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Register a new userA relativistic mixing-layer model for jets in low-luminosity radio galaxies
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We present an analytical model for jets in Fanaroff & Riley Class I (FRI) radio galaxies, in which an initially laminar, relativistic flow is surrounded by a shear layer. We apply the appropriate conservation laws to constrain the jet parameters, starting the model where the radio emission is observed to brighten abruptly. We assume that the laminar flow fills the jet there and that pressure balance with the surroundings is maintained from that point outwards. Entrainment continuously injects new material into the jet and forms a shear layer, which contains material from both the environment and the laminar core. The shear layer expands rapidly with distance until finally the core disappears, and all of the material is mixed into the shear layer. Beyond this point, the shear layer expands in a cone and decelerates smoothly. We apply our model to the well-observed FRI source 3C31 and show that there is a self-consistent solution. We derive the jet power, together with the variations of mass flux and and entrainment rate with distance from the nucleus. The predicted variation of bulk velocity with distance in the outer parts of the jets is in good agreement with model fits to VLA observations. Our prediction for the shape of the laminar core can be tested with higher-resolution imaging.
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Various radio galaxies show signs of having gone through episodic jet outbursts in the past. An example is the class of double-double radio galaxies (DDRGs). However, to follow the evolution of an individual source in real-time is impossible due to the large time scales involved. Numerical studies provide a powerful tool to investigate the temporal behavior of episodic jet outbursts in a (magneto-)hydrodynamical setting. We simulate the injection of two jets from active galactic nuclei (AGN), separated by a short interruption time. Three different jet models are compared. We find that an AGN jet outburst cycle can be divided into four phases. The most prominent phase occurs when the restarted jet is propagating completely inside the hot and inflated cocoon left behind by the initial jet. In that case, the jet-head advance speed of the restarted jet is significantly higher than the initial jet-head. While the head of the initial jet interacts strongly with the ambient medium, the restarted jet propagates almost unimpeded. As a result, the restarted jet maintains a strong radial integrity. Just a very small fraction of the amount of shocked jet material flows back through the cocoon compared to that of the initial jet and much weaker shocks are found at the head of the restarted jet. We find that the features of the restarted jet in this phase closely resemble the observed properties of a typical DDRG.
We point out that the remarkable linearity of the ultra-steep radio spectra of high redshift radio galaxies reflects a previously reported general trend for powerful radio galaxies, according to which the spectral curvature is lesser for sources having steeper spectra (measured near rest-frame 1 GHz). We argue based on existing theoretical and observational evidence that it is premature to conclude that the particle acceleration mechanism in sources having straight, ultra-steep radio spectra gives rise to an ultra-steep injection spectrum of the radiating electrons. In empirical support to this we show that the estimated injection spectral indices, available for a representative sample of 35 compact steep spectrum (CSS) radio sources are not correlated with their rest-frame (intrinsic) rotation measures, which are known to be typically large, indicating a dense environment, as is also the case for high-$z$ radio galaxies.
We aim to determine the properties of the central region of NGC 1052 using X-ray and radio data. NGC 1052 (z=0.005) has been investigated for decades in different energy bands and shows radio lobes and a low luminosity active galactic nucleus (LLAGN). We use X-ray images from Chandra and radio images from Very Large Array (VLA) to explore the morphology of the central area. We also study the spectra of the nucleus and the surrounding region using observations from Chandra and XMM-Newton. We find diffuse soft X-ray radiation and hotspots along the radio lobes. The spectrum of the circum-nuclear region is well described by a thermal plasma (T~0.6 keV) and a power law with photon index Gamma~2.3. The nucleus shows a hard power law (Gamma~1.4) modified by complex absorption. A narrow iron K-alpha line is also clearly detected in all observations, but there is no evidence for relativistic reflection. The extended emission is consistent with originating from extended jets and from jet-triggered shocks in the surrounding medium. The hard power-law emission from the nucleus and the lack of relativistic reflection supports the scenario of inefficient accretion in an Advection Dominated Accretion Flow (ADAF).
(ABRIDGED) We present here the results from new Very Long Baseline Array observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI radio galaxies. New Chandra data of two sources, viz., UGC00408 and UGC08433, are combined with the Chandra archival data of 13 sources. The 5 GHz observations of ten core-jet sources are polarization-sensitive, while the 1.6 GHz observations constitute second epoch total intensity observations of nine core-only sources. Polarized emission is detected in the jets of seven sources at 5 GHz, but the cores are essentially unpolarized, except in M87. Polarization is detected at the jet edges in several sources, and the inferred magnetic field is primarily aligned with the jet direction. This could be indicative of magnetic field shearing due to jet-medium interaction, or the presence of helical magnetic fields. The jet peak intensity $I_ u$ falls with distance $d$ from the core, following the relation, $I_ upropto d^a$, where $a$ is typically -1.5. Assuming that adiabatic expansion losses are primarily responsible for the jet intensity dimming, two limiting cases are considered: [1] the jet has a constant speed on parsec-scales and is expanding gradually such that the jet radius $rpropto d^0.4$; this expansion is however unobservable in the laterally unresolved jets at 5 GHz, and [2] the jet is cylindrical and is accelerating on parsec-scales. Accelerating parsec-scale jets are consistent with the phenomenon of magnetic driving in Poynting flux dominated jets. Chandra observations of 15 UGC FRIs detect X-ray jets in nine of them. The high frequency of occurrence of X-ray jets in this complete sample suggests that they are a signature of a ubiquitous process in FRI jets.
The nearby low-luminosity active galactic nucleus (LLAGN) NGC 4258 has a weak radio continuum emission at the galactic center. Quasi-simultaneous multi-frequency observations using the Very Large Array (VLA) from 5 GHz (6 cm) to 22 GHz (1.3 cm) showed inverted spectra in all epochs, which were intra-month variable, as well as complicated spectral features that cannot be represented by a simple power law, indicating multiple blobs in nuclear jets. Using the Nobeyama Millimeter Array (NMA), we discovered a large amplitude variable emission at 100 GHz (3 mm), which had higher flux densities at most epochs than those of the VLA observations. A James Clerk Maxwell Telescope (JCMT) observation at 347 GHz (850 micron) served an upper limit of dust contamination. The inverted radio spectrum of the nucleus NGC 4258 is suggestive of an analogy to our Galactic center Sgr A*, but with three orders of magnitude higher radio luminosity. In addition to the LLAGN M 81, we discuss the nucleus of NGC 4258 as another up-scaled version of Sgr A*.
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