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We present a new method for studying the intrinsic brightness temperatures of the parsec-scale jet cores of Active Galactic Nuclei (AGN). Our method uses observed superluminal motions and observed brightness temperatures for a large sample of AGN to constrain the characteristic intrinsic brightness temperature of the sample as a whole. To study changes in intrinsic brightness temperature, we assume that the Doppler factors of individual jets are constant in time as justified by their relatively small changes in observed flux density. We find that in their median-low brightness temperature state, the sources in our sample have a narrow range of intrinsic brightness temperatures centered on a characteristic temperature, T_int = 3 x 10^10 K, which is close to the value expected for equipartition, when the energy in the radiating particles equals the energy stored in the magnetic fields. However, in their maximum brightness state, we find that sources in our sample have a characteristic intrinsic brightness temperature greater than 2 x 10^11 K, which is well in excess of the equipartition temperature. In this state, we estimate the energy in radiating particles exceeds the energy in the magnetic field by a factor of ~ 10^5. We suggest that the excess of particle energy when sources are in their maximum brightness state is due to injection or acceleration of particles at the base of the jet. Our results suggest that the common method of estimating jet Doppler factors by using a single measurement of observed brightness temperature and/or the assumption of equipartition may lead to large scatter or systematic errors in the derived values.
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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.
The fine-scale structure and the kinematics of relativistic active galactic nuclei (AGN) jets have been studied by very-long-baseline interferometry at very high resolutions since 1998 at 2 cm wavelength for a sample of over a hundred radio sources (VLBA 2cm Survey and MOJAVE programs). Since 2007, this is being complemented by the TANAMI project, based on southern observations with the Australian LBA at 3.6 cm and 1.1 cm wavelengths. From our observation campaign, we find that most of the radio jets show linear morphologies at parsec-scales, but some of show curvature and non-radial motions. Features are observed to move at highly relativistic speeds, with Lorentz factors extending above values of 30. We also provide a brief description of the relationship of our radio findings with the AGN observations by the new Fermi Gamma-ray Space Telescope.
The sensitivity, stability, and uniformity of calibration of the VLBA has revolutionized parsec-scale polarization studies of AGN jets. Not only does polarization probe the magnetic field structures of jets, serving as a hydrodynamic tracer of shocks, bends, and shear, but polarization also probes the medium through which it propagates by encoding the signature of Faraday effects along the line of sight. I review advances made by the VLBA in studying the polarization of jets to probe their magnetic field structures, properties of the jet plasma, and properties of the external environment. This review covers both linear and circular polarization and is set in the context of outstanding questions in the field.
{Abridged} We show that the surface brightness (SB) profiles of elliptical galaxies can be parametrized using a linear superposition of 2-3 components, described by functions developed in Dhar & Williams as the 2D projections of a 3D Einasto profile. For a sample of 23 ellipticals with -24 < Mv < -15, our multi-component models span a range of up to 10^6 in SB and 10^5 in radius, have a median rms of 0.032 mag arcsec^-2, and are statistically justified at >3{sigma}. Our models indicate that i) the central component is more concentrated than the outer component; and ii) the central component of core galaxies is much more luminous, extended and concentrated than that of cuspy galaxies, with their near exponential central profiles indicating disk-like systems whose existence must be verified spectroscopically. While such central excess components are not necessarily contrary to the notion of a mass deficit in core galaxies, we show that the existence, amount, radial extent and sign of mass deficits disagree substantially in the literature, both for a given galaxy and on an average over a sample. We discuss possible implications and suggest that SMBH binaries are unlikely to be the sole mechanism for producing the large cores. We also deduce conditions under which the 3D light density can be described with a multi-component Einasto model for both cuspy and core galaxies; indicating an universality in the functional form of the 3D density of light in galaxies and dark matter in LCDM N-body haloes. Finally, we show that our result - the outer component of the SB profiles of massive galaxies has 5 < n < 8 - could imply i) a common feature of collisionless systems; and ii) that galaxies with such n for their outer component are dark matter dominated.
Large scale X-ray jets that extend to >100 kpc distances from the host galaxy indicate the importance of jets interactions with the environment on many different physical scales. Morphology of X-ray clusters indicate that the radio-jet activity of a cD galaxy is intermittent. This intermittency might be a result of a feedback and/or interactions between galaxies within the cluster. Here we consider the radiation pressure instability operating on short timescales (<10^5 years) as the origin of the intermittent behaviour. We test whether this instability can be responsible for short ages (< 10^4 years) of Compact Symmetric Objects measured by hot spots propagation velocities in VLBI observations. We model the accretion disk evolution and constrain model parameters that may explain the observed compact radio structures and over-abundance of GPS sources. We also describe effects of consequent outbursts.
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