112 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
Opacity, variability and kinematics of AGN jets
Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests
itself as a time delay between flares observed at high and low radio
frequencies. It is also responsible for the observing frequency dependent
change in size and position of the apparent base of the jet, aka the core shift
effect, detected with very long baseline interferometry (VLBI). We measure the
time delays and the core shifts in 11 radio-loud AGN to estimate the speed of
their jets without relying on multi-epoch VLBI kinematics analysis. The 158
GHz total flux density time lags are obtained using Gaussian process
regression, the core shift values are measured using VLBI observations and
adopted from the literature. A strong correlation is found between the apparent
core shift and the observed time delay. Our estimate of the jet speed is higher
than the apparent speed of the fastest VLBI components by the median
coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to
20. We derive Doppler factors, Lorentz factors and viewing angles of the jets,
as well as the corresponding de-projected distance from the jet base to the
core. The results support evidence for acceleration of the jets with bulk
motion Lorentz factor on de-projected scales
of 0.5500 parsecs.Comment: Accepted by MNRAS; 11 pages, 11 figures, 3 table
New Associations of Gamma-Ray Sources from the Fermi Second Source Catalog
We present the results of an all-sky radio survey between 5 and 9 GHz of the
fields surrounding all unassociated gamma-ray objects listed in the Fermi Large
Area Telescope Second Source Catalog (2FGL). The goal of these observations is
to find all new gamma-ray AGN associations with radio sources >10 mJy at 8 GHz.
We observed with the Very Large Array and the Australia Telescope Compact Array
the areas around unassociated sources, providing localizations of weak radio
point sources found in 2FGL fields at arcmin scales. Then we followed-up a
subset of those with the Very Long Baseline and the Long Baseline Arrays to
confirm detections of radio emission on parsec-scales. We quantified
association probabilities based on known statistics of source counts and
assuming a uniform distribution of background sources. In total we found 865
radio sources at arcsec scales as candidates for association and detected 95 of
170 selected for follow-up observations at milliarcsecond resolution. Based on
this we obtained firm associations for 76 previously unknown gamma-ray AGN.
Comparison of these new AGN associations with the predictions from using the
WISE color-color diagram shows that half of the associations are missed. We
found that 129 out of 588 observed gamma-ray sources at arcmin scales not a
single radio continuum source was detected above our sensitivity limit within
the 3-sigma gamma-ray localization. These "empty" fields were found to be
particularly concentrated at low Galactic latitudes. The nature of these
Galactic gamma-ray emitters is not yet determined.Comment: accepted for publication by ApJS, 18 pages, 10 figures, 12 tables;
full electronic versions of tables 2-8 are available as ancillary file
How Spatially Resolved Polarimetry Informs Black Hole Accretion Flow Models
The Event Horizon Telescope (EHT) Collaboration has successfully produced
images of two supermassive black holes, enabling novel tests of black holes and
their accretion flows on horizon scales. The EHT has so far published total
intensity and linear polarization images, while upcoming images may include
circular polarization, rotation measure, and spectral index, each of which
reveals different aspects of the plasma and space-time. The next-generation EHT
(ngEHT) will greatly enhance these studies through wider recorded bandwidths
and additional stations, leading to greater signal-to-noise, orders of
magnitude improvement in dynamic range, multi-frequency observations, and
horizon-scale movies. In this paper, we review how each of these different
observables informs us about the underlying properties of the plasma and the
spacetime, and we discuss why polarimetric studies are well-suited to
measurements with sparse, long-baseline coverage.Comment: Submitted for Galaxies Special Issue "From Vision to Instrument:
Creating a Next-Generation Event Horizon Telescope for a New Era of Black
Hole Science
The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets
We have compared the parsec-scale jet linear polarization properties of the
Fermi LAT-detected and non-detected sources in the complete
flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE
sources, 30 were detected by the LAT during its first three months of
operation. We find that during the era since the launch of Fermi, the
unresolved core components of the LAT-detected jets have significantly higher
median fractional polarization at 15 GHz. This complements our previous
findings that these LAT sources have higher apparent jet speeds, brightness
temperatures and Doppler factors, and are preferentially found in higher
activity states.Comment: 6 pages, 3 figures, to appear in the proceedings of "High Energy
Phenomena In Relativistic Outflows II" (Buenos Aires, Argentina, October
26-30, 2009) International Journal of Modern Physics
Time variability of the core-shift effect in the blazar 3C 454.3
Using VLBI to measure a so-called core shift effect is a common way of
obtaining estimates of the jet magnetic field strength. The VLBI core is
typically identified as the bright feature at the jet's base, and the position
of the core changes with the observed frequency, . In this work, we investigated the time variability of the
core-shift effect in the blazar 3C 454.3. We employed self-referencing analysis
of multi-frequency (5, 8, 15, 22-24, and 43 GHz) VLBA data covering 19 epochs
from 2005 until 2010. We found significant core shift variability ranging from
0.27 to 0.86 mas between 5 and 43 GHz, confirming the core-shift variability
phenomenon observed before. Time variability of the core-shift index ()
was found typically below one, with an average value of and a
standard deviation of . values were found during flaring and
quiescent states and our results indicate that commonly assumed conical jet
shape and equipartition conditions do not always hold simultaneously. Still,
these conditions are often assumed when deriving magnetic field strengths from
core shift measurements, leading to unreliable results if significantly
deviates from unity. Therefore, it is important to verify that holds
before using core shift values and the equipartition assumption to derive
physical parameters in the jets. When epochs are selected in the case
of 3C 454.3, the magnetic field estimates are indeed quite consistent, even
though the core shift varies with time. Additionally, our estimations of the
jet's magnetic flux in 3C 454.3 show that the source is indeed in the
magnetically arrested disk state. Finally, we found a good correlation of the
core position with the core flux density, , which is consistent with increased particle density
during the flares.Comment: 53 pages, 6 tables, 57 figures. Article submitted to Astronomy and
Astrophysic
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