Browsing by Author "Blanchard, J. M."
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Item The Gamma-Ray Blazar PKS 0208-512, a Multi-Wavelength Investigation(2012-06-27) Blanchard, J. M.; Lovell, J. E. J.; Dickey, J.; Ojha, R.; Kadler, M.; Nesci, R.; Edwards, P. G.; Stevens, J.; Dutka, M.; Muller, C.; Pursimo, T.The gamma-ray blazar PKS 0208-512 has shown strong periods of flaring, at all frequencies from radio to gamma-ray. This has led to its inclusion in the TANAMI project, which tracks the jets of southern AGN using VLBI as well as supporting flux density monitoring programs. Time series analysis of the light curves generated by such monitoring is presented and discussed and VLBI maps of the source are used to show the evolution in the jet. A frequency dependent lag is observed between flaring at different radio frequencies which does not appear to correspond to purely optical depth effects. Major flaring at gamma-ray frequencies appears to be preceded by a new component in the jet seen in our VLBI data.Item High resolution rapid response observations of compact radio sources with the Ceduna Hobart Interferometer (CHI)(EDP sciences, 2012-02-17) Blanchard, J. M.; Lovell, J. E. J.; Ojha, R.; Kadler, M.; Dickey, J. M.; Edwards, P. G.Context. Frequent, simultaneous observations across the electromagnetic spectrum are essential to the study of a range of astrophysical phenomena including active galactic nuclei. A key tool of such studies is the ability to observe an object when it flares i.e. exhibits a rapid and significant increase in its flux density. Aims. We describe the specific observational procedures and the calibration techniques that have been developed and tested to create a single baseline radio interferometer that can rapidly observe a flaring object. This is the only facility that is dedicated to rapid high resolution radio observations of an object south of −30 degrees declination. An immediate application is to provide rapid contemporaneous radio coverage of AGN flaring at γ-ray frequencies detected by the Fermi Gamma-ray Space Telescope. Methods. A single baseline interferometer, the Ceduna Hobart Interferometer (CHI), was formed with radio telescopes in Hobart, Tasmania and Ceduna, South Australia. A software correlator was set up at the University of Tasmania to correlate these data. Results. Measurements of the flux densities of flaring objects can be made using our observing strategy within half an hour of a triggering event. These observations can be calibrated with amplitude errors better than 15%. Lower limits to the brightness temperatures of the sources can also be calculated using CHI.Item PKS 2123−463: a confirmed γ-ray blazar at high redshift(Oxford University Press, 2012-11-21) D'Ammando, F.; Rau, A.; Schady, P.; Finke, J.; Orienti, M.; Greiner, J.; Kann, D. A.; Ojha, R.; Foley, A. R.; Stevens, J.; Blanchard, J. M.; Edwards, P. G.; Kadler, M.; Lovell, J. E. J.The flat spectrum radio quasar (FSRQ) PKS 2123−463 was associated in the first Fermi-Large Area Telescope (LAT) source catalogue with the γ-ray source 1FGL J2126.1−4603, but when considering the full first two years of Fermi observations, no γ-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123−463 was not reported in the second Fermi-LAT source catalogue. On 2011 December 14 a γ-ray source positionally consistent with PKS 2123−463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, Gamma-ray Optical/Near-Infrared Detector (GROND), Australia Telescope Compact Array (ATCA), Ceduna and Seven Dishes Karoo Array Telescope (KAT-7) observatories. Results of the localization of the γ-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, ultraviolet (UV) and X-ray data collected soon after the γ-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and γ-ray bands led to a firm identification of the γ-ray source with PKS 2123−463. A new photometric redshift has been estimated as z = 1.46 ± 0.05 using GROND and Swift Ultraviolet/Optical Telescope (UVOT) observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broad-band spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disc component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disc has a luminosity of ∼1.8 × 10⁴⁶ erg s⁻¹, and a fit to the disc emission assuming a Schwarzschild (i.e. non-rotating) black hole gives a mass of ∼2 × 10⁹ M⊙. This is the first black hole mass estimate for this source.Item Unusual flaring activity in the blazar PKS 1424−418 during 2008−2011(EDP sciences, 2014-09-16) Buson, S.; Longo, F.; Larsson, S.; Cutini, S.; Finke, J.; Ciprini, S.; Ojha, R.; D'Ammando, F.; Donato, D.; Thompson, D. J.; Desiante, R.; Bastieri, D.; Wagner, S.; Hauser, M.; Fuhrmann, L.; Dutka, M.; Müller, C.; Kadler, M.; Angelakis, E.; Zensus, J. A.; Stevens, J.; Blanchard, J. M.; Edwards, P. G.; Lovell, J. E. J.; Gurwell, M. A.; Wehrle, A. E.; Zook, A.Context. Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission. Aims. We report peculiar behavior in the bright γ-ray blazar PKS 1424−418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet. Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous γ-ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar. Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424−418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low γ-ray activity. Although the optical/γ-ray behavior of PKS 1424−418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters. Conclusions. Detailed studies of individual blazars like PKS 1424−418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN.