We present multi-epoch, parsec-scale core brightness temperature observations of 447 AGN jets from the MOJAVE and 2cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value a
nd variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of (4.1 +- 0.6)*10^10 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked (HSP) BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked (LSP) BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in gamma rays, and we find a strong correlation between gamma-ray luminosity and Doppler factor for the detected sources.
Supermassive black holes launch highly relativistic jets with velocities reaching Lorentz factors as high as $Gamma>50$. How the jets accelerate to such high velocities and where along the jet do they reach terminal velocity are open questions that a
re tightly linked to their structure, launching and dissipation mechanisms. Changes in the beaming factor along the jets could potentially reveal jet acceleration, deceleration, or bending. We aim to (1) quantify the relativistic effects in multiple radio frequencies and (2) study possible jet velocity--viewing angle variations at parsec scales. We used the state-of-the-art code Magnetron to model light curves from the University of Michigan Radio Observatory and the Mets{a}hovi Radio Observatorys monitoring programs in five frequencies covering about 25 years of observations in the 4.8-37~GHz range for 61 sources. We supplement our data set with high-frequency radio observations in the 100-340~GHz range from ALMA, CARMA, and SMA. For each frequency we estimate the Doppler factor which we use to quantify possible changes in the relativistic effects along the jets. The majority of our sources do not show any statistically significant difference in their Doppler factor across frequencies. This is consistent with constant velocity in a conical jet, as expected at parsec scales. However, our analysis reveals 17 sources where relativistic beaming changes as a function of frequency. In the majority of cases the Doppler factor increases towards lower frequencies. Only 1253-053 shows the opposite behavior. By exploring their jet properties we find that the jet of 0420-014 is likely bent across the 4.8-340~GHz range. For 0212+735 the jet is likely parabolic, and still accelerating in the 4.8-37~GHz range. We discuss possible interpretations for the trends found in the remaining sources.
Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei wi
th relativistic jets, also known as blazars, have been considered to be one of the main candidates because of their ability to accelerate particles to high energies. We study the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory (OVRO) and Metsahovi Radio Observatory blazar monitoring programs. We identify sources in our radio monitoring sample that are positionally consistent with IceCube high-energy neutrino events. We estimate their mean flux density and variability amplitudes around the neutrino arrival time, and compare these with values from random samples to establish the significance of our results. We find radio source associations within our samples with 15 high-energy neutrino events detected by IceCube. Nearly half of the associated sources are not detected in the $gamma$-ray energies, but their radio variability properties and Doppler boosting factors are similar to the $gamma$-ray detected objects in our sample, meaning that they could still be potential neutrino emitters. We find that the number of strongly flaring objects in our statistically complete OVRO samples is unlikely to be a random coincidence (at $2sigma$ level). Based on our results, we conclude that although it is clear that not all neutrino events are associated with strong radio flaring blazars, observations of large-amplitude radio flares in a blazar at the same time as a neutrino event are unlikely to be a random coincidence.
There are several methods to calculate the radiative and kinetic power of relativistic jets, but their results can differ by one or two orders of magnitude. Therefore, it is necessary to perform a calibration of the jet power, to understand the reaso
ns for these differences (whether wrong hypotheses or intrinsic source variability), and if it is possible to converge to a reliable measurement of this physical quantity. We present preliminary results of a project aimed at calibrating the power of relativistic jets in active galactic nuclei (AGN) and X-ray binaries (XRB). We started by selecting all the AGN associations with known redshift in the Fourth Fermi LAT Gamma-Ray Catalog (4FGL). We then calculated the radiative and/or kinetic powers from available data or we extracted this information from literature. We compare the values obtained for overlapping samples and highlight early conclusions.
We have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over a $sim$10 year span by the Owens Valley Radio Observatory
40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-day timescales. The sky distribution of the variable sources is strongly dependent on the line-of-sight Galactic H$alpha$ intensities from the Wisconsin H$alpha$ Mapper Survey, demonstrating the contribution of interstellar scintillation (ISS) to their interday variability. 21% of sources observed through sight-lines with H$alpha$ intensities larger than 10 rayleighs exhibit significant ISS persistent over the $sim$10 year period. The fraction of scintillators is potentially larger when considering less significant variables missed by our selection criteria, due to ISS intermittency. This study demonstrates that ISS is still important at 15 GHz, particularly through strongly scattered sight-lines of the Galaxy. Of the 20 most significant variables, 11 are observed through the Orion-Eridanus superbubble, photoionized by hot stars of the Orion OB1 association. The high-energy neutrino source TXS0506$+$056 is observed through this region, so ISS must be considered in any interpretation of its short-term radio variability. J0616$-$1041 appears to exhibit large $sim$20% interday flux density variations, comparable in magnitude to that of the very rare class of extreme, intrahour scintillators that includes PKS0405$-$385, J1819$+$3845 and PKS1257$-$326; this needs to be confirmed by higher cadence follow-up observations.
We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual br
ight features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3sigma level. Although most features within a jet typically have speeds within ~40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc de-projected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 +- 0.2. The parent jets of the brightest radio quasars have a space density of 261 +- 19 Gpc$^{-3}$ and unbeamed 15 GHz luminosities above ~$10^{24.5}$ W/Hz, consistent with FR II class radio galaxies.
Relativistic effects dominate the emission of blazar jets complicating our understanding of their intrinsic properties. Although many methods have been proposed to account for them, the variability Doppler factor method has been shown to describe the
blazar populations best. We use a Bayesian hierarchical code called {it Magnetron} to model the light curves of 1029 sources observed by the Owens Valley Radio Observatorys 40-m telescope as a series of flares with an exponential rise and decay, and estimate their variability brightness temperature. Our analysis allows us to place the most stringent constraints on the equipartition brightness temperature i.e., the maximum achieved intrinsic brightness temperature in beamed sources which we found to be $rm langle T_{eq}rangle=2.78times10^{11}Kpm26%$. Using our findings we estimated the variability Doppler factor for the largest sample of blazars increasing the number of available estimates in the literature by almost an order of magnitude. Our results clearly show that $gamma$-ray loud sources have faster and higher amplitude flares than $gamma$-ray quiet sources. As a consequence they show higher variability brightness temperatures and thus are more relativistically beamed, with all of the above suggesting a strong connection between the radio flaring properties of the jet and $gamma$-ray emission.
The radio light-curve of J1415+1320 (PKS 1413+135) shows time-symmetric and recurring U-shaped features across the cm-wave and mm-wave bands. The symmetry of these features points to lensing by an intervening object as the cause. U-shaped events in r
adio light curves in the cm-wave band have previously been attributed to Extreme scattering events (ESE). ESEs are thought to be the result of lensing by compact plasma structures in the Galactic interstellar medium, but the precise nature of these plasma structures remains unknown. Since the strength of a plasma lens evolves with wavelength $lambda$ as $lambda^2$, the presence of correlated variations at over a wide wavelength range casts doubt on the canonical ESE interpretation for J1415+1320. In this paper, we critically examine the evidence for plasma lensing in J1415+1320. We compute limits on the lensing strength, and the associated free-free opacity of the putative plasma lenses. We compare the observed and model ESE light curves, and also derive a lower limit on the lens distance based on the effects of parallax due to the Earths orbit around the Sun. We conclude that plasma lensing is not a viable interpretation for J1415+1320s light curves and that symmetric U-shaped features in the radio light curves of extragalactic sources do not present {em prima facie} evidence for ESEs. The methodology presented here is generic enough to be applicable to any plasma lensing candidate.
We report the discovery of a rare new form of long-term radio variability in the light-curves of active galaxies (AG) --- Symmetric Achromatic Variability (SAV) --- a pair of opposed and strongly skewed peaks in the radio flux density observed over a
broad frequency range. We propose that SAV arises through gravitational milli-lensing when relativistically moving features in AG jets move through gravitational lensing caustics created by $10^3-10^6 ;{rm M}_{odot}$ subhalo condensates or black holes located within intervening galaxies. The lower end of this mass range has been inaccessible with previous gravitational lensing techniques. This new interpretation of some AG variability can easily be tested and if it passes these tests, will enable a new and powerful probe of cosmological matter distribution on these intermediate mass scales, as well as provide, for the first time, micro-arcsecond resolution of the nuclei of AG --- a factor of 30--100 greater resolution than is possible with ground-based millimeter VLBI.
Blazars are known to show periods of quiescence followed by outbursts visible throughout the electromagnetic spectrum. We present a novel maximum likelihood approach to capture this bimodal behavior by examining blazar radio variability in the flux-d
ensity domain. We separate quiescent and flaring components of a sources light curve by modeling its flux-density distribution as a series of off and on states. Our modeling allows us to extract information regarding the flaring ratio, duty cycle, and the modulation index in the off-state, in the on-state, as well as throughout the monitoring period of each blazar. We apply our method to a flux-density-limited subsample from the Owens Valley Radio observatorys 15 GHz blazar monitoring program, and explore differences in the variability characteristics between BL Lacs and FSRQs as well as between $gamma$-ray detected and non-detected sources. We find that: (1) BL Lacs are more variable and have relatively larger outbursts than the FSRQs, (2) unclassified blazar candidates in our sample show similar variability characteristics as the FSRQs, and (3) $gamma$-ray detected differ from the $gamma$-ray non-detected sources in all their variability properties, suggesting a link between the production of $gamma$-rays and the mechanism responsible for the radio variability. Finally, we fit distributions for blazar flaring ratios, duty cycles, and on- and off- modulation indices that can be used in population studies of variability-dependent blazar properties.
