Jet Structure Research Articles

Incidence of afterglow plateaus in gamma-ray bursts associated with binary neutron star mergers

One of the most surprising gamma-ray burst (GRB) features discovered with the Swift X-ray telescope (XRT) is a plateau phase in the early X-ray afterglow light curves. These plateaus are observed in the majority of long GRBs, while their incidence in short GRBs (SGRBs) is still uncertain due to their fainter X-ray afterglow luminosity with respect to long GRBs. An accurate estimate of the fraction of SGRBs with plateaus is of utmost relevance given the implications that the plateau may have for our understanding of the jet structure and possibly of the nature of the binary neutron star (BNS) merger remnant. This work presents the results of an extensive data analysis of the largest and most up-to-date sample of SGRBs observed with the XRT, and for which the redshift has been measured. We find a plateau incidence of 18-37<!PCT!> in SGRBs, which is a significantly lower fraction than that measured in long GRBs (>50<!PCT!>). Although still debated, the plateau phase could be explained as energy injection from the spin-down power of a newly born magnetized neutron star (NS; magnetar). We show that this scenario can nicely reproduce the observed short GRB (SGRBs) plateaus, while at the same time providing a natural explanation for the different plateau fractions between short and long GRBs. In particular, our findings may imply that only a minority of BNS mergers generating SGRBs leave behind a sufficiently stable or long-lived NS to form a plateau. From the probability distribution of the BNS remnant mass, a fraction 18-37<!PCT!> of short GRB plateaus implies a maximum NS mass in the range $ odot

Numerical and Experimental Simulation of Supersonic Gas Outflow into a Low-Density Medium

This study is aimed at developing methods for the experimental and numerical simulation of the outflow of underexpanded gas jets into a rarefied medium. The numerical method is based on using Navier–Stokes equations in the continuum flow regime and the direct simulation Monte Carlo method in the transitional flow regime. The experimental method includes the modeling of jet flows in the LEMPUS-2 gas-dynamic setup with electron beam diagnostics for the jet density measurements. The results of the experimental modeling for the nozzles of various diameters confirm that a key parameter determining the jet structure is the Reynolds number based on the characteristic length ReL. The results of the numerical simulations agree well with the experimental data both for the maximum values of the ReL considered (approximately 30) when a barrel jet structure with Mach disks is formed and for the minimum values (approximately 4) when no Mach disks are formed. In the entire range of parameters, significant thermal nonequilibrium is observed at all jet segments where the measurements are performed.

Pressure-equilibrium semi-implicit solver for real fluids

In this study, a semi-implicit pressure-based scheme which can suppress the spurious pressure oscillations is developed for real fluid flows. Conservation properties relevant to the proposed scheme are investigated through one-dimensional numerical simulations of the nitrogen interface advection. The efficiency and validity of this scheme are carefully examined with two- and three-dimensional numerical simulations of cryogenic nitrogen jets. From the one-dimensional simulation results, the conservation properties are well conserved as the past studies. The two-dimensional simulation results show that the developed scheme can reduce the computational cost by more than 92% (13–14 times faster) compared with the conventional density-based explicit solver employing the double flux model. It is also found that, through the three-dimensional simulation results, the developed scheme can predict the structure of the nitrogen jet under both transcritical and supercritical conditions, while taking into account the effects of real fluid properties on the mixing and spatiotemporal evolution of this jet, with excellent accuracy.

Magnetic dissipation in short gamma-ray-burst jets

Aims. Short gamma-ray bursts originate when relativistic jets emerge from the remnants of binary neutron star (BNS) mergers, as observed in the first multi-messenger event GW170817–GRB 170817A, which coincided with a gravitational wave signal. Both the jet and the remnant are believed to be magnetized, and the presence of magnetic fields is known to influence the jet propagation across the surrounding post-merger environment. In the magnetic interplay between the jet and the environment itself, effects due to a finite plasma conductivity may be important, especially in the first phases of jet propagation. We aim to investigate these effects, from jet launching to its final breakout from the post-merger environment. Methods. We used the PLUTO numerical code to perform 2D axisymmetric and full 3D resistive relativistic magnetohydrodynamic (MHD) simulations, employing spherical coordinates with spatial radial stretching. We considered and compared different models for physical resistivity, which must be small but still dominating over the intrinsic numerical dissipation (which yields unwanted smearing of structures in any ideal MHD code). Stiff terms in the current density are treated with IMplicit-EXplicit Runge Kutta algorithms for time-stepping. A Synge-like gas (Taub equation of state) is also considered. All simulations were performed using an axisymmetric analytical model for both the jet propagation environment and the jet injection; we leave the case of jet propagation in a realistic environment (i.e., imported from an actual BNS merger simulation) to a future study. Results. As expected, no qualitative differences are detected due to the effect of a finite conductivity, but significant quantitative differences in the jet structure and induced turbulence are clearly seen in 2D axisymmetric simulations, and we also compare different resistivity models. We see the formation of regions with a resistive electric field parallel to the magnetic field, and nonthermal particle acceleration may be enhanced there. The level of dissipated Ohmic power is also dependent on the various recipes for resistivity. Most of the differences arise before the breakout from the inner environment, whereas once the jet enters the external weakly magnetized environment region, these differences are preserved during further propagation despite the lower grid refinement. Finally, we show and discuss the 3D evolution of the jet within the same environment in order to highlight the emergence of nonaxisymmetric features.

The putative center in NGC 1052

Context: Many active galaxies harbor powerful relativistic jets, however, the detailed mechanisms of their formation and acceleration remain poorly understood. Aims: To investigate the area of jet acceleration and collimation with the highest available angular resolution, we study the innermost region of the bipolar jet in the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC\,1052. Methods: We combined observations of NGC\,1052 taken with VLBA, GMVA, and EHT over one week in the spring of 2017. Our study is focused on the size and continuum spectrum of the innermost region containing the central engine and the footpoints of both jets. We employed a synchrotron-self absorption model to fit the continuum radio spectrum and we combined the size measurements from close to the central engine out to $ to study the jet collimation. Results: For the first time, NGC\,1052 was detected with the EHT, providing a size of the central region in-between both jet bases of $43\ perpendicular to the jet axes, corresponding to just around $250\,R_ S $ (Schwarzschild radii). This size estimate supports previous studies of the jets expansion profile which suggest two breaks of the profile at around $3 10^3\,R_ S $ and $1 10^4\,R_ S $ distances to the core. Furthermore, we estimated the magnetic field to be 1.25\,Gauss at a distance of $22\ from the central engine by fitting a synchrotron-self absorption spectrum to the innermost emission feature, which shows a spectral turn-over at $ 130\,$GHz. Assuming a purely poloidal magnetic field, this implies an upper limit on the magnetic field strength at the event horizon of $2.6 10^4\,$Gauss, which is consistent with previous measurements. Conclusions: The complex, low-brightness, double-sided jet structure in NGC\,1052 makes it a challenge to detect the source at millimeter (mm) wavelengths. However, our first EHT observations have demonstrated that detection is possible up to at least 230\,GHz. This study offers a glimpse through the dense surrounding torus and into the innermost central region, where the jets are formed. This has enabled us to finally resolve this region and provide improved constraints on its expansion and magnetic field strength.

Resolving the ubiquitous small-scale semi-permanent features of the general ocean circulation: a multiplatform observational approach

Abstract Based on twenty years of Argo and ship/animal-borne/glider hydrographic profile data, we derive a new high resolution hydrographic Atlas and associated circulation field for the oceans above 2000 dbar. Satellite altimetric observations are used to explicitly regress out eddy noise in the fit, greatly reducing one of the major sources of noise. Geostrophic shears are found from the fitted geopotential anomaly fields. Ekman velocities are estimated using satellite wind stresses. Both Argo trajectory observations at 1000 dbar and surface drifter observations are used to reference geostrophic shears derived from the Atlas hydrography. Surface drifter velocities are analyzed with an additional wind-friction term to remove the wind-related flow. Agreement between the surface geostrophic (referenced to Argo trajectories) and drifter-based surface velocity is high at both large and mesoscales, lending confidence to the derived geostrophic circulation fields. The Atlas reveals standing mesoscale eddies and meanders in western boundary systems, and the braided jet structure of the Antarctic Circumpolar Current. In the interior, the upper ocean flow consists of a highly baroclinic large-scale Sverdrup flow and smaller scale (~200 km width) semi-zonal jets, which are more barotropic (low vertical shear) and have an average zonal width of around 2000 km. These semi-zonal jets are globally ubiquitous - found in all basins pole-to-pole. The many permanent mesoscale features of the mean general circulation contrasts with that predicted by theories of the large-scale flow in simplified flat-bottomed domains. The new Atlas presents a new opportunity to benchmark modern high resolution ocean and climate models.

Optimization Study on Nozzle Selection Based on the Influence of Nozzle Parameters on Jet Flow Field Structure

Currently, the primary method for controlling red tides in the ocean involves spraying water solutions with special chemicals as solutes. High-pressure spraying results in the formation of typical jet structures. In this study, numerical simulation methods are employed to investigate the velocity variations, turbulent characteristics, and gas content distribution of jet flow fields under different initial jet flow pressures, cone angles, and nozzle diameters. Based on practical application scenarios, cluster analysis is used to explore the similarities and differences in jet equivalent diameters under different parameter conditions. The research findings indicate the following. (1) The difference of jet velocity distribution at the far field exit will be enlarged with the increase in the nozzle cone angle. When the nozzle cone angle is 4 mm, the velocity uniformity at the outlet is the best. (2) The TKE of the flow field has no consistent change law along the central axis. At the jet exit, the TKE shows an obvious multi-peak structure. (3) The gas content demonstrates a typical “double-valley” feature at the jet outlet cross-section. Increasing the initial pressure leads to a decrease in the gas content within the jet due to reduced entrainment, while the entrainment range remains largely constant. (4) Cluster analysis reveals that the similarity of jet flow width when it reaches the water surface is minimal compared to other operating conditions when the initial pressure is 0.36 MPa, the cone angle is 115°, and the nozzle diameter is 2 mm. All conditions can be categorized into two or three groups to ensure jet effectiveness. The study results provide scientific guidance for selecting spray devices for controlling red tides in the ocean.

Intraseasonal shift in the wintertime North Atlantic jet structure projected by CMIP6 models

The projected winter changes of the North Atlantic eddy-driven jet (EDJ) under climate change conditions have been extensively analysed. Previous studies have reported a squeezed and elongated EDJ. However, other changes present large uncertainties, specifically those related to the intensity and latitude. Here, the projections of the EDJ in a multimodel ensemble of CMIP6 are scrutinised by using a multiparametric description of the EDJ. The multimodel mean projects non-stationary responses of the EDJ latitude through the winter, characterised by a poleward shift in early winter and equator migration in late winter. These intraseasonal shifts (rather than a genuine narrowing) explain the previously established squeezing of the EDJ and are linked to the future changes in different drivers: the 200 hPa meridional temperature gradient and Atlantic warming hole in early winter, and the stratospheric vortex in late winter. Model biases also influence EDJ projections, contributing to the poleward shift in early winter.

Afterglow Linear Polarization Signatures from Shallow GRB Jets: Implications for Energetic GRBs

Gamma-ray bursts (GRBs) are powered by ultrarelativistic jets. The launching sites of these jets are surrounded by dense media, which the jets must cross before they can accelerate and release high-energy emission. Interaction with the medium leads to the formation of a mildly relativistic sheath around the jet, resulting in an angular structure to the jet’s asymptotic Lorentz factor and energy per solid angle, which modifies the afterglow emission. We build a semi-analytical tool to analyze the afterglow light curve and polarization signatures of jets observed from a wide range of viewing angles, and focus on ones with slowly declining energy profiles known as shallow jets. We find overall lower polarization compared to the classical top hat jet model. We provide an analytical expression for the peak polarization degree as a function of the energy profile power-law index, magnetic field configuration, and viewing angle, and show that it occurs near the light-curve break time for all viewers. When applying our tool to GRB 221009A, suspected to originate from a shallow jet, we find that the suggested jet structures for this event agree with the upper limits placed on the afterglow polarization in the optical and X-ray bands. We also find that at early times the polarization levels may be significantly higher, allowing for a potential distinction between different jet structure models and possibly constraining the magnetization in both forward and reverse shocks at that stage.

Influence of a local diabatic heating source on the midlatitude circulation

AbstractDiabatic heating due to latent heat release in the storm tracks plays an important but poorly understood role in the midlatitude circulation. To examine how localised diabatic heating affects the circulation, a dry model is used to apply midtropospheric perturbations to the radiative‐equilibrium temperature profile to which the model is relaxed. This allows us to isolate the one‐way causal influence of diabatic heating on the circulation without feedbacks of the circulation onto the heating. By applying switch‐on perturbations at various latitudes, the mechanisms by which an equilibrated state is reached are examined. In all cases, the equilibrated circulation exhibits a dynamically insensitive jet structure with eddies maintaining a region of self‐concentrated baroclinicity, latitudinally shifted away from the heating perturbation. However, the initial response is generally very different. For heating poleward of the jet, there is an initial thermal‐wind adjustment and weakened eddy heat fluxes, followed later by weakened eddy momentum fluxes that maintain an equilibrated equatorward jet shift. For heating equatorward of the jet, the evolution is more complex with an initial strengthening of the eddy heat fluxes that dominantly balance the imposed heating, as well as a weakening of the Hadley cell. Further, the initial thermal‐wind adjustment immediately modifies the subtropical critical lines and eddy momentum fluxes that encourage an 15° poleward propagation from the Subtropics to Midlatitudes, yielding an equilibrated poleward jet shift. The overall determining factor in balancing the imposed heating at equilibration is the local climatological heat budget: if the climatological eddy heat fluxes locally warm, such as in mid‐to‐high latitudes, then this effect weakens, whereas if the climatological overturning circulation locally warms, such as in the Subtropics, then this effect weakens. The insensitivity of the equilibrated jet profile to the imposed heating is remarkable given the maintenance mechanisms vary drastically according to heating latitude.

Analysis of Surface Drag Reduction Characteristics of Non-Smooth Jet Coupled Structures

To enhance the service life of shipping equipment and minimize surface wear, this study employs biomimetic principles, integrating fitted structures with jet dynamics to model three configurations: non-smooth structures, single jet structures, and non-smooth jet-coupled structures. We utilized the SST k-ω turbulence model for numerical simulations to investigate the drag reduction characteristics of these structural models. By varying the jet angle and speed, we analyzed the changes in viscous resistance, pressure differential resistance, and drag reduction rates at the wall surface. Furthermore, the mechanisms of compressive stress, velocity fields, vortex structures, and shear stress on drag-reducing surfaces were elucidated, revealing how these factors contribute to drag reduction in non-smooth jet-coupled structures. The results indicate that the non-smooth jet-coupled structure exhibits superior drag reduction performance at a main flow field velocity of 20 m/s. As the jet velocity increases, the viscous drag on the surface of the non-smooth jet-coupled structure decreases, while the pressure differential drag increases. Conversely, variations in the jet angle have a minimal effect on viscous drag but lead to a reduction in pressure differential drag. Specifically, when the jet velocity is set at 1 m/s, and the jet angle is 60°, the drag reduction achieved by the non-smooth jet-coupled structure peaks at 7.48%. Additionally, the non-smooth jet-coupled structure features a larger area characterized by low shear stress, along with an increased boundary layer thickness at the bottom; this configuration effectively reduces surface velocity and consequent viscous drag.

The Turkana Jet Diurnal Cycle in Observations and Reanalysis

Abstract The Turkana jet is an equatorial low-level jet (LLJ) in East Africa. The jet influences both flooding and droughts, and powers Africa’s largest wind farm. Much of what we know about the jet, including the characteristics of its diurnal cycle, derives from reanalysis simulations that are not constrained by radiosonde observations in the region. Here, we report the characteristics of the Turkana jet with data from a field campaign during March–April 2021: The Radiosonde Investigation for the Turkana Jet (RIFTJet). The southeasterly jet forms on average at 380 m above the surface, with mean speeds of 15.0 m s−1. The strongest low-level winds are during the night and early morning from 0300 to 0600 LT (>16 m s−1). The average wind profile retains a characteristic low-level jet structure throughout the day, with the low-level wind maximum weakening to a minimum of 10.9 m s−1 at 1500 LT. There is significant shear, of up to 1.5 m s−1 (100 m)−1 maintained through the 1000 m above the wind maximum. The diurnal cycle of the jet is associated with the nocturnal strengthening and lowering of elevated subsidence inversions, which form above the jet. Reanalysis simulations (ERA5 and MERRA-2) do not capture the daytime persistence of the jet and underestimate the speed of the jet throughout the diurnal cycle. The largest absolute errors of over 4.5 m s−1 (−35%) occur at 0900 LT. The reanalyses also fail to simulate the elevated subsidence inversions above the jet and associated dry layer in the lower troposphere.

Effects of oscillated wall on the turbulent structure and heat transfer of three-dimensional wall jet

Effects of oscillated wall on the turbulent structure and heat transfer of three-dimensional wall jet

Conversion of oxidative combustion into powerful thermal and light energy

The phenomenon of transformation of the oxidative combustion of natural coolants - coal, methane, propane, etc. - into the production of powerful light and thermal energy when their combustion zone is exposed to supersonic spiral-twisted jets of dusty plasma from a dynamic emitter has been experimentally discovered. The phenomenon occurs without emissions of harmful substances from chemical combustion that contribute to the greenhouse effect. The structure of dusty plasma jet is a self-compressing double helix with collective resonant oscillations. The phenomenon opens up a new type of energy production, connected with a conversion of coal into aluminosilicate minerals, etc.

Global climate modelling of Saturn’s atmosphere, Part V: Large-scale vortices

This paper presents an analysis of large-scale vortices in the atmospheres of gas giants, focusing on a detailed study conducted using the Saturn-DYNAMICO global climate model (GCM). Large-scale vortices, a prominent feature of gas giant atmospheres, play a critical role in their atmospheric dynamics. By employing three distinct methods – manual detection, machine learning via artificial neural networks (ANN), and dynamical detection using the Automated Eddy-Detection Algorithm (AMEDA) – we characterise the spatial, temporal, and dynamical properties of these vortices within the Saturn-DYNAMICO GCM. Our findings reveal a consistent production of vortices due to well-resolved eddy-to-mean flow interactions, exhibiting size and intensity distributions broadly in agreement with observational data. However, notable differences in vortex location, size, and concentration highlight the model’s limitations and suggest areas for further refinement. The analysis underscores the importance of zonal wind conditions in influencing vortex characteristics and suggests that more accurate modelling of giant planet vortices may require improved representation of moist convection and jet structure. This study not only provides insights into the dynamics of Saturn’s atmosphere as simulated by the GCM but also offers a framework for comparing vortex characteristics across observations and models of planetary atmospheres.

Optically Thick Jet Base and Explanation of Edge Brightening in Active Galactic Nucleus Jets

The jet cores in active galactic nuclei (AGNs) are resolved and found to harbor an edge-brightened structure where the jet base appears extended at the sides compared to its propagation axis. This peculiar phenomenon invites various explanations. We show that the photosphere of an optically thick jet base in AGNs is observed edge brightened if the jet Lorentz factor harbors an angular dependence. The jet assumes a higher Lorentz factor along the jet axis and decreases following a power law along its polar angle. For an observer near the jet axis, the jet has a lower optical depth along its propagation axis compared to off-axis regions. Higher optical depth at the outer region makes the jet photosphere appear to extend to larger radii compared to a deeper photosphere along its propagation axis. We tackle the problem both analytically and numerically, confirming the edge brightening through Monte Carlo simulations. Other than the edge brightening, the outcomes are significant as they provide a unique tool to determine the jet structure and associated parameters by their resolved observed cores. The study paves the way to explore the spectral properties of optically thick cores with structured Lorentz factors in the future.

X-ray Afterglow limits on the viewing angles of short gamma-ray bursts

Abstract The behavior of a short gamma-ray burst (sGRB) afterglow lightcurve can reveal the angular structure of the relativistic jet and constrain the observer’s viewing angle θobs. The observed deceleration time of the jet, and, therefore, the time of the afterglow peak, depends on the observer’s viewing angle. A larger viewing angle leads to a later peak of the afterglow and a lower flux at peak. We utilize the earliest afterglow detections of 58 sGRBs detected with the Neil Gehrels Swift Observatory X-ray Telescope to constrain the ratio of the viewing angle θobs to the jet’s core θc. We adopt a power-law angular jet structure in both energy E(θ)∝θ−a and Lorentz factor Γ(θ)∝θ−b beyond the core. We find that either sGRBs are viewed within θobs/θc < 1 or the initial Lorentz factor of material in their jet’s core is extremely high (Γ0 > 500). If we consider tophat jets, we constrain 90% of our sample to be viewed within θobs/θc < 1.06 and 1.15 for our canonical and conservative afterglow scenarios. For a subset of events with measurements of the jet break, we can constrain Γ0θc ≳ 30. This confirmation that cosmological sGRBs are viewed either on-axis or very close to their jet’s core has significant implications for the nature of the prompt gamma-ray production mechanism and for the rate of future sGRB detections coincident with gravitational waves (GWs), implying that they are extremely rare.

Effects of labyrinth seal with jet structure on the performance of a compressor cascade

Effects of labyrinth seal with jet structure on the performance of a compressor cascade

On the Role of the Meridional Jet and Horizontal Potential Vorticity Dipole in the Iowa Derecho of 10 August 2020

Abstract On 10 August 2020, a derecho caused widespread damage across Iowa and Illinois. Des Moines station data show that the arrival of the gust front was characterized by an abrupt shift to northerly flow, exceeding 22 m s−1 for ∼20 min. To test the hypothesis that this northerly jet is associated with a horizontal potential vorticity (PV) dipole in the lower troposphere, we investigated the structure of PV in the University of Wisconsin Nonhydrostatic Modeling System (UWNMS) and of absolute vorticity in High-Resolution Rapid Refresh (HRRR) forecast analyses. This structure is described here for the first time. The negative PV member coincides with the downdraft, while the positive PV member coincides with the updraft, with a northerly jet between. The westerly inflow jet descends anticyclonically in the downdraft, joining with northerly flow from the surface anticyclone. The resulting northerly outflow jet creates the trailing comma-shaped radar echo. The speed of propagation of the derecho is similar to the westerly wind maximum in the 3–5-km layer associated with the approaching synoptic cyclone, which acts as a steering level for resonant amplification. Idealized diagrams and 3D isosurfaces illustrate the commonality of the PV dipole/northerly jet structure. Differences in this structure among the three model states are related to low-level wind shear theory. The PV dipole coincides with the pattern of diabatic stretching tendency, which shifts westward and downward relative to the updraft/downdraft with increasing tilt. The PV dipole can contribute toward dynamical stability in a derecho. Significance Statement The purpose of this work is to investigate the structure of potential vorticity (PV) in the lower troposphere in a derecho. It is found that a northerly outflow jet occurs between an east–west-oriented horizontal PV dipole, which is described here for the first time. The negative PV member coincides with the downdraft and is inertially unstable, while the positive PV member coincides with the updraft. This work contributes toward the theory of resonant structures and longevity. The 3–5-km westerly inflow layer constitutes a steering level, which controls propagation speed despite differences in structure. The degree of westward tilt with height is related to the pattern of forcing by diabatic stretching in producing the PV dipole.

Swift J1727.8–1613 Has the Largest Resolved Continuous Jet Ever Seen in an X-Ray Binary

Multiwavelength polarimetry and radio observations of Swift J1727.8–1613 at the beginning of its recent 2023 outburst suggested the presence of a bright compact jet aligned in the north–south direction, which could not be confirmed without high-angular-resolution images. Using the Very Long Baseline Array and the Long Baseline Array, we imaged Swift J1727.8–1613 during the hard/hard-intermediate state, revealing a bright core and a large, two-sided, asymmetrical, resolved jet. The jet extends in the north–south direction, at a position angle of −0.60° ± 0.07° east of north. At 8.4 GHz, the entire resolved jet structure is ∼110(d/2.7kpc)/sini au long, with the southern approaching jet extending ∼80(d/2.7kpc)/sini au from the core, where d is the distance to the source and i is the inclination of the jet axis to the line of sight. These images reveal the most resolved continuous X-ray binary jet, and possibly the most physically extended continuous X-ray binary jet ever observed. Based on the brightness ratio of the approaching and receding jets, we put a lower limit on the intrinsic jet speed of β ≥ 0.27 and an upper limit on the jet inclination of i ≤ 74°. In our first observation we also detected a rapidly fading discrete jet knot 66.89 ± 0.04 mas south of the core, with a proper motion of 0.66 ± 0.05 mas hr−1, which we interpret as the result of a downstream internal shock or a jet–interstellar medium interaction, as opposed to a transient relativistic jet launched at the beginning of the outburst.