261 research outputs found
High-resolution polarization imaging of the Fermi blazar 3C 279
Ever since the discovery by the Fermi mission that active galactic nuclei (AGN) produce copious
amounts of high-energy emission, its origin has remained elusive. Using high-frequency radio
interferometry (VLBI) polarization imaging, we could probe the magnetic field topology of the
compact high-energy emission regions in blazars. A case study for the blazar 3C 279 reveals
the presence of multiple g -ray emission regions. Pass 8 Fermi-Large Area Telescope (LAT) data
are used to investigate the flux variations in the GeV regime; six g -ray flares were observed in
the source during November 2013 to August 2014. We use the 43 GHz VLBI data to study
the morphological changes in the jet. Ejection of a new component (NC2) during the first three
g -ray flares suggests the VLBI core as the possible site of the high-energy emission. A delay
between the last three flares and the ejection of a new component (NC3) indicates that highenergy
emission in this case is located upstream of the 43 GHz core (closer to the black hole).Accepted manuscrip
Modeling the time-dependent polarization of blazars
Linear polarization is an extremely valuable observational tool for probing the dynamic physical conditions of blazar jets. Some patterns are seen in the data, suggestive of order that can be explained by shock waves and helical magnetic field components. However, much disorder is apparent, which implies that turbulence plays a major role as well, especially in the fluctuations of flux and polarization, and perhaps particle acceleration. Here, we present some actual flux and polarization versus time data, plus simulations of model jets. We analyze the output of the simulations in a manner that can be compared with observational data. The results suggest that the ratio of turbulent to ordered magnetic fields varies with time.AST-1615796 - National Science Foundation; NASA; NNX14AQ58G; NNX15AR45
"Orphan" -ray Flares and Stationary Sheaths of Blazar Jets
Blazars exhibit flares across the entire electromagnetic spectrum. Many
-ray flares are highly correlated with flares detected at longer
wavelengths; however, a small subset appears to occur in isolation, with little
or no correlated variability at longer wavelengths. These "orphan" -ray
flares challenge current models of blazar variability, most of which are unable
to reproduce this type of behavior. Macdonald et al. have developed the Ring of
Fire model to explain the origin of orphan -ray flares from within
blazar jets. In this model, electrons contained within a blob of plasma moving
relativistically along the spine of the jet inverse-Compton scatter synchrotron
photons emanating off of a ring of shocked sheath plasma that enshrouds the jet
spine. As the blob propagates through the ring, the scattering of the ring
photons by the blob electrons creates an orphan -ray flare. This model
was successfully applied to modeling a prominent orphan -ray flare
observed in the blazar PKS 1510089. To further support the plausibility of
this model, Macdonald et al. presented a stacked radio map of PKS 1510089
containing the polarimetric signature of a sheath of plasma surrounding the
spine of the jet. In this paper, we extend our modeling and stacking techniques
to a larger sample of blazars: 3C 273, 4C 7101, 3C 279, 1055018, CTA 102,
and 3C 345, the majority of which have exhibited orphan -ray flares. We
find that the model can successfully reproduce these flares, while our stacked
maps reveal the existence of jet sheaths within these blazars.Comment: 19 pages, 27 figures, accepted for publication in ApJ. arXiv admin
note: text overlap with arXiv:1505.0123
The Megaparsec-Scale X-ray Jet of the BL Lac Object OJ287
We present an X-ray image of the BL Lacertae object OJ287 revealing a long
jet, curved by 55 degrees and extending 20", or 90 kpc from the nucleus. This
de-projects to >1 Mpc based on the viewing angle on parsec scales. Radio
emission follows the general X-ray morphology but extends even farther from the
nucleus. The upper limit to the isotropic radio luminosity, ~2E24 W/Hz, places
the source in the Fanaroff-Riley 1 (FR 1) class, as expected for BL Lac
objects. The spectral energy distribution indicates that the extended X-ray
emission is from inverse Compton scattering of cosmic microwave background
photons. In this case, the derived magnetic field is B ~ 5 microGauss, the
minimum electron energy is 7-40 m_e c^2, and the Doppler factor is delta ~ 8 in
a knot 8" from the nucleus. The minimum total kinetic power of the jet is
1-2E45 erg/s. Upstream of the bend, the width of the X-ray emission in the jet
is about half the projected distance from the nucleus. This implies that the
highly relativistic bulk motion is not limited to an extremely thin spine, as
has been proposed previously for FR 1 sources. The bending of the jet, the
deceleration of the flow from parsec to kiloparsec scales, and the knotty
structure can all be caused by standing shocks inclined by ~7 degrees to the
jet axis. Moving shocks resulting from major changes in the flow properties can
also reproduce the knotty structure, but such a model does not explain as many
of the observational details.Comment: Accepted for publication in the Astrophysical Journa
Monthly 43 GHz VLBA Polarimetric Monitoring of 3C120 over 16 Epochs: Evidence for Trailing Shocks in a Relativistic Jet
We present a 16-month sequence of monthly polarimetric 43 GHz VLBA images of
the radio galaxy 3C 120. The images probe the inner regions of the radio jet of
this relatively nearby superluminal radio galaxy at a linear resolution of 0.07
pc ( km s Mpc). We follow the
motion of a number of features with apparent velocities between 4.010.08
and . A new superluminal knot, moving at , is observed to be ejected from the core at a time
coincident with the largest flare ever observed for this source at millimeter
wavelengths. Changes in the position angle of this component, as well as a
progressive rotation of its magnetic polarization vector, suggest the presence
of a twisted (resembling a helix in projection) configuration of the underlying
jet magnetic field and jet geometry. We identify several knots that appear in
the wake of the new superluminal component, moving at proper motions
times slower than any of the other moving knots observed in 3C 120. These
features have properties similar to those of the ``trailing'' shocks seen in
relativistic, time-dependent, hydrodynamical and emission simulations of
compact jets. Such trailing compressions are triggered by pinch-mode jet-body
instabilities caused by the propagation of a strong perturbation, which we
associate with the new strong superluminal component.Comment: 4 pages, 5 figures, accepted for publication in Astrophysical Journal
Letter
Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?
We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA
observations of the radio galaxy 3C 120 revealing a systematic presence of
gradients in Faraday rotation and degree of polarization across and along the
jet. The degree of polarization increases with distance from the core and
toward the jet edges, and has an asymmetric profile in which the northern side
of the jet is more highly polarized. The Faraday rotation measure is also
stratified across the jet width, with larger values for the southern side. We
find a localized region of high Faraday rotation measure superposed on this
structure between approximately 3 and 4 mas from the core, with a peak of about
6000 rad/m^2. Interaction of the jet with the external medium or a cloud would
explain the confined region of enhanced Faraday rotation, as well as the
stratification in degree of polarization and the flaring of superluminal knots
when crossing this region. The data are also consistent with a helical field in
a two-fluid jet model, consisting of an inner, emitting jet and a sheath
containing nonrelativistic electrons. However, this helical magnetic field
model cannot by itself explain the localized region of enhanced Faraday
rotation. The polarization electric vectors, predominantly perpendicular to the
jet axis once corrected for Faraday rotation, require a dominant component
parallel to the jet axis (in the frame of the emitting plasma) for the magnetic
field in the emitting region.Comment: Accepted for publication in ApJ Letters. 4 pages (including 5
figures
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