VLBI Monitoring of the bright gamma-ray blazar PKS 0537-441
Faith Hungwe, Roopesh Ojha, Matthias Kadler, Roy Booth, Jay Blanchard, Jim Lovell, Cornelia Mueller, Moritz Boeck, TANAMI team
aa r X i v : . [ a s t r o - ph . C O ] J un Fermi meets Jansky – AGN at Radio and Gamma-RaysSavolainen, T., Ros, E., Porcas, R.W., & Zensus, J.A. (eds.)June 21–23, 2010, Bonn, Germany
VLBI Monitoring of the bright γ -ray blazar PKS 0537 − F. Hungwe , R. Ojha , M. Kadler , , , R. Booth , J. Blanchard , J. Lovell , C. M¨uller , M. B¨ock and theTANAMI team Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa NVI/United States Naval Observatory, 3450 Massachusetts Ave, NW, Washington, DC 20392-5420 Dr. Remeis-Sternwarte & ECAP, Universit¨at Erlangen-N¨urnberg, Sternwartstr. 7, 96049 Bamberg, Germany Hartebeesthoek Radio Astronomy Observatory, PO Box 443, Krugersdorp 1740, South Africa School of Mathematics & Physics, Private Bag 37, University of Tasmania, Hobart TAS 7001, Australia CRESST/NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Universities Space Research Association, 10211 Wincopin Circle, Suite 500 Columbia, MD 21044, USA
Abstract.
One of the defining characteristics of BL Lacertae objects is their strong variability across the electro-magnetic spectrum. PKS 0537 −
441 is one such object and is one of the most luminous blazars from radio to γ -raywavelengths. It was detected as a strong and highly variable source by EGRET and has been reported severaltimes to be in an active state by Fermi . It is one of the brightest γ -ray blazars detected in the southern sky so far.The TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) program is monitoringPKS 0537 −
441 at VLBI resolutions. We present 8.4 GHz and 22 GHz images of the milliarcsecond scale structure.We also present our ongoing analysis of the spectral and temporal changes in this object.
1. Introduction
PKS 0537 −
441 is a strongly variable BL Lacertae ob-ject and is one of the brightest γ -ray blazars detectedin the southern sky to date. The source is known tobe a strong intra-day variable and is sometimes classi-fied as a highly polarised quasar (Treves et al. (1993)).
15 10 5 0 −5 −10−10−5051015 10 pc
Fig. 1. . −
441 for ob-servations done in Nov 2007. x- and y-axis are labeled inmilliarcseconds and the hatched ellipse on the bottom leftrepresents the beam of the observing array. It has been a candidate for gravitational microlensing(Romero et al. (1995)) but Heidt et al. (2003) disprovethis and suggest it might be part of binary quasar.Using the Australian Long Baseline Array (LBA) andaffiliated telescopes, the Tracking Active Galactic Nucleiwith Austral Milliarcsecond Interferometry (TANAMI)program (Ojha et al. (2010)) has been monitoring south-ern sky blazars such as PKS 0537 −
441 at VLBI resolu-tions. TANAMI began observing known and possible γ -ray loud extragalactic sources south of -30 degrees abouta year before the launch of Fermi . TANAMI observationsat 8 . Fermi Gamma-Ray Space Telescope .Studying Active Galactic Nuclei (AGN) at differentwavelengths is crucial in order to understand AGN-jetsand differentiate between different models explaining thejets. TANAMI observations of PKS 0537 −
441 complementongoing observations at other wavelengths. This source ison the
Fermi /LAT monitored source list and is also in theLAT 1-year Point Source Catalog (Abdo et al. (2010c)).PKS 0537 −
441 has been reported by
Fermi /LAT tobe active about four times in the past two years. InOctober 2008 and July 2009, the
Fermi /LAT observed anincrease in γ -ray activity in the source (Tosti (2008)) &(Bastieri (2009)). In February of 2010, AGILE detected‘enhanced γ -ray emission above 100 MeV from a sourcepositionally consistent with the blazar PKS 0537 − Fermi /LAT observed increased γ -ray activity in thissource.1 F. Hungwe et. al: VLBI Monitoring of the bright γ -ray blazar PKS 0537 −
15 10 5 0 −5 −10−10−5051015 10 pc
10 5 0 -5 -101050-5
15 10 5 0 −5 −10−10−5051015 10 pc
15 10 5 0 −5 −10−10−5051015 10 pc
10 5 0 -5 -101050-5
15 10 5 0 −5 −10−10−5051015 10 pc
Fig. 2.
Images from 2 epochs of dual frequency observations of PKS 0537 − .
2. Observations and Data Reduction
VLBI observations for the epochs presented were made us-ing the Australian Long Baseline Array (LBA ) togetherwith the Hartebeesthoek 26m dish in South Africa. TheLBA consists of five telescopes, namely, Parkes (64 m),Narrabri (5x22 m), Ceduna (30 m), Hobart (26 m) andMopra (22 m). In addition, TANAMI sometimes has ac-cess to two other telescopes within Australia, the 70 m and34 m telescopes belonging to NASA’s Deep Space Networkat Tidbinbilla. These observations were made at 8 . difmap The Long Baseline Array is part of the Australia Telescopewhich is funded by the Commonwealth of Australia for opera-tion as a National Facility managed by CSIRO. interactive mode (Shepherd (1997)). For details of the ob-servations, calibration and imaging see Ojha et al. (2010).Here we present VLBI images from three epochs of obser-vations two of which have data both at X and K-band.The data used for the radio light curve was taken us-ing the University of Tasmania’s 30 m telescope. This dish,together with the 26 m dish at Hobart, have since August2007, been monitoring TANAMI sources (PI: Jim Lovell).Hobart observes at 2 . . . γ -ray observations were made using the LAT. The Fermi /LAT has a number of observing modes but spendsmost of its time in survey modes where the whole skyis scanned once every two orbits. For each event, the
Fermi /LAT measures 3 quantities, the arrival direction,the energy and the arrival time. All
Fermi /LAT data . Hungwe et. al: VLBI Monitoring of the bright γ -ray blazar PKS 0537 −
441 3 shown here were downloaded from the public website .The data were reduced using Fermi science tools usingmonthly time bins and an energy range of 100 MeV to300 GeV. GTSELECT was used to specify the time (inmission elapsed time) and energy range and the regionof interest. To make good time intervals, removing datawhen the Fermi spacecraft was over the South AtlanticAnomaly, GTMKTIME was used. Livetime cubes werecalculated using GTLTCUBE while GTEXPMAP wasused to generate exposure maps for the region of interest.GTLIKE was then used to perform the likelihood analysisof the
Fermi /LAT data. The output of GTLIKE gives theintegrated flux and the test statistic among other param-eters.
3. TANAMI images of PKS 0537 − Figure 1 shows an 8 . . . γ -ray flare observed weeksleading to October 2008. The core is unresolved at 22 GHzfor both epochs. Observations at the two frequencies al-low calculations of the spectral indices which, when usedtogether with indices from other energy bands, e.g. γ -ray,give the broadband Spectral Energy Distribution (SEDs)needed to understand Active Galactic Nuclei.The mean flux density ¯ S at the 8 . S peak ≈ . − while ¯ S total ≈ .
75 Jy. The peakflux density S peak at 8 . .
80 Jybeam − , while the total flux density S total has a standarddeviation of 0 .
45 Jy. At 22 GHz, there is a slight decrease inthe flux density from the March 2008 epoch to the August2008 epoch.Figure 2 shows the spectral index maps derived fromthe two observing frequencies, 8 . F ν ∼ ν α . The spectral in-dex maps of PKS 0537 −
441 were calculated according tothis definition where S . ≥ σ . and S ≥ σ . The color coding shows the changing of the spec-tral index from optically thin to optically thick emissionregions. The overlaid contours show the flux density distri-bution at 8.4 GHz folded with the beam of the correspond-ing 22 GHz image. The core of the source has an almostperfectly flat spectrum, while the jet has an optically thin http://fermi.gsfc.nasa.gov/cgi-bin/ssc/LAT/LATDataQuery.cgi Fermi science tools can be downloaded from http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/ spectrum with a hint of steepening with time. As we havenot modeled any optically thin components, we have notbeen able to calculate a core shift for PKS 0537 −
441 yet.
4. Radio and γ -ray Light Curves of PKS 0537-441 F l u x _ ( M EV ) pho t on . c m ^ { - } . s ^ { - } x ^ { - } Time (MJD) "correcttime.txt"
Fig. 3.
Gamma-ray light curve for PKS 0537 −
441 span-ning ∼
21 months from August 2008 to May 2010.Figures 3 and 4 show the γ -ray light curve for datafrom August 2008 to May 2010 and the radio light curvefor the period 14 April to 28 May 2010, respectively. Thepoints on the γ -ray light curve represent monthly bin sizes.Photons with energies from 100 MeV to 300 GeV were con-sidered.PKS 0537 −
441 has been reported by the Astronomer’sTelegram to be in an active state in October 2008, July2009, February 2010 and April 2010. The γ -ray light curveshows the source to be active in September 2008, July 2009and April 2010. When reducing the data, we used monthlybin sizes and this is a possible source of the disparity. Also,most telegrams are sent at the onset of increased activityand not necessarily the peak of such activity.The data for the radio light curve was taken at 6 . F. Hungwe et. al: VLBI Monitoring of the bright γ -ray blazar PKS 0537 − Table 1.
Image parameters and observation characteristics
Frequency Epoch RMS S peak S total θ maj θ min P.A.[GHz] yyyy-mm-dd [mJy beam − ] [Jy beam − ] [Jy] [mas] [mas] [ ◦ ]8.4 2007-11-10 0.37 ± .
40 4.85 5.10 2.2 0.7 128.4 2008-03-28 0.58 ± .
11 3.38 4.24 2.8 0.6 -18.4 2008-08-08 0.39 ± .
35 4.67 4.91 3.5 1.9 -322.3 2008-03-26 0.47 ± .
99 3.12 3.43 1.9 1.2 -7022.3 2008-08-08 0.47 ± .
36 3.00 3.21 1.7 1.3 -72
Fig. 4.
Radio light curve for PKS 0537 − γ -ray curve and does not show evidence forvariability. We will continue to monitor this source in theradio to look for possible connections between the radioand γ -ray lightcurves.
5. Conclusion
Multi-epoch TANAMI observations of the highly luminousblazar PKS 0537 −
441 at two radio frequencies and sin-gle dish monitoring data from the Ceduna radio telescopeare presented here in addition to a γ -ray light curve from Fermi /LAT. We have used simultaneous observations at8 . Fermi to construct SEDs and understand the be-haviour of this interesting object. Modeling of multiple epochs of data in order to understand the kinematic be-haviour is also in progress.
Acknowledgements.
FH acknowledges support from the SouthAfrican SKA Project and Hartebeesthoek Radio AstronomyObservatory. This research has been partially funded by theFermi Guest Investigator Program. This research has beenpartially funded by the Bundesministerium f¨ur Wirtschaftund Technologie under Deutsches Zentrum f¨ur Luft- undRaumfahrt grant number 50OR0808.