D. H. Roberts

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.

Milliarcsecond Polarization Structure of 24 Objects from the Pearson-Readhead Sample of Bright Extragalactic Radio Sources. II. Discussion

The observations (Paper I) of the milliarcsecond-scale linear polarization properties of compact extragalactic radio sources from the Pearson-Readhead sample are discussed. In general, the linear polarization properties correlate strongly with optical identification, and, for the sources in this sample, galaxies, quasars, and BL Lacertae objects are quite distinct. None of the compact radio sources identified with galaxies was detected in polarization on milliarcsecond scales. In contrast, most quasars and BL Lacertae objects have polarized structures, especially if they have prominent jets. We find systematic differences between quasars and BL Lacertae objects in the core polarizations, the orientations of the inferred magnetic fields in the jets, the visible lengths of the jets, and the behavior of the run of fractional polarization along the jets

Radio constraints on the nature of BL Lacertae objects and their parent population

5 GHz VLA observations of 17 BL Lac objects with bright radio cores at both high and low resolution are reported. Extended emission is detected around most objects. None of the sources observed at low resolution show evidence of giant halos on the scale of tens of arcmin. In general, the sources with the most luminous extended emission exhibit FR II characteristics in both morphology and polarization, and less luminous sources exhibit FR I characteristics. Thus, the parent population of the BL Lac objects contains both FR I and FR II radio sources. No BL Lac objects are found that clearly exhibit quasarlike polarization at milliarcsec resolution. This argues against the view that the more luminous BL Lac objects are simply an extension of the quasar/OVV population, or that most BL Lac objects are gravitationally microlensed images of distant quasars. Other properties are generally consistent with the view the BL Lac objects are normal radio galaxies whose jets make a small angle to the line of sight.

Radio Jet-Ambient Medium Interactions on Parsec Scales in the Blazar 1055+018

As part of our study of the magnetic fields of active galactic nuclei, we have recently observed a large sample of blazars with the Very Long Baseline Array. Here we report the discovery of a striking two-component jet in the source 1055+018 that consists of an inner spine with a transverse magnetic field and a fragmentary but distinct boundary layer with a longitudinal magnetic field. The polarization distribution in the spine strongly supports shocked-jet models, while that in the boundary layer suggests interaction with the surrounding medium. This behavior suggests a new way to understand the differing polarization properties of strong- and weak-lined blazars.

A survey of the milliarcsecond polarization properties of BL Lacertae objects at 5 GHz

Consideration is given to millarcsecond-resolution total-intensity and linear polarization maps at 5 GHz presented for 11 BL Lacertae objects. In every BL Lacertae object in which polarization structure was detected, the polarization position angle of the knots is nearly parallel to the VLBI structural axis. The direction of the polarization in the cores of these sources appears to be random. The preferred polarization direction in the jets is explained by the fact that plane perpendicular shock waves are common in these sources; the origin of the absence of the preferred polarization direction in the core components is unclear. These results support an early conclusion that it cannot be the case that a significant number of BL Lacertae objects are gravitationally microlensed images of more distant quasars, since the characteristic VLBI polarization structures observed in these two types of objects are very different.

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.

One-Dimensional Hydrogen Atom.

The time-independent Schrodinger equation is solved for the bound state solutions of the one-dimensional Coulomb potential, −e2/|x|. The wave functions obtained are normalizable and continuous. The energy spectrum consists of a set of discrete levels with energies equal to the Bohr energies of hydrogen, and a set of continuum levels with energies lying strictly between the discrete levels. The odd wave functions associated with the discrete levels are differentiable everywhere, but the even wave functions associated with the continuum levels have a cusp with infinite slope at the origin. The energy levels are not degenerate. Following the method of Loudon, the bound state solutions of a truncated Coulomb potential −e2/(|x|+a), a>0, are also obtained. For small a, the discrete spectrum of this potential contains: (1) energy levels which approach arbitrarily close to the discrete levels of the true Coulomb potential as a→0, with odd wave functions identical to the odd wave functions of the true Coulomb pote...

The radio time delay in the double quasar 0957 + 561

Gravitational lens models for the double quasar 0957+561 predict that there should be a time delay of roughly 1 year between the arrival of wave fronts from the two images. In order to measure this delay, the flux densities were monotored at λ = 6 cm of 0957+561A and 0957+561B since 1979. The flux curves of the two images constructed from the VLA data are presented, which are accurate to 2% of the image flux densities. The VLA data are best fitted by a time delay of 1.40 ± 0.1 yr (513 ± 40 days, A leading), with a magnification ratio (B/A) of 0.697 ± 0.003. The error estimates were obtained through Mont Carlo methods, and include 68% of the simulated solutions

Evidence for Ordered Magnetic Fields in the Quasar Environment

At a distance of 20 pc from the purported supermassive black hole powering quasars, temperatures and densities are inferred from optical observations (Osterbrock) to be ~104 K and ~104 cm-3. Here we present very long baseline interferometry radio observations revealing organized magnetic fields on the parsec scale in the hot plasma surrounding the quasar OQ 172 (1442+101). These magnetic fields rotate the plane of polarization of the radio emission coming from the core and inner jet of the quasar. The derived rotation measure (RM) is 40,000 rad m-2 in the rest frame of the quasar. Only 10 mas (a projected distance of 68 pc) from the nucleus, the jet absolute values of RM fall to less than 100 rad m-2.

Superluminal motion in the BL Lacertae object OJ 287

New observations of the total intensity structure of the BL Lacertae object OJ 287 have been made with an angular resolution of 7 x 1 mas at 6 cm. The source consists of a core and three knots in a VLBI jet at position angle of about -100 deg. Previously suspected superluminal motion in the outer two knots (based primarily on VLBI polarimetry) has been confirmed. It is suggested that the comparatively modest superluminal motions seen in BL Lacertae objects are due to systematically slower jet pattern speeds than those of other superluminal sources (mostly quasars). 16 references.