Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (glo
bal) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure and physics of the relativistic jet in this object. The RadioAstron observations at wavelengths of 18cm, 6cm, and 1.3cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02mas to 200mas. Brightness temperatures in excess of $10^{13}$,K are measured in the jet, requiring Doppler factors of $ge 100$ for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin-Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be $M_mathrm{j}approx 12$ and $etaapprox 0.33$. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations.
Very long baseline interferometry (VLBI) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching, acceleration, and collimation of relativistic outflows. The two-dimensional structure a
nd kinematics of the jet in M,87 (NGC,4486) have been studied by applying the Wavelet-based Image Segmentation and Evaluation (WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array (VLBA) observations made in January-August 2007 at 43 GHz ($lambda = 7$ mm). The WISE analysis recovers a detailed two-dimensional velocity field in the jet in M,87 at sub-parsec scales. The observed evolution of the flow velocity with distance from the jet base can be explained in the framework of MHD jet acceleration and Poynting flux conversion. A linear acceleration regime is observed up to $z_{obs} sim 2$,mas. The acceleration is reduced at larger scales, which is consistent with saturation of Poynting flux conversion. Stacked cross correlation analysis of the images reveals a pronounced stratification of the flow. The flow consists of a slow, mildly relativistic layer (moving at $beta sim 0.5,c$), associated either with instability pattern speed or an outer wind, and a fast, accelerating stream line (with $beta sim 0.92$, corresponding to a bulk Lorentz factor $gamma sim 2.5$). A systematic difference of the apparent speeds in the northern and southern limbs of the jet is detected, providing evidence for jet rotation. The angular velocity of the magnetic field line associated with this rotation suggests that the jet in M87 is launched in the inner part of the disk, at a distance $r_0 sim 5, R_mathrm{s}$ from the central engine. The combined results of the analysis imply that MHD acceleration and conversion of Poynting flux to kinetic energy play the dominant roles in collimation and acceleration of the flow in M,87.
Polarization of radio emission in extragalactic jets at a sub-milliarcsecond angular resolution holds important clues for understanding the structure of the magnetic field in the inner regions of the jets and in close vicinity of the supermassive bla
ck holes in the centers of active galaxies. Space VLBI observations provide a unique tool for polarimetric imaging at a sub-milliarcsecond angular resolution and studying the properties of magnetic field in active galactic nuclei on scales of less than 10^4 gravitational radii. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471), made at a wavelength of 18 cm (frequency of 1.6 GHz) as part of the Early Science Programme (ESP) of the RadioAstron} mission, is used here to test the polarimetric performance of the orbiting Space Radio Telescope (SRT) employed by the mission, to establish a methodology for making full Stokes polarimetry with space VLBI at 1.6 GHz, and to study the polarized emission in the target object on sub-milliarcsecond scales. Polarization leakage of the SRT at 18 cm is found to be within 9 percents in amplitude, demonstrating the feasibility of high fidelity polarization imaging with RadioAstron at this wavelength. A polarimetric image of 0642+449 with a resolution of 0.8 mas (signifying an ~4 times improvement over ground VLBI observations at the same wavelength) is obtained. The image shows a compact core-jet structure with low (~2%) polarization and predominantly transverse magnetic field in the nuclear region. The VLBI data also uncover a complex structure of the nuclear region, with two prominent features possibly corresponding to the jet base and a strong recollimation shock. The maximum brightness temperature at the jet base can be as high as 4*10^13 K.
3C 345 is one of the archetypical active galactic nuclei, showing structural and flux variability on parsec scales near a compact unresolved radio core. During the last 2 years, the source has been undergoing a period of high activity visible in the
broad spectral range, from radio through high-energy bands. We have been monitoring parsec-scale radio emission in 3C 345 during this period at monthly intervals, using the VLBA at 15, 24, and 43 GHz. Our radio observations are compared with gamma-ray emission detected by Fermi-LAT in the region including 3C 345 (1FGL J1642.5+3947). Three distinct gamma-ray events observed in this region are associated with the propagation of relativistic plasma condensations inside the radio jet of 3C 345. We report on evidence for the gamma-rays to be produced in a region of the jet of up to 40 pc (de-projected) in extent. This suggests the synchrotron self-Compton process as the most likely mechanism for production of gamma-rays in the source.
Single-zone synchrotron self-Compton and external Compton models are widely used to explain broad-band Spectral Energy Distributions (SEDs) of blazars from infrared to gamma-rays. These models bear obvious similarities to the homogeneous synchrotron
cloud model which is often applied to explain radio emission from individual components of parsec-scale radio jets. The parsec-scale core, typically the brightest and most compact feature of blazar radio jet, could be the source of high-energy emission. We report on ongoing work to test this hypothesis by deriving the physical properties of parsec-scale radio emitting regions of twenty bright Fermi blazars using dedicated 5-43 GHz VLBA observations and comparing these parameters to results of SED modeling.
The apparent position of the origin (core) of extragalactic radio jets shifts with the observing frequency, owing to synchrotron self-absorption and external absorption. One of the largest core shifts was detected by us in the quasar 0850+581 between
2 and 8 GHz. We have followed this up recently by a dedicated VLBA experiment at 5, 8, 15, 24, and 43 GHz. First results from this study enabled estimating the absolute geometry and physical conditions in the parsec-scale jet origin.
The apparent position of the core in a parsec-scale radio jet (a compact, bright emitting region at the narrow end of the jet) depends on the observing frequency, owing to synchrotron self-absorption and external absorption. While providing a tool pr
obing physical conditions in the vicinity of the core, this dependency poses problems for astrometric studies using compact radio sources. We investigated the frequency-dependent shift in the positions of the cores (core shift) observed with very long baseline interferometry (VLBI) in parsec-scale jets. We discuss related physics, as well as its effect on radio astrometry and the connection between radio and optical positions of astrometric reference objects. We searched for the core shift in a sample of 277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands using VLBI observations made in 2002 and 2003. The core shift was measured by referencing the core position to optically thin jet features whose positions are not expected to change with frequency. We present here results for 29 selected active galactic nuclei (AGN) with bright distinct VLBI jet features that can be used in differential measurements and that allow robust measurements of the shift to be made. In these AGN, the magnitude of the measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median value for the sample of 0.44 mas. Nuclear flares result in temporal variability of the shift. An average shift between the radio (4 cm) and optical (6000 Angstrom) bands is estimated to be approximately 0.1 mas, and it should be taken into account in order to provide the required accuracy of the radio-optical reference frame connection. This can be accomplished with multi-frequency VLBI measurements... (abridged)
Accurate alignment of the radio and optical celestial reference frames requires detailed understanding of physical factors that may cause offsets between the positions of the same object measured in different spectral bands. Opacity in compact extrag
alactic jets (due to synchrotron self-absorption and external free-free absorption) is one of the key physical phenomena producing such an offset, and this effect is well-known in radio astronomy (core shift). We have measured the core shifts in a sample of 29 bright compact extragalactic radio sources observed using very long baseline interferometry (VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and estimate that the average shift between radio and optical positions of distant quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional accuracy of GAIA and SIM. We suggest two possible approaches to carefully investigate and correct for this effect in order to align accurately the radio and optical positions. Both approaches involve determining a Primary Reference Sample of objects to be used for tying the radio and optical reference frames together.
We present results from a large global VLBI(Very Long Baseline Interferometry) survey of compact radio sources at 86 GHz which started in October 2001. The main goal of the survey is to increase the total number of objects accessible for future 3mm-V
LBI imaging by factors of 3-5. The survey data reach the baseline sensitivity of 0.1 Jy, and image sensitivity of better than 10 mJy/beam. To date, a total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, and only 6 sources have not been detected. Flux densities and sizes of core and jet components of all detected sources have been measured using Gaussian model fitting. From these measurements, brightness temperatures have been estimated, taking into account resolution limits of the data. Here, we compare the brightness temperatures of the cores and secondary jet components with similar estimates obtained from surveys at longer wavelengths (e.g. 15 GHz). This approach can be used to study questions related to mechanisms of initial jet acceleration (accelerating or decelerating sub-pc jets?) and jet composition (electron-positron or electron-proton plasma?).
