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Register a new userMOJAVE - XIV. Shapes and opening angles of AGN jets
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Added by
Alexander Pushkarev
Publication date
2017
fields
Physics
and research's language is
English
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We present 15 GHz stacked VLBA images of 373 jets associated with active galactic nuclei (AGN) having at least five observing epochs within a 20 yr time interval 1994-2015 from the MOJAVE programme and/or its precursor, the 2 cm VLBA Survey. These data are supplemented by 1.4 GHz single-epoch VLBA observations of 135 MOJAVE AGNs to probe larger scale jet structures. The typical jet geometry is found to be close to conical on scales from hundreds to thousands of parsecs, while a number of galaxies show quasi-parabolic streamlines on smaller scales. A true jet geometry in a considerable fraction of AGNs appears only after stacking epochs over several years. The jets with significant radial accelerated motion undergo more active collimation. We have analysed total intensity jet profiles transverse to the local jet ridgeline and derived both apparent and intrinsic opening angles of the flows, with medians of $21.5deg$ and $1.3deg$, respectively. The Fermi LAT-detected gamma-ray AGNs in our sample have, on average, wider apparent and narrower intrinsic opening angle, and smaller viewing angle than non LAT-detected AGNs. We have established a highly significant correlation between the apparent opening angle and gamma-ray luminosity, driven by Doppler beaming and projection effects.
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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 and 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.
We report on the acceleration properties of 329 features in 95 blazar jets from the MOJAVE VLBA program. Nearly half the features and three-quarters of the jets show significant changes in speed and/or direction. In general, apparent speed changes are distinctly larger than changes in direction, indicating that changes in the Lorentz factors of jet features dominate the observed speed changes rather than bends along the line of sight. Observed accelerations tend to increase the speed of features near the jet base, $lesssim 10-20$ parsecs projected, and decrease their speed at longer distances. The range of apparent speeds at fixed distance in an individual jet can span a factor of a few, indicating that shock properties and geometry may influence the apparent motions; however, we suggest that the broad trend of jet features increasing their speed near the origin is due to an overall acceleration of the jet flow out to de-projected distances of order $10^2$ parsecs, beyond which the flow begins to decelerate or remains nearly constant in speed. We estimate intrinsic rates of change of the Lorentz factors in the galaxy frame of order $dot{Gamma}/Gamma simeq 10^{-3}$ to $10^{-2}$ per year which can lead to total Lorentz factor changes of a factor of a few on the length scales observed here. Finally, we also find evidence for jet collimation at projected distances of $lesssim 10$ parsecs in the form of the non-radial motion and bending accelerations that tend to better align features with the inner jet.
Within the MOJAVE VLBA program (Monitoring of Jets in AGN with VLBA Experiments), we have accumulated observational data at 15 GHz for hundreds of jets in $gamma$-ray bright active galactic nuclei since the beginning of the Fermi scientific observations in August 2008. We investigated a time delay between the flux density of AGN parsec-scale radio emission at 15 GHz and 0.1$-$300 GeV Fermi LAT photon flux, taken from constructed light curves using weekly and adaptive binning. The correlation analysis shows that radio is lagging $gamma$-ray radiation by up to 8 months in the observers frame, while in the source frame, the typical delay is about 2-3 months. If the jet radio emission, excluding the opaque core, is considered, significant correlation is found at greater time lags. We supplement these results with VLBI kinematics and core shift data to conclude that the dominant high-energy production zone is typically located within the 15 GHz VLBA core at a distance of a few parsecs from the central nucleus.
We investigate the relation between the two modes of outflow (wind and jet) in radio-loud active galactic nuclei (AGN). For this study we have carried out a systematic and homogeneous analysis of XMM-Newton spectra of a sample of 16 suitable radio-loud Seyfert-1 AGN. The ionised winds in these AGN are parameterised through high-resolution X-ray spectroscopy and photoionisation modelling. We discover a significant inverse correlation between the column density NH of the ionised wind and the radio-loudness parameter R of the jet. We explore different possible explanations for this NH-R relation and find that ionisation, inclination, and luminosity effects are unlikely to be responsible for the observed relation. We argue that the NH-R relation is rather a manifestation of the magnetic driving mechanism of the wind from the accretion disk. Change in the magnetic field configuration from toroidal to poloidal, powering either the wind or the jet mode of the outflow, is the most feasible explanation for the observed decline in the wind NH as the radio jet becomes stronger. Our findings provide evidence for a wind-jet bimodality in radio-loud AGN and shine new light on the link between these two modes of outflow. This has far-reaching consequences for the accretion disk structure and the wind ejection mechanism.
We recently found that Gamma Ray Burst energies and luminosities, in their comoving frame, are remarkably similar. This, coupled with the clustering of energetics once corrected for the collimation factor, suggests the possibility that all bursts, in their comoving frame, have the same peak energy Epeak (of the order of a few keV) and the same energetics of the prompt emission Egamma (of the order of 2e48 erg). The large diversity of bursts energies is then due to the different bulk Lorentz factor Gamma and jet aperture angle theta_jet. We investigated, through a population synthesis code, what are the distributions of Gamma and theta_jet compatible with the observations. Both quantities must have preferred values, with log-normal best fitting distributions and <Gamma0> ~ 275 and <theta_jet> ~ 8.7 degree. Moreover, the peak values of the Gamma and theta_jet distributions must be related - theta_jet^2.5 Gamma =const: the narrower the jet angle, the larger the bulk Lorentz factor. We predict that ~6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin(theta_jet)<1/Gamma. Finally, we estimate that the local rate of GRBs is ~0.3% of all local SNIb/c and ~2.5% of local hypernovae, i.e. SNIb/c with broad absorption lines.
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