As a critical part of the Tracking Active Galactic Nuclei with Austral
Milliarcsecond Interferometry (TANAMI) program, in November 2007 the Australia
Telescope Compact Array (ATCA) started monitoring the radio spectra of a sample
of southern hemisphere active galactic nuclei (AGN) that were selected as
likely candidates for detection (as well as a control sample) by the Large Area
Telescope (LAT) aboard the Fermi Gamma Ray Space Observatory. The initial
sample was chosen based on properties determined from AGN detections by the
Energetic Gamma Ray Experiment Telescope (EGRET). Most of the initial sample
has been detected by Fermi/LAT and with the addition of new detections the
sample has grown to include 226 AGN, 133 of which have data for more than one
epoch. For the majority of these AGN, our monitoring program provides the only
dynamic radio spectra available. The ATCA receiver suite makes it possible to
observe several sources at frequencies between 4.5 and 41 GHz in a few hours,
resulting in an excellent measure of spectral index at each epoch. By examining
how the spectral index changes over time, we aim to investigate the mechanics
of radio and gamma-ray emission from AGN jets.Comment: 5 pages, 1 figure, 2012 Fermi & Jansky Proceedings - eConf C111110

Blanchard, J.

Dutka, M.

Edwards, P. G.

Hungwe, F.

Kadler, M.

Lovell, J.

Macquart, J. P.

Moin, A.

Ojha, R.

Stevens, J.

Tingay, S.

English

arXiv

As a critical part of the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI) program, in November 2007 the Australia Telescope Compact Array (ATCA) started monitoring the radio spectra of a sample of southern hemisphere active galactic nuclei (AGN) that were selected as likely candidates for detection (as well as a control sample) by the Large Area Telescope (LAT) aboard the Fermi Gamma Ray Space Observatory. The initial sample was chosen based on properties determined from AGN detections by the Energetic Gamma Ray Experiment Telescope (EGRET). Most of the initial sample has been detected by Fermi/LAT and with the addition of new detections the sample has grown to include 226 AGN, 133 of which have data for more than one epoch. For the majority of these AGN, our monitoring program provides the only dynamic radio spectra available. The ATCA receiver suite makes it possible to observe several sources at frequencies between 4.5 and 41 GHz in a few hours, resulting in an excellent measure of spectral index at each epoch. By examining how the spectral index changes over time, we aim to investigate the mechanics of radio and gamma-ray emission from AGN jets

Edwards, P.

Macquart, Jean-Pierre

espace@Curtin

2011 Fermi & Jansky: Our Evolving Understanding of AGN, St Michaels, MD, Nov. 10-12 1ATCA monitoring of gamma-ray loud AGNJ. StevensCSIRO Astronomy and Space Science, Locked Bag 194, Narrabri NSW 2390, AustraliaP.G. EdwardsCSIRO Astronomy and Space Science, PO Box 76, Epping NSW 1710, AustraliaR. OjhaNASA GSFC, Mail Code 661, Greenbelt MD 20771, USAM. KadlerUniversität Würzburg, Lehrstuhl für Astronomie,Emil-Fischer-Str. 31 Würzburg 97074, GermanyF. HungweRhodes University, P.O. Box 94 Grahamstown 6140, South AfricaM. DutkaCatholic University of America, 620 Michigan Ave., N.E. Washington DC 20064, USAS. Tingay, J.-P. Macquart, A. MoinCurtin Institute of Radio Astronomy, Curtin University, GPO Box U1987, Perth WA 6845, AustraliaJ. Lovell, J. BlanchardSchool of Mathematics & Physics, University of Tasmania, Private Bag 37, Hobart TAS 7001, AustraliaAs a critical part of the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferom-etry (TANAMI) program [19], in November 2007 the Australia Telescope Compact Array (ATCA)started monitoring the radio spectra of a sample of southern hemisphere active galactic nuclei (AGN)that were selected as likely candidates for detection (as well as a control sample) by the Large AreaTelescope (LAT) aboard the Fermi Gamma Ray Space Observatory. The initial sample was chosenbased on properties determined from AGN detections by the Energetic Gamma Ray ExperimentTelescope (EGRET). Most of the initial sample has been detected by Fermi/LAT and with the ad-dition of new detections the sample has grown to include 226 AGN, 133 of which have data for morethan one epoch. For the majority of these AGN, our monitoring program provides the only dynamicradio spectra available. The ATCA receiver suite makes it possible to observe several sources atfrequencies between 4.5 and 41 GHz in a few hours, resulting in an excellent measure of spectralindex at each epoch. By examining how the spectral index changes over time, we aim to investigatethe mechanics of radio and γ-ray emission from AGN jets.I. INTRODUCTIONResults from EGRET revealed a very strong linkbetween γ-ray emission in AGN and core-dominatedradio emission from relativistic jets, which may origi-nate from super-massive black hole and accretion disksystems [12]. Recent Fermi data have confirmed this[6]. Observations at radio wavelengths were criticallyimportant for the understanding of the γ-ray emis-sion mechanism in these AGN. In particular, the rel-ative timing of γ-ray flares and radio outbursts pro-vides constraints on the mechanisms and location ofthe flare origin [15]. The main models for γ-ray emis-sion entail inverse Compton scattering of low energyphotons to high energies by the relativistic electronpopulation in the AGN jets. The low energy pho-tons could have their origin external to the jet, per-haps in the broad line region of the AGN [10] or couldoriginate internal to the jet, being the synchrotron ra-dio/IR/optical emission from the electrons themselvesinteracting with the jet magnetic field [8].Radio flux density monitoring observations, whichallow for the evolution of the radio emission to befollowed as a function of time, may indirectly revealwhen relativistic electrons are being injected into theAGN jet. Observations at high frequencies, in theoptically-thin part of the radio spectrum, may allowus to probe deep into the structure of the radio jet, towhere the γ-ray emission is likely generated [16].II. ATCA OBSERVATIONSSince late 2007, the ATCA has undertaken a project(designated C1730) to regularly observe a number ofeConf C11111012 2011 Fermi & Jansky: Our Evolving Understanding of AGN, St Michaels, MD, Nov. 10-12southern AGN that have been detected in γ-rays. Formany sources, particularly those south of −30◦ decli-nation, this is the only project providing flux densitymonitoring data. It measures flux densities for theseAGN at frequencies between 4.5 to 41 GHz, makingit ideal for investigating the physics involved in γ-rayemission.The ATCA is ideally suited for a program such asC1730. It is a six-element interferometer located nearthe town of Narrabri in New South Wales. Each an-tenna has a 22-m diameter, with a Cassegrain focus.Each antenna has a suite of cryogenically-cooled re-ceivers, which cover 67% of the frequency range be-tween 1.1 - 50 GHz. The telescope can quickly changebetween these receivers to make observations at arange of different frequencies within a short timespan.The C1730 project uses this ability to measure thespectral index of each source it monitors, by obtain-ing flux densities at 5.5, 9, 17, 19, 38 and 40 GHzalmost simultaneously. A typical C1730 epoch willobserve 5 closely-spaced sources at two simultaneousfrequencies in 30 minutes, before switching frequenciesand reobserving them again, resulting in full spectralcoverage within 2 hours.Between the start of the C1730 project in Novem-ber 2007 and February 2009, observations were madewith the original ATCA correlator, which provided256 MHz of bandwidth, with 128 MHz at each of twosimultaneously measured frequencies. In March 2009,the ATCA correlator was upgraded to the CompactArray Broadband Backend (CABB, [21]), which pro-vides 4 GHz of bandwidth, with 2 GHz at each of twosimultaneous frequencies.The ATCA changes the locations of its antenna sev-eral times each year. Five of the six antenna are seton a 3 km long East-West track, while the sixth isheld stationary 3 km west of the western end of thistrack. This allows for configurations with baselinelengths between 31m and 6 km. Antenna can also beplaced on a short (214m) North-South track placedroughly half-way along the East-West track, and thisis done when high brightness-sensitivity is required.The C1730 project has been scheduled in various ar-ray configurations, as the sources that it observes areusually point-like.Details of the C1730 observing epochs to-date arelisted in Table I. The project commenced with a pilotstudy before Fermi was launched, to trial monitor-ing of 16 southern AGN detected by EGRET. Thissuccessful pilot study paved the way for continued ob-servations, and with the emergence of the first resultsfrom Fermi in early 2009, the source list was quadru-pled.In 2011, the source list was revised based on thefirst and second Fermi catalogs of detected AGN, andthe expanded list of TANAMI targets. The originaldeclination limit of −10◦ was changed to better matchthe TANAMI range (δ < −30◦) and reflect the largernumber of southern sources now identified by Fermi.The most recent epochs included 150 2LAC sourceswith δ < −40◦, including ∼ 30 proposed counterparts(primarily X-ray associations) that had not previouslybeen detected at radio frequencies.TABLE I: C1730 epoch list. For each epoch, the number ofsources observed is shown, along with the ATCA antennaconfiguration at the time. Details of each configurationcan be found from the ATCA website.Dates Array No. sources2007 Nov 01/02 1.5A 172008 Jan 11/12 6A 202008 Mar 14/15 1.5D 202008 Oct 13/14 6A 172008 Dec 17/18 750B 202009 Feb 22/23 EW352 452009 Apr 24/25 H168 732009 May 18/19 H214 732009 Jun 26/27 H75 732011 May 17/18 1.5B 502011 Aug 31 6B 362011 Sep 13 6B 342011 Oct 15 H75 442011 Nov 02 750C 362011 Nov 08/09 EW367 532011 Nov 27/28 1.5D 1582011 Dec 03/04 1.5D 42Our C1730 observations will contribute significantlytoward the identification of the physics of extragalac-tic Fermi sources, along with monitoring programs atother frequencies, for example:• “Fermi Large Area Telescope Observations ofthe Active Galaxy 4C +55.17: Steady, HardGamma-Ray Emission and its Implications” [18]• “Fermi Large Area Telescope Observations ofMarkarian 421: The Missing Piece of its Spec-tral Energy Distribution” [5]• “The First Fermi Multifrequency Campaign onBL Lacertae: Characterizing the Low-activityState of the Eponymous Blazar” [4]• “Insights into the High-Energy Gamma-rayEmission of Markarian 501 from Extensive Mul-tifrequency Observations in the Fermi Era” [3]C1730 observations are also occassionally madeto rapidly follow-up sources that are reported tobe flaring by Fermi, for example ATels 3788, 3734,3713, 3703, 3579 and 3394. In this way, we havea higher probability of observing “interesting” AGN,and thus examining their temporal behaviour duringeConf C11111012011 Fermi & Jansky: Our Evolving Understanding of AGN, St Michaels, MD, Nov. 10-12 3flare events. With the sensitivity that CABB provideswith its 4 GHz of bandwidth, accurate flux densitymeasurements are possible even for objects that arenot normally very bright. For example, in 20 minutesof on-source integration time, the ATCA with CABBreaches an RMS noise level of 30 uJy at 5.5 GHz, 40uJy at 9, 17 and 19 GHz, and 70 uJy at 38 and 40GHz. It is therefore possible to follow-up interest-ing flare events during periods of unscheduled time assmall as 2 hours.III. BLAZAR VARIABILITYAll data from C1730 is reduced through the samecustom pipeline, ensuring consistency over multipleepochs. Flux density calibration is achieved via theATCA primary flux density calibrators PKS B1934-638 – for frequencies lower than 25 GHz – and theplanet Uranus for higher frequencies.Early analysis has focussed on determining which ofthe AGN monitored by this program have varied, andhow. Using the method described in [7], we have found107 sources that have varied between 2007 and now.The spectral curves for these sources are shown in Fig-ure 1. The variability is characterised by a change inthe spectral turnover frequency. For sources that varyin brightness achromatically, the spectral turnover fre-quency will remain constant, but if, for example, thesource becomes disproportionately brighter at higherfrequencies, the spectral turnover may shift towardthe higher frequency end.We used the Roma-BZCAT [17], CGRaBS [14] and1LAC [1] catalogues of blazars to separate the AGN inthis survey into BL Lacs, flat-spectrum radio quasars(FSRQ) and unclassified blazars. Sources that donot appear in any of these catalogues are listed hereas “Unclassified”. Fifty-nine percent of our sourcesare blazars, while most others are flat-spectrum radiosources (FSRS, [13]).From our survey, 72% of the BL Lacs have shownspectral-index variability, along with 87% of the FS-RQs and 80% of the unclassified blazars, while 89% ofthe unclassified sources also show spectral variation.Of these variable sources, 97% are seen to have a spec-tral turnover frequency at the very highest observedfrequencies in at least one epoch. This indicates thatthe vast majority of our variable sources become pe-riodically brighter at high frequencies, but do not al-ways stay that way. Further work is required to deter-mine whether these high-frequency flux increases areassociated with γ-ray activity.IV. ONGOING WORKObservations at the ATCA continue on a regularbasis, and analysis of the bulk of the data has yetto be completed. This section describes some of theanalysis we plan to do in the near future.The ATCA receivers are capable of measuring po-larisation accurately at all the frequencies observedby this project. It will therefore be possible to exam-ine the polarisation position angle variability for theseblazars, and determine if the magnetic field directionis changing when the spectral index varies.The ATCA monitoring data provides informationcrucial in constructing the spectral energy distribu-tions (SEDs) of AGN both in their flaring and qui-escent states. Quasi-simultaneous broadband SEDsare necessary to distinguish between competing mod-els for the high-energy emission from AGN [2, 9]. TheATCA data are also being used to look for changesin spectral index associated with the ejection of VLBIcomponents from the AGN cores.V. SUMMARYThe ATCA has monitored a set of 133 AGN overmultiple epochs for four years, and will continue tomonitor even more in the future. Through a combina-tion of regular monitoring and target-of-opportunityobservations of flaring AGN, we will investigate themechanisms responsible for both radio and γ-ray emis-sion in their jets.More information can be found on the project web-site: http://goo.gl/WHxmf[1] A.A. Abdo et al. 2010, ApJ, 715, p429[2] A.A. Abdo et al. 2011, ApJ, 719, p1433[3] A.A. Abdo et al. 2011, ApJ, 727, p129[4] A.A. Abdo et al. 2011, ApJ, 730, p101[5] A.A. Abdo et al. 2011, ApJ, 736, p131[6] M. Ackermann et al. 2011, ApJ, 743, p171[7] E. Angelakis et al. 2010, in “Fermi meets Jansky”, T.Savolainen (ed)[8] S.D. Bloom & A.P. Marscher 1996, ApJ, 461, p657[9] M. Boettcher 2007, Ap&SS, 309, p95[10] C.D. Dermer & R. Schlickeiser 1993, ApJ, 416, p458[11] M. Gaylard et al. 2011, ATel #3713[12] R.C. Hartman et al. 1999, ApJ, 123, p79[13] S.E. Healey et al. 2007, ApJSS, 171, p61[14] S.E. Healey et al. 2008, ApJSS, 175, p97[15] S.G. Jorstad et al. 2001, ApJ, 556, p738[16] A. Lahteenmaki & E. Valtaoja 2003, ApJ, 590, p95[17] E. Massaro et al. 2009, A&A, 495, p691[18] W. McConville et al. 2011, ApJ, 738, p148[19] R. Ojha et al. 2010, A&A, 519, A45eConf C11111014 2011 Fermi & Jansky: Our Evolving Understanding of AGN, St Michaels, MD, Nov. 10-12[20] E. Valtaoja & H. Terasranta 1995, A&A, 297, L13[21] W.E. Wilson et al. 2011, MNRAS, 416, p832eConf C11111012011 Fermi & Jansky: Our Evolving Understanding of AGN, St Michaels, MD, Nov. 10-12 5FIG. 1: Measurements of all sources that have displayed spectral index changes during the ATCA monitoring campaign.Each epoch is shown as a different colour/symbol combination. The name of each source is coloured depending on itsclassification in the Roma-BZCAT [17], CGRaBS [14] and 1LAC [1] catalogues, and the colour legend is shown at thebottom right of the plot. The frequency and flux density scales are the same for each plot, and are defined on thelower-left.eConf C1111101

ATCA monitoring of gamma-ray loud AGN

ATCA monitoring of gamma-ray loud AGN

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