arXiv subjects

Kosuke Nishiwaki

Publications and source records attributed to Kosuke Nishiwaki.

9 recordsLinked to original sources

arXiv2026

Unravelling Turbulence and Magnetic Fields in Galaxy Clusters with SKA and XRISM

This chapter proposes a research framework to quantitatively investigate non-thermal components in the Intracluster Medium (ICM) of galaxy clusters, which are critical ingredients for governing energy transport, structure formation, and particle acceleration. Turbulence, primarily driven by cluster mergers, is the leading mechanism for re-accelerating cosmic ray electrons (forming radio halos) and amplifying magnetic fields (via the turbulent dynamo). Observational understanding of both the turbulence and magnetic fields is rapidly evolving: the high-resolution X-ray spectrometer XRISM is directly measuring the velocity properties of the thermal ICM, providing insights into the kinetic energy of turbulence. Concurrently, high-sensitivity low-frequency radio observations, including SKA pathfinders, are mapping non-thermal components and magnetic structures through diffuse synchrotron emission and high-density Faraday Rotation Measure (RM) grids. The synergy between XRISM and SKA offers a decisive paradigm shift. XRISM's velocity maps, with its high energy resolution (<7 eV FWHM), combined with SKA-Mid's capability to deliver high-resolution RM grids ($\sim 100$--$200~\rm deg^{-2}$) and high-dynamic-range imaging, will allow for the first direct, multi-wavelength comparison of the turbulent energy properties (from X-ray) and the magnetic field properties (from radio). This joint analysis will validate Magnetohydrodynamic (MHD) simulation predictions, clarify the process of turbulent energy cascade and decay, and ultimately lead to a comprehensive understanding of the co-evolution of turbulence, magnetic fields, and cosmic rays in the largest laboratories of the Universe.

astro-ph.HE↗

arXiv2026

Cosmological simulation of a radio synchrotron bridge between pre-merging galaxy clusters

Radio bridges are diffuse synchrotron emission observed between merging galaxy clusters. Recent radio observations have reported both detections and non-detections of radio bridges between clusters. The detections imply the presence of cosmic rays (CRs) and magnetic fields permeating the cosmic web that produce synchrotron emission observable with current facilities, whereas the non-detections suggest that specific physical conditions are required for their formation. We study the CR reacceleration by solenoidal turbulence in the filament connecting two massive clusters at an early stage of the merger. Our aim is to test whether this mechanism can generate diffuse emission in the inter-cluster region. We perform a cosmological magneto-hydrodynamical (MHD) simulation using the Enzo code. We improved a run-time Lagrangian tracer method implemented in Enzo, and follow the trajectories of baryonic matter using $N=\mathcal{O}(10^7)$ tracer particles. In post-processing, we conduct a parallel computation of the Fokker-Planck (FP) equation for all tracers, with cooling and reacceleration efficiencies evaluated from the local quantities recorded along each tracer trajectory. Our simulation generate a Mpc-sized radio bridge in the early stage of the cluster merger. Within a reasonable parameter range, the reacceleration model produces a broad variety of spectra. In our fiducial model, the simulated bridge matches several properties of the one found between Abell 399 and Abell 401, such as its spectral shape, intensity profile, and pixel-by-pixel correlation between radio and X-ray intensities. The inter-cluster region is filled with turbulence induced by infalling mass clumps and subsequently amplified by the approaching motion of the clusters. The CR reacceleration by the turbulence is a viable mechanism to power a Mpc-sized synchrotron emission observed as radio bridges.

astro-ph.CO↗

arXiv2025

Hadronic origin of the very high-energy gamma-ray emission from the low-luminosity AGN in NGC 4278

The Large High Altitude Air Shower Observatory has detected very high-energy (VHE) gamma rays from NGC 4278, which is known to host a low-luminosity active galactic nucleus (AGN). Having only very weak radio jets, the origin of its VHE gamma rays is unclear. In this paper we first show that NGC 4278 has a massive molecular cloud surrounding the nucleus by analyzing data taken with the Atacama Large Millimeter/submillimeter Array. We then assume that cosmic ray protons are accelerated in a radiatively inefficient accretion flow around the supermassive black hole, which diffuse into the molecular cloud and produce gamma rays and neutrinos via $pp$ interactions. We model the gamma-ray spectra and find that the observations can be explained by such hadronic processes if the AGN activity was higher in the past than at present, and the diffusion coefficient in the molecular cloud is appreciably smaller than in the Milky Way interstellar medium. We also show that although the high-energy neutrinos co-produced with the gamma rays are unlikely to be detectable even with IceCube-Gen2, the accompanying synchrotron X-ray emission due to pion-decay secondary electrons and positrons may be detectable in the future, providing a valuable test of our hadronic model.

astro-ph.HE↗

arXiv2024

Low injection rate of cosmic-ray protons in the turbulent reacceleration model of radio halos in galaxy clusters

A giant radio halo (RH) is a diffuse synchrotron emission observed on the scale of megaparsecs (Mpc), typically found in the central region of merging galaxy clusters. Its large size and steep spectrum suggest that it originates from the re-energization of an aged population of cosmic ray electrons (CREs), while the secondary leptons produced in the $pp$ hadronic collision of cosmic ray protons (CRPs) may contribute to the emission. In this study, we investigate the reacceleration model including both primary and secondary CREs, assuming that the primary CRs originate from internal galaxies. In our new method, we follow the cosmological evolution of each cluster and calculate the energy spectra and one-dimensional spatial distributions of CRs. The primary CRE model with $\sim 3$ Gyr duration of reacceleration successfully reproduces the statistical properties of the RHs observed in the recent LOFAR survey, as well as the spectrum and profile of the Coma cluster. The gamma-ray and neutrino emissions produced by reaccelerated CRPs are consistent with the upper limits. However, if the CRP injection rate is high and the secondary CREs become significant, the model with the required $\sim 3$ Gyr reacceleration overproduces the number of RHs. The limit on the CRP injection rate, $L_{\rm p} \lesssim 10^{41}$ erg/s, is significantly lower than that expected from the early starburst activity or jets from active galactic nuclei. This discrepancy requires a revision of either the model of CR supply from galaxies or the turbulent reacceleration model.

astro-ph.HE↗

arXiv2024

Particle Reacceleration by Turbulence and Radio Constraints on Multi-Messenger High-Energy Emission from the Coma Cluster

Galaxy clusters are considered to be gigantic reservoirs of cosmic rays (CRs). Some of the clusters are found with extended radio emission, which provides evidence for the existence of magnetic fields and CR electrons in the intra-cluster medium (ICM). The mechanism of radio halo (RH) emission is still under debate, and it has been believed that turbulent reacceleration plays an important role. In this paper, we study the reacceleration of CR protons and electrons in detail by numerically solving the Fokker-Planck equation, and show how radio and gamma-ray observations can be used to constrain CR distributions and resulting high-energy emission for the Coma cluster. We take into account the radial diffusion of CRs and follow the time evolution of their one-dimensional distribution, by which we investigate the radial profile of the CR injection that is consistent with the observed RH surface brightness. We find that the required injection profile is non-trivial, depending on whether CR electrons have the primary or secondary origin. Although the secondary CR electron scenario predicts larger gamma-ray and neutrino fluxes, it is in tension with the observed RH spectrum. In either scenario, we find that galaxy clusters can make a sizable contribution to the all-sky neutrino intensity if the CR energy spectrum is nearly flat.

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arXiv2023

Efficiency of turbulent reacceleration by solenoidal turbulence and its application to the origin of radio mega halos in cluster outskirts

Recent radio observations with Low-Frequency Array (LOFAR) discovered diffuse emission extending beyond the scale of classical radio halos. The presence of such mega halos indicates that the amplification of the magnetic field and acceleration of relativistic particles are working in the cluster outskirts, presumably due to the combination of shocks and turbulence that dissipate energy in these regions. Cosmological magnetohydrodynamical (MHD) simulations of galaxy clusters suggest that solenoidal turbulence has a significant energy budget in the outskirts of galaxy clusters. In this paper, we explore the possibility that this turbulence contributes to the emission observed in mega halos through second-order Fermi acceleration of relativistic particles and the magnetic field amplification by the dynamo. We focus on the case of Abell 2255 and find that this scenario can explain the basic properties of the diffuse emission component that is observed under assumptions that are used in previous literature. More specifically, we conduct a numerical follow-up, solving the Fokker--Planck equation using a snapshot of a MHD simulation and deducing the synchrotron brightness integrated along the lines of sight. We find that a volume-filling emission, ranging between 30 and almost 100% of the projected area depending on our assumptions on the particle diffusion and transport, can be detected at LOFAR sensitivities. Assuming a magnetic field $B\sim0.2μ$G, as derived from a dynamo model applied to the emitting region, we find that the observed brightness can be matched when $\sim$1% level of the solenoidal turbulent energy flux is channeled into particle acceleration.

astro-ph.HE↗

arXiv2023

High-energy neutrino constraints on cosmic-ray re-acceleration in radio halos of massive galaxy clusters

A fraction of merging galaxy clusters host diffuse radio emission in their central region, termed as a giant radio halo (GRH). The most promising mechanism of GRHs is the re-acceleration of non-thermal electrons and positrons by merger-induced turbulence. However, the origin of these seed leptons has been under debate, and either protons or electrons can be primarily-accelerated particles. In this work, we demonstrate that neutrinos can be used as a probe of physical processes in galaxy clusters, and discuss possible constraints on the amount of relativistic protons in the intra-cluster medium with the existing upper limits by IceCube. We calculate radio and neutrino emission from massive ($>10^{14}M_\odot$) galaxy clusters, using the cluster population model of Nishiwaki & Asano (2022). This model is compatible with the observed statistics of GRHs, and we find that the contribution of GRHs to the isotropic radio background observed with the ARCADE-2 experiment should be subdominant. Our fiducial model predicts the all-sky neutrino flux that is consistent with IceCube's upper limit from the stacking analysis. We also show that the neutrino upper limit gives meaningful constraints on the parameter space of the re-acceleration model, such as the electron-to-proton ratio of primary cosmic-rays and the magnetic field, and in particular the secondary scenario, where the seed electrons mostly originate from inelastic $pp$ collisions, can be constrained even in the presence of re-acceleration.

astro-ph.HE↗

arXiv2023

Non-thermal emissions from a head-tail radio galaxy in 3D magnetohydrodynamic simulations

We present magnetohydrodynamic simulations of a jet-wind interaction in a galaxy cluster and the radio to gamma-ray and the neutrino emissions from this "head-tail galaxy". Our simulation follows the evolution of cosmic-ray (CR) particle spectra with energy losses and the stochastic turbulence acceleration. We find that the reacceleration is essential to explain the observed radio properties of head-tail galaxies, in which the radio flux and spectral index do not drastically change. Our models suggest that hard X-ray emissions can be detected around the head-tail galaxy in the Perseus cluster by the hard X-ray satellites, such as FORCE, and it will potentially constrain the acceleration efficiency. We also explore the origin of the collimated synchrotron threads, which are found in some head-tail galaxies by recent high-quality radio observations. Thin and elongated flux tubes, connecting the two tails, are formed by strong backflows at an early phase. We find that these threads advect with the wind for over 300 Myr without disrupting. The radio flux from the flux tubes is much lower than the typical observed flux. An efficient CR diffusion process along the flux tubes, however, may solve this discrepancy.

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arXiv2022

Statistical Properties of Radio Halos in Galaxy Clusters and Origin of Seed Electrons for Reacceleration

One of the most promising mechanisms for producing radio halos (RHs) in galaxy clusters is the reacceleration of cosmic-ray electrons by turbulences. However, the origin of the seed electrons for the reacceleration is still poorly constrained. In the secondary scenario, most of the seed electrons are injected via collision of proton cosmic-rays, while non-thermal electrons are directly injected in the primary scenario. In this paper, we examine the two scenarios for the seed electrons with the observed statistical properties of RHs, combining two methods: following the temporal evolutions of the electron energy and radial distributions in a cluster, and the merger history of clusters. We find that the RH lifetime largely depends on the seed origin, as it could be longer than the cosmological timescale in the secondary scenario. We study the condition for the onset of RHs with the observed RH fraction and the RH lifetime we obtained, and find that long-lived RHs in the secondary scenario should be originated from major mergers with a mass ratio of $ξ\sim0.1$, while the short lifetime in the primary scenario requires more frequent onsets by minor mergers with $ξ\sim0.01$. Our simple model of the turbulence acceleration can reproduce the observed radio luminosity-mass relation. The RH luminosity functions we obtained suggest that the expected RH number count with the ASKAP survey will detect $\approx10^3$ RHs in both the scenarios.

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