1,918 research outputs found
3C 286: a bright, compact, stable, and highly polarized calibrator for millimeter-wavelength observations
(Context.) A number of millimeter and submillimeter facilities with linear
polarization observing capabilities have started operating during last years.
These facilities, as well as other previous millimeter telescopes and
interferometers, require bright and stable linear polarization calibrators to
calibrate new instruments and to monitor their instrumental polarization. The
current limited number of adequate calibrators implies difficulties in the
acquisition of these calibration observations. (Aims.) Looking for additional
linear polarization calibrators in the millimeter spectral range, in mid-2006
we started monitoring 3C 286, a standard and highly stable polarization
calibrator for radio observations. (Methods.) Here we present the 3 and 1 mm
monitoring observations obtained between September 2006 and January 2012 with
the XPOL polarimeter on the IRAM 30 m Millimeter Telescope. (Results.) Our
observations show that 3C 286 is a bright source of constant total flux with 3
mm flux density S_3mm = (0.91 \pm 0.02) Jy. The 3mm linear polarization degree
(p_3mm =[13.5\pm0.3]%) and polarization angle (chi_3mm
=[37.3\pm0.8]deg.,expressed in the equatorial coordinate system) are also
constant during the time span of our observations. Although with poorer time
sampling and signal-to-noise ratio, our 1 mm observations of 3C 286 are also
reproduced by a constant source of 1 mm flux density (S_1mm = [0.30 \pm 0.03]
Jy), polarization fraction (p_1mm = [14.4 \pm 1.8] %), and polarization angle
(chi_1mm = [33.1 \pm 5.7]deg.). (Conclusions.) This, together with the
previously known compact structure of 3C 286 -extended by ~3.5" in the sky-
allow us to propose 3C 286 as a new calibrator for both single dish and
interferometric polarization observations at 3 mm, and possibly at shorter
wavelengths.Comment: Accepted for publication in A&A. 7 pages, 4 figures, 8 tables.
Updated data sets with regard to previous version. New discussion about multi
frequency properties of the source. Section 3.3, Figures 3 and 4, and Tables
7 and 8 are ne
Synthetic synchrotron emission maps from MHD models for the jet of M87
We present self-consistent global, steady-state MHD models and synthetic
optically thin synchrotron emission maps for the jet of M87. The model consist
of two distinct zones: an inner relativistic outflow, which we identify with
the observed jet, and an outer cold disk-wind. While the former does not
self-collimate efficiently due to its high effective inertia, the latter
fulfills all the conditions for efficient collimation by the
magneto-centrifugal mechanism. Given the right balance between the effective
inertia of the inner flow and the collimation efficiency of the outer disk
wind, the relativistic flow is magnetically confined into a well collimated
beam and matches the measurements of the opening angle of M87 over several
orders of magnitude in spatial extent. The synthetic synchrotron maps reproduce
the morphological structure of the jet of M87, i.e. center-bright profiles near
the core and limb-bright profiles away from the core. At the same time, they
also show a local increase of brightness at some distance along the axis
associated to a recollimation shock in the MHD model. Its location coincides
with the position of the optical knot HST-1. In addition our best fitting model
is consistent with a number of observational constraints such as the magnetic
field in the knot HST-1, and the jet-to-counterjet brightness ratio.Comment: 9 pages, 9 figures, accepted by Ap
Rapid TeV variability in Blazars as result of Jet-Star Interaction
We propose a new model for the description of ultra-short flares from TeV
blazars by compact magnetized condensations (blobs), produced when red giant
stars cross the jet close to the central black hole. Our study includes a
simple dynamical model for the evolution of the envelope lost by the star in
the jet, and its high energy nonthermal emission through different leptonic and
hadronic radiation mechanisms. We show that the fragmented envelope of the star
can be accelerated to Lorentz factors up to 100 and radiate effectively the
available energy in gamma-rays predominantly through proton synchrotron
radiation or external inverse Compton scattering of electrons. The model can
readily explain the minute-scale TeV flares on top of longer (typical
time-scales of days) gamma-ray variability as observed from the blazar PKS
2155-304. In the framework of the proposed scenario, the key parameters of the
source are robustly constrained. In the case of proton synchrotron origin of
the emission a mass of the central black hole of , a total jet power of and a Doppler factor, of the gamma-ray emitting blobs, of
are required. Whilst for the external inverse Compton model,
parameters of
, and the are required.Comment: 25 pages, 11 figures, Submitted to Ap
3D Simulations of Relativistic Precessing Jets Probing the Structure of Superluminal Sources
We present the results of a three-dimensional, relativistic, hydrodynamic
simulation of a precessing jet into which a compact blob of matter is injected.
A comparison of synthetic radio maps computed from the hydrodynamic model,
taking into account the appropriate light travel time delays, with those
obtained from observations of actual superluminal sources shows that the
variability of the jet emission is the result of a complex combination of phase
motions, viewing angle selection effects, and non-linear interactions between
perturbations and the underlying jet and/or the external medium. These results
question the hydrodynamic properties inferred from observed apparent motions
and radio structures, and reveal that shock-in-jet models may be overly
simplistic.Comment: Accepted for publication in ApJL. 4 pages, 5 figures (4 in color
The milliarcsecond-scale jet of PKS 0735+178 during quiescence
We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae
object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous
and later observations suggests that the overall kinematic and structural
properties of the jet are greatly influenced by its activity. Time intervals of
enhanced activity, as reported before 1993 and after 2000 by other studies, are
followed by highly superluminal motion along a rectilinear jet. In contrast the
less active state in which we performed our observations, shows subluminal or
slow superluminal jet features propagating through a twisted jet with two sharp
bends of about 90 deg. within the innermost three-milliarcsecond jet structure.
Proper motion estimates from the data presented here allow us to constrain the
jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between
2 and 4.Comment: 11 pages, 12 figures. Accepted for publication in A&
Multiwavelength observations of the blazar BL Lacertae: a new fast TeV Îł-ray flare
Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Busan (South Korea). Published in Proceeding of Science.Observations of fast TeV Îł-ray flares from blazars reveal the extreme compactness of emitting regions in blazar jets. Combined with very-long-baseline radio interferometry measurements, they probe the structure and emission mechanism of the jet. We report on a fast TeV Îł-ray flare from BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and a decay time of about 36 minutes. The peak flux at >200 GeV measured with the 4-minute binned light curve is (4.2±0.6)Ă10â6photonsmâ2sâ1, or âŒ180% the Crab Nebula flux. Variability in GeV Îł-ray, X-ray, and optical flux, as well as in optical and radio polarization was observed around the time of the TeV Îł-ray flare. A possible superluminal knot was identified in the VLBA observations at 43 GHz. The flare constrains the size of the emitting region, and is consistent with several theoretical models with stationary shocks
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