Browsing by Author "Lazio, T. Joseph W."
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Item Angular Broadening of Intraday Variable AGNs. II. Interstellar and Intergalactic Scattering(IOP, 2008-01-01) Lazio, T. Joseph W.; Ojha, Roopesh; Fey, Alan L.; Kedziora-Chudczer, Lucyna; Cordes, James M.; Jauncey, David L.; Lovell, James E. J.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 exhibit centimeter-wavelength intraday variability (interstellar scintillation), while the other 25% do not show intraday variability. We find that interstellar scattering is measurable for most of these AGNs, and the typical broadening diameter is 2 mas at 1 GHz. We find that the scintillating AGNs are typically at lower Galactic latitudes than the nonscintillating AGNs, consistent with the scenario that intraday variability is a propagation effect from the Galactic interstellar medium. The magnitude of the inferred interstellar broadening measured toward the scintillating AGNs, when scaled to higher frequencies, is comparable to the diameters inferred from analyses of the light curves for the more well-known intraday variable sources. However, we find no difference in the amount of scattering measured toward the scintillating versus nonscintillating AGNs. A consistent picture is one in which the scintillation results from localized regions (‘‘clumps’’) distributed throughout the Galactic disk, but that individually make little contribution to the angular broadening. Of the 58 AGNs observed, 37 (64%) have measured redshifts. At best, a marginal trend is found for scintillating (nonscintillating) AGNs to have smaller (larger) angular diameters at higher redshifts. We also use our observations to try to constrain the possibility of intergalactic scattering. While broadly consistent with the scenario of a highly turbulent intergalactic medium, our observations do not place significant constraints on its properties.Item Milliarcsecond Structure of Microarcsecond Sources(Proceedings of Science, 2007-06-12) Ojha, Roopesh; Lazio, T. Joseph W.; Fey, Alan L.; Jauncey, David L.; Lovell, James E. J.; Johnston, Kenneth J.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 determined by the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Both single frequency (8.4 GHz) observations to compare morphologies, and multi-frequency observations (from 330 MHz through 22 GHz) to study scattering behaviour have been carried out. Though scintillation probes the structure of quasars on scales of tens of microarcseconds, information at the milliarcsecond scale probed by these VLBI observations is proving invaluable in furthering our understanding of this phenomenon. Also, the common occurence of scintillation in compact radio sources might be the ultimate limitation for VLBI astrometry (which among other things defines the International Celestial Reference Frame) in the sense that the final precision of reference frame source positions are likely to be influenced by the presence of scintillation.Item Scatter Broadening of Scintillating and Nonscintillating AGNs. I. A Multifrequency VLBA Survey(IOP, 2006-09-01) Ojha, Roopesh; Fey, Alan L.; Lazio, T. Joseph W.; Jauncey, David L.; Lovell, James E. J.; Kedziora-Chudczer, LucynaMultiwavelength Very Long Baseline Array observations of a sample of 49 extragalactic radio sources selected from the Micro-Arcsecond Scintillation-Induced Variability survey are presented. These observations are intended to provide a sizeable, uniform data set for the investigation of angular broadening of scintillating sources. These data will be used to compare and contrast the scattering behavior of scintillating and nonscintillating radio sources in subsequent analysis. We confirm earlier analyses that found scintillating sources to be more core-dominated than nonscintillating sources.