145 research outputs found
RoboPol: First season rotations of optical polarization plane in blazars
We present first results on polarization swings in optical emission of
blazars obtained by RoboPol, a monitoring program of an unbiased sample of
gamma-ray bright blazars specially designed for effective detection of such
events. A possible connection of polarization swing events with periods of high
activity in gamma rays is investigated using the dataset obtained during the
first season of operation. It was found that the brightest gamma-ray flares
tend to be located closer in time to rotation events, which may be an
indication of two separate mechanisms responsible for the rotations. Blazars
with detected rotations have significantly larger amplitude and faster
variations of polarization angle in optical than blazars without rotations. Our
simulations show that the full set of observed rotations is not a likely
outcome (probability ) of a random walk of the
polarization vector simulated by a multicell model. Furthermore, it is highly
unlikely () that none of our rotations is physically
connected with an increase in gamma-ray activity.Comment: 16 pages, 9 figure
RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
We use results of our 3 year polarimetric monitoring program to investigate
the previously suggested connection between rotations of the polarization plane
in the optical emission of blazars and their gamma-ray flares in the GeV band.
The homogeneous set of 40 rotation events in 24 sources detected by {\em
RoboPol} is analysed together with the gamma-ray data provided by {\em
Fermi}-LAT. We confirm that polarization plane rotations are indeed related to
the closest gamma-ray flares in blazars and the time lags between these events
are consistent with zero. Amplitudes of the rotations are anticorrelated with
amplitudes of the gamma-ray flares. This is presumably caused by higher
relativistic boosting (higher Doppler factors) in blazars that exhibit smaller
amplitude polarization plane rotations. Moreover, the time scales of rotations
and flares are marginally correlated.Comment: 12 pages, 16 figures, accepted to MNRA
Compact Symmetric Objects -- I Towards a Comprehensive Bona Fide Catalog
Compact Symmetric Objects (CSOs) are jetted Active Galactic Nuclei (AGN) with
overall projected size <1 kpc. The classification was introduced to distinguish
these objects from the majority of compact jetted-AGN in centimeter wavelength
very long baseline interferometry observations, where the observed emission is
relativistically boosted towards the observer. The original classification
criteria for CSOs were: (i) evidence of emission on both sides of the center of
activity, and (ii) overall size <1 kpc. However some relativistically boosted
objects with jet axes close to the line of sight appear symmetric and have been
mis-classified as CSOs, thereby undermining the CSO classification. This is
because two essential CSO properties, pointed out in the original papers, have
been neglected: (iii) low variability, and (iv) low apparent speeds along the
jets. As a first step towards creating a comprehensive catalog of ``bona fide''
CSOs, we identify 79 bona fide CSOs, including 15 objects claimed as confirmed
CSOs here for the first time, that match the CSO selection criteria. This
sample of bona fide CSOs can be used for astrophysical studies of CSOs without
contamination by mis-classified CSOs. We show that the fraction of CSOs in
complete flux density limited AGN samples with S >700 mJy is
between % and %.Comment: 28 pages, 9 figures, 3 tables, accepted for publicatio
Compact Symmetric Objects -- II Confirmation of a Distinct Population of High-Luminosity Jetted Active Galaxies
Compact Symmetric Objects (CSOs) are compact (<1 kpc), jetted Active Galactic
Nuclei (AGN), whose jet axes are not aligned close to the line of sight, and
whose observed emission is not predominantly relativistically boosted towards
us. Two classes of CSOs have previously been identified: approximately one
fifth are edge-dimmed and designated as CSO 1s, while the rest are edge
brightened and designated as CSO 2s. This paper focuses almost exclusively on
CSO 2s. Using complete samples of CSO 2s we present three independent lines of
evidence, based on their relative numbers, redshift distributions, and size
distributions, which show conclusively that the vast majority (> 99%) of CSO 2s
do not evolve into larger-scale radio sources. These CSO 2s belong to a
distinct population of jetted-AGN, which should be characterized as
``short-lived'' compared to the classes of larger jetted-AGN, as opposed to
``young''. We show that there is a sharp upper cutoff in the CSO 2 size
distribution at pc. The distinct differences between most CSO 2s
and other jetted-AGN provides a crucial new time domain window on the formation
and evolution of relativistic jets in AGN and the supermassive black holes that
drive them.Comment: 29 pages, 10 figures, 7 tables, accepted for publicatio
Localizing the γ-ray emitting region in the blazar TXS 2013+370
Aims. The γ-ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales.
Methods. TXS 2013+370 was monitored during 2002–2013 with VLBI at 15, 22, 43, and 86 GHz, which allowed us to image the jet base with an angular resolution of ≥0.4 pc. By employing CLEAN imaging and Gaussian model-fitting, we performed a thorough kinematic analysis at multiple frequencies, which provided estimates of the jet speed, orientation, and component ejection times. Additionally, we studied the jet expansion profile and used the information on the jet geometry to estimate the location of the jet apex. VLBI data were combined with single-dish measurements to search for correlated activity between the radio, mm, and γ-ray emission. For this purpose, we employed a cross-correlation analysis, supported by several significance tests.
Results. The high-resolution VLBI imaging revealed the existence of a spatially bent jet, described by co-existing moving emission features and stationary features. New jet features, labeled as A1, N, and N1, are observed to emerge from the core, accompanied by flaring activity in radio/mm- bands and γ-rays. The analysis of the transverse jet width profile constrains the location of the mm core to lie ≤2 pc downstream of the jet apex, and also reveals the existence of a transition from parabolic to conical jet expansion at a distance of ∼54 pc from the core, corresponding to ∼1.5 × 106 Schwarzschild radii. The cross-correlation analysis of the broad-band variability reveals a strong correlation between the radio-mm and γ-ray data, with the 1 mm emission lagging ∼49 days behind the γ-rays. Based on this, we infer that the high energy emission is produced at a distance of the order of ∼1 pc from the jet apex, suggesting that the seed photon fields for the external Compton mechanism originate either in the dusty torus or in the broad-line region
RoboPol: the optical polarization of gamma-ray-loud and gamma-ray-quiet blazars
We present average R-band optopolarimetric data, as well as variability parameters, from the first and second RoboPol observing season. We investigate whether gamma-ray-loud and gamma-ray-quiet blazars exhibit systematic differences in their optical polarization properties. We find that gamma-ray-loud blazars have a systematically higher polarization fraction (0.092) than gamma-ray-quiet blazars (0.031), with the hypothesis of the two samples being drawn from the same distribution of polarization fractions being rejected at the 3σ level. We have not found any evidence that this discrepancy is related to differences in the redshift distribution, rest-frame R-band luminosity density, or the source classification. The median polarization fraction versus synchrotron-peak-frequency plot shows an envelope implying that high-synchrotron-peaked sources have a smaller range of median polarization fractions concentrated around lower values. Our gamma-ray-quiet sources show similar median polarization fractions although they are all low-synchrotron-peaked. We also find that the randomness of the polarization angle depends on the synchrotron peak frequency. For high-synchrotron-peaked sources, it tends to concentrate around preferred directions while for low-synchrotron-peaked sources, it is more variable and less likely to have a preferred direction. We propose a scenario which mediates efficient particle acceleration in shocks and increases the helical B-field component immediately downstream of the shock
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