arXiv subjects

Uri Keshet

Publications and source records attributed to Uri Keshet.

At least 19 recordsLinked to original sources

arXiv2026

Fermi bubbles detected in $\sim$100 TeV neutrinos

The Fermi bubbles have been identified as a collimated bipolar outflow emanating from the Milky-Way center, tracing strong forward shocks which extend $\sim10$ kpc from the Galactic disk, originating from a $\sim10^{56\pm1}$ erg outburst. These shocks are sufficiently strong, extended, and energetic to produce a detectable flux of $\gtrsim 10$ TeV neutrinos, especially in their denser north and east, but early IceCube data were insufficient for identifying the signal. We find that IceCube high-energy starting events (HESE 12-year) correlate with the $\textit{Fermi}$-LAT sky map outside the Galactic plane ($>3\sigma$). Testing for neutrinos coincident with the bubble shells, localized using $\textit{eROSITA}$ data, we detect ($>4\sigma$) both bubbles at high ($|b|>30^{\circ}$) latitudes, with a local excess ($>5\sigma$) mainly in their X-ray bright eastern shells. The signal matches the anticipated secondaries of relativistic ions carrying $\sim 10^{54.5}$ erg (with factor $\sim3$ uncertainty) in each bubble, shock-accelerated to $>$PeV energies. The results verify the strength of the shocks, suggesting an ion acceleration efficiency of order $\sim10\%$. We also present preliminary evidence for neutrinos from the even larger shells of the eROSITA bubbles, which encapsulate their younger Fermi-bubble counterparts, carrying a similar energy and confined by shocks nearly as strong.

astro-ph.HE↗

arXiv2026

Galaxy-cluster-stacked Fermi-LAT, part IV: $\sim70$ GeV WIMP annihilation lines

The strongest constraints on the velocity-dependent ($p$-wave) annihilation of weakly interacting massive particle (WIMP) dark matter were derived from the deep potential wells of galaxy clusters. Even weaker signals can be extracted from sufficient aggregated clusters, by cross-correlating $\gamma$-rays with large-scale structure tracers or stacking over extensive cluster catalogs. Three independent such analyses show a similar triad of emission lines in Fermi-LAT data, around 70, 40, and 13 GeV, emerging from featureless spectra in each hemisphere upon cross-correlation with eROSITA maps, and in stacked MCXC, eROSITA, and DESI catalog clusters only once boosted to the cluster frame. These lines fit the anticipated $\chi\chi\to\gamma\gamma$, $\gamma Z$, and $\gamma h$ annihilation channels of a $\sim70$ GeV WIMP $\chi$, detected by composite matched filters at trial-corrected global $Z$-scores reaching $5.6\sigma$ (cross-correlations) and $2.3\sigma$ (stacking), with intrinsic $\sim10^{[-20,-19]}$ cm$^3$ s$^{-1}$ channel cross-sections. High-resolution spectra establish six lines and an unresolved ($2$--$3$ line) feature in total, naturally aligned with the anticipated nine channels of two cross-annihilating WIMPs of masses $67.3_{-0.1}^{+0.1}$ and $71.4_{-0.1}^{+0.2}$ GeV (profile-likelihood bounds; $_{-5\%}^{+3\%}$ systematic; $5.3\sigma$). The Galactic-center GeV excess is broadly consistent with the corresponding broad annihilation continuum.

astro-ph.HE↗

arXiv2026

Nested Fermi and eROSITA bubbles require very similar $\sim10^{56}$ erg collimated Galactic-center outbursts; their asymmetry indicates an eastern density gradient

Observations indicate two nested pairs of extended bipolar bubbles emanating from the Milky-Way center - the $|b|\sim80^\circ$ latitude eROSITA bubbles (RBs), encompassing the smaller, $|b|\sim 50^{\circ}$ Fermi bubbles (FBs) - and classify the edges of both bubble pairs as strong forward shocks. Identifying each bubble pair as driven by a distinct, collimated outburst, we evolve these bubbles and constrain their origin using a stratified 1D model verified by a suite of 2D and 3D hydrodynamic simulations which reproduce X-ray observations. While the RBs are at the onset of slowdown, the FBs are still expanding ballistically into the RB-shocked medium. Observational constraints indicate that both RB and FB outbursts had (up to factor $\sim2$-$4$ uncertainties) $\sim4^\circ$ half-opening angles and $\sim 2000$ km s$^{-1}$ velocities $100$ pc from their base, carrying $\sim10^{56}$ erg. The FBs and RBs could thus arise from identical outbursts separated by $\sim10$ Myr; their longitudinal asymmetry favors an eastern ambient-density gradient over western wind suggestions.

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arXiv2026

Mach $>3$ shocks at the tips of both eROSITA bubbles

eROSITA substantiated earlier indications that Loop-I is the northern part of an extended bipolar Galactic-bubble structure, but the southern bubble was not established in nonthermal emission and the shock strength was not robustly measured in either bubble. After using eROSITA data to map the bubble edges, we analyzed edge-adjacent radio and $\gamma$-ray data to remove foregrounds, test if the southern bubble can be detected in nonthermal emission, and measure the corresponding high-latitude spectra of both bubbles. Data were stacked parallel to the eROSITA bubble edges traced by an edge detector, in the same method used previously to pick up weak signals in the smaller, nested Fermi bubbles; the detected brightness jumps were then used to measure the spectrum. We detect ($>5\sigma$) both bubble tips in both radio and $\gamma$-rays, and find a radio spectrum corresponding to high, Mach $3$-$5$ shocks. The southern bubble is fainter, by $\sim$an order of magnitude in radio, its edge propagating into a medium roughly half as dense. The results indicate that these eROSITA bubbles are older, evolved counterparts of the Fermi bubbles, arising from an earlier collimated high-energy outburst from the Galactic center.

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arXiv2025

Cavitation instability in unmagnetized relativistic pair shocks

We investigate the formation of plasma cavities in unmagnetized relativistic pair shocks and demonstrate that these cavities emerge naturally as a nonlinear consequence of asymmetric Weibel instability. We provide an intuitive physical picture and a minimal fluid model that captures the essential features of this cavitation instability and compare them with PIC results. This mechanism may provide the missing link between kinetic Weibel turbulence and macroscopic magnetic fields in astrophysical shocks.

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arXiv2025

Galaxy-cluster-stacked Fermi-LAT III: substructure and radio-relic counterparts

Faint $\gamma$-ray signatures emerge in Fermi-LAT data stacked scaled to the characteristic $R_{500}$ radii of MCXC galaxy clusters. This third paper in a series shows a $4.3\sigma$ excess of discrete 4FGL-DR4 catalog $\gamma$-ray sources at the $r<1.5R_{500}$ radii of 205 clusters, coincident with an $r\sim R_{500}$ diffuse $2.6\sigma$ excess of 1-100 GeV emission from 75 high-latitude clusters. The source excess becomes highly ($>5\sigma$) significant when considering the substantial ($3.4\sigma$) and unexpectedly rapid quenching of $\gamma$-ray sources just inside the virial shock. The excess sources show radial, spectral, and luminosity distributions better matching radio-relic counterparts or substructure than present tentative classifications as blazar-candidates. Their spectral distribution is bimodal: flat-spectrum sources are consistent with enhanced hadronic emission behind weak, Mach $\sim2$ shocks, while softer sources may be phoenix counterparts.

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arXiv2025

Relativistic ions with power-law spectra explain radio phoenixes

Radio phoenixes are filamentary sources in the intracluster medium (ICM) of galaxy clusters, often extending over $>100$ kpc, arising from fossil radio lobes. Their soft, curved spectrum is widely attributed to aged relativistic electrons recently accelerated or compressed, but at high frequencies is shown to approach a power-law. Moreover, the full, curved spectrum is naturally reproduced by secondary $e^{\pm}$ from a pure power-law spectrum of relativistic ions, radiating in highly-magnetized filaments; this model provides a better fit to all phoenixes, with only three free parameters. Weaker magnetization shifts the curvature to low frequencies, explaining pure power-law phoenixes. Hadronic high-curvature phoenixes require $e^{\pm}$ heating, by a factor $\gtrsim 15$ if at ICM pressure. The $\sim$keV Compton- and $\sim$GeV $\pi^0$-decay-peaked counterparts of hadronic phoenixes may be detectable as non-thermal X-rays and $\gamma$-rays.

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arXiv2025

Galaxy-cluster-stacked Fermi-LAT II: extended central hadronic signal

Faint $\gamma$-ray signatures emerge in Fermi-LAT data stacked scaled to the characteristic $\theta_{500}$ angles of MCXC galaxy clusters. After Paper I of this series thus discovered virial shocks, later supported in other bands, this second paper focuses on cluster cores. Stacking $1$-$100$ GeV source-masked data around clusters shows a significant ($4.7\sigma$ for 75 clusters) and extended central excess, inconsistent with central point sources. The resolved signal is best fit ($3.7\sigma$ TS-test) as hadronic emission from a cosmic-ray ion (CRI) distribution that is flat both spectrally ($p\equiv1-d\ln u/d\ln E=2.0\pm0.3$) and spatially (CRI-to-gas index $\sigma=0.1\pm0.4$), carrying an energy density $du(0.1\theta_{500})/d\ln E=10^{-13.6\pm0.5}$ erg cm$^{-3}$ at $E=100$ GeV energy; insufficient resolution would raise $p$ and $\sigma$. Such CRI match the long-predicted distribution needed to power diffuse intracluster radio emission in its various forms (mini-halos, giant halos, standard relics, their transitional forms, and mega-halos), disfavoring models that invoke electron (re)acceleration in weak shocks or turbulence. Stringent upper limits on residual $\gamma$-ray emission, e.g. from dark-matter annihilation, are imposed.

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arXiv2024

Macroscopic magnetization of primordial plasma by virial shocks

Galaxy-cluster virial (structure-formation accretion) shock observations are shown to imply $\gtrsim1\%$ magnetization of a layer extending $\gtrsim10^{16}$ Debye lengths downstream, challenging the modelling of high Alfv\'en-Mach collisionless shocks. Unlike similar shocks in supernova remnants or relativistic shocks in $\gamma$-ray burst afterglows, where macroscopic magnetized layers were detected but purportedly attributed to preexisting or non-resonant cosmic-ray streaming-seeded substructure, the upstream of strong virial shocks is both weakly magnetized and pristine. Hence, some mechanism must generate large-scale and possibly self-similar magnetic sub-structure out of the accreted primordial plasma; such a mechanism may dominate other high-Mach shock systems, too.

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arXiv2024

Radially polarized synchrotron from galaxy-cluster virial shocks

Radio-to-$\gamma$-ray signals, recently found narrowly confined near the characteristic $2.4R_{500}$ scaled radii of galaxy clusters and groups, have been associated with their virial (structure-formation accretion) shocks based on spectro-spatial characteristics. By stacking high-latitude GMIMS radio data around MCXC galaxy clusters, we identify ($3\sigma$--$4\sigma$) excess radially polarized emission at the exact same scaled radius, providing directional support, and indicating tangential magnetic fields induced by the shocked inflow. The results suggest a strong mass dependence, a flat energy spectrum, and a high polarization fraction, consistent with synchrotron emission from electrons accelerated by strong virial shocks. The narrow radial range of such stacked virial-shock signals suggests that although the shocks are theorized to have diverse, irregular morphologies, they share similar $\sim2.4R_{500}$ minimal radii.

astro-ph.CO↗

arXiv2024

Coma cluster $\gamma$-ray and radio emission is consistent with a secondary electron origin for the radio halo

Observations of diffuse, non-thermal radio emission spanning several megaparsecs have been documented in over 100 galaxy clusters. This emission, classified as giant radio halos (GHs), mini halos, and radio relics based mainly on their location and morphology, is interpreted as synchrotron radiation and implies the presence of relativistic electrons and magnetic fields in the intra-cluster medium (ICM). GHs were initially thought to be generated by secondary electrons resulting from inelastic $p+p\rightarrow X+\pi^{\pm}$ collisions. However, recent literature has leaned towards primary-electron turbulent (re)acceleration models, partly due to claimed upper limits on the $\gamma$-ray emission from $\pi^0$ decay. We demonstrate that the observed GH and $\gamma$-ray flux in the Coma cluster are consistent with a secondary origin for the GH across a broad range of magnetic field values. Although the constraints on magnetic field configuration are not stringent, they align well with previous estimates for Coma. Within this magnetic field range, the energy density of cosmic-ray protons (CRp) constitutes a few percent to tens of percent of the ICM energy density, as predicted and observed for a sample of radio-emitting galaxy clusters. Notably, we detect a rise in the ratio of CRp to ICM energy densities towards the outer regions of the cluster. This phenomenon was anticipated to arise from either adiabatic compression of CRp accelerated by accretion shocks or, more likely, from strong CRp diffusion.

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arXiv2024

Galaxy cluster virial-shock sources in eROSITA catalogs

Following the recent identification of discrete ROSAT and radio sources associated with the virial shocks of MCXC clusters and groups, we examine if the early eROSITA-DE data release (EDR) shows virial-shock X-ray sources within its $140$ deg$^2$ field. EDR catalog sources are stacked and radially binned around EDR catalog clusters and groups. The properties of the excess virial-shock sources are inferred statistically by comparing the virial-shock region to the field. An excess of X-ray sources is found narrowly localized at the $2.0<r/R_{500}<2.25$ normalized radii, just inside the anticipated virial shocks, of the resolved 532 clusters, for samples of both extended ($3\sigma$ for 534 sources) or bright ($3.5\sigma$ for 5820 sources; $4\sigma$ excluding the low cluster-mass quartile) sources. The excess sources are on average extended ($\sim 100$ kpc), luminous ($L_X\simeq 10^{43-44}$ erg s$^{-1}$), and hot ($\sim$keV), consistent with infalling gaseous halos crossing the virial shock. The results agree with the stacked ROSAT-MCXC signal, showing the higher $L_X$ anticipated at EDR redshifts and a possible dependence upon host mass. Localized virial-shock spikes in the distributions of discrete radio, X-ray, and probably also $\gamma$-ray sources are new powerful probes of accretion from the cosmic web, with strong constraints anticipated with future all-sky catalogs such as by eROSITA.

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arXiv2024

Excess cataloged X-ray and radio sources at galaxy-cluster virial shocks

We detect a highly significant excess of X-ray (2RXS) and radio (NVSS, GMRT, VLSSr) catalog sources when stacked around MCXC galaxy clusters and groups, narrowly confined within $\lesssim100\mathrm{\,kpc}$ of the $\sim2.4 R_{500}$ virial shock radius (inferred from previous continuum stacking), with similar X-ray ($\sim4\sigma$ for $443$ clusters) and radio ($\sim4\sigma$ for $485$ clusters) characteristics ($>5\sigma$ joint). The excess sources show $10-100$ kpc scales, $L_X(0.1-2.4\mbox{ keV})\simeq10^{42-43}\mathrm{\,erg\,s^{-1}}$ or $\nu L_\nu(\nu=1.4\mbox{ GHz}) \simeq 10^{40-41}\mathrm{\,erg\,s^{-1}}$ luminosities, and a preferentially radial radio-polarization. The narrow localization and properties of the excess identify these sources not as AGN, often invoked speculatively for excess X-ray sources at cluster outskirts, but rather as infalling gaseous clumps interacting with the virial shock, probably galactic halos and possibly outflow remnants. The local excess of such discrete, radio-to-$\gamma$-ray sources around an object can probe its virial shock also at high redshifts and sub-cluster scales.

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arXiv2024

Detection of polarized Fermi-bubble synchrotron and dust emission

The elusive polarized microwave signal from the Fermi bubbles is disentangled from the more extended polarized lobes, which similarly emanate from the Galactic plane but stretch farther west of the bubbles. The projected ~20% synchrotron polarization reveals magnetic fields preferentially parallel to the bubble edges, as expected downstream of a strong shock. The projected ~20% polarization of thermal dust emission is similarly oriented, constraining grain alignment in an extreme environment. We argue that the larger lobes arise from an older Galactic-center, likely supermassive black-hole, outburst.

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arXiv2023

Radio halos and relics from extended cosmic-ray ion distributions with strong diffusion in galaxy clusters

A joint hadronic model is shown to quantitatively explain the observations of diffuse radio emission from galaxy clusters in the form of minihalos, giant halos, relics, and their hybrid, transitional stages. Cosmic-ray diffusion of order $D\sim 10^{31\text{--}32}\text{ cm}^2\text{ s}^{-1}$, inferred independently from relic energies, the spatial variability of giant-halo spectra, and the spectral evolution of relics, reproduces the observed spatio-spectral distributions, explains the recently discovered mega-halos as enhanced peripheral magnetization, and quenches electron (re)acceleration by weak shocks or turbulence. For instance, the hard-to-soft evolution along secondary-electron diffusion explains both the soft spectra in most halo peripheries and relic downstreams, and the hard spectra in most halo centres and relic edges, where the photon index can reach $α\simeq -0.5$ regardless of the Mach number $\mathcal{M}$ of the coincident shock. Such spatio-spectral modeling, recent $γ$-ray observations, and additional accumulated evidence are thus shown to support a previous claim (Keshet 2010) that the seamless transitions among minihalos, giant halos, and relics, their similar energetics, integrated spectra, and delineating discontinuities, the inconsistent $\mathcal{M}$ inferred from radio vs. X-rays in leptonic models, and additional observations, all indicate that these diffuse radio phenomena are manifestations of the same cosmic-ray ion population, with no need to invoke less natural alternatives.

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arXiv2023

Fermi-bubble bulk and edge analysis reveals dust, cooling breaks, and cosmic-ray diffusion, facilitating a self-consistent model

The full, radio to $\gamma$-ray spectrum of the Fermi bubbles is shown to be consistent with standard strong-shock electron acceleration at the bubble edge, without the unnatural energy cutoffs and unrealistic electron cooling of previous studies, if the ambient interstellar radiation is strong; the $\gamma$-ray cooling break should then have a microwave counterpart, undetected until now. Indeed, a broadband bubble-edge analysis uncovers a pronounced downstream dust component, which masked the anticipated $\sim35$ GHz spectral break, and the same overall radio softening consistent with Kraichnan diffusion previously reported in $\gamma$-rays.

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arXiv2023

Quasi-spiral solution to the mixed intracluster medium and the universal entropy profile of galaxy clusters

Well-resolved galaxy clusters often show a large-scale quasi-spiral structure in deprojected density $ρ$ and temperature $T$ fields, delineated by a tangential discontinuity known as a cold front, superimposed on a universal radial entropy profile with a linear $K(r)\propto Tρ^{-2/3}\propto r$ adiabat. We show that a spiral structure provides a natural quasi-stationary solution for the mixed intracluster medium (ICM), introducing a modest pressure spiral that confines the locally buoyant or heavy plasma phases. The solution persists in the presence of uniform or differential rotation, and can accommodate both an inflow and an outflow. Hydrodynamic adiabatic simulations with perturbations that deposit angular momentum and mix the plasma thus asymptote to a self-similar spiral structure. We find similar spirals in Eulerian and Lagrangian simulations of 2D and 3D, merger and offset, clusters. The discontinuity surface is given in spherical coordinates $\{r,θ,ϕ\}$ by $ϕ(r,θ)\propto Φ(r)$, where $Φ$ is the gravitational potential, combining a trailing spiral in the equatorial ($θ=π/2$) plane and semicircles perpendicular to the plane, in resemblance of a snail shell. A local convective instability can develop between spiral windings, driving a modified global instability in sublinear $K(r)$ regions; evolved spirals thus imprint the observed $K\propto r$ onto the ICM even after they dissipate. The spiral structure brings hot and cold phases to close proximity, suggesting that the observed fast outflows could sustain the structure even in the presence of radiative cooling.

astro-ph.CO↗

arXiv2023

Synchrotron emission from virial shocks around stacked OVRO-LWA galaxy clusters

Galaxy clusters accrete mass through large scale, strong, structure-formation shocks. Such a virial shock is thought to deposit fractions $ξ_e$ and $ξ_B$ of the thermal energy in cosmic-ray electrons (CREs) and magnetic fields, respectively, thus generating a leptonic virial ring. However, the expected synchrotron signal was not convincingly established until now. We stack low-frequency radio data from the OVRO-LWA around the 44 most massive, high latitude, extended MCXC clusters, enhancing the ring sensitivity by rescaling clusters to their characteristic, $R_{500}$ radii. Both high (73 MHz) and co-added low ($36\text{--}68\text{ MHz}$) frequency channels separately indicate a significant ($4\text{--}5σ$) excess peaked at $(2.4 \text{--} 2.6) R_{500}$, coincident with a previously stacked Fermi $γ$-ray signal interpreted as inverse-Compton emission from virial-shock CREs. The stacked radio signal is well fit (TS-test: $4$--$6σ$ at high frequency, $4$--$8σ$ at low frequencies, and $8$--$10σ$ joint) by virial-shock synchrotron emission from the more massive clusters, with $\dot{m}ξ_eξ_B\simeq (1\text{--}4)\times 10^{-4}$, where $\dot{m}\equiv \dot{M}/(MH)$ is the dimensionless accretion rate for a cluster of mass $M$ and a Hubble constant $H$. The inferred CRE spectral index is flat, $p \simeq 2.0 \pm 0.2$, consistent with acceleration in a strong shock. Assuming equipartition or using $\dot{m}ξ_e\sim0.6\%$ inferred from the Fermi signal yields $ξ_B\simeq (2\text{--}9)\%$, corresponding to $B \simeq (0.1\text{--}0.3)~μ\text{G}$ magnetic fields downstream of typical virial shocks. Preliminary evidence suggests non-spherical shocks, with factor $2$--$3$ elongations.

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