arXiv subjects

Uri Keshet

Publications and source records attributed to Uri Keshet.

At least 37 records · Page 2Linked to original sources

arXiv2021

Nonlinear electromagnetic-wave interactions in pair plasma: (I) Non-relativistic regime

This paper is the first in a series devoted to the numerical study of nonlinear interactions of electromagnetic waves with plasma. We start with non-magnetized pair plasmas, where the primary processes are induced (Compton) scattering and the filamentation instability. In this paper, we consider waves in which electron oscillations are non-relativistic. Here, the numerical results can be compared to analytical theory, facilitating the development of appropriate numerical tools and framework. We distill the analytic theory, reconciling plasma and radiative transfer pictures of induced scattering and developing in detail the kinetic theory of modulation/filamentation instability. We carry out homogeneous numerical simulations using the particle-in-cell codes EPOCH and Tristan-MP, for both monochromatic waves and wave packets. We show that simulations of both processes are consistent with theoretical predictions, setting the stage for analyzing the highly nonlinear regime.

astro-ph.HE↗

arXiv2021

Fermi bubbles: the collimated outburst needed to explain forward-shock edges

The bipolar, nonthermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modeling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at $z\simeq100$ pc heights in our model) with a half-opening angle $θ\simeq4^\circ$, a normalized velocity $β_{-2}\equiv v/(0.01c)\gtrsim 0.4$, and an energy $E\gtrsim2β_{-2}^2\times 10^{55}$ erg, launched $\mathbb{T}\simeq 3.3β_{-2}^{-1}$ Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller $E/(β_{-2}θ)^{2}$, and a comparable $\mathbb{T}$, and follow a ballistic stage that must reach a height $z_{s}\gtrsim 5$ kpc.

astro-ph.HE↗

arXiv2021

Protruding bullet heads indicating dark matter pull

A clump moving through the intracluster medium of a galaxy cluster can drive a bow shock trailed by a bullet-like core. In some cases, such as in the prototypical Bullet cluster, X-rays show a gas bullet with a protruding head and pronounced shoulders. We point out that these features, while difficult to explain without dark matter (DM), naturally arise as the head of the slowed-down gas is gravitationally pulled forward toward its unhindered DM counterpart. X-ray imaging thus provides a unique, robust probe of the offset, collisionless DM, even without gravitational lensing or other auxiliary data. Numerical simulations and a toy model suggest that the effect is common in major mergers, is often associated with a small bullet-head radius of curvature, and may lead to distinct bullet morphologies, consistent with observations.

astro-ph.CO↗

arXiv2020

Maximally-hard spectra from diffusive shock-acceleration in pulsar-wind nebulae

The processes leading to the exceptionally hard radio spectra of pulsar-wind nebulae (PWNe) are not yet understood. Radio photon spectral indices among $29$ PWNe from the literature show an approximately normal, $α=0.2\pm0.2$ distribution. We present $\sim 3σ$ evidence for a distinct sub-population of PWNe, with a hard spectrum $α=0.01\pm0.06$ near the termination shock and significantly softer elsewhere, possibly due to a recent evacuation of the shock surroundings. Such spectra, especially in the hard sub-population, suggest a Fermi process, such as diffusive shock acceleration, at its extreme, $α=0$ limit. We show that this limit is approached for sufficiently anisotropic small-angle scattering, enhanced on either side of the shock for particles approaching the shock front. In the upstream, the spectral hardening is mostly associated with an enhanced energy gain, possibly driven by the same beamed particles crossing the shock. Downstream, the main effect is a diminished escape probability, but this lowers the acceleration efficiency to $\lesssim25\%$ for $α=0.3$ and $\lesssim1\%$ for $α=0.03$.

astro-ph.HE↗

arXiv2020

Compressible potential flows around round bodies: Janzen-Rayleigh expansion inferences

The subsonic, compressible, potential flow around a hypersphere can be derived using the Janzen-Rayleigh expansion (JRE) of the flow potential in even powers of the incident Mach number $\mathcal{M}_\infty$. JREs were carried out with terms polynomial in the inverse radius $r^{-1}$ to high orders in two dimensions (2D), but were limited to order $\mathcal{M}_\infty^4$ in three dimensions (3D). We derive general JRE formulae to arbitrary order, adiabatic index, and dimension. We find that powers of $\ln(r)$ can creep into the expansion, and are essential in 3D beyond order $\mathcal{M}_\infty^4$. Such terms are apparently absent in the 2D disk, as we confirm up to order $\mathcal{M}_\infty^{100}$, although they do show in other dimensions (e.g. at order $\mathcal{M}_\infty^2$ in 4D) and in non-circular 2D bodies. This suggests that the disk, which was extensively used to study basic flow properties, has additional symmetry. Our results are used to improve the hodograph-based approximation for the flow in front of a sphere. The symmetry-axis velocity profiles of axisymmetric flows around different prolate spheroids are approximately related to each other by a simple, Mach-independent scaling.

physics.flu-dyn↗

arXiv2020

Diffusive shock acceleration in $N$ dimensions

Collisionless shocks are often studied in two spatial dimensions (2D), to gain insights into the 3D case. We analyze diffusive shock acceleration for an arbitrary number $N\in\mathbb{N}$ of dimensions. For a non-relativistic shock of compression ratio $\mathcal{R}$, the spectral index of the accelerated particles is $s_E=1+N/(\mathcal{R}-1)$; this curiously yields, for any $N$, the familiar $s_E=2$ (i.e., equal energy per logarithmic particle energy bin) for a strong shock in a mono-atomic gas. A precise relation between $s_E$ and the anisotropy along an arbitrary relativistic shock is derived, and is used to obtain an analytic expression for $s_E$ in the case of isotropic angular diffusion, affirming an analogous result in 3D. In particular, this approach yields $s_E = (1+\sqrt{13})/2 \simeq 2.30$ in the ultra-relativistic shock limit for $N=2$, and $s_E(N\to\infty)=2$ for any strong shock. The angular eigenfunctions of the isotropic-diffusion transport equation reduce in 2D to elliptic cosine functions, providing a rigorous solution to the problem; the first function upstream already yields a remarkably accurate approximation. We show how these and additional results can be used to promote the study of shocks in 3D.

astro-ph.HE↗

arXiv2020

Advection by large-scale spiral flows in galaxy clusters

The intracluster medium of a galaxy cluster often shows an extended quasi-spiral structure, accentuated by tangential discontinuities known as cold fronts (CFs). These discontinuities are thought to isolate between low-entropy, high-metallicity gas inside (i.e., below) the CF that was lifted from the center of the cluster by some radial factor $f_i$, and high-entropy, low-metallicity gas outside the CF that was pushed inward by a factor $f_o$. We find broad support for such a picture, by comparing the entropy and metallicity discontinuities with the respective azimuthal averages, using newly deprojected thermal profiles in clusters A2029, A2142, A2204 and Centaurus, supplemented by deprojected CFs from the literature. In particular, the mean advection factors $f_K$ and $f_Z$, inferred respectively from entropy and metallicity, strongly correlate ($\mathcal{R}=0.7^{+0.2}_{-0.3}$) with each other, consistent with large-scale advection. However, unlike sloshing simulations, in which the inside/outside phases are an inflow/outflow settling back to equilibrium after a violent perturbation, our results are more consistent with an outflow/inflow, with the fast, Mach $\mathcal{M}_i\sim0.8$ gas inside the CF being a rapidly heated or mixed outflow, probably originating from the cD galaxy, and gas outside the CF being a $\mathcal{M}_o\sim0.03$, slowly-cooling inflow. In particular, entropy indicates an outside advection factor $f_{Ko}\simeq 1.33\pm0.04$ that is approximately constant in all CFs, gauging the distance traversed by inflowing gas within a cooling time. In contrast, $1.1\lesssim f_{Ki}\lesssim 2.5$ and $1\lesssim f_Z\lesssim 17$ vary considerably among clusters, and strongly correlate ($3.1σ\text{-}4.2σ$) with the virial mass, $f_{Ki}\propto M_{200}^{0.14\pm0.07}$ and $f_Z\propto M_{200}^{1.4\pm0.4}$, suggesting that each cluster sustains a quasi-steady spiral flow.

astro-ph.HE↗

arXiv2020

Deprojecting galaxy-cluster cold fronts: evidence for bulk, magnetised spiral flows

Tangential discontinuities known as cold fronts (CFs) are abundant in groups and clusters of galaxies (GCs). The relaxed, spiral-type CFs were initially thought to be isobaric, but a significant, $10\%$--$20\%$ jump in the thermal pressure $P_t$ was reported when deprojected CFs were stacked, interpreted as missing $P_t$ below the CFs (i.e. at smaller radii $r$) due to a locally-enhanced nonthermal pressure $P_{nt}$. We report a significant ($\sim4.3σ$) deprojected jump in $P_t$ across a single sharp CF in the Centaurus cluster. Additional seven CFs are deprojected in the GCs A2029, A2142, A2204, and Centaurus, all found to be consistent (stacked: $\sim1.9σ$) with similar pressure jumps. Combining our sample with high quality deprojected CFs from the literature indicates pressure jumps at significance levels ranging between $2.7σ$ and $5.0σ$, depending on assumptions. Our nominal results are consistent with $P_{nt}\simeq (0.1\mbox{--}0.3)P_t$ just below the CF. We test different deprojection and analysis methods to confirm that our results are robust, and show that without careful deprojection, an opposite pressure trend may incorrectly be inferred. Analysing all available deprojected data, we also find: (i) a nearly constant CF contrast $q$ of density and temperature within each GC, monotonically increasing with the GC mass $M_{200}$ as $q\propto M_{200}^{0.23\pm0.04}$; (ii) hydrostatic mass discontinuities indicating fast bulk tangential flows below all deprojected CFs, with a mean Mach number $\sim0.76$; and (iii) the newly deprojected CFs are consistent (stacked: $\sim2.9σ$) with a $1.25^{+0.09}_{-0.08}$ metallicity drop across the CF. These findings suggest that GCs quite generally harbor extended spiral flows.

astro-ph.HE↗

arXiv2020

Coincident SZ and $γ$-ray signals from cluster virial shocks

Virial shocks around galaxy clusters are expected to show a cutoff in the thermal Sunyaev-Zel'dovich (SZ) signal, coincident with a leptonic ring. However, until now, leptonic virial signals were reported only in Coma and in stacked Fermi-LAT clusters, and an SZ virial shock signal was reported only in A2319. We point out that a few clusters --- presently Coma, A2319, and A2142 --- already show a sharp drop in Planck SZ pressure near the virial radius, coincident with a weak LAT $γ$-ray excess. These signatures are shown to trace the virial shocks of the clusters, at joint medium to high confidence levels. The electron acceleration rates inferred from $γ$-rays are consistent with previous measurements. The combined signal allows a separate measurement of the $\sim0.5\%$ acceleration efficiency and of the accretion rate. Lower limits of order a few are imposed on the shock Mach numbers.

astro-ph.CO↗

arXiv2019

Diffusive acceleration in relativistic shocks: particle feedback

The spectral index $s$ of particles diffusively accelerated in a relativistic shock depends on the unknown angular diffusion function $\mathcal{D}$, which itself depends on the particle distribution function $f$ if acceleration is efficient. We develop a relaxation code to compute $s$ and $f$ for an arbitrary functional $\mathcal{D}$ that depends on $f$. A local $\mathcal{D}(f)$ dependence is motivated and shown, when rising (falling) upstream, to soften (harden) $s$ with respect to the isotropic case, shift the angular distribution towards upstream (downstream) directions, and strengthen (weaken) the particle confinement to the shock; an opposite effect on $s$ is found downstream. However, variations in $s$ remain modest even when $\mathcal{D}$ is a strong function of $f$, so the standard, isotropic-diffusion results remain approximately applicable unless $\mathcal{D}$ is both highly anisotropic and not a local function of $f$. A mild, $\sim 0.1$ softening of $s$, in both 2D and 3D, when $\mathcal{D}(f)$ rises sufficiently fast, may be indicated by ab-initio simulations.

astro-ph.HE↗

arXiv2019

Diffusive shock-acceleration: breakdown of spatial diffusion and isotropy

We point out that particles accelerated in a non-relativistic shock of compression ratio $r$ attain the standard, $p=(r+2)/(r-1)$ spectral index only under certain conditions. Previous derivations of the spectrum, based on the approximations of spatial diffusion or negligible anisotropy, are shown to be unjustified for a general scattering function. We explain and demonstrate numerically that in contrast to previous claims, $p$ can substantially deviate from the standard result for anisotropic scattering. We prove analytically that the standard approach is nevertheless valid in the limit of an isotropic medium. Additional spectral modifications, for example by motions of scattering modes at intermediate optical depths from the shock, are discussed.

astro-ph.HE↗

arXiv2018

Detection of virial shocks in stacked Fermi-LAT clusters

Galaxy clusters are thought to grow by accreting mass through large-scale, strong, yet elusive, virial shocks. Such a shock is expected to accelerate relativistic electrons, thus generating a spectrally-flat leptonic virial-ring. However, until now, only the nearby Coma cluster has shown evidence for a $γ$-ray virial ring. We stack Fermi-LAT data for the 112 most massive, high latitude, extended clusters, enhancing the ring sensitivity by rescaling clusters to their virial radii and utilizing the expected flat energy spectrum. In addition to a central unresolved, hard signal (detected at the $\sim 5.8σ$ confidence level), probably dominated by AGN, we identify (at the $5.8σ$ confidence level) a bright, spectrally-flat $γ$-ray ring at the expected virial shock position. The ring signal implies that the shock deposits $\sim 0.6\%$ (with an interpretation uncertainty factor $\sim2$) of the thermal energy in relativistic electrons over a Hubble time. This result, consistent with the Coma signal, validates and calibrates the virial shock model, and indicates that the cumulative emission from such shocks significantly contributes to the diffuse extragalactic $γ$-ray and low-frequency radio backgrounds.

astro-ph.HE↗

arXiv2017

Detection of virial shocks in stacked Fermi-LAT clusters

In the hierarchical paradigm of structure formation, galaxy clusters are the largest objects ever to virialize. They are thought to grow by accreting mass through large scale, strong virial shocks. Such a collisionless shock is expected to accelerate relativistic electrons, thus generating a spectrally flat leptonic virial ring. However attempts to detect virial rings have all failed, leaving the shock paradigm unconfirmed. Here we identify a virial $γ$-ray signal by stacking Fermi-LAT data for 112 clusters, enhancing the ring sensitivity by rescaling clusters to their virial radii and utilizing the anticipated spectrum. In addition to a central unresolved, hard signal (detected at the nominal $5.8σ$ confidence level), probably dominated by active galactic nuclei, we identify ($5.9σ$) a bright, spectrally flat $γ$-ray ring at the expected shock position. It corresponds to $\sim 0.6\%$ (with an uncertainty factor $\sim2$) thermal energy deposition in relativistic electrons over a Hubble time. This result validates the shock paradigm, calibrates its parameters, and indicates that the cumulative emission from such shocks significantly contributes to the diffuse extragalactic $γ$-ray and radio backgrounds.

astro-ph.HE↗

arXiv2017

Analytic study of 1D diffusive relativistic shock acceleration

Diffusive shock acceleration (DSA) by relativistic shocks is thought to generate the $dN/dE\propto E^{-p}$ spectra of charged particles in various astronomical relativistic flows. We show that for test particles in one dimension (1D), $p^{-1}=1-\ln\left[γ_d(1+β_d)\right]/\ln\left[γ_u(1+β_u)\right]$, where $β_u$ ($β_d)$ is the upstream (downstream) normalized velocity, and $γ$ is the respective Lorentz factor. This analytically captures the main properties of relativistic DSA in higher dimensions, with no assumptions on the diffusion mechanism. Unlike 2D and 3D, here the spectrum is sensitive to the equation of state even in the ultra-relativistic limit, and (for a J{ü}ttner-Synge equation of state) noticeably hardens with increasing $1<γ_u<57$, before logarithmically converging back to $p(γ_u\to\infty)=2$. The 1D spectrum is sensitive to drifts, but only in the downstream, and not in the ultra-relativistic limit.

astro-ph.HE↗

arXiv2017

IceCube constraints on the Fermi Bubbles

We analyze the IceCube four-year neutrino data in search of a signal from the Fermi bubbles. No signal is found from the bubbles or from their dense shell, even when taking into account the softer background. This imposes a conservative $ξ_i<8\%$ upper limit on the cosmic-ray ion (CRI) acceleration efficiency, and an $η\equiv ξ_e/ξ_i \gtrsim0.006$ lower limit on the electron-to-ion ratio of acceleration efficiencies (at the $2σ$ confidence level). For typical $ξ_i$, a signal should surface once the number of IceCube neutrinos increases by $\sim$an order of magnitude, unless there is a $<$PeV cutoff on the CRI spectrum.

astro-ph.HE↗

arXiv2017

Preliminary evidence for a virial shock around the Coma galaxy cluster

Galaxy clusters, the largest gravitationally bound objects in the Universe, are thought to grow by accreting mass from their surroundings through large-scale virial shocks. Due to electron acceleration in such a shock, it should appear as a $γ$-ray, hard X-ray, and radio ring, elongated towards the large-scale filaments feeding the cluster, coincident with a cutoff in the thermal Sunyaev-Zel'dovich (SZ) signal. However, no such signature was found until now, and the very existence of cluster virial shocks has remained a theory. We find preliminary evidence for a large, $\sim 5$ Mpc minor axis $γ$-ray ring around the Coma cluster, elongated towards the large scale filament connecting Coma and Abell 1367, detected at the nominal $2.7σ$ confidence level ($5.1σ$ using control signal simulations). The $γ$-ray ring correlates both with a synchrotron signal and with the SZ cutoff, but not with Galactic tracers. The $γ$-ray and radio signatures agree with analytic and numerical predictions, if the shock deposits $\sim 1\%$ of the thermal energy in relativistic electrons over a Hubble time, and $\sim 1\%$ in magnetic fields. The implied inverse-Compton and synchrotron cumulative emission from similar shocks can significantly contribute to the diffuse extragalactic $γ$-ray and low frequency radio backgrounds. Our results, if confirmed, reveal the prolate structure of the hot gas in Coma, the feeding pattern of the cluster, and properties of the surrounding large scale voids and filaments. The anticipated detection of such shocks around other clusters would provide a powerful new cosmological probe.

astro-ph.CO↗

arXiv2017

X-ray to gamma-ray virial shock signal from the Coma cluster

Following evidence for an east--west elongated virial ring around the Coma galaxy cluster in a $\sim220$ GeV VERITAS mosaic, we search for corresponding signatures in $>$GeV $γ$-rays from Fermi-LAT, and in soft, $\sim0.1$ keV X-rays from ROSAT. For the ring elongation and orientation inferred from VERITAS, we find a $3.4σ$ LAT excess, and detect ($>5σ$) the expected signature in ROSAT bands R1 and R1+R2. The significances of both LAT and ROSAT signals are maximal near the VERITAS ring parameters. The intensities of the ROSAT, Fermi, and VERITAS signals are consistent with the virial shock depositing $\sim0.3\%$ (with an uncertainty factor of $\sim3$) of its energy over a Hubble time in a nearly flat, $p\equiv - d\ln N_e/d\ln E\simeq 2.0$--$2.2$ spectrum of cosmic-ray electrons. The sharp radial profiles of the LAT and ROSAT signals suggest preferential accretion in the plane of the sky, as indicated by the distribution of neighboring large-scale structure. The X-ray signal gauges the compression of cosmic-rays as they are advected deeper into the cluster.

astro-ph.HE↗

arXiv2017

Fermi bubbles: high latitude X-ray supersonic shell

The nature of the bipolar, $γ$-ray Fermi bubbles (FB) is still unclear, in part because their faint, high-latitude X-ray counterpart has until now eluded a clear detection. We stack ROSAT data at varying distances from the FB edges, thus boosting the signal and identifying an expanding shell behind the southwest, southeast, and northwest edges, albeit not in the dusty northeast sector near Loop I. A Primakoff-like model for the underlying flow is invoked to show that the signals are consistent with halo gas heated by a strong, forward shock to $\sim$keV temperatures. Assuming ion--electron thermal equilibrium then implies a $\sim10^{56}$ erg event near the Galactic centre $\sim7$ Myr ago. However, the reported high absorption-line velocities suggest a preferential shock-heating of ions, and thus more energetic ($\sim 10^{57}$ erg), younger ($\lesssim 3$ Myr) FBs.

astro-ph.HE↗