Terminal Hotspot Research Articles

Nonsteady-State Electric Circuit in Electrophoresis on Paper: Thermal Consideration of Electrophoretic Lateral-Flow Assays.

Nonsteady-state behaviors are not expected in electric circuits that lack significant capacitance, inductivity, and/or active feedback. Here, we report that electrophoresis on paper─used, e.g., to electrophoretically driven lateral-flow immunoassays (LFIA)─can create a nonsteady-state electric circuit. We studied electrophoresis on 50 × 4 mm nitrocellulose membrane strips utilized in LFIA. The voltage was applied to strip termini immersed in reservoirs with a running buffer. If the electric power of this circuit exceeded approximately 0.5 W, neither the electric current nor the temperature map reached their steady states on a multiminute time scale. The current grew slowly to its maximum and then slowly decreased. The temperature map evolved slowly, with one or more hot spots appearing and disappearing gradually in different positions on the strip. The slow evolution of a temperature map led to the occurrence of a terminal hot spot in which the strip burned. No chaotic behavior was observed, i.e., time dependences of both the current and temperature map were reproducible. We analyzed major processes involved in paper-based electrophoresis and explained the nonsteady-state behavior. Unlike ordinary electric circuits with metal conductors, paper-based electrophoresis involves two slow processes: (i) intense buffer evaporation from hot spots and (ii) buffer supply from the reservoirs by an interplay of the capillary penetration and the electroosmotic flow. These processes affect heat generation and/or dissipation on the strip and, accordingly, the resistivity profile. The slow evolution of the resistivity profile is responsible for the nonsteady-state behavior. The results of our computer modeling support this explanation. The hot spots may have a destructive effect on electrophoretically driven LFIA. To avoid denaturation of immunoreagents, experimentalists should empirically confirm that spatiotemporal temperature maps are compatible with the developed assay.

A Kpc-scale radio polarization study of PG BL Lacs with the uGMRT

ABSTRACT We present here uGMRT band 4 (∼650 MHz) polarization images of 8 BL Lac objects belonging to the Palomar-Green (PG) ‘blazar’ sample. A large fraction of the sources ($\sim 63~{{\ \rm per\ cent}}$) reveal core-halo radio structures with most of the polarization detected in the inner core-jet regions. PG1101 + 385 and PG2254 + 075 exhibit a ‘spine-sheath structure’ in polarization. The core-halo and ‘spine-sheath’ structures are consistent with the Unified Scheme suggestion that BL Lacs are the pole-on beamed counterparts of Fanaroff–Riley (FR) type I radio galaxies. PG1418 + 546 and PG0851 + 203 (OJ287) show the presence of terminal hotspots similar to FR type II radio galaxies. They were also found to be low-spectrally peaked BL Lacs, supportive of the ‘blazar envelope’ scenario for BL Lacs and quasars. Fractional polarization ranges from 1 to 13 per cent in the cores and 2 to 26 per cent in the inner jets/lobes of the sample BL Lacs. Compared to the varied radio morphology of quasars from the PG ‘blazar’ sample, the BL Lacs appear to be less diverse. A comparison of the inferred core magnetic (B-) field structures on arcsec- (kpc-) scales w.r.t. the Very Long Baseline Interferometry jet direction does not reveal any preferred orientation, suggesting that if large-scale ordered B-fields exist, they do so on scales smaller than probed by the current observations. However, the presence of polarized emission on arcsec-scales suggests that any mixing of thermal plasma with the synchrotron emitting plasma is insufficient to fully depolarize the emission via the internal depolarization process.

A physically-motivated perspective of Fanaroff–Riley classification of radio galaxies

A small subset of extragalactic double radio sources, termed HYbrid MOrpholgy Radio Sources (HYMORS), is distinguished by a very unusual, hybrid morphology in terms of Fanaroff–Riley (FR) classification. In HYMORS, one radio lobe appears edge-darkened (FR I), while the other shows a well-defined emission peak near its outer edge (edge-brightened, FR II). Such sources are rare, but critical for constraining the mechanism responsible for FR dichotomy, a widely debated issue in extragalactic astrophysics. Here, we highlight the need for caution in assigning FR type, in view of some upcoming observational campaigns to confirm HYMORS among the candidates. To illustrate this, we highlight the cases of three radio sources, which have been perceived to be HYMORS, including the radio galaxy 0500 $$+$$ 630 (4C $$+$$ 63.07), which has been claimed to be a good, original example of a HYMORS, with a FR I western lobe and a FR II eastern lobe marked by a prominent terminal hot spot. However, its recent VLASS map at 3 GHz has revealed that the western lobe actually extends much farther out than reported and terminates in a well-defined emission peak. This implies that the source is a regular FR II radio galaxy and not a HYMORS. We also provide a brief perspective of the HYMORS phenomenon and underscore the need to confirm a FR I classification by ruling out additional FR II characteristics, such as an inward lobe-widening and spectral steepening, as well as a lack of prominent radio jet within the lobe.

New mechanisms for forming multiple hotspots in radio jets

ABSTRACTHotspots of radio galaxies are regions of shock-driven particle acceleration. Multiple hotspots have long been identified as potential indicators of jet movement or precession. Two frequent explanations describe a secondary hotspot as either the location of a prior jet termination point or a deflected backflow-driven shock: the so-called dentist’s drill and splatter-spot models. We created high-resolution simulations of precessing jets with a range of parameters. In addition to the existing mechanisms, our results show three additional mechanisms for multiple hotspot formation: (1) the splitting of a large terminal hotspot into passive and active components; (2) jet stream splitting resulting in two active hotspots; and (3) dynamic multiple hotspot complexes that form as a result of jet termination in a turbulent cocoon, linked here to rapid precession. We show that these distinct types of multiple hotspots are difficult to differentiate in synthetic radio maps, particularly hotspot complexes that can easily be mistaken for the jet itself. We discuss the implication for hypothesized binary supermassive black hole systems where jet precession is a key component of the morphology, and show a selection of potential precession candidates found using the Low-Frequency Array Two-metre Sky Survey Data Release 2.

Subarcsecond view on the high-redshift blazar GB 1508+5714 by the International LOFAR Telescope

Context. Studies of the most distant active galactic nuclei (AGNs) allow us to test our current understanding of the physics present in radio-jetted AGNs across a range of environments, and probe their interactions with these environments. The decrease in apparent luminosity with distance is the primary difficulty to overcome in the study of these distant AGNs, which requires highly sensitive instruments. Aims. Our goal is to employ new long wavelength radio data to better parametrise the broad-band spectral energy distribution (SED) of GB 1508+5714, a high-redshift (z = 4.30) AGN. Its high redshift, high intrinsic luminosity and classification as a blazar allow us to test emission models that consider the efficient cooling of jet electrons via inverse Compton losses in interactions with the dense cosmic microwave background (CMB) photon field at high redshifts. A significant detection of this effect in GB 1508+5714 may partly explain the apparent sparsity of high-redshift radio galaxies in wide-field surveys, detections of this kind are only becoming possible with the current generation of Square Kilometre Array (SKA) precursors. Methods. We used the LOw-Frequency ARray (LOFAR) to image the long wavelength radio emission around the high-redshift blazar GB 1508+5714 on arcsecond scales at frequencies between 128 and 160 MHz. This allowed us to compare the spatially resolved structure with higher frequency observations, and construct spectral index maps to study the spectral properties of the different components. Results. The LOFAR image shows a compact unresolved core and two resolved emission regions around 2 arcsec to the east and to the west of the radio core. We find structure consistent with previous Very Large Array (VLA) observations, as well as a previously unreported emission region to the east. The region in the west shows a spectral index of −1.2−0.2+0.4 while the region in the east indicates a spectral index of ≲−1.1. The radio core features aflat spectral index of 0.02 ± 0.01. Conclusions. We interpret the arcsecond-scale radio structure of GB 1508+5714 as a FR II-like radio galaxy at a small viewing angle, and the western component as the region containing the approaching jet’s terminal hot spot while the eastern diffuse component near the core can be interpreted as the counter-hot spot region. Our SED modelling shows that a scenario featuring significant quenching effects caused by interaction with the CMB provides a good description of the data, and notably explains the suppressed radio emission.

Herscheldiscovery of far-infrared emission from the hotspot D in the radio galaxy Cygnus A

ABSTRACTThe far-infrared counterpart of hotspot D, the terminal hotspot of the eastern jet hosted by the radio galaxy Cygnus A, is detected with Herschel Aperture photometry of the source performed in 5 photometric bands covering the wavelength range of 70–350 $\mu$m. After removing the contamination from another nearby hotspot, E, the far-infrared intensity of hotspot D is derived as 83 ± 13 and 269 ± 66 mJy at 160 and 350 $\mu$m, respectively. Since the far-infrared spectrum of the object smoothly connects to the radio one, the far-infrared emission is attributed to the synchrotron radiation from the radio-emitting electron population. The radio-to-near-infrared spectrum is confirmed to exhibit a far-infrared break feature at the frequency of $\nu _\mathrm{br}=2.0^{+1.2}_{-0.8} \times 10^{12}$ Hz. The change in energy index at the break (Δα = 0.5) is interpreted as the impact of radiative cooling on an electron distribution sustained by continuous injection from diffusive shock acceleration. By ascribing the derived break to this cooling break, the magnetic field, B, in the hotspot is determined as a function of its radius, R within a uniform one-zone model combined with the strong relativistic shock condition. An independent B–R constraint is obtained by assuming the X-ray spectrum is wholly due to synchrotron self-Compton emission. By combining these conditions, the two parameters are tightly determined as B = 120–150 μG and R = 1.3–1.6 kpc. A further investigation into the two conditions indicates the observed X-ray flux is highly dominated by the synchrotron self-Compton emission.

LOFAR measures the hotspot advance speed of the high-redshift blazar S5 0836+710

Context. The emission and proper motion of the terminal hotspots of active galactic nucleus (AGN) jets can be used as a powerful probe of the intergalactic medium. However, measurements of hotspot advance speeds in active galaxies are difficult, especially in the young universe, because of the low angular velocities and the low brightness of distant radio galaxies. Aims. Our goal is to study the termination of an AGN jet in the young universe and to deduce physical parameters of the jet and the intergalactic medium. Methods. We used the LOw Frequency ARray (LOFAR) to image the long-wavelength radio emission of the high-redshift blazar S5 0836+710 on arcsecond scales between 120 MHz and 160 MHz. Results. The LOFAR image shows a compact unresolved core and a resolved emission region about 1.5 arcsec to the southwest of the radio core. This structure is in general agreement with previous higher-frequency radio observations with the Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Very Large Array (VLA). The southern component shows a moderately steep spectrum with a spectral index of about ≳ − 1, and the spectral index of the core is flat to slightly inverted. In addition, we detect for the first time a resolved steep-spectrum halo with a spectral index of about −1 surrounding the core. Conclusions. The arcsecond-scale radio structure of S5 0836+710 can be understood as a Faranoff–Riley (FR) II radio galaxy observed at a small viewing angle. The southern component can be interpreted as the region of the approaching jet’s terminal hotspot, and the halo like diffuse component near the core can be interpreted as the counter-hotspot region. From the differential Doppler boosting of both features, we can derive the hotspot advance speed to (0.01 − 0.036) c. At a constant advance speed, the derived age of the source would exceed the total lifetime of such a powerful FR II radio galaxy substantially. Thus, the hotspot advance speed must have been higher in the past, in agreement with a scenario in which the originally highly relativistic jet has lost collimation as a result of instability growth and has transformed into an only mildly relativistic flow. Our data suggest that the density of the intergalactic medium around this distant (z = 2.22) AGN could be substantially higher than the values typically found in less distant FR II radio galaxies.

Protein intrinsic disorder in Arabidopsis NAC transcription factors: transcriptional activation by ANAC013 and ANAC046 and their interactions with RCD1.

Protein ID (intrinsic disorder) plays a significant, yet relatively unexplored role in transcription factors (TFs). In the present paper, analysis of the transcription regulatory domains (TRDs) of six phylogenetically representative, plant-specific NAC [no apical meristem, ATAF (Arabidopsis transcription activation factor), cup-shaped cotyledon] TFs shows that the domains are present in similar average pre-molten or molten globule-like states, but have different patterns of order/disorder and MoRFs (molecular recognition features). ANAC046 (Arabidopsis NAC 046) was selected for further studies because of its simple MoRF pattern and its ability to interact with RCD1 (radical-induced cell death 1). Experiments in yeast and thermodynamic characterization suggest that its single MoRF region is sufficient for both transcriptional activation and interaction with RCD1. The remainder of the large regulatory domain is unlikely to contribute to the interaction, since the domain and truncations thereof have similar affinities for RCD1, which are also similar for ANAC013-RCD1 interactions. However, different enthalpic and entropic contributions to binding were revealed for ANAC046 and ANAC013, suggestive of differences in binding mechanisms. Although substitution of both hydrophobic and acidic residues of the ANAC046 MoRF region abolished binding, substitution of other residues, even with α-helix-breaking proline, was less disruptive. Together, the biophysical analyses suggest that RCD1-ANAC046 complex formation does not involve folding-upon-binding, but rather fuzziness or an unknown structure in ANAC046. We suggest that the ANAC046 regulatory domain functions as an entropic chain with a terminal hot spot interacting with RCD1. RCD1, a cellular hub, may be able to interact with many different TFs by exploiting their ID-based flexibility, as demonstrated for its interactions with ANAC046 and ANAC013.

Microquasar jets in the supernova remnant G11.2–0.3

The radio and X-ray fine structure of the supernova remnant G11.2-0.3 is analyzed using VLA and Chandra data. A possible pair of bipolar X-ray jets inside the remnant shell was reported earlier; caps or terminal X-ray hot spots of these jets have now been discovered outside the shell. These hot spots lie on the jet axis, which is oriented in position angle 66° (measured from North through East). The proper motions of them are measured, and age is determined to be more than 160 years. The estimated physical parameters of the jets agree with the parameters of micro quasar jets. Three broad gaps with opening angles of 9° have been revealed in the radio shell, as well as radio bulges with opening angles of 2°–4° displaying tube-like structures at the shell periphery. Signs of the motion of material along position angle 124.5° have been found in the shell, suggesting the past activity of another pair of bipolar centrally symmetrical jets that were older than those first detected. These structural features provide direct evidence that the shell has been subject to multiple perforation by expanding fragments of matter from the supernova explosion, and by one or two pairs of bipolar jets with different orientations.

AGN JET KINETIC POWER AND THE ENERGY BUDGET OF RADIO GALAXY LOBES

Recent results based on the analysis of radio galaxies and their hot X-ray emitting atmospheres suggest that non-radiating particles dominate the energy budget in the lobes of FRI radio galaxies, in some cases by a factor of more than 1000, while radiating particles dominate the energy budget in FRII radio galaxy lobes. This implies a significant difference in the radiative efficiency of the two morphological classes. To test this hypothesis, we have measured the kinetic energy flux for a sample of 3C FRII radio sources using a new method based on the observed parameters of the jet terminal hotspots, and compared the resulting Q(jet) - L(radio) relation to that obtained for FRI radio galaxies based on X-ray cavity measurements. Contrary to expectations, we find approximate agreement between the Q(jet) - L(radio) relations determined separately for FRI and FRII radio galaxies. This result is ostensibly difficult to reconcile with the emerging scenario in which the lobes of FRI and FRII radio galaxies have vastly different energy budgets. However, a combination of lower density environment, spectral ageing and strong shocks driven by powerful FRII radio galaxies may reduce the radiative efficiency of these objects relative to FRIs and couteract, to some extent, the higher radiative efficiency expected to arise due to the lower fraction of energy in non-radiating particles. An unexpected corollary is that extrapolating the Q(jet) - L(radio) relation determined for low power FRI radio galaxies provides a reasonable approximation for high power sources, despite their apparently different lobe compositions.

VLBI OBSERVATIONS OF 10 COMPACT SYMMETRIC OBJECT CANDIDATES: EXPANSION VELOCITIES OF HOT SPOTS

Observations of ten Compact Symmetric Objects ({\rm CSO}) candidates have been made with the Very Long Baseline Array at 8.4 GHz in 2005 and with a combined Chinese and European VLBI array at 8.4 GHz in 2009. The 2009 observations incorporate for the first time the two new Chinese telescopes at Miyun and Kunming for international astrophysical observations. The observational data, in combination with archival VLBA data from previous epochs, have been used to derive the proper motions of the VLBI components. Because of the long time baseline of $\sim$16 years of the VLBI data sets, the expansion velocities of the hot spots can be measured at an accuracy as high as $\sim$1.3 $\mu$as yr$^{-1}$. Six of the ten sources are identified as CSOs with a typical double or triple morphology on the basis of both spectral index maps and their mirror-symmetry of proper motions of the terminal hot spots. The compact double source J1324+4048 is also identified as a CSO candidate. Among the three remaining sources, J1756+5748 and J2312+3847 are identified as core-jet sources with proper motions of their jet components relating to systemic source expansion. The third source J0017+5312 is likely also a core-jet source, but a robust detection of a core is needed for an unambiguous identification. The kinematic ages of the CSOs derived from proper motions range from 300 to 2500 years. The kinematic age distribution of the CSOs confirm an overabundance of compact young CSOs with ages less than 500 years. CSOs with known kinematic ages may be used to study the dynamical evolution of extragalactic radio sources at early stages.

MODELING X–RAY EMISSION OF A STRAIGHT JET: PKS 0920-397

We summarize a study of PKS 0920-397 using our 42 ks Chandra observation in conjunction with our ATCA 20GHz image, and HST/ACS F814W and F475W images. We investigate the hypothesis that the jet X–ray emission is due to inverse-Compton (IC) scattering on the cosmic microwave background (CMB) from the same population of relativistic electrons that give rise to the radio emission. To calculate parameters intrinsic to the source, one must finesse the fact that we do not know the true angle of the jet to our line of sight. Typical assumptions are that the Doppler factor equals the bulk Lorentz factor, or that the Lorentz factor takes some fixed numerical value. While giving useful estimates, neither assumption can be exact in general. We try different constraints to determine the jet quantities. It is plausible that the kinetic flux is constant along the jet, prior to a terminal hotspot or lobe, and with minimal bending of the jet. Alternatively, because PKS 0920-397 appears straight in projection on the sky, we might assume the jet maintains a constant angle to our line of sight. Either approach gives bulk Lorentz factors of 6 to 8, with kinetic energy flux of order 1046 erg s-1, and with the jet at an angle 2° to 4° from our line of sight.

Evolution of and High‐Energy Emission from GHz‐Peaked Spectrum Sources

Here we discuss evolution and broad-band emission of compact (< kpc) lobes in young radio sources. We propose a simple dynamical description for these objects, consisting of a relativistic jet propagating into a uniform gaseous medium in the central parts of an elliptical host. In the framework of the proposed model, we follow the evolution of ultrarelativistic electrons injected from a terminal hotspot of a jet to expanding lobes, taking into account their adiabatic energy losses as well as radiative cooling. This allows us to discuss the broad-band lobe emission of young radio sources. In particular, we argue that the observed spectral turnover in the radio synchrotron spectra of these objects cannot originate from the synchrotron self-absorption process but is most likely due to free-free absorption effects connected with neutral clouds of interstellar medium engulfed by the expanding lobes and photoionized by active centers. We also find a relatively strong and complex high-energy emission component produced by inverse-Compton up-scattering of various surrounding photon fields by the lobes electrons. We argue that such high energy radiation is strong enough to account for several observed properties of GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In addition, this emission is expected more » to extend up to GeV (or possibly even TeV) photon energies and can thus be probed by several modern {gamma}-ray instruments. In particular, we suggest that GPS radio galaxies should constitute a relatively numerous class of extragalactic sources detected by GLAST. « less

3D SIMULATIONS OF RADIO GALAXY EVOLUTION IN CLUSTER MEDIA

We present a set of high-resolution 3D MHD simulations exploring the evolution of light, supersonic jets in cluster environments. We model sets of high- and low-Mach jets entering both uniform surroundings and King-type atmospheres and propagating distances more than 100 times the initial jet radius. Through complimentary analyses of synthetic observations and energy flow, we explore the detailed interactions between these jets and their environments. We find that jet cocoon morphology is strongly influenced by the structure of the ambient medium. Jets moving into uniform atmospheres have more pronounced backflow than their non-uniform counterparts, and this difference is clearly reflected by morphological differences in the synthetic observations. Additionally, synthetic observations illustrate differences in the appearances of terminal hotspots and the x-ray and radio correlations between the high- and low-Mach runs. Exploration of energy flow in these systems illustrates the general conversion of kinetic to thermal and magnetic energy in all of our simulations. Specifically, we examine conversion of energy type and the spatial transport of energy to the ambient medium. Determination of the evolution of the energy distribution in these objects will enhance our understanding of the role of AGN feedback in cluster environments.

Witnessing the Gradual Slowdown of Powerful Extragalactic Jets: The X-Ray-Optical-Radio Connection

A puzzling feature of the Chandra-detected quasar jets is that their X-ray emission decreases faster along the jet than their radio emission, resulting in an outward-increasing radio-to-X-ray ratio. In some sources this behavior is so extreme that the radio emission peak is located clearly downstream of that of the X-rays. This is a rather unanticipated behavior given that the inverse Compton nature of the X-rays and the synchrotron radio emission are attributed to roughly the same electrons of the jet's nonthermal electron distribution. In this Letter we show that this morphological behavior can result from the gradual deceleration of a relativistic flow and that the offsets in peak emission at different wavelengths carry the imprint of this deceleration. This notion is consistent with another recent finding, namely, that the jets feeding the terminal hot spots of powerful radio galaxies and quasars are still relativistic with Lorentz factors Γ ~ 2-3. The picture of the kinematics of powerful jets emerging from these considerations is that they remain relativistic as they gradually decelerate from kiloparsec scales to the hot spots, where, in a final collision with the intergalactic medium, they slow down rapidly to the subrelativistic velocities of the hot spot advance speed.

Simple Multiresolution Filtering and the Spectra of Radio Galaxies and Supernova Remnants

ABSTRACTA simple method is explored for examining the structures and spectral index distributions on different scales in Galactic and extragalactic radio sources, based on a multiresolution filtering technique suggested by Starck, Murtagh, & Bijaoui. Multiresolution filtering, whether in this simple algorithm or more sophisticated ones, is also useful for a wide variety of image analysis purposes. For spectral index studies, the filtering is complementary to other methods and is especially useful in separating large‐scale and small‐scale features with different spectral distributions. In this paper, filtered spectral distributions are produced for several sources. In the extragalactic sources, different spectral gradients are found for narrow and broad features as a function of distance from the terminal hot spots. This complicates the standard analyses used to understand relativistic particle acceleration and losses. We show that the high‐field regions can yield either flatter or steeper spectra than low‐field regions and be either brighter or fainter, depending on the particle history. In our supernova remnant example, Cas A, the spectra of the fine‐scale features are found, in the mean, to be flatter than those on larger scales. In addition, a clear asymmetry is found in the spectral indices of the largest scale emission and is related to other physical properties of the remnant.

Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Three‐Dimensional Flows

We introduce a simple and economical but effective method for including relativistic electron transport in multidimensional simulations of radio galaxies. The method is designed to follow explicitly diffusive acceleration at shocks, and, in smooth flows, second-order Fermi acceleration, plus adiabatic and synchrotron losses for electrons in the energy range responsible for radio emission in these objects. We are able to follow both the spatial and the energy (momentum) distributions of the electrons, so that direct synchrotron emission properties can be modeled in time-dependent simulated flows of this type for the first time. That feature is essential if simulations are to bridge successfully the fundamental physical gap between flow dynamics and observed emissions. As an initial step toward that goal, we present results from some axisymmetric MHD simulations of Mach 20 light jet flows. These explicitly explore the effects of shock acceleration, as well as adiabatic expansion and synchrotron in smooth flows. The simulations demonstrate the importance of the fact that even for steady inflows jet terminal shocks are not simple, steady plane structures. Most importantly, this should play a very major role in determining the properties of synchrotron emission within the terminal hot spot and in the lobes generated by the jet back flow. In fact, the outflows are inherently complex, because of the basic driven character of a jet flow. Consequently, the nonthermal electron population emerging from the jet may encounter a wide range of shock types and strengths, as well as of magnetic field environments. We may expect to find a complex range in synchrotron spectral and brightness patterns associated with terminal hot spots and lobes. These include the possibility of steep spectral gradients (of either sign) within hot spots, the potential in lobes for islands of flat-spectrum electrons within steeper spectral regions (or the reverse), and spectral gradients coming from the dynamical history of a given flow element rather than from synchrotron of the embedded electrons. Finally, synchrotron aging in the lobes tends to proceed more slowly than one would estimate from regions of high emissivity. This is a consequence of the fact that those regions are ordinarily places where the magnetic fields are the strongest, so that the instantaneous rates of energy loss are atypical of the full history of the electron population. This feature supports earlier suggestions that nonuniform field structures may help to explain why dynamical ages of FR II sources often seem to be greater than the apparent age of the electrons radiating in the lobes, as measured in terms of spectral steepening, or the absence thereof.

Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Axisymmetric Flows

We introduce a simple and economical but effective method for including relativistic electron transport in multidimensional simulations of radio galaxies. The method is designed to follow explicitly diffusive acceleration at shocks, and, in smooth flows, second-order Fermi acceleration, plus adiabatic and synchrotron losses for electrons in the energy range responsible for radio emission in these objects. We are able to follow both the spatial and the energy (momentum) distributions of the electrons, so that direct synchrotron emission properties can be modeled in time-dependent simulated flows of this type for the first time. That feature is essential if simulations are to bridge successfully the fundamental physical gap between flow dynamics and observed emissions. As an initial step toward that goal, we present results from some axisymmetric MHD simulations of Mach 20 light jet flows. These explicitly explore the effects of shock acceleration, as well as adiabatic expansion and synchrotron aging in smooth flows. The simulations demonstrate the importance of the fact that even for steady inflows jet terminal shocks are not simple, steady plane structures. Most importantly, this should play a very major role in determining the properties of synchrotron emission within the terminal hot spot and in the lobes generated by the jet back flow. In fact, the outflows are inherently complex, because of the basic "driven" character of a jet flow. Consequently, the nonthermal electron population emerging from the jet may encounter a wide range of shock types and strengths, as well as of magnetic field environments. We may expect to find a complex range in synchrotron spectral and brightness patterns associated with terminal hot spots and lobes. These include the possibility of steep spectral gradients (of either sign) within hot spots, the potential in lobes for islands of flat-spectrum electrons within steeper spectral regions (or the reverse), and spectral gradients coming from the dynamical history of a given flow element rather than from synchrotron aging of the embedded electrons. Finally, synchrotron "aging" in the lobes tends to proceed more slowly than one would estimate from regions of high emissivity. This is a consequence of the fact that those regions are ordinarily places where the magnetic fields are the strongest, so that the instantaneous rates of energy loss are atypical of the full history of the electron population. This feature supports earlier suggestions that nonuniform field structures may help to explain why dynamical ages of FR II sources often seem to be greater than the apparent age of the electrons radiating in the lobes, as measured in terms of spectral steepening, or the absence thereof.

High dynamic range VLA observations of eight core-dominated quasars

view Abstract Citations (44) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High Dynamic Range VLA Observations of Eight Core-Dominated Quasars Kollgaard, R. I. ; Wardle, J. F. C. ; Roberts, D. H. Abstract The Very Large Array has been used at 5 GHz to make high dynamic range observations of eight quasars with prominent cores. All exhibit one-sided jets, and all but one show evidence of diffuse halos. The luminosity of the extended emission by itself is sufficient for these to be Fanaroff-Riley Class II radio sources. This interpretation is confirmed by the polarization of the extended structure: the inferred magnetic field is parallel to the jet axis in all cases, and in all but one instance turns to be nearly perpendicular to the jet at its outer end. We identify these latter structures as classical terminal hotspots. Both the total intensity data and especially the polarization data strongly support the notion that these quasars are similar to the classical lobe-dominated quasars, but are oriented with jet axes close to the line of sight. If this is so, then the relatively high degrees of polarization observed in the terminal hotspots appear to require that the downstream fluid velocities in the hotspots are mildly relativistic, in the range 0.2 <= v/c <= 0.8. This, in turn, implies that the jets are at least moderately relativistic over their entire length. Publication: The Astronomical Journal Pub Date: October 1990 DOI: 10.1086/115579 Bibcode: 1990AJ....100.1057K Keywords: Luminous Intensity; Polarization (Waves); Quasars; Stellar Cores; Very Large Array (Vla); Emission Spectra; Halos; Stellar Luminosity; Stellar Spectra; Astrophysics; QUASARS full text sources ADS | data products NED (12) SIMBAD (8)

Numerical Observations of a Restarting Radio Jet

An MHD computation of a restarting jet is presented and compared with observations of classical double radio sources. Assuming ideal MHD and an adiabatic equation of state, it is found that a “partial” jet exists for about 25% of the time, and that the terminal hotspot dissipates quickly compared to other physical time scales once the outflow of the jet is terminated.