Dual-Frequency VSOP Observations of AO 0235+164

AO 0235+164 is a very compact, flat spectrum radio source identified as a BL Lac object at a redshift of z=0.94. It is one of the most violently variable extragalactic objects at both optical and radio wavelengths. The radio structure of the source revealed by various ground-based VLBI observations is dominated by a nearly unresolved compact component at almost all available frequencies. Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP mission in January-February 1999. The array of the Japanese HALCA satellite and co-observing ground radio telescopes in Australia, Japan, China and South Africa allowed us to study AO 0235+164 with an unprecedented angular resolution at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural properties of the source. The 5-GHz observations led to an estimate of T_B > 5.8 x 10^{13} K for the rest-frame brightness temperature of the core, which is the highest value measured with VSOP to date.Comment: 8 pages, 8 figures, to appear in Publ. Astron. Soc. Japa

Doppler Boosting, Superluminal Motion, and the Kinematics of AGN Jets

We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution. Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to gamma ~30 and intrinsic luminosity up to ~10^26 W/Hz. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to ~5x10^13 K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are ~2x10^11 K, i.e., closer to equipartition.Comment: 10 pages, 12 color figures; proceedings of the 5th Stromlo Symposium: Disks, Winds, and Jets - from Planets to Quasars; accepted in Astrophysics & Space Scienc

MOJAVE: monitoring of jets in active galactic nuclei with VLBA experiments. V. Multi-epoch VLBA images

We present images from a long-term program (MOJAVE: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz Very Long Baseline Array (VLBA) images of a complete flux-density-limited sample of 135 AGNs above declination –20°, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample-selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond

A multifrequency analysis of radio variability of blazars

We have carried out a multifrequency analysis of the radio variability of blazars, exploiting the data obtained during the extensive monitoring programs carried out at the University of Michigan Radio Astronomy Observatory (UMRAO, at 4.8, 8, and 14.5 GHz) and at the Metsahovi Radio Observatory (22 and 37 GHz). Two different techniques detect, in the Metsahovi light curves, evidences of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023, 2200+420, and 2251+158). For the last three sources consistent periods are found also at the three UMRAO frequencies and the Scargle (1982) method yields an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz periodicities of 0224+671 and 0945+408 (which were less extensively monitored at Metsahovi and for which we get a significant false-alarm probability) are not confirmed by the UMRAO database, where some indications of ill-defined periods about a factor of two longer are retrieved. We have also investigated the variability index, the structure function, and the distribution of intensity variations of the most extensively monitored sources. We find a statistically significant difference in the distribution of the variability index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in the sense that the former objects are more variable. For both populations the variability index steadily increases with increasing frequency. The distribution of intensity variations also broadens with increasing frequency, and approaches a log-normal shape at the highest frequencies. We find that variability enhances by 20-30% the high frequency counts of extragalactic radio-sources at bright flux densities, such as those of the WMAP and Planck surveys.Comment: A&A accepted. 12 pages, 16 figure

Rapid physical and biological particle mixing on an intertidal sandflat

Sediment mixing processes were investigated using inert tracer experiments, benthic macrofaunal community analysis, and surveys of ray feeding pits to quantify the relative rates and controls of physical and biological reworking on Debidue Flat, an intertidal sandflat in South Carolina. Sediment reworking on Debidue Flat was rapid, with both advective and biodiffusive mixing operating over different vertical spatial scales. Physical reworking by tidal currents dominated initial transport of the tracer in the top 5-10 cm on timescales of ~30 days. Although the exact mechanism of tracer transport is unclear, it is most likely due to active fluidization of surface sediments during stages of the tide followed by a density-driven settling of tracer resulting in a steady downward transport to the depth of bedform reworking. Biodiffusive mixing was evident throughout the sampled interval (~30 cm) and dominated reworking at depths greater than 10 cm. Estimated biodiffusive mixing coefficients (Db) were high all year (0.15-0.28 cm2d-1), and were comparable to values reported for coastal bioturbated muds. The haustoriid amphipod Pseudohaustorius caroliniensis was most likely responsible for tracer dispersal in the 10 -30 cm interval based on its distribution, abundance, size, and observed burrow structures. Ray pit excavation and infilling were seasonal disturbances that contributed ~12-22% to spatially averaged advective transport rates but were locally intense and capable alone of turning over the entire upper ~15 cm of the flat in ~100 -1000 d. We propose that the mixing processes on Debidue Flat promote an unconstricted, open sediment matrix that maintains the high permeability required for the rapid porewater exchange to 25 cm noted for this system. Thus, in addition to redistributing organic substrates, physical and biological particle mixing play important roles in controlling permeability of flat deposits and quantification of these processes is important to understand controls on permeability and biogeochemical cycling of solutes in sandy systems

Parsec-Scale Blazar Monitoring: Flux and Polarization Variability

We present analysis of the flux 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, 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 (abridged).Comment: 35 pages, 30 figures, accepted by the Astrophysical Journal (Changes from original version: typos corrected and a clarification in terminology

A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum

III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare where the source has brightened more than twenty-fold within less than two years. A broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA observations at 43 GHz confirm this double structure and these sizes. Time scale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenario with very compact hotspots. Similar millimeter-peaked spectrum (MPS) sources could have escaped our attention because of their low flux density at typical survey frequencies and their strong variability.Comment: ApJ Letters, in press, (AAS)LaTeX, 3 figures, available at http://www2.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#iiizw2 or in a few weeks at http://www.mpifr-bonn.mpg.de/staff/falcke/publications.html#iiizw

Detection of a multi-shell planetary nebula around the hot subdwarf O-type star 2MASS J19310888+4324577

(Abridged) The origin of hot subdwarf O-type stars (sdOs) remains unclear since their discovery in 1947. Among others, a post-Asymptotic Giant Branch (post-AGB) origin is possible for a fraction of sdOs. We are involved in a comprehensive ongoing study to search for and to analyze planetary nebulae (PNe) around sdOs with the aim of establishing the fraction and properties of sdOs with a post-AGB origin. We use deep Halpha and [OIII] images of sdOs to detect nebular emission and intermediate resolution, long-slit optical spectroscopy of the detected nebulae and their sdO central stars. These data are complemented with other observations for further analysis of the detected nebulae. We report the detection of an extremely faint, complex PN around 2MASS J19310888+4324577 (2M1931+4324), a star classified as sdO in a binary system. The PN shows a bipolar and an elliptical shell, whose major axes are oriented perpendicular to each other, and high-excitation structures outside the two shells. WISE archive images show faint, extended emission at 12 and 22 microns in the inner nebular regions. The internal nebular kinematics is consistent with a bipolar and a cylindrical/ellipsoidal shell, in both cases with the main axis mainly perpendicular to the line of sight. The nebular spectrum only exhibits Halpha, Hbeta and [OIII]4959,5007 emission lines, but suggests a very low-excitation ([OIII]/Hbeta = 1.5), in strong contrast with the absence of low-excitation emission lines. The spectrum of 2M1931+4324 presents narrow, ionized helium absorptions that confirm the previous sdO classification and suggest an effective temperature >= 60000 K. The binary nature of 2M1931+4324, its association with a complex PN, and several properties of the system provide strong support for the idea that binary central stars are a crucial ingredient in the formation of complex PNe.Comment: 8 pages, 7 figures, accepted in Astronomy and Astrophysic