Edge Of Disk Research Articles

Experimental study of a free-surface circular liquid sheet using planar laser-induced fluorescence

This work presents an experimental study of various spatiotemporal events leading to the primary atomization of a radially expanding circular liquid sheet. A cylindrical liquid (water) jet orthogonally made to impinge on the horizontally placed cone-disc deflector to produce a thin free-surface circular liquid sheet from the peripheral edge of the deflector disc. High-speed planar laser-induced fluorescence (PLIF) at 6 kHz framing rate was carried out to capture the atomizing liquid sheet in the side and front views at various jet Weber numbers ($${\text{We}}_{{{\text{jet}}}}$$) over a range of 994 < $${\text{We}}_{{{\text{jet}}}}$$ < 8029. The side view PLIF comprising of three fields of view to capture the complete atomization process of expanding liquid sheet in a time-resolved manner starting from capturing radial wave features on liquid sheet down to the sheet breakup, ligament formation, and droplet production. The azimuthal wave features were captured in the front view PLIF experiments. The quantitative information of different spatiotemporal features was extracted from experimental data using rigorous image processing methods. Advanced feature extraction and transient analysis methods like proper orthogonal decomposition (POD) was applied to obtain the dominant flow features associated with sheet dynamics. The POD analysis showed that the most dominant phenomenon in the flow field for $${\text{We}}_{{{\text{jet}}}}$$ < 1500 is found to be the oscillation of the liquid sheet, the radial wave propagation in the radial direction for 1500 ≤ $${\text{We}}_{{{\text{jet}}}}$$ < 2000, and sheet breaks up at the outer edge for $${\text{We}}_{{{\text{jet}}}}$$ ≥ 2000. It is shown that the dominancy of each of these phenomena changes with $${\text{We}}_{{{\text{jet}}}}$$. Statistical analysis-based ligament length and thickness ($$L_{{{\text{lig}}}}$$ and $$t_{{{\text{lig}}}}$$) follow $${\text{We}}_{{{\text{jet}}}}^{ - 0.6}$$ and droplet diameter ($$D_{32}$$ and $$D_{10}$$) follow $${\text{We}}_{{{\text{jet}}}}^{ - 0.33}$$ power law. Further, the direct correlations obtained showed that the droplet diameter is a stronger function of ligament thickness compared to ligament length. Schematic of optics, laser light and camera arrangements for side view PLIF experiments of a circular liquid sheet. Three FOVs adopted for side view PLIF experiments at different radial locations; RA= 280 mm for A = 100 mm × 100 mm, RB = 360 mm for B = 80 mm × 80 mm, and RC = 420 mm for C = 40 mm × 40 mm Direct imaging of azimuthal waves of liquid sheet using front view PLIF experimental technique

Debris Disks in Multiplanet Systems: Are Our Inferences Compromised by Unseen Planets?

Abstract Resolved debris disk features (e.g., warps, offsets, edges and gaps, azimuthal asymmetries, radially thickened rings, scale heights) contain valuable information about the underlying planetary systems, such as the posited planet’s mass, semimajor axis, and other orbital parameters. Most existing models assume a single planet is sculpting the disk feature, but recent observations of mature planetary systems (e.g., by radial velocity surveys or Kepler) have revealed that many planets reside in multiplanet systems. Here we investigate if/how planet properties inferred from single-planet models are compromised when multiple planets reside in the system. For each disk feature, we build a two-planet model that includes a planet b with fixed parameters and a planet c with a full range of possible parameters. We investigate these two-planet systems and summarize the configurations for which assuming a single planet (i.e., planet b) leads to significantly flawed inferences of that planet’s properties. We find that although disk features are usually primarily dominated by a single planet, when using single-planet models, we are at risk of misinterpreting planet properties by orders of magnitude in extreme cases. Specifically, we are at high risk of misinterpreting planet properties from disk warps; at moderate risk from disk edges and gaps, radially thickened rings, and scale height features; and at low risk from host star–disk center offsets and azimuthal asymmetries. We summarize situations where we can infer the need to use a multiplanet model instead of a single-planet one from disk morphology dissimilarities.

Ring Morphology with Dust Coagulation in Protoplanetary Disks

Abstract Tidal interactions between the embedded planets and their surrounding protoplanetary disks are often postulated to produce the observed complex dust substructures, including rings, gaps, and asymmetries. In this Letter, we explore the consequences of dust coagulation on the dust dynamics and ring morphology. Coagulation of dust grains leads to dust size growth that, under typical disk conditions, produces faster radial drifts, potentially threatening the dust ring formation. Utilizing 2D hydrodynamical simulations of protoplanetary disks that include a full treatment of dust coagulation, we find that if the planet does not open a gap quickly enough, the formation of an inner ring is impeded due to dust coagulation and subsequent radial drift. Furthermore, we find that a “buildup” of submillimeter-sized grains often appears in the dust emission at the outer edge of the dust disk.

A dynamic black hole corona in an active galaxy through X-ray reverberation mapping

X-ray reverberation echoes are assumed to be produced in the strongly distorted spacetime around accreting supermassive black holes. This signal allows us to spatially map the geometry of the inner accretion flow - a region which cannot yet be spatially resolved by any telescope - and provides a direct measure of the black hole mass and spin. The reverberation timescale is set by the light travel path between the direct emission from a hot X-ray corona and the reprocessed emission from the inner edge of the accretion disc. However, there is an inherent degeneracy in the reverberation signal between black hole mass, inner disc radius and height of the illuminating corona above the disc. Here, we use a long X-ray observation of the highly-variable active galaxy, IRAS 13224-3809, to track the reverberation signal as the system evolves on timescales of a day. With the inclusion of all the relativistic effects, modelling reveals that the height of the X-ray corona increases with increasing luminosity, providing a dynamic view of the inner accretion region. This simultaneous modelling allows us to break the inherent degeneracies and obtain an independent timing-based estimate for the mass and spin of the black hole. The uncertainty on black hole mass is comparable to the leading optical reverberation method, making X-ray reverberation a powerful technique, particularly for sources with low optical variability.

The Detection of Dust Gap-ring Structure in the Outer Region of the CR Cha Protoplanetary Disk

Abstract We observe the dust continuum at 225 GHz and CO isotopologue (12CO, 13CO, and C18O) J = 2–1 emission lines toward the CR Cha protoplanetary disk using the Atacama Large Millimeter/submillimeter Array. The dust continuum image shows a dust gap-ring structure in the outer region of the dust disk. A faint dust ring is also detected around 120 au beyond the dust gap. The CO isotopologue lines indicate that the gas disk is more extended than the dust disk. The peak brightness temperature of the 13CO line shows a small bump around 130 au, while 12CO lines do not. We investigate two possible mechanisms for reproducing the observed dust gap-ring structure and a gas temperature bump. First, the observed gap structure can be opened by a Jupiter-mass planet using the relation between the planet mass and the gap depth and width. Meanwhile, the radiative transfer calculations based on the observed dust surface density profile show that the observed dust ring could be formed by dust accumulation at the gas temperature bump, that is, the gas pressure bump produced beyond the outer edge of the dust disk.

Micro Electrochemical Machining of Array Micro-Grooves Using In-Situ Disk Electrode Fabricated by Micro-WEDM.

This paper develops an array micro-grooves manufacturing method using micro electrochemical machining (ECM) with disk electrode, which is prepared by in-situ micro wire electrical discharge machining (WEDM). This technology focuses on the difficulty of array structure manufacture in micro-electro-mechanical systems (MEMS). A micro-ECM system is built based on the micro-WEDM machine to achieve high precision processing of the array micro-grooves. Since micro-WEDM has good performance in high precision machining of the rotating structure, single and multi-edge disk electrodes can be fabricated in-situ using graphite. The as-prepared disk tool electrode is directly used for micro-electrochemical milling of the array micro-grooves without disassembling away from the device, which avoids the positioning error caused by the re-clamping of the disk electrode. With the advantages of high surface quality and no electrode loss, micro-ECM improves the manufacture performance of the micro-parts. Through wire path optimization, the shape accuracy of the disk edge is improved. After the research of the micro-ECM parameters, the process is improved, and finally, the high precision array micro-grooves are obtained. This method combines the advantages of micro-WEDM and disk electrode micro-ECM milling, and it is convenient for large-scale manufacture of array micro-structures on micro-parts and MEMS.

MHD simulation of accretion processes in magnetic cataclysmic variable EX Hya

A hierarchical numerical model for investigation of the accretion processes in the magnetic cataclysmic variable EX Hya has been described. The coarsest level of the hierarchy corresponds to the accretion disk formation due to the mass transfer. The intermediate level describes the structure of the white dwarf magnetosphere. The finest level corresponds to the formation of the accretion curtain at the inner edge of the accretion disk. The model is based on the equations of the modified MHD, where a strong external magnetic field acts as an efficient fluid with which the plasma interacts. Results of simulations show that the accretion in EX Hya proceeds via accretion columns that have curtain-like shapes. These columns are very thin and the accretion regions cover a small fraction of the accretor surface. Numerical results agree with the parameters of the EX Hya system as determined by Semena et al. (2014).

Midplane temperature and outer edge of the protoplanetary disk around HD 163296

Knowledge of the midplane temperature of protoplanetary disks is one of the key ingredients in theories of dust growth and planet formation. However, direct measurement of this quantity is complicated, and often depends on the fitting of complex models to data. In this paper we demonstrate a method to directly measure the midplane gas temperature from an optically thick molecular line if the disk is moderately inclined. The only model assumption that enters is that the line is very optically thick, specifically in the midplane region where we wish to measure the temperature. Freeze-out of the molecule onto dust grains could thwart this. However, in regions that are expected to be warm enough to avoid freeze-out, this method should work. We apply the method to the CO 2–1 line channel maps of the disk around HD 163296. We find that the midplane temperature between 100 and 400 au drops only mildly from 25 K down to 18 K. While we see no direct evidence of the midplane being optically thin due to strong CO depletion by freeze-out, we cannot rule it out either. The fact that the inferred temperatures are close to the expected CO freeze-out temperature could be an indication of this. Incidently, for the disk around HD 163296 we also find dynamic evidence for a rather abrupt outer edge of the disk, suggestive of outside-in photoevaporation or truncation by an unseen companion.

Mathematical modeling of the grain trajectory in the workspace of the sheller with rotating decks

The purpose of the work is to determine the possibility of controlling the operation of cereal grain husks by changing the angular rotation speed of the disk (rotor) and deck to provide optimal conditions for peeling (removal of the flower shell from the grain). For this purpose, the movement of grain in the working space of the husk between the disk and the rotating deck is considered. The mathematical model of grain movement in the working space of the husk, taking into account the real aerodynamics of the rotating air flow, where the efficiency of peeling is determined by the speed and direction of the grain flight, is presented. To study the nature of the influence of structural and regime parameters of the desiccant on the efficiency of work, on the basis of the obtained mathematical model, numerical calculations and graphical dependences are made. On the basis of the analysis of the received graphs it is revealed that the direction of the grain impact on the deck is strongly influenced by the direction of the grain velocity vector at the moment of its tearing from the disk edge and the deck rotation angle speed. At the same time, the right angle impact of the grain can be achieved at low speeds of the deck rotation, and the speed of the grain during the impact is mainly determined by the speed of the disk rotation and practically does not depend on the speed of the deck rotation.

Formation of moon systems around giant planets

The four major satellites of Jupiter, known as the Galilean moons, and Saturn’s most massive satellite, Titan, are believed to have formed in a predominantly gaseous circum-planetary disk during the last stages of formation of their parent planet. Pebbles from the protoplanetary disk are blocked from flowing into the circumplanetary disk by the positive pressure gradient at the outer edge of the planetary gap, so the gas drag assisted capture of planetesimals should be the main contributor to the delivery of solids onto circum-planetary disks. However, a consistent framework for the subsequent accretion of the moons remains to be built. Here, we use numerical integrations to show that most planetesimals that are captured within a circum-planetary disk are strongly ablated due to the frictional heating they experience, thus supplying the disk with small dust grains, whereas only a small fraction “survives” their capture. We then constructed a simple model of a circum-planetary disk supplied by ablation, where the flux of solids through the disk is at equilibrium with the ablation supply rate, and we investigate the formation of moons in such disks. We show that the growth of satellites is mainly driven by accretion of the pebbles that coagulate from the ablated material. The pebble-accreting protosatellites rapidly migrate inward and pile up in resonant chains at the inner edge of the circum-planetary disk. We propose that dynamical instabilities in these resonant chains are at the origin of the different architectures of Jupiter’s and Saturn’s moon systems. The assembly of moon systems through pebble accretion can therefore be seen as a down-scaled manifestation of the same process that forms systems of super-Earths and terrestrial-mass planets around solar-type stars and M-dwarfs.

Dynamical evidence for an early giant planet instability

The dynamical structure of the Solar System can be explained by a period of orbital instability experienced by the giant planets. While a late instability was originally proposed to explain the Late Heavy Bombardment, recent work favors an early instability. Here we model the early dynamical evolution of the outer Solar System to self-consistently constrain the most likely timing of the instability. We first simulate the dynamical sculpting of the primordial outer planetesimal disk during the accretion of Uranus and Neptune from migrating planetary embryos during the gas disk phase, and determine the separation between Neptune and the inner edge of the planetesimal disk. We performed simulations with a range of (inward and outward) migration histories for Jupiter. We find that, unless Jupiter migrated inwards by 10 AU or more, the instability almost certainly happened within 100 Myr of the start of Solar System formation. There are two distinct possible instability triggers. The first is an instability that is triggered by the planets themselves, with no appreciable influence from the planetesimal disk. About half of the planetary systems that we consider have a self-triggered instability. Of those, the median instability time is ∼4Myr. Among self-stable systems – where the planets are locked in a resonant chain that remains stable in the absence of a planetesimal’s disk– our self-consistently sculpted planetesimal disks nonetheless trigger a giant planet instability with a median instability time of 37–62 Myr for a reasonable range of migration histories of Jupiter. The simulations that give the latest instability times are those that invoked long-range inward migration of Jupiter from 15 AU or beyond; however these simulations over-excited the inclinations of Kuiper belt objects and are inconsistent with the present-day Solar System. We conclude on dynamical grounds that the giant planet instability is likely to have occurred early in Solar System history.

Jellyfish: Resolving the Kinematics of Extreme Ram-pressure Stripping at z ∼ 0.3* †

Abstract We present and discuss results from the first spatially resolved kinematic study of ram-pressure stripping of a massive late-type galaxy at intermediate redshifts. Our target, the spectacular “jellyfish” galaxy, A1758N_JFG1, was previously identified as a fast-moving member of the equal-mass merger A1758N (z = 0.28), with a star formation rate of 48 M ☉ yr−1, far above the galaxy main sequence. Integral field unit (IFU) data obtained by us unambiguously confirm ram-pressure stripping as the physical mechanism driving the optical morphology and high star formation rate of this system by revealing extended [O ii]λ3727 Å emission up to 40 kpc (in projection) downstream, as well as an ordered radial-velocity field generated by (a) conservation of angular momentum of the interstellar gas stripped from the edge of the galactic disk and (b) drag forces exerted by the intracluster medium on the “tentacles” of stripped material. We find no evidence of significant nuclear activity in A1758N_JFG1, although an AGN might, at this early stage of the stripping process, be obscured by high column densities of gas and dust near the galactic core. Finally, our exploration of possible trajectories of A1758N_JFG1 found solutions consistent with the notions (a) that the A1758N merger proceeds along an axis that is substantially inclined with respect to the plane of the sky and (b) that A1758N_JFG1 participated in the merger, rather than having been accreted independently from the field.

The Exoplanet Population Observation Simulator. II. Population Synthesis in the Era of Kepler

Abstract The collection of planetary system properties derived from large surveys such as Kepler provides critical constraints on planet formation and evolution. These constraints can only be applied to planet formation models, however, if the observational biases and selection effects are properly accounted for. Here we show how epos, the Exoplanet Population Observation Simulator, can be used to constrain planet formation models by comparing the Bern planet population synthesis models to the Kepler exoplanetary systems. We compile a series of diagnostics, based on occurrence rates of different classes of planets and the architectures of multiplanet systems within 1 au, that can be used as benchmarks for future and current modeling efforts. Overall, we find that a model with 100-seed planetary cores per protoplanetary disk provides a reasonable match to most diagnostics. Based on these diagnostics we identify physical properties and processes that would result in the Bern model more closely matching the known planetary systems. These are as follows: moving the planet trap at the inner disk edge outward; increasing the formation efficiency of mini-Neptunes; and reducing the fraction of stars that form observable planets. We conclude with an outlook on the composition of planets in the habitable zone, and highlight that the majority of simulated planets smaller than 1.7 Earth radii in this zone are predicted to have substantial hydrogen atmospheres. The software used in this paper is available online for public scrutiny at https://github.com/GijsMulders/epos.

Effects of single- and dual-blockage disks on swirling coaxial jets at high annulus Reynolds numbers

Using the laser-assisted smoke wire flow visualization technique, particle image velocimetry, and concentration detection method, this study examines the effects of two circular disks, arranged in tandem, on the flow structures, turbulence properties, and mixing characteristics of swirling double-concentric jets at high annulus Reynolds numbers. The smoke flow patterns, velocity vector fields, streamlines, normalized velocity contours, fluctuation intensity contours, vorticity contours, and distributions of carbon dioxide are presented and discussed. When a single-blockage disk is installed at the exit of swirling double-concentric jets, two pairs of counter-rotating vortices are formed in the near wake. The central jet fluids penetrate the recirculation zone so that no stagnation point, and therefore no reverse flow induced by annular jet flow, is found along the central axis. This causes high values of carbon dioxide concentration along the central axis. When a second circular disk is installed a short distance downstream of the disk at the jet exit, the axial momentum of the central jet transfers radially, turns downstream at the edge of the downstream disk, and then merges with the swirling annular flow. For this dual-disk case, a large reverse-flow zone is observed in the near wake in the condition of high annulus Reynolds and swirl numbers. Large turbulence intensity region is therefore induced downstream disks. Consequently, the mixing capability of dual-disk-controlled swirling double-concentric jets is significant improved compared to that of the single-blockage disk.

The 2017 Failed Outburst of GX 339–4: Relativistic X-Ray Reflection near the Black Hole Revealed by NuSTAR and Swift Spectroscopy

Abstract We report on the spectroscopic analysis of the black hole binary GX 339−4 during its recent 2017–2018 outburst, observed simultaneously by the Swift and NuSTAR observatories. Although during this particular outburst the source failed to make state transitions, and despite Sun constraints during the peak luminosity, we were able to trigger four different observations sampling the evolution of the source in the hard state. We show that even for the lowest-luminosity observations the NuSTAR spectra show clear signatures of X-ray reprocessing (reflection) in an accretion disk. Detailed analysis of the highest signal-to-noise spectra with our family of relativistic reflection models relxill indicates the presence of both broad and narrow reflection components. We find that a dual-lamppost model provides a superior fit when compared to the standard single lamppost plus distant neutral reflection. In the dual-lamppost model two sources at different heights are placed on the rotational axis of the black hole, suggesting that the narrow component of the Fe K emission is likely to originate in regions far away in the disk, but still significantly affected by its rotational motions. Regardless of the geometry assumed, we find that the inner edge of the accretion disk reaches a few gravitational radii in all our fits, consistent with previous determinations at similar luminosity levels. This confirms a very low degree of disk truncation for this source at luminosities above ∼1% Eddington. Our estimates of R in reinforce the suggested behavior for an inner disk that approaches the innermost regions as the luminosity increases in the hard state.

Visualization of features of atomizing circular liquid sheet using simultaneous Volume Laser-Induced Fluorescence

Present work is an experimental investigation of primary atomization of a radially expanding circular liquid sheet produced from a cylindrical vertical water jet issuing out from the convergent nozzle and impinging orthogonally on a horizontally placed cone-disc deflector below the nozzle. The radially expanding thin circular water sheet formed from the peripheral edge of deflector disc. The atomization process of expanding liquid sheet is captured simultaneously for the first time in the top and side views using a high-speed Volume Laser-Induced Fluorescent (VLIF). The experiments were conducted at 6 kHz framing rate for various jet Weber numbers (Wejet) over a range of 996 <Wejet < 7480. The top view VLIF revealed interesting spatiotemporal features of atomizing sheet such as outer edge thickness, liquid threads, ligaments and droplets and formation of perforations punctures (perforations) on the local sites of liquid sheet. Moreover, the evolution of the perforations leading to the breakup into ligaments and droplets was also captured in top view VLIF as well. Results show the liquid sheet outer edge (rim) thickness is unaffected by change in Wejet, whereas the rim velocity is found linearly increases with increase in Wejet. The liquid threads observed at the rim are subjected to end-pinching effect (as Ohnesorge number, Oh ≪ 1) when detached (because of Plateau-Rayleigh instability) from the rim. Ligament size in terms of length (Llig) and thickness (tlig) are measured at two different locations; (i) at rim (~100 mm from nozzle axis) and (ii) at about 130 mm downstream from the rim. Ligaments measured at downstream of the rim were found to be smaller than those measured at the rim. Further downstream, the droplets are formed whose size decreased with increase in jet Weber number (Wejet). Three different sites of origin of droplets namely rim, perforations edges and liquid bridge are identified. The variation of droplet diameter that produced from the rim with the Wejet is found to follow D10~Wejet-1/4 and D32~Wejet-1/5. Various features originating from the perforations are captured in top view VLIF and quantified for the first time using rigorous image processing methods. These features are perforation diameter, perforation’s front and rear edge thicknesses and velocities, liquid bridge, droplets from perforation via ligaments and via formation of liquid bridges. The perforations are formed close to sheet breakup and number of occurrence of perforations increases with increase in Wejet. The rear edge of perforation is seen thicker than the front edge because of larger accumulation of water in the rear edge. Consequently, the rear edge velocity is seen lower than the front edge velocity during the life span of perforations from the origin to their rupture into ligaments and droplets. Liquid bridge formed by merging/coalescing of two perforations is also observed. It is found that liquid bridge thickness increased with increase in Wejet. The droplets originating from the perforations are weak function of Wejet.

A bizarre Eocene dasyatoid batomorph (Elasmobranchii, Myliobatiformes) from the Bolca Lagerst\xe4tte (Italy) reveals a new, extinct body plan for stingrays

In the last few years, the detailed revision of the Eocene cartilaginous fishes (Chondrichthyes) from the Bolca Lagerstätte (Italy) has provided new insights into the fish biodiversity of the western Tethys. The morphological analysis of three previously undescribed specimens from the Pesciara deposit of Bolca revealed the existence of a new stingray taxon, †Lessiniabatis aenigmatica gen. et sp. nov., which is unique among the myliobatiform batoids in having the following unique combination of characters: low number of vertebrae posterior to the pelvic girdle (65–68); thoracolumbar synarcual extending backward beyond the pelvic girdle; tail extremely short not protruding from the posterior edge of the pectoral disc; radials proximally fused to each other; pelvic girdle extremely small and strongly arched; dorsal and caudal fins absent; tail stings and cartilaginous tail rod absent; and teeth of dasyatoid morphology with smooth enameloid surface. The phylogenetic analysis suggests that †Lessiniabatis gen. nov. is deeply nested within the benthic stingrays (Dasyatoidea) representing the sister to all dasyatids and potamotrygonids. Its unique anatomy clearly reveals the existence of a new hitherto unknown body plan experimented by benthic stingrays, whose evolution can be possibly linked to the adaptive fish radiation in the aftermath of the end-Cretaceous extinction.

Detailed Comparative Analysis of Interaction of a Supersonic Flow with a Transverse Gas Jet at High Pressure Ratios

The interaction of a 3D supersonic turbulent gas flow with a transverse sonic jet injected from the wall has been studied in detail both numerically and experimentally. However, the main drawback of such studies is the lack of detailed description of formation and propagation of vortex structures for moderate and large parameters n (ratio of pressure in the jet to pressure in the flow). Analysis performed in this study is aimed at revelation and detailed explanation of mechanisms of formation of vortices behind the injected sonic jet in a supersonic oncoming flow depending on n for improving the effectiveness of mixing of the jet with the flow. As initial equations, we have used 3D Favre-averaged Navier–Stokes equations closed by the k–ω model of turbulence; these equations are solved using the algorithm based on the essentially nonoscillatory scheme of the third approximation order. We have demonstrated the presence of the following vortex structures known from a number of theoretical publications: two oppositely rotating vortices in front of the jet, a horseshoe vortex; and two pairs formed in the mixing zone between the jet and the flow (one in the wake behind the jet and the other on the lateral line of the jet). We have determined the pressure ratios for which extra pairs of vortices appear (one pair emerges at the Mach disk edge as a result of interaction of a retarded flow of the jet behind the Mach disk with a high-velocity ascending flow behind the barrel and the other pair is formed due to the interaction of the ascending jet flow with the incoming main gas flow). As a result of comparative analysis, the pressure ratios for which a clear pattern of additional horn vortices is observed near the wall in the region behind the jet, have been determined. The dependence of the slope of the bow shock on the pressure ratio has been plotted. It is found that the pressure distribution at the wall in front of the jet in the symmetry plane is in satisfactory agreement with experimental data.

The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636–53 in the frequency-energy space

ABSTRACT We present for the neutron-star low-mass X-ray binary 4U 1636–53, and for the first time for any source of kilohertz quasi-periodic oscillations (kHz QPOs), the two-dimensional behaviour of the fractional rms amplitude of the kHz QPOs in the parameter space defined by QPO frequency and photon energy. We find that the rms amplitude of the lower kHz QPO increases with energy up to $\sim \!\!12$ keV and then decreases at higher energies, while the rms amplitude of the upper kHz QPO either continues increasing or levels off at high energies. The rms amplitude of the lower kHz QPO increases and then decreases with frequency, peaking at $\sim \!\!760$ Hz, while the amplitude of the upper kHz QPO decreases with frequency, with a local maximum at around $\sim \!\!770$ Hz, and is consistent with becoming zero at the same QPO frequency, $\sim \!\!1400$ Hz, in all energy bands, thus constraining the neutron-star mass at $M_{NS} \le 1.6 \, \mathrm{M}_{\odot }$, under the assumption that this QPO reflects the Keplerian frequency at the inner edge of the accretion disc. We show that the slope of the rms energy spectrum is connected to the changing properties of the kHz QPOs in different energy bands as its frequencies change. Finally, we discuss a possible mechanism responsible for the radiative properties of the kHz QPOs and, based on a model in which the QPO arises from oscillations in a Comptonizing cloud of hot electrons, we show that the properties of the kHz QPOs can constrain the thermodynamic properties of the inner accretion flow.

Mechanical Bonding of Aluminum Hybrid Alloy Systems through High‐Pressure Torsion

The present study demonstrates an innovative approach of utilizing high‐pressure torsion (HPT) processing for the mechanical bonding of dissimilar metals during the microstructural refinement process. This processing approach has been developed recently for introducing unique alloy systems with improving physical and mechanical properties. Accordingly, the present study focuses specifically on the microstructural evolution and development in micro‐mechanical responses in the mechanically bonded Al‐Mg and Al‐Cu hybrid alloy systems when synthesized by HPT processing for very high number of turns up to 60 under 6.0 GPa at room temperature. The microstructural and hardness evaluations confirm the capability of the HPT procedure for the formation of heterostructures with extreme hardness at the disk peripheries and with low hardness at the disk centers in these processed alloy systems. Nanoindentation measurements demonstrate that both hybrid alloy systems exhibit excellent plasticity at the disk edges, where the hardness is the highest. There is a considerable potential for applying the solid‐state reaction through the HPT process for the bonding of dissimilar metals as a manufacturing technique and for the development of hybrid alloy systems.