| Scientists at Max Planck Institute for Radioastronomie discover a double helical structure inside an extragalactic jet, providing direct evidence for the presence of Kelvin--Helmholtz instability in the relativistic plasma of the jet. | |
| Abstract
One of the most intriguing features observed in the active galactic nuclei (AGN) are relativistic plasma outflows (jets) that originate in the immediate vicinity of the center of activity and propagate at distances that are sometimes more than a hundred times larger than the size of the host galaxy. Observations of jets in AGN probe the behavior of extremely relativistic matter in the Universe, and provide a unique, remote ``laboratory'' for studying the most powerful cosmic phenomena such as supermassive black holes and extragalactic accretion disks. In a paper that will be published in Science on October 10, Andrei Lobanov and Anton Zensus (scientists from Max Planck Institute for Radioastronomy, Bonn, Germany) present first direct evidence for the presence of plasma instability in an extragalactic relativistic jet. Their finding is based on analysis of radio interferometric observations of the quasar 3C273 made with the orbiting radio telescope, HALCA and an array of ground telescopes. In the image of 3C273, the emission across the jet is resolved, revealing two threadlike patterns that form a double helix inside the jet. The MPIfR scientists have shown that this double helical structure is consistent with Kelvin-Helmholtz (K-H) instability developing in a light jet with Lorentz factor of 2 and Mach number of 3.5. This description reproduces in detail the internal structure of the jet on scales of up 300 parsecs (1 parsec = 3.26 light years). |
Scientists at the Max Planck Institut for Radioastronomy in Bonn have applied linear perturbation analysis of Kelvin-Helmholtz instability to model the observed double helical structure of the jet in the quasar 3C273 and have shown that the model (outlined with the red and blue lines) describes well the intrinsic structure of the jet on scales of up to 300 parsecs. (PostScript, 1.2MB ; GIF, 69kB) |
| Details
Studying the origin and development of plasma instability in extragalactic jets is crucial for understanding the nature of relativistic outflows from AGN. K-H instability is the most likely type of instability to be found in extragalactic jets. This instability originates from transverse velocity gradients or discontinuities due to interaction between the relativistic plasma of the outflow and (typically) sub-relativistic surrounding material. Recent theoretical works have predicted that K-H instability should produce complex, three-dimensional ribbonlike and threadlike patterns inside a relativistic jet. In these ribbons and threads, an order of magnitude increase of particle density and radio emissivity can be expected. In view of these findings, observational studies of internal structure of parsec-scale jets become particularly important for understanding the role of plasma instabilities in relativistic flows. Such studies require observations at extremely high angular resolution, which can be satisfied at present only by radio wavelength observations using very long baseline interferometry (VLBI). |
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| Space VLBI observations
In VLBI observations, signals from a celestial object are recorded simultaneously at a number of participating telescopes located worldwide. These signals are later combined together at one of the specially designed computer centers (correlators) which determine the correlations between the recordings from different telescopes. Further analysis of these correlations yields images with such an unprecedented resolution that could only be reached by a telescope with the diameter equal to the longest of the distances between the radio telescopes participating in the VLBI observation. For a typical observing wavelength of 6 centimeters, VLBI observations made with intercontinental arrays of radio telescopes reach a resolution of about 2 milliarcseconds. The space VLBI mission VSOP (VLBI Space Observatory Programme, led by the Japanese Institute of Space and Astronautical Science, has complemented ground VLBI arrays with an orbiting 8-meter antenna on board of the satellite HALCA. The VSOP observations provide 3-4 times better resolution, compared with ground VLBI observations at similar frequencies. For relatively close AGN, the resolution of the VSOP is sufficient for resolving the internal structure of their jets.
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Figure 1. Intrinsic structure of the jet in the quasar 3C273. The image obtained from an observation made at a wavelength of 6 cm with the Japanese led space VLBI mission VSOP (sketched in the lower left corner). The image has revealed a spectacular double helix inside the jet which is produced by Kelvin-Helmholtz instability developing in the relativistic plasma. This is the first direct observational evidence for the presence of plasma instability in an extragalactic relativistic jet. (PostScript, 0.5MB; GIF, 62kB) |
| The quasar 3C273
The quasar 3C273 is one of the closest, most luminous, and best studied AGN, with a prominent, relativistic outflow observed in the X--ray, optical and radio wavebands. The relativistic jet observed in the quasar 3C273 is one--sided, with no signs of emission on the counter--jet side at dynamic ranges of up to 16,000:1. This is evidence for strong relativistic boosting in an intrinsically double--sided outflow powered by an accretion disk around a central black hole. The structure and kinematics of such outflows are typically explained in terms of shock waves and Kelvin--Helmholtz instability. Internal structure of the jet in 3C273
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