127 research outputs found
The core shift effect in the blazar 3C 454.3
Opacity-driven shifts of the apparent VLBI core position with frequency (the
"core shift" effect) probe physical conditions in the innermost parts of jets
in active galactic nuclei. We present the first detailed investigation of this
effect in the brightest gamma-ray blazar 3C454.3 using direct measurements from
simultaneous 4.6-43 GHz VLBA observations, and a time lag analysis of 4.8-37
GHz lightcurves from the UMRAO, CrAO, and Metsahovi observations in 2007-2009.
The results support the standard Konigl model of jet physics in the VLBI core
region. The distance of the core from the jet origin r_c(nu), the core size
W(nu), and the lightcurve time lag DT(nu) all depend on the observing frequency
nu as r_c(nu)~W(nu)~ DT(nu)~nu^-1/k. The obtained range of k=0.6-0.8 is
consistent with the synchrotron self-absorption being the dominating opacity
mechanism in the jet. The similar frequency dependence of r_c(nu) and W(nu)
suggests that the external pressure gradient does not dictate the jet geometry
in the cm-band core region. Assuming equipartition, the magnetic field strength
scales with distance r as B = 0.4(r/1pc)^-0.8 G. The total kinetic power of
electron/positron jet is about 10^44 ergs/s.Comment: Accepted for publication in MNRAS; 10 pages, 6 figure
RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz
Polarization of radio emission in extragalactic jets at a sub-milliarcsecond
angular resolution holds important clues for understanding the structure of the
magnetic field in the inner regions of the jets and in close vicinity of the
supermassive black holes in the centers of active galaxies. Space VLBI
observations provide a unique tool for polarimetric imaging at a
sub-milliarcsecond angular resolution and studying the properties of magnetic
field in active galactic nuclei on scales of less than 10^4 gravitational
radii. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471),
made at a wavelength of 18 cm (frequency of 1.6 GHz) as part of the Early
Science Programme (ESP) of the RadioAstron} mission, is used here to test the
polarimetric performance of the orbiting Space Radio Telescope (SRT) employed
by the mission, to establish a methodology for making full Stokes polarimetry
with space VLBI at 1.6 GHz, and to study the polarized emission in the target
object on sub-milliarcsecond scales. Polarization leakage of the SRT at 18 cm
is found to be within 9 percents in amplitude, demonstrating the feasibility of
high fidelity polarization imaging with RadioAstron at this wavelength. A
polarimetric image of 0642+449 with a resolution of 0.8 mas (signifying an ~4
times improvement over ground VLBI observations at the same wavelength) is
obtained. The image shows a compact core-jet structure with low (~2%)
polarization and predominantly transverse magnetic field in the nuclear region.
The VLBI data also uncover a complex structure of the nuclear region, with two
prominent features possibly corresponding to the jet base and a strong
recollimation shock. The maximum brightness temperature at the jet base can be
as high as 4*10^13 K.Comment: Accepted for publication in A&A, 10 pages, 6 figure
An oversized magnetic sheath wrapping around the parsec-scale jet in 3C 273
In recent studies, several AGN have exhibited gradients of the Faraday
Rotation Measure (RM) transverse to their parsec-scale jet direction. Faraday
rotation likely occurs as a result of a magnetized sheath wrapped around the
jet. In the case of 3C 273, using Very Long Baseline Array multi-epoch
observations at 5, 8 and 15 GHz in 2009--2010, we observe that the jet RM has
changed significantly towards negative values compared with that previously
observed. These changes could be explained by a swing of the parsec-scale jet
direction which causes synchrotron emission to pass through different portions
of the Faraday screen. We develop a model for the jet-sheath system in 3C 273
where the sheath is wider than the single-epoch narrow relativistic jet. We
present our oversized sheath model together with a derived wide jet full
intrinsic opening angle and magnetic field
strength G and thermal particle density
at the wide jet--sheath boundary 230 pc
downstream (deprojected) from its beginning. Most of the Faraday rotation
occurs within the innermost layers of the sheath. The model brings together the
jet direction swing and long-term RM evolution and may be applicable to other
AGN jets that exhibit changes of their apparent jet direction.Comment: 10 pages, 9 figures. Accepted to the Astrophysical Journa
Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron II. Observations of 3C 273 at minimum activity
RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R
satellite, launched in 2011, performing Space Very Long Baseline Interferometry
(SVLBI) observations supported by a global ground array of radio telescopes.
With an apogee of about 350 000 km, it is offering for the first time the
possibility to perform {\mu}as-resolution imaging in the cm-band. We present
observations at 22 GHz of 3C 273, performed in 2014, designed to reach a
maximum baseline of approximately nine Earth diameters. Reaching an angular
resolution of 0.3 mas, we study a particularly low-activity state of the
source, and estimate the nuclear region brightness temperature, comparing with
the extreme one detected one year before during the RadioAstron early science
period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study
the kinematics of the jet in a 1.5-year time window. We find that the nuclear
brightness temperature is two orders of magnitude lower than the exceptionally
high value detected in 2013 with RadioAstron at the same frequency (1.4x10^13
K, source-frame), and even one order of magnitude lower than the equipartition
value. The kinematics analysis at 43 GHz shows that a new component was ejected
2 months after the 2013 epoch, visible also in our 22 GHz map presented here.
Consequently this was located upstream of the core during the brightness
temperature peak. These observations confirm that the previously detected
extreme brightness temperature in 3C 273, exceeding the inverse Compton limit,
is a short-lived phenomenon caused by a temporary departure from equipartition.
Thus, the availability of interferometric baselines capable of providing
{\mu}as angular resolution does not systematically imply measured brightness
temperatures over the known physical limits for astrophysical sources.Comment: Accepted for publication in A&
Broad-band properties of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies
We report about recent updates of broad-band properties of radio-loud
narrow-line Seyfert 1 galaxies.Comment: 5 pages, no figures. Proceedings 28th Texas Symposium on Relativistic
Astrophysics, Geneva (Switzerland), 13-18 December 201
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
Probing the innermost regions of AGN jets and their magnetic fields with radioastron. I. Imaging BL LACERTAE at 21 μm as resolution
We present the first polarimetric space very long baseline interferometry (VLBI) imaging observations at 22 GHz. BL Lacertae was observed in 2013 November 10 with the RadioAstron space VLBI mission, including a ground array of 15 radio telescopes. The instrumental polarization of the space radio telescope is found to be less than 9%, demonstrating the polarimetric imaging capabilities of RadioAstron at 22 GHz. Ground-space fringes were obtained up to a projected baseline distance of 7.9 Earth diameters in length, allowing us to image the jet in BL Lacertae with a maximum angular resolution of 21 μas, the highest achieved to date. We find evidence for emission upstream of the radio core, which may correspond to a recollimation shock at about 40 μas from the jet apex, in a pattern that includes other recollimation shocks at approximately 100 and 250 μas from the jet apex. Polarized emission is detected in two components within the innermost 0.5 mas from the core, as well as in some knots 3 mas downstream. Faraday rotation analysis, obtained from combining RadioAstron 22 GHz and ground-based 15 and 43 GHz images, shows a gradient in rotation measure and Faraday-corrected polarization vector as a function of position angle with respect to the core, suggesting that the jet in BL Lacertae is threaded by a helical magnetic field. The intrinsic de-boosted brightness temperature in the unresolved core exceeds K, suggesting, at the very least, departure from equipartition of energy between the magnetic field and radiating particles.This research has been supported by the Spanish Ministry of Economy and Competitiveness grant AYA2013-40825-P, by the Russian Foundation for Basic Research (projects 13-02-12103, 14-02-31789, and 15-02-00949), and St. Petersburg University research grant 6.38.335.2015. The research at Boston University (BU) was funded in part by NASA Fermi Guest Investigator grant NNX14AQ58G. Y.M. acknowledges support from the ERC Synergy Grant >BlackHoleCam-Imaging the Event Horizon of Black Holes> (Grant 610058). Part of this work was supported by the COST Action MP1104 >Polarization as a tool to study the Solar System and beyond.> The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the Russian Federal Space Agency, in collaboration with partner organizations in Russia and other countries.Peer Reviewe
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