J. F. C. Wardle

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.

Detection and Measurement of Parsec-Scale Circular Polarization in Four AGNs

We present five epochs of 15 GHz VLBA observations of 13 AGNs. These observations were specially calibrated to detect parsec-scale circular polarization and our calibration techniques are discussed and analyzed in detail. We obtained reliable detections of parsec-scale circular polarization in the radio jets of four AGNs: 3C 84, PKS 0528+134, 3C 273, and 3C 279. For each of these objects our detections are at the level of approximately 0.3%–1% local fractional circular polarization. Each individual detection has a significance in the range of 3 to 10 σ. Our observations are consistent across multiple epochs (and different calibration techniques) in the sign and magnitude of the circular polarization observed. 3C 273 and 3C 279 both undergo core outbursts during our observations and changes in the circular polarization of both sources are correlated with these outbursts. In general, we observe the circular polarization to be nearly coincident with the strong VLBI cores of these objects; however, in 3C 84 the circular polarization is located a full milliarcsecond south of the source peak, and in the 1996.73 epoch of 3C 273 the circular polarization is predominately associated with the newly emerging jet component. Our observations support the theoretical conclusion that emission of circular polarization is a sensitive function of opacity, being strongest when the optical depth is near unity. Circular polarization may be produced as an intrinsic component of synchrotron radiation or by the Faraday conversion of linear to circular polarization. Our single-frequency observations do not easily distinguish between these possible mechanisms, but independent of mechanism, the remarkable consistency across epoch of the sign of the observed circular polarization suggests the existence of a long-term, stable, unidirectional magnetic field. Single-dish observations of 3C 273 and 3C 279 at 8 GHz by Hodge and Aller suggest that this stability may persist for decades in our frame of observation.

Parsec-Scale Circular Polarization Observations of 40 Blazars

We present circular polarization results from a 5 GHz survey of the parsec-scale polarization properties of 40 active galactic nuclei (AGNs) made with the VLBA. We find 11 circular polarization detections at the 3 σ level or higher. This nearly quadruples the number of sources detected in circular polarization at VLBI resolution. We find no correlation between fractional linear and circular polarization across our sample. A likely explanation is external Faraday depolarization in the cores of AGNs which reduces linear polarization but leaves circular polarization unchanged. In comparing ours and other recent results to observations made ~20 years ago, we find that, in five of six cases, sources have the same sign of circular polarization today as they did 20 years ago. This suggests the presence of a long-term property of the jets, such as the polarity of a net magnetic flux, which is stable on timescales much longer than those of individual outbursts.

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A∗

Magnetic fields near the event horizon Astronomers have long sought to examine a black holes event horizon—the boundary around the black hole within which nothing can escape. Johnson et al. used sophisticated interferometry techniques to combine data from millimeter-wavelength telescopes around the world. They measured polarization just outside the event horizon of Sgr A*, the supermassive black hole at the center of our galaxy, the Milky Way. The polarization is a signature of ordered magnetic fields generated in the accretion disk around the black hole. The results help to explain how black holes accrete gas and launch jets of material into their surroundings. Science, this issue p. 1242 Magnetic fields around the event horizon of a supermassive black hole have been probed. Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.

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.

OBSERVATIONAL EVIDENCE AGAINST BIREFRINGENCE OVER COSMOLOGICAL DISTANCES

We show that recent radio and optical observations of polarized radiation from well-resolved high redshift quasars and radio galaxies rule out the cosmological rotation of the plane of polarization claimed recently by Nodland and Ralston [Phys. Rev. Lett. 78, 3043 (1997)]. A least squares fit to the radio data has a slope only 2% of their claimed effect.