Roopesh Ojha

Electron–positron jets associated with the quasar 3C279

A long-standing question in extragalactic astrophysics is the composition of the relativistic jets of plasma that stream from the nuclei of quasars and active galaxies—do they consist of a ‘normal’ (electron–proton) plasma, or a ‘pair’ (electron–positron) plasma? Distinguishing between these possibilities is crucial for understanding the physical processes occurring close to the putative supermassive black holes that are believed responsible for the jets. Here we report the detection of circularly polarized radio emission from the jets of the archtypal quasar 3C279. The circular polarization is produced by Faraday conversion, which requires the energy distribution of the radiating particles to extend to very low energies, indicating that electron–positron pairs are an important component of the jet plasma. Similar detections in three other radio sources suggest that, in general, extragalactic radio jets are composed mainly of an electron–positron plasma.

Parsec-Scale Blazar Monitoring: Flux and Polarization Variability

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.

AST/RO Observations of Atomic Carbon near the Galactic Center

We present a coarsely sampled map of the region |l| ≤ 2°, |b| ≤ 01 in the 492 GHz (3P1 → 3P0) fine-structure transition of neutral carbon, observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The distribution of [C I] emission is similar on the large scale to that of CO J = 1 → 0. On average, the ratio of the integrated intensities, I[C I]/I12CO, is higher in the Galactic disk than in the Galactic center region. This result is accounted for by the absorption of 12CO within the clouds located in the outer Galactic disk. The ratio I[C I]/I12CO is surprisingly uniform over the variety of environments near the Galactic center. On average, [C I] is optically thin [or as optically thin as 13CO (J = 1 → 0)], even in the dense molecular clouds of the Galactic center region.

PARSEC-SCALE BLAZAR MONITORING: THE DATA

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.

Detection of Parsec-Scale Circular Polarization with the VLBA

Circular polarization has been detected in 3C 279 and 3C 84 at multiple epochs, using the VLBA at 15 GHz. This is the first detection of circular polarization in a compact radio source at VLBI resolution. In the case of 3C 279 the polarization is most likely produced by the conversion of linear to circular polarization by low energy relativistic electrons in the emitting region. If this is so, the absence of Faraday rotation by the same electrons suggests that the jet consists of an electron-positron plasma.

Proper motions, flux and polarization variability from a parsec scale blazar monitoring program

Abstract We review the results from a polarization sensitive, six-epoch (at about 2 month intervals), dual frequency (22 and 15 GHz) VLBA program that monitored rapid changes in the structure and polarization of 12 highly active AGN jets. Our analysis of the proper motion, polarization and flux variability of VLBI ‘components’, is in the context of some key questions addressed at this meeting including, dependence of jet velocities on distance from nucleus, structure and evolution of jet magnetic fields and jet morphology.