144 research outputs found
A connection between -ray and parsec-scale radio flares in the blazar 3C 273
We present a comprehensive 5-43 GHz VLBA study of the blazar 3C 273 initiated
after an onset of a strong -ray flare in this source. We have analyzed
the kinematics of new-born components, light curves, and position of the
apparent core to pinpoint the location of the -ray emission. Estimated
location of the -ray emission zone is close to the jet apex, 2 pc to 7
pc upstream from the observed 7 mm core. This is supported by ejection of a new
component. The apparent core position was found to be inversely proportional to
frequency. The brightness temperature in the 7 mm core reached values up to at
least K during the flare. This supports the dominance of particle
energy density over that of magnetic field in the 7 mm core. Particle density
increased during the radio flare at the apparent jet base, affecting
synchrotron opacity. This manifested itself as an apparent core shuttle along
the jet during the 7 mm flare. It is also shown that a region where optical
depth decreases from to spans over several parsecs along
the jet. The jet bulk flow speed estimated at the level of 12c on the basis of
time lags between 7 mm light curves of stationary jet features is 1.5 times
higher than that derived from VLBI apparent kinematics analysis.Comment: Accepted for publication in MNRAS. 17 pages, 15 figures, 10 tables,
with supplementary materials attache
RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz
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 black 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.Comment: Accepted for publication in A&A, 10 pages, 6 figure
The core shift effect in the blazar 3C 454.3
Opacity-driven shifts of the apparent VLBI core position with frequency (the
"core shift" effect) probe physical conditions in the innermost parts of jets
in active galactic nuclei. We present the first detailed investigation of this
effect in the brightest gamma-ray blazar 3C454.3 using direct measurements from
simultaneous 4.6-43 GHz VLBA observations, and a time lag analysis of 4.8-37
GHz lightcurves from the UMRAO, CrAO, and Metsahovi observations in 2007-2009.
The results support the standard Konigl model of jet physics in the VLBI core
region. The distance of the core from the jet origin r_c(nu), the core size
W(nu), and the lightcurve time lag DT(nu) all depend on the observing frequency
nu as r_c(nu)~W(nu)~ DT(nu)~nu^-1/k. The obtained range of k=0.6-0.8 is
consistent with the synchrotron self-absorption being the dominating opacity
mechanism in the jet. The similar frequency dependence of r_c(nu) and W(nu)
suggests that the external pressure gradient does not dictate the jet geometry
in the cm-band core region. Assuming equipartition, the magnetic field strength
scales with distance r as B = 0.4(r/1pc)^-0.8 G. The total kinetic power of
electron/positron jet is about 10^44 ergs/s.Comment: Accepted for publication in MNRAS; 10 pages, 6 figure
An oversized magnetic sheath wrapping around the parsec-scale jet in 3C 273
In recent studies, several AGN have exhibited gradients of the Faraday
Rotation Measure (RM) transverse to their parsec-scale jet direction. Faraday
rotation likely occurs as a result of a magnetized sheath wrapped around the
jet. In the case of 3C 273, using Very Long Baseline Array multi-epoch
observations at 5, 8 and 15 GHz in 2009--2010, we observe that the jet RM has
changed significantly towards negative values compared with that previously
observed. These changes could be explained by a swing of the parsec-scale jet
direction which causes synchrotron emission to pass through different portions
of the Faraday screen. We develop a model for the jet-sheath system in 3C 273
where the sheath is wider than the single-epoch narrow relativistic jet. We
present our oversized sheath model together with a derived wide jet full
intrinsic opening angle and magnetic field
strength G and thermal particle density
at the wide jet--sheath boundary 230 pc
downstream (deprojected) from its beginning. Most of the Faraday rotation
occurs within the innermost layers of the sheath. The model brings together the
jet direction swing and long-term RM evolution and may be applicable to other
AGN jets that exhibit changes of their apparent jet direction.Comment: 10 pages, 9 figures. Accepted to the Astrophysical Journa
Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron II. Observations of 3C 273 at minimum activity
RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R
satellite, launched in 2011, performing Space Very Long Baseline Interferometry
(SVLBI) observations supported by a global ground array of radio telescopes.
With an apogee of about 350 000 km, it is offering for the first time the
possibility to perform {\mu}as-resolution imaging in the cm-band. We present
observations at 22 GHz of 3C 273, performed in 2014, designed to reach a
maximum baseline of approximately nine Earth diameters. Reaching an angular
resolution of 0.3 mas, we study a particularly low-activity state of the
source, and estimate the nuclear region brightness temperature, comparing with
the extreme one detected one year before during the RadioAstron early science
period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study
the kinematics of the jet in a 1.5-year time window. We find that the nuclear
brightness temperature is two orders of magnitude lower than the exceptionally
high value detected in 2013 with RadioAstron at the same frequency (1.4x10^13
K, source-frame), and even one order of magnitude lower than the equipartition
value. The kinematics analysis at 43 GHz shows that a new component was ejected
2 months after the 2013 epoch, visible also in our 22 GHz map presented here.
Consequently this was located upstream of the core during the brightness
temperature peak. These observations confirm that the previously detected
extreme brightness temperature in 3C 273, exceeding the inverse Compton limit,
is a short-lived phenomenon caused by a temporary departure from equipartition.
Thus, the availability of interferometric baselines capable of providing
{\mu}as angular resolution does not systematically imply measured brightness
temperatures over the known physical limits for astrophysical sources.Comment: Accepted for publication in A&
A connection between Îł-ray and parsec-scale radio flares in the blazar 3C 273
We present a comprehensive 5-43 GHz VLBA study of the blazar 3C 273 initiated after an onset of a strong gamma-ray flare in this source. We have analysed the kinematics of newborn components, light curves and position of the apparent core to pinpoint the location of the gamma-ray emission. Estimated location of the gamma-ray emission zone is close to the jet apex, 2-7 pc upstream from the observed 7 mm core. This is supported by ejection of a new component. The apparent core position was found to be inversely proportional to frequency. The brightness temperature in the 7 mm core reached values up to at least 1013 K during the flare. This supports the dominance of particle energy density over that of magnetic field in the 7 mm core. Particle density increased during the radio flare at the apparent jet base, affecting synchrotron opacity. This manifested itself as an apparent core shuttle along the jet during the 7 mm flare. It is also shown that a region where optical depth decreases from tau similar to 1 to tau << 1 spans over several parsecs along the jet. The jet bulk flow speed estimated at the level of 12c on the basis of time lags between 7 mm light curves of stationary jet features is 1.5 times higher than that derived from very long baseline interferometry apparent kinematics analysis
Broad-band properties of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies
We report about recent updates of broad-band properties of radio-loud
narrow-line Seyfert 1 galaxies.Comment: 5 pages, no figures. Proceedings 28th Texas Symposium on Relativistic
Astrophysics, Geneva (Switzerland), 13-18 December 201
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