonshorttimescales.Our
longer PT baselines do not overlap much with the shorter VLA-PT link Her A image can also be a useful model for
ones.Anotherreasonforthepresenceoftheseartifactscould futurecalibration.
be the limitation in deconvolution induced by the fact that
the uv coverage for the PT link is relatively poor near the
equator.
ACKNOWLEDGMENTS
The spectral differences between different regions at
We acknowledge Paddy Leahy and Rick Perley for help-
highfrequencyaresmalleratlowfrequency.Thisisevidence
ful discussions. Nectaria Gizani would like to acknowledge
of spectral curvature confirming the multiple outburst in-
the Naval Research Laboratory travel and maintenance
terpretationusedbyGizaniandLeahy(2003),tointerprete
grant, with which her travel to the US and data reduction
thepresenceofregionswithdifferentage(withyoungerand
was made possible. NG acknowledges the grant PRAXIS
older material) apparent at the spectral index at high fre-
XXI/BPD/18860/98 from the Funda¸c˜ao para a Ciˆencia e
quencies.
a Tecnologia, Portugal for her post-doctoral fellowship of
Thesteepspectrum,selfabsorbedcoreofHerculesAis 2002–2003, during which the observations and most of the
absent at 74 MHz. At 325 MHz is barely visible. We sug- data reduction of the current work were made; Also the
gest that its subsequent dissapearance at lower frequencies 2004–2007 post-doctoral grant SFRH /BPD/11551/2002,
is because of spectral curvature occuring in these frequen- from the same Institute during which this work was pub-
cies.Wesuggestthatthelikelycandidateoftheoriginofthe lished. NG also acknowledges the State Scholarships Foun-
low frequency spectral curvature of the core is synchrotron dation(IKY),Greece,forher2003-2004post-doctoralgrant
self-absorption. undercontract332duringwhichthecurrentpaperwaswrit-
8 Nectaria A. B. Gizani, A. Cohen and N. E. Kassim
0 2 4
05 00 45
30
15
) 00
0
0
0
2
J04 59 45
(
N
O
TI 30
A
N
LI 15
C
E
D
00
58 45
30
15
16 51 16 14 12 10 08 06 04 02 00
RIGHT ASCENSION (J2000)
Figure5.Thebest74MHzmapwiththePTantenna(contours)overlayedontopofthe325MHzmap(grey-scale).Thedynamicrange
inthe imageis 360:1 withrms noise 0.2Jybm−1. The beam size(25.12 9.75 arcsec, PA = 30.95◦) is shown inthe lower left-hand
∼ ×
corner.Contours areseparatedbyfactors√2,andstartingat 2Jy/beam.Coordinates areasinFigure1.
±
-2.0 -1.5 -1.0 -0.5
05 00 30
15
00
0)
0
0
2
N (J 04 59 45
O
ATI 30
N
LI
C
E 15
D
00
58 45
30
16 51 14 12 10 08 06 04 02 00
RIGHT ASCENSION (J2000)
Figure 6. Grey-scale of the spectral index map between 74 and 325 MHz at 25.12 9.75 arcsec resolution with PA = 30.95◦. The
grey-scaleruns from 2.5 α 0.5. Contours areof the 325 MHzmapat the 74 M×Hzresolution, separated by factors of 2 starting
at 81mJybeam−1−. ≤ ≤−
≃
First Results of the 74 MHz VLA-Pie Town Link.Hercules A at Low Frequencies 9
ten. Finally NG is grateful to the Departamento de F´isica
oftheFaculdadedeCiˆenciasoftheUniversidadedeLisboa,
Portugalforallowinghertouseitsinstallationstocarryout
this task.
Basic research in astronomy at the NRL is funded by
the Office of Naval Research. The National Radio Astron-
omy Observatory is a facility of theNational ScienceFoun-
dation operated under cooperative agreement by Associ-
ated Universities, Inc. This research has made use of the
NASA/IPACExtragalactic Database (NED)which isoper-
ated bytheJet Propulsion Laboratory,Caltech, undercon-
tract with the National Aeronautics and Apace Adminis-
tration. This research has also made use of NASA’s Astro-
physicsData System.
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