Variability of the Centimeter-Submillimeter Spectrum and Polarization of 3C 273 during Outburst

Original article can be found at: http://www.journals.uchicago.edu/ApJ/--Copyright University of Chicago Press/ AASCentimeter to submillimeter total flux and polarization monitoring data are used to investigate the nature of a prominent flare in the quasar 3C 273 during 1995/6. After removal of the quiescent level, the resulting “flare spectra" are well fitted by a simple homogeneous synchrotron source model, which in turn allows the movement of the self-absorption turnover to be tracked during the flare. Both the flare amplitude/time delay relationship and the overall spectral evolution are qualitatively consistent with existing models. The early evolution of the spectrum is best determined and is shown to be in excellent agreement with the Compton stage of the Marscher & Gear shock model. However, the polarization behavior during the flare is different at millimeter and centimeter wavelengths and the observations are difficult to reconcile with a simple transverse shock. They are, however, consistent with a conical shock for which the observed polarization properties vary with distance along the jet. Such variations may be caused, for example, by a change in cone angle owing to disruption caused by the growing component of the magnetic field parallel to the jet axis or by a moderate change in viewing angle.Peer reviewe

Polarimetric Observations of 15 AGNs at High Frequencies

Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificWe have obtained total and polarized intensity images of 15 AGNs with the VLBA at 7 mm at 17 epochs from 25/26 March 1998 to 14 April 2001. The VLBA observations are accompanied at many epochs by simultaneous mea- surements of polarization at 1.35/0.85 mm as well as less frequent simultaneous optical polarization measurements. We discuss the similarities and complexities of polarization behavior at different frequencies along with the VLBI properties

Compact extragalactic radio sources

SIGLEAvailable from British Library Document Supply Centre- DSC:D58590/86 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Multi-frequency Polarization Properties of Blazars

Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificWe have obtained total and polarized intensity images of 15 AGNs with the VLBA at 7 mm at 17 epochs, accompanied at many epochs by nearly simultaneous measurements of polarization at 1.35/0.85 mm and at optical wavelengths. Analysis of the data shows an obvious connection between the polarized emission at high frequencies and the most strongly polarized features in parsecscale jets. The largest deviations between polarization parameters at different frequencies are detected when a new compact knot emerges from the core, and are likely caused by very short time-scale (<1 week) polarization variability during the events

Jet Kinematics of AGNs at High Radio Frequencies

Original paper can be found at: http://www.astrosociety.org/pubs/cs/348.html--Copyright Astronomical Society of the PacificWe have completed a 3 yr program of polarization monitoring of 15 active galactic nuclei with the Very Long Baseline Array at 7 mm wavelength. At some epochs the images are accompanied by nearly simultaneous polarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. We determine apparent velocities in the jets of all sources in the sample. We suggest a new method to estimate Doppler factor of the jet components based on their VLBI properties. This allows us to derive the Lorentz factors and viewing angles of superluminal knots and opening angles of the jets

The Evolution of the Centimeter-Submillimeter Spectrum of 3C 345 during Outburst

Monitoring data between 375 and 4.8 GHz are presented for the blazar 3C 345 between 1989 and 1995, during which period the source flared. The multifrequency light curves are used to construct snap-shot spectra that span the synchrotron self-absorption turnover frequency. After subtraction of the underlying quiescent level, the flare spectrum is isolated and followed during the outburst. The evolution of the turnover frequency (ν_m_ with turnover flux (S_m_) is examined and compared with the predictions of the Marscher & Gear shocked-jet model. We find that the flare spectrum is well fitted by a homogeneous synchrotron curve for at least 2 yr after the initial rise in flux. The high-frequency optically thin section of this spectrum is observed to steepen with time. The extent of this steepening suggests that little reacceleration is occurring in the shock. We find that a power law is a remarkably good approximation to the observed trend of S_m_ with ν_m_. The turn over moves smoothly toward lower frequency with time, as expected from an emitting region that is expanding. Furthermore, this movement is observed to slow down with time, as predicted by the model. Initially, the turnover flux rises and then decays with a power-law index of ~1.0. This decay is interrupted by a second rise, which also decays with an index of ~1.0. It is found that the initial rise corresponds to the expansion phase, with the jet bending toward the line of sight rather than to the Compton or synchrotron phases of the model. Furthermore, the decays can only be reconciled with the model if the jet is assumed to be nonadiabatic or if it curves away from the line of sight during this period. The implied magnetic field orientation is parallel to the shock front, as expected from compression of the underlying field. This finding is supported by the total polarization data that suggest that the outbursts depolarize the total emission, which, during quiescent periods, is significantly polarized in a direction approximately perpendicular (B_parallel_) to the jet axis.Peer reviewe

Cryptic species within the cosmopolitan desiccation-tolerant moss Grimmia laevigata

The common cushion moss Grimmia laevigata (Bridel) Bridel grows on bare rock in a broad range of environments on every continent except Antarctica. As such, it must harbor adaptations to a remarkably broad set of environmental stresses, the extremes of which can include very high temperatures, prolonged nearly complete desiccation, and high ultraviolet B (UVB) exposure. Yet, like many mosses, G. laevigata shows very little morphological variability across its cosmopolitan range. This presents an evolutionary puzzle, the solution to which lies in understanding the phylogeographic structure of this morphologically simple organism. Here we report the results of an analysis of amplified fragment length polymorphisms (AFLPs) in G. laevigata, focusing on individuals from the California Floristic Province. We found evidence that populations within California constitute two distinct geographically overlapping cryptic species. Each clade harbors multiple private alleles, indicating they have been genetically isolated for some time. We suggest that the existence of cryptic species within G. laevigata, in combination with its life history, growth habits, and extreme desiccation tolerance, makes this moss an ideal research tool and a candidate for a biological indicator of climate change and pollution