1,797 research outputs found
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
Unexpected High Brightness Temperature 140 PC from the Core in the Jet of 3C 120
We present 1.7, 5, 15, 22 and 43 GHz polarimetric multi-epoch VLBA
observations of the radio galaxy 3C 120. The higher frequency observations
reveal a new component, not visible before April 2007, located 80 mas from the
core (which corresponds to a deprojected distance of 140 pc), with a brightness
temperature about 600 times higher than expected at such distances. This
component (hereafter C80) is observed to remain stationary and to undergo small
changes in its brightness temperature during more than two years of
observations. A helical shocked jet model - and perhaps some flow acceleration
- may explain the unusually high Tb of C80, but it seems unlikely that this
corresponds to the usual shock that emerges from the core and travels
downstream to the location of C80. It appears that some other intrinsic process
in the jet, capable of providing a local burst in particle and/or magnetic
field energy, may be responsible for the enhanced brightness temperature
observed in C80, its sudden appearance in April 2007, and apparent
stationarity.Comment: 5 pages, 5 figures Accepted to be published in ApJ Letter
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
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
Location of gamma-ray Flare Emission in the Jet of the BL Lacertae Object OJ287 more than 14pc from the Central Engine
We combine time-dependent multi-waveband flux and linear polarization
observations with sub-milliarcsecond-scale polarimetric images at lambda=7mm of
the BL Lacertae-type blazar OJ287 to locate the gamma-ray emission in prominent
flares in the jet of the source >14pc from the central engine. We demonstrate a
highly significant correlation between the strongest gamma-ray and
millimeter-wave flares through Monte-Carlo simulations. The two reported
gamma-ray peaks occurred near the beginning of two major mm-wave outbursts,
each of which is associated with a linear polarization maximum at millimeter
wavelengths. Our Very Long Baseline Array observations indicate that the two
mm-wave flares originated in the second of two features in the jet that are
separated by >14 pc. The simultaneity of the peak of the higher-amplitude
gamma-ray flare and the maximum in polarization of the second jet feature
implies that the gamma-ray and mm-wave flares are co-spatial and occur >14 pc
from the central engine. We also associate two optical flares, accompanied by
sharp polarization peaks, with the two gamma-ray events. The multi-waveband
behavior is most easily explained if the gamma-rays arise from synchrotron
self-Compton scattering of optical photons from the flares. We propose that
flares are triggered by interaction of moving plasma blobs with a standing
shock. The gamma-ray and optical emission is quenched by inverse Compton losses
as synchrotron photons from the newly shocked plasma cross the emission region.
The mm-wave polarization is high at the onset of a flare, but decreases as the
electrons emitting at these wavelengths penetrate less polarized regions.Comment: Accepted for Publication in The Astrophysical Journal Letters. 6
pages, including 5 figure
Collective Evidence for Inverse Compton Emission from External Photons in High-Power Blazars
We present the first collective evidence that Fermi-detected jets of high
kinetic power (L_kin) are dominated by inverse Compton emission from
upscattered external photons. Using a sample with a broad range in orientation
angle, including radio galaxies and blazars, we find that very high power
sources (L_kin > 10^45.5 erg s^{-1}) show a significant increase in the ratio
of inverse Compton to synchrotron power (Compton dominance) with decreasing
orientation angle, as measured by the radio core dominance and confirmed by the
distribution of superluminal speeds. This increase is consistent with beaming
expectations for external Compton (EC) emission, but not for synchrotron self
Compton (SSC) emission. For the lowest power jets (L_kin < 10^43.5 erg s^{-1}),
no trend between Compton and radio core dominance is found, consistent with
SSC. Importantly, the EC trend is not seen for moderately high power flat
spectrum radio quasars with strong external photon fields. Coupled with the
evidence that jet power is linked to the jet speed (Kharb et al. 2010), this
finding suggests that external photon fields become the dominant source of seed
photons in the jet comoving frame only for the faster and therefore more
powerful jets.Comment: Accepted for Publication in ApJ Letters, 5 pages, 4 figure
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