Now showing items 1-20 of 117

    • Electron-positron jets associated with the quasar 3C279 

      Wardle, J.F.C.; Homan, D.C.; Ojha, R.; Roberts, D.H. (Brandeis University, 1998)
      A long outstanding problem in extragalactic astrophysics is the composition of the relativistic jets of plasma streaming from the nuclei of quasars and active galaxies - whether it is predominantly a "normal" (electron-proton) ...
    • AST/RO Observations of Atomic Carbon near the Galactic Center 

      Ojha, Roopesh; Stark, Antony A.; Hsieh, Henry H.; Lane, Adair P.; Chamberlin, Richard A.; Bania, Thomas M.; Bolatto, Alberto D.; Jackson, James M.; Wright, Gregory A. (IOP, 2001-02-10)
      We present a coarsely sampled map of the region |l| ≤ 2°, |b| ≤ 0°1 in the 492 GHz (³P₁ → ³P₀) fine-structure transition of neutral carbon, observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). ...
    • Accurate Astrometry of 22 Southern Hemisphere Radio Sources 

      Fey, Alan L.; Ojha, Roopesh; Jauncey, David L.; Johnston, Kenneth J.; Reynolds, John E.; Lovell, James E. J.; Tzioumis, Anastasios K.; Quick, Jonathan F. H.; Nicolson, George D.; Ellingsen, Simon P.; McCulloch, Peter M.; Koyama, Yasuhiro (IOP, 2003-12-03)
      Milliarcsecond-accurate radio positions for 22 southern hemisphere extragalactic sources are reported. These positions are derived from Mark III very long baseline interferometry observations made between 2003 February and ...
    • The Impact of Interstellar Scintillation on Astrometric VLBI 

      Jauncey, David; Lovell, James; Ojha, Roopesh; Fey, Alan L.; Koyama, Yasuhiro; Bignall, Hayley E.; Kedziora-Chudczer, Lucyna; Macquart, Jean-Pierre; Rickett, Barney J.; Titov, Oleg; Tzioumis, Anastasios K. (2004)
      We review the evidence demonstrating the interstellar scintillation (ISS) is the principal mechanism responsible for the intra-day variability (IDV) seen at cm wavelengths in many compact, flat-spectrum extra-galactic radio ...
    • Astrometry of 25 Southern Hemisphere Radio Sources from a VLBI Short-Baseline Survey 

      Fey, Alan L.; Ojha, Roopesh; Reynolds, John E.; Ellingsen, Simon P.; McCulloch, Peter M.; Jauncey, David L.; Johnston, Kenneth J. (IOP, 2004-11)
      Milliarcsecond-accurate radio positions for 25 southern hemisphere extragalactic sources are reported. These positions are derived from Mark III Very Long Baseline Interferometry observations made between 2003 September ...
    • MASIV:The Microarcsecond Scintillation-Induced Variability Survey 

      Lovell, J.E.J.; Jauncey, D.L.; Senkbeil, C.; Shabala, S.; Bignall, H.E.; Pursimo, T.; Ojha, R.; Macquart, J.-P.; Rickett, B.J.; Dutka, M.; Kedziora-Chudczer, L. (Astronomical Society of the Pacific, 2007-01-21)
      We are undertaking a large-scale 5 GHz VLA survey of the northern sky to search for rapid intra-day variability (IDV). Over four observing epochs we found that 56% of the sources showed variability on timescales of hours ...
    • Milliarcsecond Structure of Microarcsecond Sources 

      Ojha, Roopesh; Lazio, T. Joseph W.; Fey, Alan L.; Jauncey, David L.; Lovell, James E. J.; Johnston, Kenneth J. (Proceedings of Science, 2007-06-12)
      We present the key findings of an ongoing program of Very Long Baseline Interferometry (VLBI) observations of scintillating and non-scintillating flat-spectrum extragalactic radio sources whose scintillation status was ...
    • The MicroArcsecond Scintillation-Induced Variability (MASIV) Survey: Detection of Angular Broadening at High Redshift? 

      Pursimo, T.; Ojha, R.; Jauncey, D.; Lovell, J.; Dutka, M.; Bignall, H.; Macquart, J.-P.; Rickett, B.; Kedziora-Chudczer, L. (Astronomical Society of the Pacific, 2008)
      Interstellar scintillation has been shown to be primarily responsible for intra-day variability exhibited by extragalactic sources at centimeter wavelengths. The recent MASIV survey has shown that a very high proportion ...
    • Angular Broadening of Intraday Variable AGNs. II. Interstellar and Intergalactic Scattering 

      Lazio, T. Joseph W.; Ojha, Roopesh; Fey, Alan L.; Kedziora-Chudczer, Lucyna; Cordes, James M.; Jauncey, David L.; Lovell, James E. J. (IOP, 2008-01-01)
      We analyze a sample of 58 multiwavelength, Very Long Baseline Array observations of active galactic nuclei (AGNs) to determine their scattering properties. Approximately 75% of the sample consists of AGNs that ...
    • Enormous disc of cool gas surrounding the nearby powerful radio galaxy NGC 612 (PKS 0131−36) 

      Emonts, B. H. C.; Morganti, R.; Oosterloo, T. A.; Holt, J.; Tadhunter, C. N.; Van Der Hulst, J. M.; Ojha, R.; Sadler, E. M. (Oxford University Press, 2008-05-09)
      We present the detection of an enormous disc of cool neutral hydrogen (H I) gas surrounding the S0 galaxy NGC 612, which hosts one of the nearest powerful radio sources (PKS 0131−36). Using the Australia Telescope Compact ...
    • The Second Realization of the International Celestial Reference Frame by Very Long Baseline Interferometry 

      Ojha, R.; et al. (International Earth Rotation and Reference Systems Service, 2009)
      This Technical Note describes the generation by an international team of the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30 years of Very Long Baseline ...
    • Intraday Radio Variability and Micro-Arcsecond Resolution 

      Bignall, Hayley E.; Jauncey, David L.; Kedziora-Chudczer, Lucyna; Lovell, James E. J.; Macquart, Jean-Pierre; Ojha, Roopesh; Rickett, Barnaby J. (Astronomical Society of the Pacific, 2009)
      Intraday variability (IDV) of AGN at centimeter wavelengths has been shown to be predominantly due to interstellar scintillation (ISS). Recent VLBI observations have shown that IDV sources are more compact than non-IDVs ...
    • The Spectral Energy Distribution of Fermi Bright Blazers 

      Ojha, Roopesh; Vasileiou, V.; Celik, O.; et al. (The American Astronomical Society, 2010-05-13)
      e have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, ...
    • High-Resolution Observations of Centaurus A 

      Muller, Cornelia; Blanchard, J.; Bock, M.; Carpenter, B.; Dutka, M.; Furst, F.; Grafe, C.; Großberger, C.; Kadler, M.; Krauß, F.; Markowitz, A.; Ojha, R.; Perucho, M.; Pottschmidt, K.; Ros, E.; Rothschild, R.; Schulz, R.; Trustedt, J.; Wilms, J.; the TANAMI Collaboration (European Space Agency, 2011)
      The closest active galaxy Centaurus A is regularly observed at milliarcsecond resolution within the framework of the Southern Hemisphere AGN monitoring program TANAMI. Additional multiwavelength observations up to gamma-ray ...
    • Dual-frequency VLBI study of Centaurus A on sub-parsec scales. The highest-resolution view of an extragalactic jet 

      Müller, C.; Kadler, M.; Ojha, R.; Wilms, J.; Böck, M.; Edwards, P. G.; Fromm, C. M.; Hase, H.; Horiuchi, S.; Katz, U.; Lovell, J. E. J.; Plötz, C.; Pursimo, T.; Richers, S.; Ros, E.; Rothschild, R. E.; Taylor, G. B.; Tingay, S. J.; Zensus, J. A. (EDP sciences, 2011-05-20)
      Context. Centaurus A is the closest active galactic nucleus. High resolution imaging using Very Long Baseline Interferometry (VLBI) enables us to study the spectral and kinematic behavior of the radio jet–counterjet system ...
    • DUAL-FREQUENCY OBSERVATIONS OF 140 COMPACT, FLAT-SPECTRUM ACTIVE GALACTIC NUCLEI FOR SCINTILLATION-INDUCED VARIABILITY 

      Koay, J. Y.; Macquart, J.-P.; Rickett, B. J.; Bignall, H. E.; Lovell, J. E. J.; Reynolds, C.; Jauncey, D. L.; Pursimo, T.; Kedziora-Chudczer, L.; Ojha, R. (IOP, 2011-08-30)
      The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in interstellar scintillation (ISS) for sources at redshifts z gsim 2, indicating an apparent increase in angular diameter or a ...
    • Characterization of long baseline calibrators at 2.3 GHz 

      Hungwe, F.; Ojha, R.; Booth, R. S.; Bietenholz, M. F.; Collioud, A.; Charlot, P.; Boboltz, D.; Fey, A. L. (Oxford University Press, 2011-12-08)
      We present a detailed multi-epoch analysis of 31 potential Southern hemisphere radio calibrators that were originally observed as part of a programme to maintain the International Celestial Reference Frame. At radio ...
    • ATCA monitoring of gamma-ray loud AGN 

      Stevens, J.; Edwards, P. G.; Ojha, R.; Kadler, M.; Hungwe, F.; Dutka, M.; Tingay, S.; Macquart, J. P.; Moin, A.; Lovell, J.; Blanchard, J. (2012-11-12)
      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 ...
    • FERMI LARGE AREA TELESCOPE DETECTION OF GRAVITATIONAL LENS DELAYED γ-RAY FLARES FROM BLAZAR B0218+357 

      Cheung, C. C.; Larsson, S.; Scargle, J. D.; Amin, M. A.; Blandford, R. D.; Bulmash, D.; Chiang, J.; Ciprini, S.; Corbet, R. H. D.; Falco, E. E.; Marshall, P. J.; Wood, D. L.; Ajello, M.; Bastieri, D.; Chekhtman, A.; D'Ammando, F.; Giroletti, M.; Grove, J. E.; Lott, B.; Ojha, R.; Orienti, M.; Perkins, J. S.; Razzano, M.; Smith, A. W.; Thompson, D. J.; Wood, K. S. (IOP, 2014-01-30)
      Using data from the Fermi Large Area Telescope (LAT), we report the first clear γ-ray measurement of a delay between flares from the gravitationally lensed images of a blazar. The delay was detected in B0218+357, a known ...
    • Exceptional NIR flare in gamma-ray loud FSRQ PKS B1424-418 

      Nesci, R.; Ojha, R.; Kadler, M. (2015-01-26)
      The gamma-ray loud flat spectrum radio quasar PKS 1424-418 has been steadily increasing its luminosity since August 2017 and is now (2018-01-21) at its historic recorded maximum of J=13.73 +/- 0.05. These observations were ...