arXiv subjects

Dongsu Ryu

Publications and source records attributed to Dongsu Ryu.

At least 37 records · Page 2Linked to original sources

arXiv2020

Modeling of Cosmic-Ray Production and Transport and Estimation of Gamma-Ray and Neutrino Emissions in Starburst Galaxies

Starburst galaxies (SBGs) with copious massive stars and supernova (SN) explosions are the sites of active cosmic-ray production. Based on the predictions of nonlinear diffusive shock acceleration theory, we model the cosmic-ray proton (CRP) production by both pre-SN stellar winds (SWs) and supernova remnants (SNRs) from core-collapse SNe inside the starburst nucleus. Adopting different models for the transport of CRPs, we estimate the $γ$-ray and neutrino emissions due to $pp$ collisions from nearby SBGs such as M82, NGC253, and Arp220. We find that with the current $γ$-rays observations by Fermi-LAT, Veritas, and H.E.S.S., it would be difficult to constrain CRP production and transport models. Yet, the observations are better reproduced with (1) the combination of the single power-law (PL) momentum distribution for SNR-produced CRPs and the diffusion model in which the CRP diffusion is mediated by the strong Kolmogorov-type turbulence of $δB/B\sim1$, and (2) the combination of the double PL model for SNR-produced CRPs and the diffusion model in which the scattering of CRPs is controlled mostly by self-excited waves rather than the pre-existing turbulence. The contribution of SW-produced CRPs could be substantial in Arp220, where the star formation rate is higher and the slope of the initial mass function would be flatter. We suggest that M82 and NGC253 might be detectable as point sources of high-energy neutrinos in the upcoming KM3NET and IceCube-Gen2, when optimistic models are applied. Future observations of neutrinos as well as $γ$-rays would provide constraints for the production and diffusion of CRPs in SBGs.

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arXiv2020

Limiting the shock acceleration of cosmic-ray protons in the ICM

Observations of large-scale radio emissions prove the existence of shock accelerated cosmic-ray electrons in galaxy clusters, while the lack of detected $γ$-rays limits the acceleration of cosmic-ray protons in galaxy clusters. This challenges our understanding of how diffusive shock acceleration works. In this work, we couple the most updated recipes for shock acceleration in the intracluster medium to state-of-the-art magneto-hydrodynamical simulations of massive galaxy clusters. Furthermore, we use passive tracer particles to follow the evolution of accelerated cosmic-rays. We show that when the interplay between magnetic field topology and the feedback from accelerated cosmic rays is taken into account, the latest developments of particle acceleration theory give results which are compatible with observational constraints.

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arXiv2020

Reconstruction of Radio Relics and X-ray Tails in an Off-axis Cluster Merger: Hydrodynamical Simulations of A115

Although a merging galaxy cluster is a useful laboratory to study many important astrophysical processes and fundamental physics, only limited interpretations are possible without careful analysis of the merger scenario. However, the study is demanding because a thorough comparison of multi-wavelength observations with detailed numerical simulations is required. In this paper, we present such a study for the off-axis binary merger A115. The system possesses a number of remarkable observational features, but no convincing merger scenario, explaining the shape and location of the radio relic in harmony with the orientation of the cometary X-ray tails, has been presented. Our hydrodynamical simulation, with adaptive mesh refinement, suggests that the cometary X-ray tail of A115 might be a slingshot tail and can arise $\sim0.3$ Gyrs after the impact and before the two subclusters reach their apocenters. This scenario can predict the location and orientation of the giant radio relic, which is parallel to the northern X-ray tail. In addition, our study indicates that diffusive shock acceleration alone cannot generate the observed radio power unless aided by stronger magnetic fields and/or more significant presence of fossil electrons.

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arXiv2020

Gamma-ray and Neutrino Emissions due to Cosmic-Ray Protons Accelerated at Intracluster Shocks in Galaxy Clusters

We examine the cosmic-ray protons (CRp) accelerated at collisionless shocks in galaxy clusters using cosmological structure formation simulations. We find that in the intracluster medium (ICM) within the virial radius of simulated clusters, only $\sim7$\% of shock kinetic energy flux is dissipated by the shocks that are expected to accelerate CRp, that is, supercritical, quasi-parallel ($Q_\parallel$) shocks with sonic Mach number $M_s\ge2.25$. The rest is dissipated at subcritical shocks and quasi-perpendicular shocks, both of which may not accelerate CRp. Adopting the diffusive shock acceleration (DSA) model recently presented in Ryu et al. (2019), we quantify the DSA of CRp in simulated clusters. The average fraction of the shock kinetic energy transferred to CRp via DSA is assessed at $\sim(1-2)\times10^{-4}$. We also examine the energization of CRp through reacceleration using a model based on the test-particle solution. Assuming that the ICM plasma passes through shocks three times on average through the history of the universe and that CRp are reaccelerated only at supercritical $Q_\parallel$-shocks, the CRp spectrum flattens by $\sim0.05-0.1$ in slope and the total amount of CRp energy increases by $\sim40-80$\% from reacceleration. We then estimate diffuse $γ$-ray and neutrino emissions, resulting from inelastic collisions between CRp and thermal protons. The predicted $γ$-ray emissions from simulated clusters lie mostly below the upper limits set by Fermi-LAT for observed clusters. The neutrino fluxes towards nearby clusters would be $\lesssim10^{-4}$ of the IceCube flux at $E_ν=1$ PeV and $\lesssim10^{-6}$ of the atmospheric neutrino flux in the energy range of $E_ν\leq1$ TeV.

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arXiv2019

Electron Firehose Instabilities in High-$β$ ICM shocks

The preacceleration of electrons through reflection and shock drift acceleration (SDA) is essential for the diffusive shock acceleration (DSA) of nonthermal electrons in collisionless shocks. Previous studies suggested that, in weak quasi-perpendicular ($Q_\perp$) shocks in the high-$β$ ($β=P_{\rm gas}/P_{\rm B}$) intracluster medium (ICM), the temperature anisotropy due to SDA-reflected electrons can drive the electron firehose instability, which excites oblique nonpropagating waves in the shock foot. In this paper, we investigate, through a linear analysis and particle-in-cell (PIC) simulations, the firehose instabilities driven by an electron temperature anisotropy (ETAFI) and also by a drifting electron beam (EBFI) in $β\sim100$ ICM plasmas. The EBFI should be more relevant in describing the self-excitation of upstream waves in $Q_\perp$-shocks, since backstreaming electrons in the shock foot behave more like an electron beam rather than an anisotropic bi-Maxwellian population. We find that the basic properties of the two instabilities, such as the growth rate, $γ$, and the wavenumber of fast-growing oblique modes are similar in the ICM environment, with one exception; while the waves excited by the ETAFI are nonpropagating ($ω_r=0$), those excited by the EBFI have a non-zero frequency ($ω_r\neq0$). However, the frequency is small with $ω_r<γ$. Thus, we conclude that the interpretation of previous studies for the nature of upstream waves based on the ETAFI remains valid in $Q_\perp$-shocks in the ICM.

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arXiv2019

Chandra Observations of the Spectacular A3411-12 Merger Event

We present deep Chandra observations of A3411-12, a remarkable merging cluster that hosts the most compelling evidence for electron re-acceleration at cluster shocks to date. Using the $Y_X-M$ scaling relation, we find $r_{500} \sim 1.3$ Mpc, $M_{500} = (7.1 \pm 0.7) \times 10^{14} \ M_{\rm{\odot}}$, $kT=6.5\pm 0.1$ keV, and a gas mass of $M_{\rm g,500} = (9.7 \pm 0.1) \times 10^{13} M_\odot$. The gas mass fraction within $r_{500}$ is $f_{\rm g} = 0.14 \pm 0.01$. We compute the shock strength using density jumps to conclude that the Mach number of the merging subcluster is small ($M \leq 1.15_{-0.09}^{+0.14}$). We also present pseudo-density, projected temperature, pseudo-pressure, and pseudo-entropy maps. Based on the pseudo-entropy map we conclude that the cluster is undergoing a mild merger, consistent with the small Mach number. On the other hand, radio relics extend over Mpc scale in the A3411-12 system, which strongly suggests that a population of energetic electrons already existed over extended regions of the cluster.

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arXiv2019

Propagation of ultra-high-energy cosmic rays in the magnetized cosmic web

A high concentration of ultra-high-energy cosmic ray (UHECR) events, called a hotspot, was reported by the Telescope Array (TA) experiment, but its origin still remains unsolved. One of the obstacles is that there is no astronomical object, which could be the source, behind the TA hotpot. In an effort to understand the origin of the TA hotspot, we suggested a model based on the magnetized cosmic web structure. The UHECRs were produced from sources in the Virgo cluster and were initially confined by cluster magnetic fields for a certain period. Next, some of them preferentially escaped to and propagated along filaments. Eventually, they were scattered by filament magnetic fields, and come to us. To examine the model, we followed the propagation trajectories of UHE protons in a simulated universe with clusters, filaments, and voids, by employing a number of models for cosmic magnetic fields. In this study, we present some of the initial results, such as the ratio between the particles directly escaping from the clusters to the voids and particles escaping from the clusters to the filaments. We also discuss the feasibility of our model for the origin of the hotspot by examining the trajectories of the UHE protons.

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arXiv2019

Turbulence Dynamo in Galaxy Clusters

The existence of microgauss magnetic fields in galaxy clusters have been established through observations of synchrotron radiation and Faraday rotation. They are conjectured to be generated via small-scale dynamo by turbulent flow motions in the intracluster medium (ICM). Some of giant radio relics, on the other hand, show the structures of synchrotron polarization vectors, organized over the scales of $\sim$ Mpc, challenging the turbulence origin of cluster magnetic fields. Unlike turbulence in the interstellar medium, turbulence in the ICM is subsonic. And it is driven sporadically in highly stratified backgrounds, when major mergers occur during the hierarchical formation of clusters. To investigate quantitatively the characteristics of turbulence dynamo in such ICM environment, we performed a set of turbulence simulations using a high-order-accurate, magnetohydrodynamic (MHD) code. We find that turbulence dynamo could generate the cluster magnetic fields up to the observed level from the primordial seed fields of $10^{-15}$ G or so within the age of the universe, if the MHD description of the ICM could be extended down to $\sim$ kpc scales. However, highly organized structures of polarization vectors, such as those observed in the Sausage relic, are difficult to be reproduced by the shock compression of turbulence-generated magnetic fields. This implies that the modeling of giant radio relics may require the pre-existing magnetic fields organized over $\sim$ Mpc scales.

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arXiv2019

A Diffusive Shock Acceleration Model for Protons in Weak Quasi-parallel Intracluster Shocks

Low sonic Mach number shocks form in the intracluster medium (ICM) during the formation of the large-scale structure of the universe. Nonthermal cosmic-ray (CR) protons are expected to be accelerated via diffusive shock acceleration (DSA) in those ICM shocks, although observational evidence for the $γ$-ray emission of hadronic origin from galaxy clusters has yet to be established. Considering the results obtained from recent plasma simulations, we improve the analytic test-particle DSA model for weak quasi-parallel ($Q_\parallel$) shocks, previously suggested by \citet{kang2010}. In the model CR spectrum, the transition from the postshock thermal to CR populations occurs at the injection momentum, $p_{\rm inj}$, above which protons can undergo the full DSA process. As the shock energy is transferred to CR protons, the postshock gas temperature should decrease accordingly and the subshock strength weakens due to the dynamical feed of the CR pressure to the shock structure. This results in the reduction of the injection fraction, although the postshock CR pressure approaches an asymptotic value when the CR spectrum extends to the relativistic regime. Our new DSA model self-consistently accounts for such behaviors and adopts better estimations for $p_{\rm inj}$. With our model DSA spectrum, the CR acceleration efficiency ranges $η\sim10^{-3}-0.01$ for supercritical, $Q_\parallel$-shocks with sonic Mach number $2.25\lesssim M_{\rm s}\lesssim5$ in the ICM. Based on \citet{ha2018b}, on the other hand, we argue that proton acceleration would be negligible in subcritical shocks with $M_{\rm s}<2.25$.

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arXiv2019

Plasma 2020 - Intracluster Medium Plasmas

Galaxy clusters are the largest and most massive bound objects resulting from cosmic hierarchical structure formation. Baryons account for somewhat more than 10% of that mass, with roughly 90% of the baryonic matter distributed throughout the clusters as hot ($T>1$ keV), high-$β$, very weakly collisional plasma; the so-called "intracluster medium" (ICM). Cluster mergers, close gravitational encounters and accretion, along with violent feedback from galaxies and relativistic jets from active galactic nuclei, drive winds, gravity waves, turbulence and shocks within the ICM. Those dynamics, in turn, generate cluster-scale magnetic fields and accelerate and mediate the transport of high-energy charged particles. Kinetic-scale, collective plasma processes define the basic character and fundamental signatures of these ICM phenomena, which are observed primarily by X-ray and radio astronomers.

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arXiv2019

Shock Waves and Energy Dissipation in Magnetohydrodynamic Turbulence

Shock waves play an important role in turbulent astrophysical media by compressing the gas and dissipating the turbulent energy into the thermal energy. We here study shocks in magnetohydrodynamic turbulence using high-resolution simulations. Turbulent Mach numbers of $\mathcal{M}_\text{turb}=0.5-7$ and initial magnetic fields of plasma beta $β_0=0.1 - 10$ are considered, targeting turbulences in interstellar and intracluster media. Specifically, we present the statistics of fast and slow shocks, such as the distribution of shock Mach numbers ($M_\text{s}$) and the energy dissipation at shocks, based on refined methodologies for their quantifications. While most shocks form with low $M_\text{s}$, strong shocks follow exponentially decreasing distributions of $M_\text{s}$. More shocks appear for larger $\mathcal{M}_\text{turb}$ and larger $β_0$. Fast shock populations dominate over slow shocks if $β_0\gg1$, but substantial populations of slow shocks develop in the cases of $β\lesssim1$, i.e., strong background fields. The shock dissipation of turbulent energy occurs preferentially at fast shocks with $M_\text{s}\lesssim$ a few to several, and the dissipation at strong shocks shows exponentially decreasing functions of $M_\text{s}$. The energy dissipation at shocks, normalized to the energy injection, $ε_\text{shock}/ε_\text{inj}$, is estimated to be in the range of $\sim0.1-0.5$, except for the case of $\mathcal{M}_\text{turb}=0.5$ and $β_0=0.1$ where the shock dissipation is negligible. The fraction decreases with $\mathcal{M}_\text{turb}$; it is close to $\sim0.4-0.6$ for $\mathcal{M}_\text{turb}=0.5$, while it is $\sim0.1-0.25$ for $\mathcal{M}_\text{turb}=7$. The rest of the turbulent energy is expected to dissipate through the turbulent cascade.

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arXiv2019

Electron Preacceleration in Weak Quasi-perpendicular Shocks in High-beta Intracluster Medium

Giant radio relics in the outskirts of galaxy clusters are known to be lit up by the relativistic electrons produced via diffusive shock acceleration (DSA) in shocks with low sonic Mach numbers, $M_{\rm s}\lesssim3$. The particle acceleration at these collisionless shocks critically depends on the kinetic plasma processes that govern the injection to DSA. Here, we study the preacceleration of suprathermal electrons in weak, quasi-perpendicular ($Q_\perp$) shocks in the hot, high-$β$ ($β= P_{\rm gas}/P_{\rm B}$) intracluster medium (ICM) through two-dimensional particle-in-cell simulations. \citet{guo2014a,guo2014b} showed that in high-$β$ $Q_\perp$-shocks, some of incoming electrons could be reflected upstream and gain energy via shock drift acceleration (SDA). The temperature anisotropy due to the SDA-energized electrons then induces the electron firehose instability (EFI), and oblique waves are generated, leading to a Fermi-like process and multiple cycles of SDA in the preshock region. We find that such electron preacceleration is effective only in shocks above a critical Mach number $M_{\rm ef}^*\approx2.3$. This means that in ICM plasmas, $Q_\perp$-shocks with $M_{\rm s}\lesssim2.3$ may not efficiently accelerate electrons. We also find that even in $Q_\perp$-shocks with $M_{\rm s}\gtrsim2.3$, electrons may not reach high enough energies to be injected to the full Fermi-I process of DSA, because long-wavelength waves are not developed via the EFI alone. Our results indicate that additional electron preaccelerations are required for DSA in ICM shocks, and the presence of fossil relativistic electrons in the shock upstream region may be necessary to explain observed radio relics.

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arXiv2019

Filaments of Galaxies as a Clue to the Origin of Ultra-High-Energy Cosmic Rays

Ultra-high-energy cosmic rays (UHECRs) are known to come from outside of our Galaxy, but their origin still remains unknown. The Telescope Array (TA) experiment recently identified a high concentration in the arrival directions of UHECRs with energies above $5.7 \times 10^{19} eV$, called hotspot. We here report the presence of filaments of galaxies, connected to the Virgo Cluster, on the sky around the hotspot, and a statistically significant correlation between hotspot events and the filaments. With 5-year TA data, the maximum significance of binomial statistics for the correlation is estimated to be 6.1 $σ$ at correlation angle 3.4 degree. The probability that the above significance appears by chance is $\sim 2.0 \times 10^{-8}$ (5.6 $σ$). Based on this finding, we suggest a model for the origin of TA hotspot UHECRs; they are produced at sources in the Virgo Cluster, and escape to and propagate along filaments, before they are scattered toward us. This picture requires the filament magnetic fields of strength $\gtrsim 20$ nG, which need to be confirmed in future observations.

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arXiv2018

Proton Acceleration in Weak Quasi-parallel Intracluster Shocks: Injection and Early Acceleration

Collisionless shocks with low sonic Mach numbers, $M_{\rm s} \lesssim 4$, are expected to accelerate cosmic ray (CR) protons via diffusive shock acceleration (DSA) in the intracluster medium (ICM). However, observational evidence for CR protons in the ICM has yet to be established. Performing particle-in-cell simulations, we study the injection of protons into DSA and the early development of a nonthermal particle population in weak shocks in high $β$ ($\approx 100$) plasmas. Reflection of incident protons, self-excitation of plasma waves via CR-driven instabilities, and multiple cycles of shock drift acceleration are essential to the early acceleration of CR protons in supercritical quasi-parallel shocks. We find that only in ICM shocks with $M_{\rm s} \gtrsim M_{\rm s}^*\approx 2.25$, a sufficient fraction of incoming protons are reflected by the overshoot in the shock electric potential and magnetic mirror at locally perpendicular magnetic fields, leading to efficient excitation of magnetic waves via CR streaming instabilities and the injection into the DSA process. Since a significant fraction of ICM shocks have $M_{\rm s} < M_{\rm s}^*$, CR proton acceleration in the ICM might be less efficient than previously expected. This may explain why the diffuse gamma-ray emission from galaxy clusters due to proton-proton collisions has not been detected so far.

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arXiv2018

The Contribution of Stellar Winds to Cosmic Ray Production

Massive stars blow powerful stellar winds throughout their evolutionary stages from the main sequence to Wolf-Rayet phases. The amount of mechanical energy deposited in the interstellar medium by the wind from a massive star can be comparable to the explosion energy of a core-collapse supernova that detonates at the end of its life. In this study, we estimate the kinetic energy deposition by massive stars in our Galaxy by considering the integrated Galactic initial mass function and modeling the stellar wind luminosity. The mass loss rate and terminal velocity of stellar winds during the main sequence, red supergiant, and Wolf-Rayet stages are estimated by adopting theoretical calculations and observational data published in the literature. We find that the total stellar wind luminosity due to all massive stars in the Galaxy is about $L_w\approx 1.1\times 10^{41}$ erg/s, which is about 1/4 of the power of supernova explosions, $L_{SN} \approx 4.8\times 10^{41}$ erg/s. If we assume that $\sim 1-10$ % of the wind luminosity could be converted to Galactic cosmic rays (GCRs) through collisonless shocks such as termination shocks in stellar bubbles and superbubbles, colliding-wind shocks in binaries, and bow-shocks of massive runaway stars, stellar winds might be expected to make a significant contribution to GCR production, though lower than that of supernova remnants.

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arXiv2018

Effects of Alfvenic Drift on Diffusive Shock Acceleration at Weak Cluster Shocks

Non-detection of $γ$-ray emission from galaxy clusters has challenged diffusive shock acceleration (DSA) of cosmic-ray (CR) protons at weak collisionless shocks that are expected to form in the intracluster medium. As an effort to address this problem, we here explore possible roles of Alfvén waves self-excited via resonant streaming instability during the CR acceleration at parallel shocks. The mean drift of Alfvén waves may either increase or decrease the scattering center compression ratio, depending on the postshock cross-helicity, leading to either flatter or steeper CR spectra. We first examine such effects at planar shocks, based on the transport of Alfvén waves in the small amplitude limit. For the shock parameters relevant to cluster shocks, Alfvénic drift flattens the CR spectrum slightly, resulting in a small increase of the CR acceleration efficiency, $η$. We then consider two additional, physically motivated cases: (1) postshock waves are isotropized via MHD and plasma processes across the shock transition and (2) postshock waves contain only forward waves propagating along with the flow due to a possible gradient of CR pressure behind the shock. In these cases, Alfvénic drift could reduce $η$ by as much as a factor of 5 for weak cluster shocks. For the canonical parameters adopted here, we suggest $η\sim10^{-4}-10^{-2}$ for shocks with sonic Mach number $M_{\rm s}\approx2-3$. The possible reduction of $η$ may help ease the tension between non-detection of $γ$-rays from galaxy clusters and DSA predictions.

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arXiv2018

Properties of Merger Shocks in Merging Galaxy Clusters

X-ray shocks and radio relics detected in the cluster outskirts are commonly interpreted as shocks induced by mergers of sub-clumps. We study the properties of merger shocks in merging galaxy clusters, using a set of cosmological simulations for the large-scale structure formation of the universe. As a representative case, we here focus on the simulated clusters that undergo almost head-on collisions with mass ratio $\sim2$. Due to the turbulent nature of the intracluster medium, shock surfaces are not smooth, but composed of shocks with different Mach numbers. As the merger shocks expand outward from the core to the outskirts, the average Mach number, $\left $, increases in time. We suggest that the shocks propagating along the merger axis could be manifested as X-ray shocks and/or radio relics. The kinetic energy through the shocks, $F_ϕ$, peaks at $\sim1$ Gyr after their initial launching, or at $\sim1-2$ Mpc from the core. Because of the Mach number dependent model adopted here for the cosmic ray (CR) acceleration efficiency, their CR-energy-weighted Mach number is higher with $\left< M_s \right>_{\rm CR}\sim3-4$, compared to the kinetic-energy-weighted Mach number, $\left _ϕ\sim2-3$. Most energetic shocks are to be found ahead of the lighter dark matter (DM) clump, while the heavier DM clump is located in the opposite side of clusters. Although our study is limited to the merger case considered, the results such as the means and variations of shock properties and their time evolution could be compared with the observed characteristics of merger shocks, constraining interpretations of relevant observations.

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arXiv2017

Rapid X-ray variations of the Geminga pulsar wind nebula

A recent study by Posselt et al. (2017) reported the deepest X-ray investigation of the Geminga pulsar wind nebula (PWN) by using \emph{Chandra X-ray Observatory}. In comparison with previous studies of this system, a number of new findings have been reported and we found these suggest the possible variabilities in various components of this PWN. This motivates us to carry out a dedicated search for the morphological and spectral variations of this complex nebula. We have discovered variabilities on timescales from a few days to a few months from different components of the nebula. The fastest change occurred in the circumstellar environment at a rate of 80 per cent of the speed of light. One of the most spectacular results is the wiggling of a half light-year long tail as an extension of the jet, which is significantly bent by the ram pressure. The jet wiggling occurred at a rate of about 20 per cent of the speed of light. This twisted structure can possibly be a result of a propagating torsional Alfven wave. We have also found evidence of spectral hardening along this tail for a period of about nine months.

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