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1.
《New Astronomy Reviews》1999,43(8-10):691-694
Very Long Baseline Interferometry polarisation observations provide information about the magnetic-field structure of the parsec-scale jets in active galactic nuclei. 5-GHz VLBI polarisation observations of the compact BL Lacertae object 1803+784 were obtained in July 1998 with a global ground array plus the orbiting HALCA (VSOP) antenna. The extra resolution provided by baselines to the orbiting antenna has revealed a smoothly bent jet structure in 1803+784, with the magnetic field transverse all along the jet. This may indicate the presence of relativistic shocks, or possibly of a helical magnetic field surrounding the VLBI jet.  相似文献   

2.
A comparison between low-frequency space very long baseline interferometry (VLBI) and high-frequency ground-based VLBI images can, in principle, be used to detect small variations in rotation measure (RM) on fine angular scales inaccessible to ground arrays alone. This paper reports an attempt to perform such a comparison using the jet in the quasar 3C 380. Observations made with the VSOP antenna HALCA together with a ground array at wavelength 1.6 GHz provide total intensity and polarization images of comparable resolution to those from the ground array alone at 5 GHz. The results provide an image showing derotated magnetic vector position angle of somewhat higher resolution than that available earlier. The results show variations in an RM around component A of the order of 10 rad m−2 that could not have been detected with the ground array alone. It is concluded that satellite VLBI observations provide a promising means to study the distribution of matter and magnetic fields around parsec-scale jets.
The ground observations used here follow the steady outward drift of component A, which has approximately doubled its distance from the core since the first observations in 1982. They also reveal total intensity and polarization structure associated with a bright knot 0.7 arcsec from the core which is reminiscent of that expected for a conical shock wave.  相似文献   

3.
We have used the very large Jodrell Bank VLA Astrometric Survey/Cosmic Lens All-Sky Survey 8.4-GHz surveys of flat-spectrum radio sources to test the hypothesis that there is a systematic alignment of polarization position angle vectors on cosmological scales of the type claimed by Hutsemékers et al. The polarization position angles of 4290 sources with polarized flux density ≥1 mJy have been examined. They do not reveal large-scale alignments either as a whole or when split in half into high-redshift  ( z ≥ 1.24)  and low-redshift subsamples. Nor do the radio sources which lie in the specific areas covered by Hutsemékers et al. show any significant effect. We have also looked at the position angles of parsec-scale jets derived from very long baseline interferometry observations and again find no evidence for systematic alignments. Finally, we have investigated the correlation between the polarization position angle and those of the parsec-scale jets. As expected, we find that there is a tendency for the polarization angles to be perpendicular to the jet angles. However, the difference in jet and polarization position angles does not show any systematic trend in different parts of the sky.  相似文献   

4.
We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (υ app = cβ app) and the effective Doppler boosting factor (δ eff) of the relativistically moving radio knot. A novel aspect of these analytical computations of β-δ diagrams is that they are made for parsec-scale jets having a conical shape, with modest opening angles (ω up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor, Γ), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis is surrounded by a slower moving sheath, possibly arising from a velocity shear.  相似文献   

5.
Homan & Lister have recently published circular polarization (CP) detections for 34 objects in the MOJAVE sample – a set of bright, compact active galactic nuclei (AGN) being monitored by the Very Long Baseline Array at 15 GHz. We report the detection of 15-GHz parsec-scale CP in two more AGN (3C 345 and 2231+114), and confirm the MOJAVE detection of CP in 1633+382. It is generally believed that the most likely mechanism for the generation of this CP is Faraday conversion of linear polarization (LP) to CP. A helical jet magnetic field ( B field) geometry can facilitate this process – linearly polarized emission from the far side of the jet is converted to CP as it passes through the magnetized plasma at the front side of the jet on its way towards the observer. In this case, the sign of the generated CP is essentially determined by the pitch angle and helicity of the helical B field. We have determined the pitch-angle regimes and helicities of the helical jet B fields in eight AGN for which parsec-scale CP has been detected, and used them to predict the expected CP signs for these AGN if the CP is generated via conversion in these helical fields. We have obtained the intriguing result that our predictions agree with the observed signs in all eight cases, provided that the longitudinal B field components in the jets correspond to south magnetic poles. This clearly non-random pattern demonstrates that the observed CP in AGN is directly associated with the presence of helical jet B fields. These results suggest that helical B fields are ubiquitous in AGN jets.  相似文献   

6.
空间VLBI的研究现状   总被引：2，自引：0，他引：2  
沈志强 《天文学进展》1998,16(2):117-134
评述了过去近三十年中空间VLBI的发展．其内容包括空间VLBI的简短发展历史,空间VLBI与地面VLBI的不同、空间VLBI的目前状况和空间VLBI的未来展望。着重介绍了作为首次空间VLBI的VSOP的各个方面情况,并给出了VSOP的连续谱、谱线普查样本,供参考。  相似文献   

7.
We present the results of multifrequency (15 + 8 + 5 GHz) polarization Very Long Baseline Array (VLBA) observations of the three BL Lacertae objects 0745+241, 1418+546 and 1652+398 together with 5-GHz VLBI Space Observatory Programme (VSOP) observations of 1418+546 and 1.6- and 5-GHz VSOP observations of the blazar 1055+018. The jets of all these sources have polarization structure transverse to the jet axis, with the polarization E vectors aligned with the jet along the jet spine and 'sheaths' of orthogonal E vectors at one or both edges of the jet. The presence of polarization aligned with the jet near the 'spine' may indicate that the jets are associated with helical B fields that propogate outward with the jet flow; the presence of orthogonal polarization near the edges of the jet may likewise be a consequence of a helical jet B field, or may be owing to an interaction with the ambient medium on parsec scales. We have tentatively detected interknot polarization in 1055+018 with E aligned with the local jet direction, consistent with the possibility that the jet of this source is associated with a helical B field.  相似文献   

8.
We report on centimeter VLA and VLBI observations of the giant, low power radio galaxy 1144+35. On the parsec scale, we see a complex jet component moving away from the center of activity at 2.7h50−1 c. We detect a faint parsec-scale counter-jet and derive a jet velocity of 0.95c and an angle to the line of sight of 25°, consistent with an intrinsically symmetric ejection. These findings lend credence to the claim that even the jets of low-power radio galaxies start out relativistically.  相似文献   

9.
Collimated jets are believed to be an essential ingredient of the star formation process, and we are now able for the first time to test observationally the theories for their formation and propagation. The major advances achieved in recent years are reviewed, regarding the observed morphology, kinematics and excitation properties of jets, from the parsec-scale `giant outflows' down to the `microjets' from T Tauri stars. High angular resolution images and spectra have provided valuable estimates of jet diameter, space velocity, temperature, ionization fraction, electron and total density, both along and across the flow. We can thus calculate key physical quantites, as the shock excitation parameters, or the mass and momentum fluxes in the flow. The results obtained appear to validate the popular magneto-centrifugal models for jet launching, although some important issues are still under debate, as to the cause of knotty structures, observed wind thermal properties, and the dynamical relationship between jets and molecular outflows. Among the most interesting recent findings, we mention the observed indications for jet rotation, with inferred toroidal velocities consistent with the prescribed angular momentum balance between infall and outflow.  相似文献   

10.
We present extensive multi-epoch VLBI observations of the quasar 3C 380 which reveal a bent parsec-scale radio jet with complex substructure which exhibits superluminal motion out to ∼100 pc from the active core. The general characteristics of the jet and its motion are similar to those in several other well-studied superluminal sources, most notably a rapid increase in the opening angle at a projected distance of a few tens of parsecs from the core and apparent acceleration along the jet. This acceleration could be a simple kinematic effect associated with bulk flow at a constant speed, but at a varying angle to the line of sight. The jet is well resolved in the transverse direction for the majority of its length and resembles numerical simulations of a jet disrupted by the rapid growth of sinusoidal 'Kelvin–Helmholtz' modes. Our maps also reveal extremely rapid localized variations in brightness which have not yet been seen in other nuclear jets. We suggest that phase effects, arising from the intersection of oblique shocks, may be the cause of these dramatic changes. 3C 380 is often classified as a compact steep spectrum (CSS) source; however, in contrast to the majority of CSS sources, it is likely that 3C 380 is simply a powerful FRII source seen approximately end-on.  相似文献   

11.
External Faraday rotation has been detected in both the core and the parsec-scale jet of BL Lac in a four-frequency very long baseline interferometry (VLBI) experiment. This unexpected result indicates the presence of significant amounts of thermal gas close to the nucleus of this object. The rotation measure (RM) in the jet components is constant, and differs from the currently accepted Galactic RM, indicating that this value (−205 rad m−2) is not applicable to the components in the parsec-scale jet. The similarity of the RM in these jet components leads us to suspect that the jet RM is caused by a foreground screen in our Galaxy, although we cannot rule out a combination of Galactic RM and RM local to the jet. If the jet RM is due solely to the Galaxy, this would mean that the currently accepted value of the foreground RM (−205 rad m−2 ) is not correct, either because the value changed between 1982 and 1997, or because the assumption of no intrinsic source rotation was incorrect, as it was at our later epoch of observation. Our observations suggest a value of     .
After correcting for the foreground RM, the core value is −427 rad m−2, which is unexpected since, owing to the weakness of their line emission, BL Lac objects are often assumed to be depleted in gas. The core RM appears to be variable, probably because of the presence of at least two polarized components close to the core the relative contributions of which vary with time.  相似文献   

12.
The paper summarises the results to date of an ongoing multi-epoch, multi-frequency polarisation VLBI study of a complete sample of northern 1-Jy BL Lac objects. We discuss the physical origin of the characteristic polarisation properties of the core and jet structures as revealed by both general trends for the sample as a whole and more detailed observations of individual objects. It has been known for a decade that the distribution of the core polarisation position angles at moderate centimeter wavelengths is bimodal, with the corepolarisation angle being either aligned with or perpendicular to the direction of the inner VLBI jet. A preliminary analysis of the VLBI core spectral indices from 15–5 GHz suggests that this is the result of optical depth effects and that the two peaks in the distribution correspond to cores dominated by optically thin and thick regions of emission. This interpretation is supported by the results of quasi-simultaneous VLBI and optical polarisation observations, and also by VSOP polarisation observations of OJ287. Evidence for the presence of helical or toroidal magnetic fields associated with the VLBIjets is discussed. The collected results are discussed in the context of their possible implications for circular-polarisation studies of AGN. We are now beginning a search for parsec-scale circular polarisation in the complete sample BL Lac objects and hope to have our first results by June 2003.  相似文献   

13.
We present the results from a campaign to monitor the relatively low redshift (z=0.3) circumpolar superluminal quasar 1928+738 with VSOP during the first Announcement of Opportunity period. The four epochs of data show that there have been substantial structural changes in this source near the core on the time-scale of a few months.  相似文献   

14.
Several versions of the shock-in-jet model of the blazars' variability have been proposed in recent years. This family of models resorts to the interaction between a relativistic shock and a feature in the parsec-scale jet associated to an AGN in order to explain the origin of the rapid variability recently observed in many extragalactic radio sources. The nature of the involved features varies from one particular model to another. Here I discuss a possible origin for this variety of subparsec-sized features in the frame of the two-fluid model of extragalactic jets. Macroscopic Kelvin-Helmholtz instabilities are shown to be capable of producing the required plurality of morphologies in the innermost part of the beams, in such a way that the different models have particular ranges of applicability according to the differencies in the parameters of the flows.  相似文献   

15.
It has been two decades since astronomers first discovered that accretion disks around young stars drive highly collimated supersonic jets. Thanks to concerted efforts to understand emission line ratios from jets, we know that velocity variations dominate the heating within these flows, and motions in stellar jets, now observed in real time, are primarily radial. The fluid dynamics of the cooling zones can be complex, with interacting shocks, clumps, and instabilities that could benefit from insights into the physics that only experiments can provide. Recent laboratory experiments have reproduced jets with velocities and Mach numbers similar to those within stellar jets, and the field seems poised to make significant advances by connecting observations and theories with experiments. This article points out several aspects of stellar jets that might be clarified by such experiments.  相似文献   

16.
We study the bending of jets in binary stellar systems. A compact companion accretes mass from the slow wind of the mass-losing primary star, forms an accretion disc and blows two opposite jets. These fast jets are bent by the slow wind. Disregarding the orbital motion, we find the dependence of the bending angle on the properties of the slow wind and the jets. Bending of jets is observed in planetary nebulae which are thought to be the descendants of interacting binary stars. For example, in some of these planetary nebulae, the two bubbles (lobes) which are inflated by the two opposite jets are displaced to the same side of the symmetry axis of the nebula. Similar displacements are observed in bubble pairs in the centre of some clusters and groups of galaxies. We compare the bending of jets in binary stellar systems with that in clusters of galaxies.  相似文献   

17.
We performed three dimensional resistive magnetohydrodynamic simulations to study the magnetic reconnection using an initially shearing magnetic field configuration(force free field with a current sheet in the middle of the computational box).It is shown that there are two types of reconnection jets:the ordinary reconnection jets and fan-shaped jets,which are formed along the guide magnetic field.The fan-shaped jets are significantly different from the ordinary reconnection jets which are ejected by magneti...  相似文献   

18.
We study the spectral energy distribution of gamma rays and neutrinos in the precessing microquasar SS433 as a result of pp interactions within its dark jets. Gamma-ray absorption due to interactions with matter of the extended disc and of the star is found to be important, as well as absorption caused by the ultraviolet and mid-infrared radiation from the equatorial envelopment. We analyse the range of precessional phases for which this attenuation is at a minimum and the chances for detection of a gamma-ray signal are enhanced. The power of relativistic protons in the jets, a free parameter of the model, is constrained by HEGRA data. This imposes limits on the gamma-ray fluxes to be detected with instruments such as GLAST, VERITAS and MAGIC II. A future detection of high-energy neutrinos with cubic kilometre telescopes such as IceCube would also yield important information about acceleration mechanisms that may take place in the dark jets. Overall, the determination of the ratio of gamma-ray to neutrino flux will result in a key observational tool to clarify the physics of heavy jets.  相似文献   

19.
We study the influence of the matter content of extragalactic jets on their morphology, dynamics and emission properties. For this purpose we consider jets of extremely different compositions, including pure leptonic and baryonic plasmas. Our work is based on two-dimensional relativistic hydrodynamic simulations of the long-term evolution of powerful extragalactic jets propagating into a homogeneous environment. The equation of state used in the simulations accounts for an arbitrary mixture of electrons, protons and electron–positron pairs. Using the hydrodynamic models, we have also computed synthetic radio maps and the thermal bremsstrahlung X-ray emission from their cavities.
Although there is a difference of about three orders of magnitude in the temperatures of the cavities inflated by the simulated jets, we find that both the morphology and the dynamic behaviour are almost independent of the assumed composition of the jets. Their evolution proceeds in two distinct epochs. During the first one, multidimensional effects are unimportant and the jets propagate ballistically. The second epoch starts when the first larger vortices are produced near the jet head, causing the beam cross-section to increase and the jet to decelerate. The evolution of the cocoon and cavity is in agreement with a simple theoretical model. The beam velocities are relativistic  ( Γ ≃4)  at kiloparsec scales, supporting the idea that the X-ray emission of several extragalactic jets may be due to relativistically boosted CMB photons. The radio emission of all models is dominated by the contribution of the hotspots. All models exhibit a depression in the X-rays surface brightness of the cavity interior, in agreement with recent observations.  相似文献   

20.
We report the results from a 6-yr, multi-epoch very long baseline interferometry monitoring of the Seyfert galaxy NGC 3079. We have observed NGC 3079 during eight epochs between 1999 and 2005 predominantly at 5 GHz, but covering the frequency range of 1.7 to 22 GHz. Using our data and observations going back to 1985, we find that the separation of two of the three visible nuclear radio components underwent two decelerations. At the time of these decelerations, the flux density of one of the components increased by factors of 5 and 2, respectively. We interpret these events as a radio jet component undergoing compression, possibly as a result of a collision with interstellar medium material. This interpretation strongly supports the existence of jets surrounded by a clumpy medium of dense clouds within the first few parsec from the central engine in NGC 3079. Moreover, based on recently published simulations of jet interactions with clumpy media, this scenario is able to explain the nature of two additional regions of ageing synchrotron material detected at the lower frequencies as by-products of such interactions, and also the origin of the kpc-scale super-bubble observed in NGC 3079 as the result of the spread of the momentum of the jets impeded from propagating freely. The generalization of this scenario provides an explanation why jets in Seyfert galaxies are not able to propagate to scales of kpc as do jets in radio-loud AGN.  相似文献   

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