arXiv subjects

Uri Keshet

Publications and source records attributed to Uri Keshet.

At least 55 records · Page 3Linked to original sources

arXiv2017

Fermi bubble edges: spectrum and diffusion function

Current measurements of the $γ$-ray Fermi bubbles (FB) are based on model-dependent tracers, carry substantial systematic uncertainties, and are at some tension with each other. We show that gradient filters pick out the FB edges, which are found to smoothly connect to the bipolar X-ray structure emanating from the Galactic center, thus supporting the interpretation of the FBs as a Galactic-scale phenomenon. The sharp edges facilitate a direct, model-free measurement of the peripheral FB spectrum. The result is strikingly similar to the full FB-integrated spectrum, softened by a power law of index $η\simeq (0.2\mbox{--}0.3)$. This is naturally explained, in both hadronic and leptonic models, if cosmic rays are injected at the edge, and diffuse away preferentially at higher energies $E$. The inferred, averaged diffusion function in the (more plausible) leptonic model, $D(E)\simeq 10^{29.5}(E/10\mbox{ GeV})^{0.48\pm0.02}\mbox{ cm}^2\mbox{ s}^{-1}$, is consistent with estimates for Kraichnan-like turbulence. Our results, in particular the minute spatial variations in $η$, indicate that the FB edge is a strong, Mach $\gtrsim5$, forward shock.

astro-ph.HE↗

arXiv2016

Compressible flow in front of an axisymmetric blunt object

Compressible flows around blunt objects have diverse applications, but present analytic treatments are inaccurate and limited to narrow parameter regimes. We show that the flow in front of an axisymmetric body is accurately derived analytically using a low order expansion of the perpendicular gradients in terms of the parallel velocity. This reproduces both subsonic and supersonic flows measured and simulated for a sphere, including the transonic regime and the bow shock properties.

astro-ph.IM↗

arXiv2016

Compressible flow in front of an axisymmetric blunt object: analytic approximation and astrophysical implications

Compressible flows around blunt objects have diverse applications, but current analytic treatments are inaccurate and limited to narrow parameter regimes. We show that the gas-dynamic flow in front of an axisymmetric blunt body is accurately derived analytically using a low order expansion of the perpendicular gradients in terms of the parallel velocity. This reproduces both subsonic and supersonic flows measured and simulated for a sphere, including the transonic regime and the bow shock properties. Some astrophysical implications are outlined, in particular for planets in the solar wind and for clumps and bubbles in the intergalactic medium. The bow shock standoff distance normalized by the obstacle curvature is $\sim 2/(3g)$ in the strong shock limit, where $g$ is the compression ratio. For a subsonic Mach number $M$ approaching unity, the thickness $δ$ of an initially weak, draped magnetic layer is a few times larger than in the incompressible limit, with amplification $\sim ({1+1.3M^{2.6}})/({3δ})$.

astro-ph.EP↗

arXiv2015

Self-similar ultra-relativistic jetted blast wave

Following a suggestion that a directed relativistic explosion may have a universal intermediate asymptotic, we derive a self-similar solution for an ultra-relativistic jetted blast wave. The solution involves three distinct regions: an approximately paraboloid head where the Lorentz factor $γ$ exceeds $\sim1/2$ of its maximal, nose value; a geometrically self-similar, expanding envelope slightly narrower than a paraboloid; and an axial core in which the (cylindrically, henceforth) radial flow $u$ converges inward towards the axis. Most ($\sim 80\%$) of the energy lies well beyond the leading, head region. Here, a radial cross section shows a maximal $γ$ (separating the core and the envelope), a sign reversal in $u$, and a minimal $γ$, at respectively $\sim 1/6$, $\sim1/4$, and $\sim3/4$ of the shock radius. The solution is apparently unique, and approximately agrees with previous simulations, of different initial conditions, that resolved the head. This suggests that unlike a spherical relativistic blast wave, our solution is an attractor, and may thus describe directed blast waves such as in the external shock phase of a $γ$-ray burst.

astro-ph.HE↗

arXiv2015

A Dynamically Driven, Universal Thermal Profile of Galaxy Groups and Clusters

Large scale structures such as groups and clusters of galaxies show a universal, nearly linear entropy radial profile $K(r)$. Using deprojected 16 clusters and 12 groups from the literature, we find that $K\propto r^{0.96\pm0.01}$, consistent with the mean power-law index $(0.9\mbox{--}1.1)$ of previous studies. A similarly good fit is given by a $τ\propto r^{0.72\pm0.01}$ ratio between cooling and free-fall times. Both profiles slightly flatten at small radii, as $τ$ becomes of order unity. The entropy profile is usually attributed to self-similar shock accretion (shown to be inconsistent with the data), to non-standard heat conduction, or to turbulent heating. We argue that a dynamical mechanism is needed to sustain such a universal profile, oblivious to the temperature peak at the edge of the core and to the virial shock at the outskirts, and robust to the presence of ongoing cooling, merger, and active galactic nucleus (AGN) activity. In particular, we show that such a profile can be naturally obtained in a spiral flow, which is likely to underlie most galaxy aggregates according to the ubiquitous spiral patterns and cold fronts observed. Generalizing a two-phase spiral flow model out to the virial radius surprisingly reproduces the thermal profile. A generalized Schwarzschild criterion indicates that observed spiral patterns must involve a convective layer, which may regulate the thermal profile.

astro-ph.CO↗

arXiv2015

Magnetohydrodynamics using path or stream functions

Magnetization in highly conductive plasmas is ubiquitous to astronomical systems. Flows in such media can be described by three path functions $Λ_α$, or, for a steady flow, by two stream functions $λ_κ$ and an additional field such as the mass density $ρ$, velocity $v$, or travel time $Δt$. While typical analyses of a frozen magnetic field $\boldsymbol{B}$ are problem-specific and involve nonlocal gradients of the fluid element position $\boldsymbol{x}(t)$, we derive the general, local (in $Λ$ or $λ$ space) solution $\boldsymbol{B}=(\partial\boldsymbol{x}/\partialΛ_α)_t \tilde{B}_αρ/\tildeρ$, where Lagrangian constants denoted by a tilde are directly fixed at a boundary hypersurface $\tilde{H}$ on which $\boldsymbol{B}$ is known. For a steady flow, $\tildeρ\boldsymbol{B}/ρ=(\partial\boldsymbol{x}/\partialλ_κ)_{Δt}\tilde{B}_κ+\boldsymbol{v}\tilde{B}_3/\tilde{v}$; here the electric field $\boldsymbol{E}\sim(\tilde{B}_2\boldsymbol{\nabla}λ_1-\tilde{B}_1\boldsymbol{\nabla}λ_2)/\tildeρ$ depends only on $λ_κ$ and the boundary conditions. Illustrative special cases include compressible axisymmetric flows and incompressible flows around a sphere, showing that viscosity and compressibility enhance the magnetization and lead to thicker boundary layers. Our method is especially useful for directly computing electric fields, and for addressing upstream magnetic fields that vary in spacetime. We thus estimate the electric fields above heliospheres and magnetospheres, compute the draping of magnetic substructure around a planetary body, and demonstrate the resulting inverse polarity reversal layer. Our analysis can be incorporated into existing hydrodynamic codes that are based on stream or path functions. (Abridged)

astro-ph.GA↗

arXiv2015

Shock Revival in Core-Collapse Supernovae: A Phase-Diagram Analysis

We examine the conditions for the revival of the stalled accretion shock in core-collapse supernovae, in the context of the neutrino heating mechanism. We combine one dimensional simulations of the shock revival process with a derivation of a quasi-stationary approximation, which is both accurate and efficient in predicting the flow. In particular, this approach is used to explore how the evolution of the system depends on the shock radius, $R_S$, and velocity, $V_S$ (in addition to other global properties of the system). We do so through a phase space analysis of the shock acceleration, $a_S$, in the $R_S-V_S$ plane, shown to provide quantitative insights into the initiation of runaway expansion and its nature. In the particular case of an initially stationary ($V_S=0,\;a_S=0$) profile, the prospects for an explosion can be reasonably assessed by the initial signs of the partial derivatives of the shock acceleration, in analogy to a linear damped/anti-damped oscillator. If $\partial a_S/\partial R_S<0$ and $\partial a_S/\partial V_S>0$, runaway expansion will likely occur after several oscillations, while if $\partial a_S/\partial R_S>0$, runaway expansion will commence in a non-oscillatory fashion. These two modes of runaway correspond to low and high mass accretion rates, respectively. We also use the quasi-stationary approximation to assess the advection-to-heating timescale ratio in the gain region, often used as an explosion proxy. Indeed, this ratio does tend to $\sim1$ in conjunction with runaway conditions, but neither this unit value nor the specific choice of the gain region as a point of reference appear to be distinct conditions in this regard.

astro-ph.HE↗

arXiv2015

Relativistic Shocks: Particle Acceleration and Magnetization

We review the physics of relativistic shocks, which are often invoked as the sources of non-thermal particles in pulsar wind nebulae (PWNe), gamma-ray bursts (GRBs), and active galactic nuclei (AGN) jets, and as possible sources of ultra-high energy cosmic-rays. We focus on particle acceleration and magnetic field generation, and describe the recent progress in the field driven by theory advances and by the rapid development of particle-in-cell (PIC) simulations. In weakly magnetized or quasi parallel-shocks (where the magnetic field is nearly aligned with the flow), particle acceleration is efficient. The accelerated particles stream ahead of the shock, where they generate strong magnetic waves which in turn scatter the particles back and forth across the shock, mediating their acceleration. In contrast, in strongly magnetized quasi-perpendicular shocks, the efficiencies of both particle acceleration and magnetic field generation are suppressed. Particle acceleration, when efficient, modifies the turbulence around the shock on a long time scale, and the accelerated particles have a characteristic energy spectral index of ~ 2.2 in the ultra-relativistic limit. We discuss how this novel understanding of particle acceleration and magnetic field generation in relativistic shocks can be applied to high-energy astrophysical phenomena, with an emphasis on PWNe and GRB afterglows.

astro-ph.HE↗

arXiv2014

Strong Magnetization Measured in the Cool Cores of Galaxy Clusters

Tangential discontinuities, seen as X-ray edges known as cold fronts (CFs), are ubiquitous in cool-core galaxy clusters. We analyze all 17 deprojected CF thermal profiles found in the literature, including three new CFs we tentatively identify (in clusters A2204 and 2A0335). We discover small but significant thermal pressure drops below all nonmerger CFs, and argue that they arise from strong magnetic fields below and parallel to the discontinuity, carrying 10%-20% of the pressure. Such magnetization can stabilize the CFs, and explain the CF-radio minihalo connection.

astro-ph.CO↗

arXiv2012

The missing asymptotic sector of rotating black-hole spectroscopy

The rotation of a black hole splits its spectrum in two, yet only one sector is known in the highly-damped regime. We find the second, at least partly oblate sector, with quasinormal modes approaching the total reflection frequencies ω(n>>1) ~ mΩ- 2πiT(n-s), where Ωand T are the horizon angular velocity and temperature, s is the field spin, and m is an azimuthal eigenvalue. Some physical implications are discussed.

gr-qc↗

arXiv2012

Critical Conditions for Core-Collapse Supernovae

The explosion of a core-collapse supernova can be approximated by the breakdown of steady-state solutions for accretion onto a proto-neutron star (PNS). We analytically show that as the neutrino luminosity exceeds a critical value L_c, the neutrinosphere pressure exceeds the hydrostatic limit even for an optimal shock radius R. This yields L_c \propto M^2 T^2 (with logarithmic corrections) and R \propto M/T, in agreement with numerical results, where M, T are the PNS mass, neutrino temperature. The near-critical flow can be approximated as a ballistic shell on top of an isothermal layer.

astro-ph.SR↗

arXiv2012

Spiral flows in cool-core galaxy clusters

We argue that bulk spiral flows are ubiquitous in the cool cores (CCs) of clusters and groups of galaxies. Such flows are gauged by spiral features in the thermal and chemical properties of the intracluster medium, by the multi-phase properties of CCs, and by X-ray edges known as cold fronts. We analytically show that observations of piecewise-spiral fronts impose strong constraints on the CC, implying the presence of a cold, fast flow, which propagates below a hot, slow inflow, separated by a slowly rotating, trailing, quasi spiral, tangential discontinuity surface. This leads to the nearly logarithmic spiral pattern, two-phase plasma, ρ\sim r^{-1} density (or T \sim r^{0.4} temperature) radial profile, and ~100kpc size, characteristic of CCs. By advecting heat and mixing the gas, such flows can eliminate the cooling problem, provided that a feedback mechanism regulates the flow. In particular, we present a quasi-steady-state model for an accretion-quenched, composite flow, in which the fast phase is an outflow, regulated by active galactic nucleus bubbles, reproducing the observed low star formation rates and explaining some features of bubbles such as their R_b \propto r size. The simplest two-component model reproduces several key properties of CCs, so we propose that all such cores harbor a spiral flow. Our results can be tested directly in the next few years, for example by ASTRO-H.

astro-ph.CO↗

arXiv2012

Interlayer Registry Determines the Sliding Potential of Layered Metal Dichalcogenides: The case of 2H-MoS2

We provide a simple and intuitive explanation for the interlayer sliding energy landscape of metal dichalcogenides. Based on the recently introduced registry index (RI) concept, we define a purely geometrical parameter which quantifies the degree of interlayer commensurability in the layered phase of molybdenum disulphide (2HMoS2). A direct relation between the sliding energy landscape and the corresponding interlayer registry surface of 2H-MoS2 is discovered thus marking the registry index as a computationally efficient means for studying the tribology of complex nanoscale material interfaces in the wearless friction regime.

cond-mat.mtrl-sci↗

arXiv2010

Using Radio Halos and Minihalos to Measure the Distributions of Magnetic Fields and Cosmic-Rays in Galaxy Clusters

Some galaxy clusters show diffuse radio emission in the form of giant halos (GHs) on Mpc scales or minihalos (MHs) on smaller scales. Comparing VLA and XMM radial profiles of several such clusters, we find a universal linear correlation between radio and X-ray surface brightness, valid in both types of halos. It implies a halo central emissivity nu*j_nu=10^{-31.4+-0.2}*(n/10^-2cm^-3)^2*(T/T_0)^{0.2+-0.5} erg/s/cm^3, where T and T_0 are the local and central temperatures, and n is the electron number density. We argue that the tight correlation and the scaling of j_nu, combined with morphological and spectral evidence, indicate that both GHs and MHs arise from secondary electrons and positrons, produced in cosmic-ray ion (CRI) collisions with a strongly magnetized, B >3 muG intracluster gas. When the magnetic energy density drops below that of the microwave background, the radio emission weakens considerably, producing halos with a clumpy morphology (e.g. RXCJ2003.5-2323 and A2255) or a distinct radial break. We thus measure a magnetic field B=3 muG at a radius r~110 kpc in A2029 and r~50 kpc in Perseus. The spectrum of secondaries, produced from hadronic collisions of ~20 GeV CRIs, reflects the energy dependence of the collision cross section. We use the observed spectra of halos, in particular where they steepen with increasing radius or frequency, to (i) measure B~10(nu/700 MHz) muG, with nu the spectral break frequency; (ii) identify a correlation between the average spectrum and the central magnetic field; and (iii) infer a CRI spectral index s <-2.7 and energy fraction xi_p~10^{-3.6+-0.2} at particle energies above 10 GeV. Our results favor a model where CRIs diffuse away from their sources (which are probably supernovae, according to a preliminary correlation with star formation), whereas the magnetic fields are generated by mergers in GHs and by core sloshing in MHs.

astro-ph.CO↗

arXiv2010

Common origin for radio relics and halos: galaxy cluster-wide, homogeneous cosmic-ray distribution, and evolving magnetic fields

Some galaxy clusters show diffuse radio emission in the form of peripheral relics (so far attributed to primary, shock-(re)accelerated electrons) or central halos. Analysing radio and X-ray data from the literature, we find new connections between halos and relics, such as a universal linear relation between their peak radio brightness and the gas column density. Our results indicate that halos, relics, and halo-relic bridges in a cluster, all arise from the same, homogeneous cosmic ray (CR) ion (CRI) distribution. We analytically derive the signature of synchrotron emission from secondary electrons and positrons (CREs) produced in hadronic CRI collisions, for an arbitrary magnetic field evolution. In our model, flat spectrum halos (both giant and minihalos) arise from steady-state magnetic fields, whereas relics and steep halos reflect recent or irregular magnetic growth. This naturally explains the properties of halos, relics, and the connections between them, without invoking particle (re)acceleration in weak shocks or turbulence. We find CRI energy densities in the range u_p~10^-[12.4,13.3]erg cm^-3, with a spectral index s_p=-2.20+-0.05, and identify a ~0.1 magnetic energy fraction in some halos and behind relics, as far as 2Mpc from the cluster's centre. The CRI homogeneity suggests strong CR diffusion, D(100GeV)>~10^32cm^2s^-1. The strong magnetisation imposes strict upper limits on >10GeV CRE (re)acceleration in weak shocks (efficiency <10^-4) and turbulence; indeed, each weak shock slightly lowers the energy fraction of flat CRs.

astro-ph.HE↗

arXiv2010

Dynamics and Magnetization in Galaxy Cluster Cores Traced by X-ray Cold Fronts

Cold fronts (CFs) - density and temperature plasma discontinuities - are ubiquitous in cool cores of galaxy clusters, where they appear as X-ray brightness edges in the intracluster medium, nearly concentric with the cluster center. We analyze the thermodynamic profiles deprojected across core CFs found in the literature. While the pressure appears continuous across these CFs, we find that all of them require significant centripetal acceleration beneath the front. This is naturally explained by a tangential, nearly sonic bulk flow just below the CF, and a tangential shear flow involving a fair fraction of the plasma beneath the front. Such shear should generate near-equipartition magnetic fields on scales ~<50 pc from the front, and could magnetize the entire core. Such fields would explain the apparent stability of cool-core CFs and the recently reported CF-radio minihalo association.

astro-ph.CO↗

arXiv2010

Cold Fronts from Shock Collisions

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of collisions between trailing shocks. Such a collision typically results in a spherical, factor ~1.4-2.7 density/temperature discontinuity. These CFs may be found as far as the virial shock, unlike in other CF formation models. As a demonstration of this effect, we use one dimensional simulations where halo reverberations involving periodic collisions between the virial shock and outgoing secondary shocks exist. These collisions yield a distinctive, concentric geometric sequence of CFs which trace the expansion of the virial shock.

astro-ph.CO↗

arXiv2010

Cold Fronts by Merging of Shocks

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of merging between two shocks propagating in the same direction. Such shock mergers typically result in a quasi-spherical, factor ~1.4-2.7 discontinuity in density and in temperature. These CFs may be found as far out as the virial shock, unlike what is expected in other CF formation models. As a demonstration of this effect, we use one dimensional simulations of clusters and show that shock induced cold fronts form when perturbations such as explosions or mergers occur near the cluster's centre. Perturbations at a cluster's core induce periodic merging between the virial shock and outgoing secondary shocks. These collisions yield a distinctive, concentric, geometric sequence of CFs which trace the expansion of the virial shock.

astro-ph.CO↗