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Register a new userRadioAstron reveals a spine-sheath jet structure in 3C 273
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We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine/sheath structure observed by Space-VLBI with RadioAstron in 3C 273
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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 $alpha_mathrm{int}=2.1^circ$ and magnetic field strength $B_{||}=3$ $mu$G and thermal particle density $N_mathrm{e}=125~mathrm{cm}^{-3}$ 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.
Since its discovery in 1963, 3C273 has become one of the most widely studied quasars with investigations spanning the electromagnetic spectrum. While much has been discovered about this historically notable source, its low-frequency emission is far less well understood. Observations in the MHz regime have traditionally lacked the resolution required to explore small-scale structures that are key to understanding the processes that result in the observed emission. In this paper we use the first sub-arcsecond images of 3C273 at MHz frequencies to investigate the morphology of the compact jet structures and the processes that result in the observed spectrum. Using the full complement of LOFARs international stations, we produce $0.31 times 0.21$ arcsec images of 3C273 at 150 MHz to determine the jets kinetic power, place constraints on the bulk speed and inclination angle of the jets, and look for evidence of the elusive counter-jet at 150 MHz. Using ancillary data at GHz frequencies, we fit free-free absorption (FFA) and synchrotron self-absorption (SSA) models to determine their validity in explaining the observed spectra. The images presented display for the first time that robust, high-fidelity imaging of low-declination complex sources is now possible with the LOFAR international baselines. We show that the main small-scale structures of 3C273 match those seen at higher frequencies and that absorption is present in the observed emission. We determine the kinetic power of the jet to be in the range of $3.5 times 10^{43}$ - $1.5 times 10^{44}$ erg s$^{-1}$ which agrees with estimates made using higher frequency observations. We derive lower limits for the bulk speed and Lorentz factor of $beta gtrsim 0.55$ and $Gamma geq 1.2$ respectively. The counter-jet remains undetected at $150$ MHz, placing a limit on the peak brightness of $S_mathrm{cj_150} < 40$ mJy beam$^{-1}$.
We present UBVRIJHK polarization and flux density observations of the quasar 3C 273 obtained during a time of outburst over two weeks in 1988 February. We have modelled these data with two power law components, each with wavelength-independent position angle. These components are roughly perpendicular. The steeper-spectrum component has higher infrared polarized flux density, with the electric vector approximately transverse to the projected direction of the VLBI jet. The K-band polarized flux density and position angle, and possibly the spectral index of the two components, are correlated with a time lag of less than a day. We explain our results in terms of a shocked jet model with two nearly co-spatial components: a shock component with magnetic field approximately perpendicular to the jet and the other with magnetic field approximately parallel to the jet.
We have developed a quasi-analytical model for the production of radiation in strong-line blazars, assuming a spine-sheath jet structure. The model allows us to study how the spine and sheath spectral components depend on parameters describing the geometrical and physical structure of the blazar zone. We show that typical broad-band spectra of strong-line blazars can be reproduced by assuming the magnetization parameter to be of order unity and reconnection to be the dominant dissipation mechanism. Furthermore, we demonstrate that the spine-sheath model can explain why gamma-ray variations are often observed to have much larger amplitudes than the corresponding optical variations. The model is also less demanding of jet power than one-zone models, and can reproduce the basic features of extreme gamma-ray events.
We present results from four recent Chandra monitoring observations of the jet in 3C 273 using the ACIS detector, obtained between November 2003 and July 2004. We find that the X-ray emission comes in two components: unresolved knots that are smaller than the corresponding optically emitting knots and a broad channel that is about the same width as the optical interknot region. We compute the jet speed under the assumption that the X-ray emission is due to inverse Compton scattering of the cosmic microwave background, finding that the dimming of the jet X-ray emission to the jet termination relative to the radio emission may be due to bulk deceleration.
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