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 n a J 1 2February2008 2 1 ABSTRACT v 0 We present the results of the first successful observations of the Pie Town link 7 with the Very Large Array (VLA) at 74 MHz on Hercules A. The improvement in 4 resolution from 25 arcsec to 10 arcsec resolves the helical- and ring-like features seen 1 athigherfrequencies.Wealsopresentnewhighdynamicrangeimagesofthispowerful 0 radio galaxy at 325 MHz. Our low frequency observations confirm the multiple out- 5 burst interpretation of the spectral index differences at high frequencies. Comparison 0 / 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 05 01 00 00 30 00) 00 0 2 J N ( O TI04 59 30 A N LI C E D 00 58 30 00 16 51 16 14 12 10 08 06 04 02 00 50 58 05 01 00 RIGHT ASCENSION (J2000) 00 30 0) 0 00 0 2 J N ( O TI04 59 30 A N LI C E D 00 58 30 00 16 51 16 14 12 10 08 06 04 02 00 50 58 RIGHT ASCENSION (J2000) 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 05 01 00 00 30 ) 0 0 0 00 2 J ( N O TI A04 59 30 N LI C E D 00 58 30 16 51 16 14 12 10 08 06 04 02 00 RIGHT ASCENSION (J2000) 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 05 01 00 00 30 0) 0 0 2 J N ( 00 O TI A N LI04 59 30 C E D 00 58 30 16 51 15 10 05 00 50 55 RIGHT ASCENSION (J2000) 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. 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