Search

We present the results of our study of cross-correlations between long-term multi-band observations of the radio variability of the blazar 3C 279. More than a decade (2008-2022) of radio data were collected at seven different frequencies ranging from 2 GHz to 230 GHz. The multi-band radio light curves show variations in flux, with the prominent flare features appearing first at higher-frequency and later in lower-frequency bands. This behavior is quantified by cross-correlation analysis, which finds that the emission at lower-frequency bands lags that at higher-frequency bands. Lag versus frequency plots are well fit by straight lines with negative slope, typically ~-30 day/GHz. We discuss these flux variations in conjunction with the evolution of bright moving knots seen in multi-epoch VLBA maps to suggest possible physical changes in the jet that can explain the observational results. Some of the variations are consistent with the predictions of shock models, while others are better explained by a changing Doppler beaming factor as the knot trajectory bends slightly, given a small viewing angle to the jet., Comment: Submitted revised version to MNRAS journal, 11 pages, 6 figures, 4 tables

We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$\gamma$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral response' method. With more than ten years of data obtained with weekly/daily sampling intervals, most of the PSDs cover ~2-4 decades in temporal frequency; moreover, in the optical band, the PSDs cover ~6 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse Compton spectral components, in general, exhibit red-noise (slope ~2) and flicker-noise (slope ~1) type variability, respectively; (2) the slopes of $\gamma$-ray variability PSDs obtained using a 3-hr integration bin and a 3-weeks total duration exhibit a range between ~1.4 and ~2.0 (mean slope = 1.60$\pm$0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales $\leq$100\, days at $\gamma$-ray frequencies as compared to longer wavelengths, in general (except between $\gamma$-ray and optical frequencies for PKS 1510$-$089); (4) the normalization of intranight optical PSDs for 3C\,279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS\,1510$-$089., Comment: ApJ accepted. Comments are welcome

On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 percent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure which develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C IV line emission (Sloan Digital Sky Survey, SDSS) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the $\gamma$-ray emission, with radio lagging the $\gamma$-rays. Based on the $\gamma$-ray variability timescale, we constrain the $\gamma$-ray emission zone to the BLR (30-200 $r_{\rm g}$) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for $\gamma$-ray production via external Compton scattering. The neutrino is most likely produced by proton-proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR., Comment: 35 pages, 33 figures, 3 tables; accepted by the MNRAS Main Journal

The University of Michigan 26-m paraboloid was dedicated to obtaining linear polarization and total flux density observations of blazars from the mid-1960s until June 2012 providing an unprecedented record tracking centimeter-band variability over decades at 14.5, 8.0, and 4.8 GHz for both targeted objects and members of flux-limited samples. In the mid-1970s through the mid-1980s, and during the last decade of the program, observations were additionally obtained of circular polarization for a small sample of radio-bright (S>5Jy), active sources. Key program results include evidence supporting class-dependent differences in the magnetic field geometry of BL Lac and QSO jets, identification of linear polarization changes temporally associated with flux outbursts supporting a shock-in-jet scenario, and determination of the spectral evolution of the Stokes V amplitude and polarity for testing proposed models. Recent radiative transfer modeling during large flares supports a jet scenario with a kinetically-dominated, relativistic flow at parsec scales with embedded turbulent magnetic fields and dynamically-weak ordered components which may be helical; the circular polarization observations are consistent with linear-to-circular mode conversion within this turbulent jet environment., Comment: 8 pages, 4 figures, Proceedings of the conference "Polarised Emission from Astrophysical Jets", June 12-16, 2017, Ierapetra, Greece, eds. E. Angelakis, M. Boettcher, and J.-L. Gomez

Since the mid-1980s the shock-in-jet model has been the preferred paradigm to explain radio-band flaring in blazar jets. We describe our radiative transfer model incorporating relativistically-propagating shocks, and illustrate how the 4.8, 8, and 14.5 GHz linear polarization and total flux density data from the University of Michigan monitoring program, in combination with the model, constrain jet flow conditions and shock attributes. Results from strong Fermi-era flares in 4 blazars with widely-ranging properties are presented. Additionally, to investigate jet evolution on decadal time scales we analyze 3 outbursts in OT 081 spanning nearly 3 decades and find intrinsic changes attributable to flow changes at a common spatial location, or, alternatively, to a change in the jet segment viewed. The model's success in reproducing these data supports a scenario in which relativistic shocks compress a plasma with an embedded passive, initially-turbulent magnetic field, with additional ordered magnetic field components, one of which may be helical., Comment: Accepted for Proceedings of "Blazars through Sharp Multi-Wavelength Eyes", Malaga, Spain, May 30 - June 3, 2016, to appear in Galaxies (MDPI AG, Switzerland)

We analyze the shock-in-jet models for the gamma-ray flaring blazars 0420-014, OJ 287, and 1156+295 presented in Aller et al. (2014, Paper I), quantifying how well the modeling constrains internal properties of the flow (low energy spectral cutoff, partition between random and ordered magnetic field), the flow dynamics (quiescent flow speed and orientation), and the number and strength of the shocks responsible for radio-band flaring. We conclude that well-sampled, multifrequency polarized flux light curves are crucial for defining source properties. We argue for few, if any, low energy particles in these flows, suggesting no entrainment and efficient energization of jet material, and for approximate energy equipartition between the random and ordered magnetic field components, suggesting that ordered field is built by non-trivial dynamo action from the random component, or that the latter arises from a jet instability that preserves the larger-scale, ordered flow. We present evidence that the difference between orphan radio-band (no gamma-ray counterpart) and non-orphan flares is due to more complex shock interactions in the latter case., Comment: 15 pages, 10 figures; accepted for publication in Astrophysical Journal

As part of a program to investigate jet flow conditions during GeV gamma-ray flares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band total flux density and linear polarization monitoring observations to constrain radiative transfer models incorporating propagating shocks orientated at an arbitrary angle to the flow direction. We describe the characteristics of the model, illustrate how the data are used to constrain the models, and present results for three program sources with diverse characteristics: PKS 0420-01, OJ 287, and 1156+295. The modeling of the observed spectral behavior yields information on the sense, strength and orientation of the shocks producing the radio-band flaring; on the energy distribution of the radiating particles; and on the observer's viewing angle with respect to the jet independent of VLBI data. We present evidence that, while a random component dominates the jet magnetic field, a distinguishing feature of those radio events with an associated gamma-ray flare is the presence of a weak but non-negligible ordered magnetic field component along the jet axis., Comment: 6 pages, 4 figures. To appear in the proceedings of "The Innermost Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spain

In September 2012, the high-synchrotron-peaked (HSP) blazar Markarian 421 underwent a rapid wideband radio flare, reaching nearly twice the brightest level observed in the centimeter band in over three decades of monitoring. In response to this event we carried out a five epoch centimeter- to millimeter-band multifrequency Very Long Baseline Array (VLBA) campaign to investigate the aftermath of this emission event. Rapid radio variations are unprecedented in this object and are surprising in an HSP BL Lac object. In this flare, the 15 GHz flux density increased with an exponential doubling time of about 9 days, then faded to its prior level at a similar rate. This is comparable with the fastest large-amplitude centimeter-band radio variability observed in any blazar. Similar flux density increases were detected up to millimeter bands. This radio flare followed about two months after a similarly unprecedented GeV gamma-ray flare (reaching a daily E>100 MeV flux of (1.2 +/- 0.7)x10^(-6) ph cm^(-2) s^(-1)) reported by the Fermi Large Area Telescope (LAT) collaboration, with a simultaneous tentative TeV detection by ARGO-YBJ. A cross-correlation analysis of long-term 15 GHz and LAT gamma-ray light curves finds a statistically significant correlation with the radio lagging ~40 days behind, suggesting that the gamma-ray emission originates upstream of the radio emission. Preliminary results from our VLBA observations show brightening in the unresolved core region and no evidence for apparent superluminal motions or substantial flux variations downstream., Comment: 5 pages, 8 figures. Contributed talk at the meeting "The Innermost Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spain. Updated to correct author list and references

We carried out intra-day variability (IDV) observations from August 2005 to January 2010 with the Urumqi 25m radio telescope for a dozen IDV sources including the quasar 0917+624. This target exhibited pronounced centimeter-band, intra-day variability during the 1980s--1990s, but its strong IDV phase ceased in 2000. The source showed no IDV in the majority of the Urumqi observing sessions, although weak IDV activity was detected in some. Multifrequency UMRAO data for 0917+624 show that the spectral index is steeper during the weak and non-IDV phases than during the strong IDV phase, supporting the idea that the size of the scintillating component may be enlarged in the weak/non IDV phases., Comment: 4 pages, 2 figures, accepted for publication in Astrophysics and Space Science

We have used single-dish centimeter-band, multi-frequency linear polarization and total flux density monitoring observations obtained by UMRAO at 14.5, 8, and 4.8 GHz during gamma-ray flaring to constrain propagating shock-in-jet models; this procedure allows us to probe jet conditions at or near the presumed parsec-scale gamma-ray emission site within the relativistic jet. Results are presented for the blazar 0420-014 during the gamma-ray flare detected by the LAT which peaked in late January 2010. Our work identifies the passage of three forward-moving transverse shocks during the radio-band flaring, a shock Lorentz factor of 12, and an observer's viewing angle of 2 degrees and sets limits on the energy distribution of the radiating particles., Comment: 2012 Fermi Symposium proceedings - eConf C121028

The broad-line radio galaxy 3C 111 exhibited a major flux density outburst in 2007. Here, we present imaging and preliminary kinematic results of the jet, based on three millimetre-VLBI observations (86 GHz) using the Global Millimeter VLBI Array (GMVA) covering one year just after the radio flare. The GMVA data allow us to study this outburst with unprecedented image fidelity at highest (sub-parsec) resolution. On these scales, the outburst is resolved into a complex series of plasma components forming an intriguing bent structure. Within 1 mas from the jet base, ejections vary in position angle and components move with an apparent velocity of ~3.7c, significantly slower than the maximum velocity observed with cm-VLBI on scales beyond 1 mas., Comment: 6 pages, 2 figures, Conference Proceedings of "11th European VLBI Network Symposium & Users Meeting", to be published in Proceedings of Science

We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with 0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7 mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary "core" and in the superluminal knot, both parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes., Comment: Accepted for Publication in the Astrophysical Journal Letters. 7 pages (including 5 figures). Minor corrections with regard to previous version, as proposed by the referee

The `shock in jet' model for cm-waveband blazar variability is revisited, allowing for arbitrary shock orientation with respect to the jet flow direction, and both random and ordered magnetic field. It is shown that oblique shocks can explain events with swings in polarization position angle much less than the 90 deg. associated with transverse structures, while retaining the general characteristics of outbursts, including spectral behavior and level of peak percentage polarization. Models dominated by a force-free, minimum energy magnetic field configuration (essentially helical) display a shallow rise in percentage polarization and frequency dependent swing in polarization position angle not in agreement with the results of single-dish monitoring observations, implying that the field is predominantly random in the quiescent state. Outbursts well-explained by the `shock in jet' model are present during gamma-ray flaring in several sources, supporting the idea that shock events are responsible for activity from the radio to gamma-ray bands., Comment: 19 pages, 8 figures, accepted for publication in ApJ

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 111 between 2004 and 2010 at X-ray (2.4--10 keV), optical (R band), and radio (14.5, 37, and 230 GHz) wave bands, as well as multi-epoch imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the six years of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. This shows a clear connection between the radiative state near the black hole, where the X-rays are produced, and events in the jet. The X-ray continuum flux and Fe line intensity are strongly correlated, with a time lag shorter than 90 days and consistent with zero. This implies that the Fe line is generated within 90 light-days of the source of the X-ray continuum. The power spectral density function of X-ray variations contains a break, with steeper slope at shorter timescales. The break timescale of 13 (+12,-6) days is commensurate with scaling according to the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). The data are consistent with the standard paradigm, in which the X-rays are predominantly produced by inverse Compton scattering of thermal optical/UV seed photons from the accretion disk by a distribution of hot electrons --- the corona --- situated near the disk. Most of the optical emission is generated in the accretion disk due to reprocessing of the X-ray emission. The relationships that we have uncovered between the accretion disk and the jet in 3C 111, as well as in the FR I radio galaxy 3C 120 in a previous paper, support the paradigm that active galactic nuclei and Galactic BHXRBs are fundamentally similar, with characteristic time and size scales proportional to the mass of the central black hole, Comment: Accepted for publication in ApJ. 18 pages, 17 figures, 11 tables (full machine readable data-tables online in ApJ website)

The properties of blazar variability in the radio band are studied using the unique combination of temporal resolution from single dish monitoring and spatial resolution from VLBA imaging; such measurements, now available in all four Stokes parameters, together with theoretical simulations, identify the origin of radio band variability and probe the characteristics of the radio jet where the broadband blazar emission originates. Outbursts in total flux density and linear polarization in the optical-to-radio bands are attributed to shocks propagating within the jet spine, in part based on limited modeling invoking transverse shocks; new radiative transfer simulations allowing for shocks at arbitrary angle to the flow direction confirm this picture by reproducing the observed centimeter-band variations observed more generally, and are of current interest since these shocks may play a role in the gamma-ray flaring detected by Fermi. Recent UMRAO multifrequency Stokes V studies of bright blazars identify the spectral variability properties of circular polarization for the first time and demonstrate that polarity flips are relatively common. All-Stokes data are consistent with the production of circular polarization by linear-to-circular mode conversion in a region that is at least partially self-absorbed. Detailed analysis of single-epoch, multifrequency, all-Stokes VLBA observations of 3C 279 support this physical picture and are best explained by emission from an electron-proton plasma., Comment: 6 pages, 5 figures, uses, jaa.sty. Invited talk presented at the conference Multifrequency Variability of Blazars, Guangzhou, China, September 22-24, 2010. To appear in J. Astrophys. Astr

The quasar B0605-085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. We study the possible periodicity of B0605-085 in the total flux-density, spectra and opacity changes in order to compare it with jet kinematics on parsec scales. We have analyzed archival total flux-density variability at ten frequencies (408 MHz, 4.8 GHz, 6.7 GHz, 8 GHz, 10.7 GHz, 14.5 GHz, 22 GHz, 37 GHz, 90 GHz, and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-density light curves. In addition, spectral evolution and changes of the opacity have been analyzed. We found a period in multi-frequency total flux-density light curves of 7.9+-0.5 yrs. Moreover, a quasi-stationary jet component C1 follows a prominent helical path on a similar time scale of 8 years. We have also found that the average instantaneous speeds of the jet components show a clear helical pattern along the jet with a characteristic scale of 3 mas. Taking into account average speeds of jet components, this scale corresponds to a time scale of about 7.7 years. Jet precession can explain the helical path of the quasi-stationary jet component C1 and the periodical modulation of the total flux-density light curves. We have fitted a precession model to the trajectory of the jet component C1, with a viewing angle phi=2.6+-2.2 degrees, aperture angle of the precession cone Omega=23.9+-1.9 degrees and fixed precession period (in the observers frame) P = 7.9 yrs., Comment: 14 pages, 16 figures, 5 tables, accepted for publication in A&A

In the 1990s a comparison of sparse EGRET measurements with single-dish flux density monitoring from the Metsahovi and UMRAO programs established a temporal connection between the onset of flaring at radio band and the occurrence of gamma-ray activity. Correlations between the emergence of new VLBI components from the core, flares in linearly polarized radio flux, and gamma-ray activity in bright EGRET-detected blazars supported a picture in which the gamma-ray and the radio band emission arises in the same shocked region of the jet, with the high energy emission produced via inverse Compton scattering by the synchrotron-emitting electrons in the jet. Quantitative tests of this scenario, however, were hampered by insufficient temporal sampling of the data and the simple nature of the models adopted. The extensive data from Fermi coupled with the wealth of well-sampled radio band data from old as well as new programs such as the F-GAMMA project now permit statistical studies for large numbers of sources, including weak HBLs, and detailed analyses of individual highly-active class members. I summarize progress in understanding the origin of the gamma-ray emission using these new measurements. I focus on three areas: attempts to isolate the physical site of the high energy emission using time delay information; investigation of the emission process using the characteristics of the variability; and quantitative tests of the shock model picture using high-time-sampled multifrequency linear polarization data, VLBP imaging, and new models of propagating oblique relativistic shocks incorporating detailed radiative transfer calculations., Comment: Invited Review at "Fermi Meets Jansky - AGN in Radio and Gamma-Rays", Bonn, Germany, June 21 - 23, 2010

We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510-089, concentrating on eight major gamma-ray flares that occurred during the interval 2009.0-2009.5. The gamma-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wavebands varied by an order of magnitude. The optical polarization vector rotated by 720 degrees during a 5-day period encompassing six of these flares. This culminated in a very bright, roughly 1 day, optical and gamma-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the "core") seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at gamma-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid gamma-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons., Comment: 10 pages of text + 5 figures, to be published in Astrophysical Journal Letters in 2010

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. Consistent with this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the radio variations. This implies that, in this radio galaxy, the radiative state of accretion disk plus corona system, where the X-rays are produced, has a direct effect on the events in the jet, where the radio emission originates. The X-ray power spectral density of 3C 120 shows a break, with steeper slope at shorter timescale and the break timescale is commensurate with the mass of the central black hole based on observations of Seyfert galaxies and black hole X-ray binaries. These findings provide support for the paradigm that black hole X-ray binaries and active galactic nuclei are fundamentally similar systems, with characteristic time and size scales linearly proportional to the mass of the central black hole. The X-ray and optical variations are strongly correlated in 3C 120, which implies that the optical emission in this object arises from the same general region as the X-rays, i.e., in the accretion disk-corona system. We numerically model multi-wavelength light curves of 3C 120 from such a system with the optical-UV emission produced in the disk and the X-rays generated by scattering of thermal photons by hot electrons in the corona. From the comparison of the temporal properties of the model light curves to that of the observed variability, we constrain the physical size of the corona and the distances of the emitting regions from the central BH., Comment: Accepted for publication in the Astrophysical Journal. 28 pages, 21 figures, 2 tables

We report the discovery of an extremely curved jet in the radio-loud quasar PKS2136+141. Multi-frequency Very Long Baseline Array (VLBA) images show a bending jet making a turn-around of 210 degrees in the plane of the sky, which is, to our knowledge, the largest ever observed change in the position angle of an astrophysical jet. Images taken at six different frequencies, from 2.3 to 43 GHz, reveal a spiral-like trajectory, which is likely a sign of an intrinsic helical geometry. A space-VLBI image, taken with the HALCA satellite at 5 GHz and having comparable resolution to our ground-based 15 GHz data, confirms that the bend is a frequency-independent structure. VLBA monitoring data at 15 GHz, covering eight years of observations, show knots in the jet clearly deviating from ballistic motion, which suggests that the bending may be caused by a growing helical Kelvin-Helmholtz normal mode. The jet appearance suggests a helical wave at a frequency well below the "resonant" frequency of the jet, which indicates that the wave is driven by a periodic perturbation at the base of the jet. We fit the observed structure in the source with a helical twist, and we find that a simple isothermal model with a constant wave speed and wavelength gives a good fit. The measured apparent velocities indicate some degree of acceleration along the jet, which together with an observed change in the apparent half-opening angle of the jet allow us to estimate the changes in the angle between the local jet direction and our line of sight. We suggest that the jet in PKS2136+141 is distorted by a helical Kelvin-Helmholtz normal mode externally driven into the jet (e.g. by precession), and that our line of sight falls within the opening angle of the helix cone., Comment: Accepted for publication in The Astrophysical Journal, 15 pages, uses emulateapj.cls

About twenty years of radio observations in five bands (from 4.8 to 37 GHz) of the BL Lac object GC 0109+224 (S2 0109+22, RGB J0112+227), are presented and analysed together with the optical data. Over the past ten years this blazar has exhibited enhanced activity. There is only weak correlation between radio and optical flares delays, usually protracted on longer timescales in the radio with respect to the optical. In some cases no radio flare counterpart was observed for the optical outbursts. The radio variability, characterised by peaks superposition, shows hints of some characteristic timescales (around the 3-4 years), and a fluctuation mode between the flickering and the shot noise. The reconstructed spectral energy distribution, poorly monitored at high energies, is preliminarily parameterised with a synchrotron-self Compton description. The smooth synchrotron continuum, peaked in the near-IR-optical bands, strengthens the hypothesis that this source could be an intermediate blazar. Moreover the intense flux in millimetre bands, and the optical and X-ray brightness, might suggest a possible detectable gamma-ray emission., Comment: 9 pages, 7 figures, 2 tables. Typeset with a LaTex2e-AMSLaTex code prepared by the author (using mn2e class, and natbib, hyperref, graphicx, packages). Accepted for publication in MNRAS

We introduce an algorithm for applying a cross-wavelet transform to analysis of quasiperiodic variations in a time-series, and introduce significance tests for the technique. We apply a continuous wavelet transform and the cross-wavelet algorithm to the Pearson-Readhead VLBI survey sources using data obtained from the University of Michigan 26-m parabloid at observing frequencies of 14.5, 8.0, and 4.8 GHz. Thirty of the sixty-two sources were chosen to have sufficient data for analysis, having at least 100 data points for a given time-series. Of these thirty sources, a little more than half exhibited evidence for quasiperiodic behavior in at least one observing frequency, with a mean characteristic period of 2.4 yr and standard deviation of 1.3 yr. We find that out of the thirty sources, there were about four time scales for every ten time series, and about half of those sources showing quasiperiodic behavior repeated the behavior in at least one other observing frequency., Comment: Revised version, accepted by ApJ. 17 pages, 13 figures, color figures included as gifs, seperate from the text. The addition of statistical significance tests has resulted in modifying the technique and results, but the broad conclusion remain the same. A high resolution version may be found at http://www.astro.lsa.umich.edu/obs/radiotel/prcwdata.html

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)

We examine the coincidence of times of high $\gamma$-ray flux and ejections of superluminal components from the core in EGRET blazars based on a VLBA monitoring program at 22 and 43 GHz from November 1993 to July 1997. In 23 cases of $\gamma$-ray flares for which sufficient VLBA data exist, 10 of the flares (in 8 objects) fall within 1$\sigma$ uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730-130) two successive $\gamma$-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations which show that if the number of coincidences $\ge$ 10 the radio and $\gamma$-ray events are associated with each other at >99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the $\gamma$-ray flares suggests that the $\gamma$-ray events occur in the superluminal radio knots. This implies that the $\gamma$-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine., Comment: 23 pages, 3 tables, 5 figures, submitted to ApJ

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#iiizw2

Results from a long-term program to quantify the range of behavior of the cm-wavelength total flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations with the University of Michigan 26-m telescope operating at 14.5, 8.0, and 4.8 GHz are presented; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. The BL Lacs are found to be more highly variable in total flux density than the QSOs, exhibiting changes that are often nearly-simultaneous and of comparable amplitude at 14.5 and 4.8 GHz in contrast to the behavior in the QSOs and supporting the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure function analyses of the flux observations quantify that a characteristic timescale is identifiable in only 1/3 of the BL Lacs. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the EVPA is present; when compared with the VLBI structural axis, no preferred position angle difference is identified. The polarized flux typically exhibits variability with timescales of months to a few years and shows the signature of a propagating shock during several resolved outbursts. The observations indicate that the source emission is predominately due to evolving source components and support the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets. The differences in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in jet stability., Comment: 1 LaTex (aastex) file, 21 postscript figure files, 2 external LaTex table files. To appear in the Astrophysical Journal

We use a continuous wavelet transform to analyze more than two decades of data for the BL~Lac object OJ~287 acquired as part of the UMRAO variability program. We find clear evidence for a persistent modulation of the total flux and polarization with period $\sim 1.66$ years, and for another signal that dominates activity in the 1980s with period $\sim 1.12$ years. The relationship between these two variations can be understood in terms of a `shock-in-jet' model, in which the longer time scale periodicity is associated with an otherwise quiescent jet, and the shorter time scale activity is associated with the passage of a shock; the different periodicities of these two components may reflect different internal conditions of the two flow domains, leading to different wave speeds, or different contractions of a single underlying periodicity, due to the different Doppler factors of the two flow components. We suggest that the modulation arises from a wave driven by some asymmetric disturbance close to the central engine. The periodic behavior in polarization exhibits excursions in $U$ which correspond to a direction $\sim 45^{\circ}$ from the VLBI jet axis. This behavior is not explained by the random walk in the $Q$-$U$ plane which is expected from models in which a pattern of randomly aligned magnetic field elements propagate across the visible portion of the flow, and suggests a small amplitude, cyclic variation in the flow direction in that part of the flow that dominates cm-wavelength emission., Comment: Text: 22 pages, PostScript line figures: 10 pages, GIF color figures: 7. High quality PostScript versions of the GIF images may be obtained from the "Wavelets/First Results" section of http://www.astro.lsa.umich.edu/users/hughes/, or by anon. ftp from file get/hughes/waves.tar.gz at ra.astro.lsa.umich.edu. Submitted to Ap. J

We present a study of the z = 2.07 quasar PKS 0528+134, which has been detected as an extraordinarily luminous gamma-ray source. Its radio properties are highly variable in both total and polarized flux density. Milliarcsecond-scale maps from global very long base interferometry (VLBI) experiments, an X-ray spectrum from ROSAT Position Sensitive Proportional Counter (PSPC) observations, and light curves in total flux density and polarization are used to investigate the geometry, radiation mechanism, and physical environment of the emission region in the source. The VLBI images reveal a bent jet extending toward the northeast on parsec scales, with less intense knots of emission appearing on the opposite side of the brightest spot. The position of the core usually found in such sources is unclear. The polarization angle is stable despite strong variability in polarized flux density and indicates that the magnetic field is aligned with the jet axis as defined by our 8.4 GHz image. The ROSAT X-ray flux density of PKS 0528+134 in 1991 March is measured to be 1.6 micro Jy at 1 keV, with a very steep spectral (`energy') index sigma(sub x) approximately equal to 2.2. The X-ray observations reveal the presence of cold gas along the line of sight significantly in excess of that present in the Galaxy. A strong radio flare began within two months of the first observation of a high flux of gamma-rays from PKS 0528+134 by Hunter et al. Using the geometry and spectral chacateristics determined by our VLBI observations, a synchrotron self-Compton calculation indicates that relativistic bulk motion is required in PKS 0528+134, with an estimated Doppler beaming factor delta approximately greater than 4.3, similar to the value delta approximately greater than 7 required to explain the low optical depth of the gamma-rays to photon-photon pair production. We suggest that the core activity of PKS 0528+134 is sporadic in nature, with the nonthermal outburst starting in 1991 representing a violent period involving ejection of relativistic plasma along a compact jet pointed almost directly toward us.

We present radio to submillimeter-wave continuum spectra of 44 bright, compact extragalactic radio sources with flat spectra at centimeter wavelengths ('blazars'). Infrared J, H, and K flux densities are added to the spectra of six of these objects. These spectra are useful in comparisons of x-ray and gamma-ray measurements with the multiwaveband properties of blazars. A number of the objects have been detected as strong, hard gamma-ray sources by the Compton Gamma Ray Observatory (CGRO). The millimeter-wave spectra of the gamma-ray bright blazars we observe are flatter on average than for the sample as a whole.