Jet Side Research Articles

Hybrid Simulation of Magnetosheath Jet‐Driven Bow Waves

AbstractHigh‐speed jets (HSJs) are commonly observed in the Earth's magnetosheath. The HSJs can drive shock‐like bow waves when compressing the ambient plasma, which are important for the HSJ's evolution and the energization of charged particles. Here we present the first two‐dimensional hybrid simulation of the formation and evolution of jet‐driven bow waves. The simulated bow waves exhibit localized enhanced magnetic field and ion density, with their peaks separated by the order of ion inertial length. The bow waves are formed when a super‐magnetosonic HSJ encounters a magnetic structure with the magnetic field nearly perpendicular to the HSJ's velocity. The magnetic field structure acts as an obstacle to deflect and decelerate the jet, causing the pile up of ions on the jet side and the compression of the magnetic structure on the downstream side. Our study explains the observed properties of bow waves, and helps to better understand the evolution of HSJs.

RAD@home citizen science discovery of an active galactic nucleus spewing a large unipolar radio bubble on to its merging companion galaxy

ABSTRACT Active galactic nucleus (AGN) feedback during galaxy merger has been the most favoured model to explain black hole–galaxy co-evolution. However, how the AGN-driven jet/wind/radiation is coupled with the gas of the merging galaxies, which leads to positive feedback, momentarily enhanced star formation, and subsequently negative feedback, a decline in star formation, is poorly understood. Only a few cases are known where the jet and companion galaxy interaction leads to minor off-axis distortions in the jets and enhanced star formation in the gas-rich minor companions. Here, we briefly report one extraordinary case, RAD12, discovered by RAD@home citizen science collaboratory, where for the first time a radio jet–driven bubble (∼ 137 kpc) is showing a symmetric reflection after hitting the incoming galaxy which is not a gas-rich minor but a gas-poor early-type galaxy in a major merger. Surprisingly, neither positive feedback nor any radio lobe on the counter jet side, if any, is detected. It is puzzling if RAD12 is a genuine one-sided jet or a case of radio lobe trapped, compressed and re-accelerated by shocks during the merger. This is the first imaging study of RAD12 presenting follow-up with the Giant Metrewave Radio Telescope, archival MeerKAT radio data and Canada-France-Hawaii Telescope optical data.

Dynamical modelling and emission signatures of a candidate dual AGN with precessing radio jets

ABSTRACT In this paper, we have modelled the dynamical and emission properties (in the presence of radiative losses and diffusive shock acceleration) of an observed S-shaped radio source (2MASX J12032061+131931) due to a precessing jet. In this regard, we have performed high-resolution 3D magnetohydrodynamic (MHD) simulations of a precessing jet in a galactic environment. We show the appearance of a distinct S-shape with two bright hotspots when the bow shock region weakens over time. The formed morphology is sensitive to the parameter selections. The increased interaction between the helical jet and the ambient medium and the deceleration of the jet due to MHD instabilities also greatly affect the resulting structure. Hence, kinematic models must be corrected for these deceleration effects in order to adequately predict the precession parameters. The synthetic spectral index map shows that the jet side and leading edges possess relatively steeper spectral index values than the jet ridge lines, whereas the hotspots show flat spectral index values. The jets are also found to be highly linearly polarized (up to 76 per cent) and the magnetic field lines, in general, follow the jet locus which is formed due to the jet-ambient medium interaction. Diffusive shocks, in this context, keep the structure active during its course of evolution. Furthermore, we have demonstrated that these galaxies deviate significantly from the ‘equipartition’ approximation leading to a discrepancy in their spectral and dynamical age.

A CFD study of gas and particle motion in an aerosol generator operated by desublimation process

In this work, a novel aerosol generator for submicron particles with a core-shell structure is presented. The operation process starts with the sublimation by heat of an organic material, copper phthalocyanine (CuPc), to form an under saturated gas flow. Then, the adiabatic expansion through a Laval nozzle is used to form a supersaturated free jet. Finally, by employing an active cooling and the injection of titanium dioxide (TiO2) core particles, on the free jet side, the aerosol formation is enabled by the desublimation process. The Euler–Lagrange CFD simulations of the gas and particle flow revealed the rotating backflow of the core particles inside the cooled expansion chamber. Hence, the core particles are transported into the conical free jet region emerging from the Laval nozzle where the highest supersaturation of CuPc occurs. In addition to the numerical simulations, the resulting structure and size of the generated particles were experimentally investigated by employing scanning mobility particle sizer and a scanning electron microscope analysis. The surface of modified TiO2 core particles showed a punctual coating with desublimated CuPc.

NGC 3894: a young radio galaxy seen by Fermi-LAT

Context. According to radiative models, radio galaxies may produce γ-ray emission from the first stages of their evolution. However, very few such galaxies have been detected by the Fermi Large Area Telescope (LAT) so far. Aims. NGC 3894 is a nearby (z = 0.0108) object that belongs to the class of compact symmetric objects (CSOs, i.e., the most compact and youngest radio galaxies), which is associated with a γ-ray counterpart in the Fourth Fermi-LAT source catalog. Here we present a study of the source in the γ-ray and radio bands aimed at investigating its high-energy emission and assess its young nature. Methods. We analyzed 10.8 years of Fermi-LAT data between 100 MeV and 300 GeV and determined the spectral and variability characteristics of the source. Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz over a period of 35years were used to study the radio morphology of NGC 3894 and its evolution. Results. NGC 3894 is detected in γ-rays with a significance >9σ over the full period, and no significant variability has been observed in the γ-ray flux on a yearly time-scale. The spectrum is modeled with a flat power law (Γ = 2.0 ± 0.1) and a flux on the order of 2.2 × 10−9 ph cm−2 s−1. For the first time, the VLBA data allow us to constrain with high precision the apparent velocity of the jet and counter-jet side to be βapp, NW = 0.132 ± 0.004 and βapp, SE = 0.065 ± 0.003, respectively. Conclusions. Fermi-LAT and VLBA results favor the youth scenario for the inner structure of this object, with an estimated dynamical age of 59 ± 5 years. The estimated range of viewing angle (10° < θ < 21°) does not exclude a possible jet-like origin of the γ-ray emission.

Numerical simulation and optimization of hydrodynamics in a novel integral multi-jet spout-fluidized bed

The hydrodynamics in a novel integral multi-jet spout-fluidized bed was simulated using both Eulerian-Eulerian approach and the kinetic theory of granular flows. The effect of side jet number and side jet width on gas-solid two phase flow in the integral multi-jet spout-fluidized bed (IMJSFB) was investigated. The CFD results show that when the number of side jet is greater than 1 (N = 2–5), a wave-like spout and stable spout structure is formed in the IMJSFB. Compared with simulation results in the single nozzle spouted bed, the profiles of particles' velocity and concentration along the radial direction in the IMJSFB become relatively flatter. The integral uniformity of particle velocity distribution in spouted bed can be enhanced by increasing δ/Di. The particle flow factor (η) decreases first and then increases with the increase of δ/Di, and η reaches its minimum value when δ/Di = 0.208. On the whole, there is a set of optimal design parameters of side jet (N = 4, δ/Di = 0.208), at which, the fluidization quality particle phase in spout-fluidized bed reaches the optimum state while the bypass fluidizing air supply auxiliary equipment is omitted. The fluidization quality of particles in annulus can be significantly enhanced in the IMJSFB while the bypass fluidizing air supply auxiliary equipment is omitted.

Ignition of ultra-lean premixed H2/air using multiple hot turbulent jets generated by pre-chamber combustion

A detailed investigation on the ignition characteristics of ultra-lean premixed H2/air mixtures by multiple hot turbulent jets in a dual combustion chamber system was carried out. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition processes inside the main combustion chamber. The focus was on the effects of the spark location and fuel/air equivalence ratio within the pre-chamber on the ignition pattern of the main-chamber mixture. The results show that multiple jets resulted in similar lean flammability limit of H2/air as the single jets, given that the total nozzle areas are the same. However, the ignition probability improved significantly near the lean flammability limit using multi-jets compared to single jets. Additionally, depending on the spark location and pre-chamber equivalence ratio, either the side jet, the middle jet, or all the jets can initiate ignition in the main chamber. The ignition characteristics of straight and angled multi-jets were compared, and angled jets produced shorter main chamber burn time. Numerical modeling of the flame propagation process within the pre-chamber was carried out to explain the behavior of individual jets in a multi-jet system. It was found that depending on the pre-chamber spark position, the flame shape inside the pre-chamber changes drastically. The effective L/D ratio governs the flame dynamics in the pre-chamber and subsequently determines which jet (side, middle, or all) will first ignite the main chamber.

АЛЬТЕРНАТИВНОЕ ТОПЛИВО ДЛЯ АВТОТРАНСПОРТА НИГЕРИИ НА ОСНОВЕ ПАЛЬМОВОГО МАСЛА

The paper presents the results of the analysis on using vegetable oils as fuel for diesel engines. Possibility of using palm oil as alternative fuel is considered. Physicochemical properties of plant oil and the requirements for diesel fuel are given. Changes in physicochemical parameters of composite fuel are investigated. Palm oil is proposed to be used as a biological additive to diesel fuel and organic peroxide - as a reagent. The results of mixing process analysis, taking into account the differences between physical properties of palm oil and diesel fuel characteristics, are presented. The analysis was based on mathematical investigation. The effect of physical properties (density, dynamic viscosity, surface tension) of palm oil on the characteristics of mixing process has been examined. Changes in palm oil density, higher values of surface tension and dynamic viscosity have been observed. It was established that with increased density, surface tension and viscosity of palm oil, the average diameter of the drops increases, which should increase not only the ignition delay period, but also temperature sensitivity to the ignition process. With that the fuel jet dispersion angle and side surface decrease. A conclusion was made that in order to maintain the effectiveness of the diesel engine working cycle when switching to palm oil fuel, there is an obvious need for radical measures to substantially change its structure and fuel feed system. Palm oil is proposed to be used as an additive to diesel fuel and organic peroxide - as a reagent.

SPATIALLY RESOLVED HCN ABSORPTION FEATURES IN THE CIRCUMNUCLEAR REGION OF NGC 1052

ABSTRACT We present the first VLBI detection of HCN molecular absorption in the nearby active galactic nucleus NGC 1052. Utilizing the 1 mas resolution achieved by the Korean VLBI Network, we have spatially resolved the HCN absorption against a double-sided nuclear jet structure. Two velocity features of HCN absorption are detected significantly at the radial velocity of 1656 and 1719 km s−1, redshifted by 149 and 212 km s−1 with respect to the systemic velocity of the galaxy. The column density of the HCN molecule is estimated to be 1015–1016 cm−2, assuming an excitation temperature of 100–230 K. The absorption features show high optical depth localized on the receding jet side, where the free–free absorption occurred due to the circumnuclear torus. The size of the foreground absorbing molecular gas is estimated to be on approximately one-parsec scales, which agrees well with the approximate size of the circumnuclear torus. HCN absorbing gas is likely to be several clumps smaller than 0.1 pc inside the circumnuclear torus. The redshifted velocities of the HCN absorption features imply that HCN absorbing gas traces ongoing infall motion inside the circumnuclear torus onto the central engine.

Large eddy simulation of a turbulent spray jet generated by high-pressure injection: impact of the in-nozzle flow

ABSTRACTThe turbulence properties of a gas spray jet generated through the injection of a high-pressure atomising spray were studied in the context of the Euler–Lagrange formulation using a large eddy simulation (LES) approach. The study's main aim was to investigate nozzle flow effects on flow and turbulence statistics for a two-phase jet flow in the near and far fields of the spray. The study investigated the injection of an existing in-nozzle flow at the spray inlet. Simulation results were compared with measurements obtained for non-evaporating sprays under quasi-steady conditions, which, in practice, correspond to a long-duration injection. Both simulated high-injection pressure cases showed a good agreement with experimental data. A two-stage evolution of the spray was observed. The flow of the gas jet first developed along the spray cone, followed by the appearance of highly vortical structures around and within the spray. Cavitation in the nozzle flow resulted in an observed asymmetry of the generated gas spray jet: on the gas spray jet side of the nozzle (relative to the cavitation side), stronger jet spreading occurred, and droplets were smaller and slower. The spray jet typically exhibited an ellipsoidal cross-sectional shape. The paper provides a characterisation of the asymmetry that persisted for the duration of the spray evolution (at least for the distances considered here).

DETERMINATION OF CENTRAL ENGINE POSITION AND ACCRETION DISK STRUCTURE IN NGC 4261 BY CORE SHIFT MEASUREMENTS

We report multifrequency phase-referenced observations of the nearby radio galaxy NGC 4261, which has prominent two-sided jets, using the Very Long Baseline Array at 1.4-43 GHz. We measured radio core positions showing observing frequency dependences (known as "core shift") in both approaching jets and counter jets. The limit of the core position as the frequency approaches infinity, which suggests a jet base, is separated by 82$\pm$16 ${\mu}$as upstream in projection, corresponding to (310$\pm$60)Rs (Rs: Schwarzschild radius) as a deprojected distance, from the 43 GHz core in the approaching jet. In addition, the innermost component at the counter jet side appeared to approach the same position at infinity of the frequency, indicating that cores on both sides are approaching the same position, suggesting a spatial coincidence with the central engine. Applying a phase referencing technique, we also obtained spectral index maps, which indicate that emission from the counter jet is affected by free-free absorption (FFA). The result of the core shift profile on the counter jet also requires FFA because the core positions at 5-15GHz cannot be explained by a simple core shift model based on synchrotron self-absorption (SSA). Our result is apparently consistent with the SSA core shift with an additional disk-like absorber over the counterjet side. Core shift and opacity profiles at the counter jet side suggest a two-component accretion: a radiatively inefficient accretion flow at the inner region and a truncated thin disk in the outer region. We proposed a possible solution about density and temperature profiles in the outer disk on the basis of the radio observation.

LES of turbulent jet in cross-flow: Part 1 – A numerical validation study

The paper presents results of a LES based numerical simulation of the turbulent jet-in-cross-flow (JICF) flowfield, with Reynolds number based on cross-flow velocity and jet diameter Re=2400 and jet-to-cross-flow velocity ratio of R=3.3. The JICF flow case has been investigated in great detail, involving conduction of two independent precursor simulations, prior to the main JICF simulation, as the considered case has turbulent inflow conditions on both jet and cross-stream side. The LES results are directly compared to pointwise Laser Doppler Anemometry (LDA) measurements, showing a very good agreement on the level of various statistical quantities in all flow regions but the immediate jet-to-cross-flow exhaustion zone. Several LES computations involving grids of up to 15million grid points have been conducted, showing no improvement in the agreement between numerical results and measurements, possibly indicating a LDA measurement problem in this particular region.

Numerical Simulation of Turbulent Flow and Heat Transfer in an Anti- Gravity Blind Duct with Tangential Entry Jet

This numerical study investigates the flow and heat transfer characteristics in a vertical square blind duct with the coolant fed by a tangential entry jet. The detailed Nusselt number (Nu) distributions over the five constituent walls of the blind duct are calculated at duct (jet) Reynolds numbers Re(Rej) of 5000(20000), 7000(28000) and 10000(40000) us- ing the commercial CFD Star CD code. As an attempt to explore the buoyancy effect on heat transfer performances, three different heat fluxes, which vary the gravitational Grashof numbers (Grg) at the fixed Re, are imposed on each duct wall to vary the buoyancy levels. The jet-induced flow phenomena in the blind duct exhibit various heat transfer impacts on the five duct walls over which the different near-wall flow structures are generated. This is demonstrated by cross-examining the detailed Nu distributions and the area-averaged Nusselt number ( Nu ) over the five duct walls at the tested Re and Grg. The cross-plane swirls induced by the tangential entry jet together with the impinging jet flows considerably elevate the Heat Transfer Enhancement (HTE) performances in the blind duct. Within the parametric conditions simulated, the ratios of Nu to the Dittus-Boelter levels (Nu� ) over the jet wall, back wall, impingement wall, side wall and end wall are re- spectively raised to 3-5.7, 2.8-5.6, 3-5.8, 2.7-4.8 and 3.1-6.1; while the HTE ratios ( Nu /Nu� ) over these duct walls con- sistently decrease as Re increases.

Determination of a Local Bulk Temperature Based Heat Transfer Coefficient for the Wetted Surfaces in a Single Inline Row Impingement Channel

High performance turbine airfoils are typically cooled with a combination of internal cooling channels and impingement. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, most current studies use the plenum temperature as the reference temperature in heat transfer calculations. This presents some difficulty to designers who need to determine heat transfer rates based on the local bulk temperatures. This paper examines three different methods to determining the local bulk temperature in a steady state impingement channel heat transfer experiment. The various methods will be compared based on their ease of application as well as their accuracy in describing the results. One method proves to be the most accurate, while another proves to be more easily implemented. The methods are compared for a single case previously studied, on a 15 hole, single row impingement channel, with dimensions of X/D=5, Y/D=4, Z/D=1 and 3, and an average jet based Reynolds number of 17,000 and 45,000. Effects due to the choice of the reference temperature in heat transfer calculations are shown to cause significant variations in the calculated heat transfer coefficients. These results point to a transition between different flow regimes in the post-impingement flow.

A decelerating jet observed by the EVN and VLBA in the X-ray transient XTE J1752−223

Abstract The recently discovered Galactic X-ray transient XTE J1752−223 entered its first known outburst in 2010, emitting from the X-ray to the radio regimes. Its general X-ray properties were consistent with those of a black hole candidate in various spectral states, when ejection of jet components is expected. To verify this, we carried out very long baseline interferometry (VLBI) observations. The measurements were carried out with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February. The images at the first three epochs show a moving jet component that is significantly decelerated by the last epoch, when a new jet component appears that is likely to be associated with the receding jet side. The overall picture is consistent with an initially mildly relativistic jet, interacting with the interstellar medium or with swept-up material along the jet. The brightening of the receding ejecta at the final epoch can be well explained by initial Doppler deboosting of the emission in the decelerating jet.

An H I absorbing circumnuclear disk in Cygnus A

We present Very Long Baseline Array (VLBA) HI absorption observations of the core region of the powerful radio galaxy Cygnus A. These data show both broad (FWHM = 231 \pm 21 km/s) and narrow (FWHM <30 km/s) velocity width absorption components. The broad velocity absorption shows high opacity on the counter jet, low opacity against the core and no absorption on the jet side. We argue that these results are most naturally explained by a circumnuclear HI absorbing disk orientated roughly perpendicular to the jet axis. We estimate that the HI absorbing gas lies at a radius of ~80 pc has a scale height of about 20 pc, density n > 10^{4} cm^{-3} and total column density in the range 10^{23}-10^{24} cm^{-2}. Models in which the HI absorption is primarily from an atomic or a molecular gas phase can both fit our data. Modelling taking into account the effective beam shows that the broad HI absorbing gas component does not cover the radio core in Cygnus A and therefore does not contribute to the gas column that blocks our view of the hidden quasar nucleus. If however Cygnus A were observed from a different direction, disk gas on ~100 pc radius scales would contribute significantly to the nuclear column density, implying that in some radio galaxies gas on these scales may contribute to the obscuration of the central engine. We argue that the circumnuclear torus in Cygnus A contains too little mass to power the AGN over > 10^{7} yr but that material in the outer HI absorbing gas disk can provide a reservoir to fuel the AGN and replenish torus clouds. The second narrow HI absorption component is significantly redshifted (by 186km/s) with respect to the systemic velocity and probably traces infalling gas which will ultimately fuel the source. [abridged]

STUDYING THE INTERACTION BETWEEN MICROQUASAR JETS AND THEIR ENVIRONMENTS

In high-mass microquasars (HMMQ), strong interactions between jets and stellar winds at binary system scales could occur. In order to explore this possibility, we have performed numerical two-dimensional hydrodynamical simulations of jets crossing the dense stellar material to study how the jet will be affected by these interactions. We find that the jet head generates strong shocks in the wind. These shocks reduce the jet advance speed, and compress and heat up the jet and wind material. In addition, strong recollimation shocks can occur where pressure balance between the jet side and the surrounding medium is reached. All this, together with jet bending, could lead to the destruction of jets with power &lt; 1036 erg/s . The conditions around the outflow shocks would be convenient for accelerating particles up to ~ TeV energies. These accelerated particles could emit via synchrotron and inverse Compton (IC) scattering if they were leptons, and via hadronic processes if they were hadrons.

DETECTION OF A RELIC X-RAY JET IN CYGNUS A

We present a 200 ks Chandra ACIS-I image of Cygnus A, and discuss a long linear feature seen in its counterlobe. This feature has a nonthermal spectrum and lies on the line connecting the brighter hotspot on the approaching side and the nucleus. We therefore conclude that this feature is (or was) a jet. However, the outer part of this X-ray jet does not trace the current counterjet observed in radio. No X-ray counterpart is observed on the jet side. Using light-travel time effects we conclude that this X-ray 50 kpc linear feature is a relic jet that contains enough low-energy plasma (γ ~ 103) to inverse-Compton scatter cosmic microwave background photons, producing emission in the X-rays.

Effect of rear slant angle on flow structures, and pollutant dispersion and concentration fields in the wake of the studied model vehicle

Experimental investigations of the interaction effect of rear slant angle (i.e., α = 25° and 60°) and heated air exhaust jet condition (i.e., U j and T j ) on flow structures, and pollutant (i.e., scalar) dispersion and concentration fields in the near-wake region of two simplified scale-model vehicles at Reynolds number Re h = 5.8 × 10 4 were performed in a closed-circuit wind tunnel using laser Doppler anemometry (LDA) and the cold wire thermometry. The results show that the behaviors of flow structures and pollutant dispersion in the near-wake region of the studied model vehicles are highly dependent on the interaction effect of U j / U ∞ and α. For the low value of U j / U ∞ (i.e., the driving mode from 30 km h −1 to 70 km h −1), the wake structure of the two studied model vehicles is almost similar to the case without introducing the vehicular exhaust jet flow, U j / U ∞ = 0, but the scalar concentration is neither sent straight to the wake region nor trapped by the recirculation zone. For higher value of U j / U ∞ (i.e., the driving mode of 10 km h −1), the jet flow perturbation along the central plane of the vehicular exhaust tailpipe causes the disappearance of the lower vortex in the recirculation zone for the studied model vehicle A ( α = 25°) while it causes the stretching of the recirculation zone for the studied model vehicle B ( α = 60°). The contaminant is mainly straight trajectory along its jet exhaust flow axis. Outside the recirculation zone, the flow field becomes symmetrical and its flow structure depends only on the rear slant angle of the studied model vehicle. The scalar concentration field of the studied model vehicle B is characterized by a two-dimensional flow structure whatever the studied driving modes are. The distribution of the mean normalized temperature excess field (i.e., scalar concentration field) conforms to the velocity vector field for both studied model vehicles. For the studied model vehicle A, the scalar concentration is drawn to the edge of the trailing vortex at the vehicular exhaust jet side. This phenomenon causes a significant reduction of the scalar concentration level whatever the studied driving modes are.

A multiband study of Hercules A -- II. Multifrequency Very Large Array imaging

We have mapped the powerful radio galaxy Hercules A at six frequencies spanning 1295 to 8440 MHz using the Very Large Array (VLA) in all four configurations. Here we discuss the structure revealed in total intensity, spectral index, polarization, and projected magnetic field. Our observations clearly reveal the relation between the bright jets, prominent rings, bulbous outer lobes and faint bridge that make up the radio source. The jets and rings form a coherent structure with a dramatically flatter spectrum than the surrounding lobes and bridge, strongly suggesting that they represent a recently renewed outburst from the active nucleus. The spectrum of the lobes is also steeper than in typical radio sources, and steepens further towards the centre. The compact core is optically thin and also has a remarkably steep spectrum (α≃−1.2). There is some evidence that the old lobe material has been swept up and compressed ahead of the new outburst. We interpret the dramatic asymmetry in the bright structure, and more subtle differences between diffuse lobe structures, in terms of relativistic beaming combined with front-to-back light-travel delays, which mean that we view the two lobes at different stages of the outburst. After correcting for Faraday rotation the projected magnetic field closely follows the edge of the lobes, the jets and the rings; the field pattern in the two lobes is broadly similar. We confirm a strong asymmetry in depolarization and Faraday rotation, with the jet side being the less depolarized and with the flatter spectrum, consistent with general correlations between these asymmetries. The spectral index asymmetry is clearly present in the ‘old’ lobe material and so, at least in this case, is not caused by beaming; but it can be understood in terms of the light-travel delay.