Browsing by Author "Aller, Hugh D."
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Item Parsec-Scale Blazar Monitoring: Flux and Polarization Variability(IOP, 2002-03-20) Homan, Daniel C.; Ojha, Roopesh; Wardle, John F. C.; Roberts, David H.; Aller, Margo F.; Aller, Hugh D.; Hughes, Philip A.We present analysis of the flux density and polarization variability of parsec-scale radio jets from a dual-frequency, six-epoch, VLBA polarization experiment monitoring 12 blazars. The observations were made at 15 and 22 GHz at bimonthly intervals over 1996. Here we analyze the flux density, fractional polarization, and polarization position angle behavior of core regions and jet features, considering both the linear trends of these quantities with time and more rapid fluctuations about the linear trends. The dual frequency nature of the observations allows us to examine spectral evolution, to separate Faraday effects from changes in magnetic field order, and also to deduce empirical estimates for the uncertainties in measuring properties of VLBI jet features (see the Appendix). Our main results include the following: On timescales >=1 yr, we find that jet features generally decayed in flux, with older features decaying more slowly than younger features. Using the decay rates of jet features from six sources, we find I ∝ R⁻¹·³±⁰·¹. Short-term fluctuations in flux tended to be fractionally larger in core regions than in jet features, with the more compact core regions having the larger fluctuations. We find significant spectral index changes in the core regions of four sources. Taken together these are consistent with an outburst-ejection cycle for new jet components. Jet features from one source showed a significant spectral flattening over time. Jet features either increased in fractional polarization with time or showed no significant change, with the smallest observed changes in the features at the largest projected radii. Increasing magnetic field order explains most of the increasing fractional polarization we observed. Only in the case of 3C 273 is there evidence of a feature emerging from behind a Faraday depolarizing screen. We find a number of significant polarization angle rotations, including two very large (>=180°) rotations in the core regions of OJ 287 and J1512-09. In general, polarization angle changes were of the same magnitude at both observing bands and cannot be explained by Faraday rotation. The observed polarization angle changes most likely reflect underlying changes in magnetic field structure. In jet features, four of the five observed rotations were in the direction of aligning the magnetic field with the jet axis, and coupled with the tendency of jet features to show a fractional polarization increase, this suggests increasing longitudinal field order.Item Parsec-Scale Blazar Monitoring: Proper Motions(IOP, 2001-03-10) Homan, Daniel C.; Ojha, Roopesh; Wardle, John F. C.; Roberts, David H.; Aller, Margo F.; Aller, Hugh D.; Philip A. HughesWe present proper motions obtained from a dual-frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 and 22 GHz at bimonthly intervals over 1996. A total of 10 of the 11 sources for which proper motion could be reliably determined are superluminal. Only J2005+77 has no superluminal components. Three sources (OJ 287, J1224+21, and J1512-09) show motion faster than 10h⁻¹c, requiring γpattern of at least 10h⁻¹ (H0 = 100h km s⁻¹ Mpc⁻¹). We compare our results to those in the literature and find motions outside the previously observed range for four sources. While some jet components exhibit significant nonradial motion, most motion is radial. In at least two sources there are components moving radially at significantly different structural position angles. In five of six sources (3C 120, J1224+21, 3C 273, 3C 279, J1512-09, and J1927+73) that have multiple components with measurable proper motion, the innermost component is significantly slower than the others, suggesting that acceleration occurs in the jet. In the motions of individual components we observe at least one decelerating motion and two "bending" accelerations that tend to align their motions with larger scale structure. We also discuss in detail our procedures for obtaining robust kinematical results from multifrequency VLBI data spanning several epochs.Item Parsec-Scale Blazar Monitoring: The Data(IOP, 2004-01-01) Ojha, Roopesh; Homan, Daniel C.; Roberts, David H.; Wardle, John F. C.; Aller, Margo F.; Aller, Hugh D.; Hughes, Philip A.We present the images and modeling data obtained from a dual frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 and 22 GHz at evenly spaced, bimonthly intervals over 1996. The advent of the VLBA makes possible a data set with reliable calibration as well as regular and frequent temporal sampling. Detection of circular polarization, proper motion studies, and flux and polarization variability in the sample are some of the topics that such a data set makes available for robust investigation.Item VLBA Polarimetry: A Faraday Screen in 3C 273(Astronomical Society of the Pacific Conference Series, 2005) Ojha, Roopesh; Homan, Daniel C.; Wardle, John F. C.; Roberts, David H.; Aller, Hugh D.; Aller, Margo F.; Hughes, Philip A.The polarization of superluminal radio components moving at the base of the VLBI jet in 3C273 are tracked as part of a six epoch monitoring campaign at 15 and 22 GHz. The components rise sharply in fractional polarization as they move outward, suggesting either (1) an increase in intrinsic B-field order or (2) an emergence from behind a Faraday depolarizing screen. We prefer the second possibility and discuss a model that provides depolarization with little observable Faraday rotation.