arXiv subjects

Katsuaki Asano

Publications and source records attributed to Katsuaki Asano.

At least 37 records · Page 2Linked to original sources

arXiv2023

Gamma rays from a reverse shock with turbulent magnetic fields in GRB 180720B

Gamma-ray bursts (GRBs) are the most electromagnetically luminous cosmic explosions. They are powered by collimated streams of plasma (jets) ejected by a newborn stellar-mass black hole or neutron star at relativistic velocities (near the speed of light). Their short-lived (typically tens of seconds) prompt $γ$-ray emission from within the ejecta is followed by long-lived multi-wavelength afterglow emission from the ultra-relativistic forward shock. This shock is driven into the circumburst medium by the GRB ejecta that are in turn decelerated by a mildly-relativistic reverse shock. Forward shock emission was recently detected up to teraelectronvolt-energy $γ$-rays, and such very-high-energy emission was also predicted from the reverse shock. Here we report the detection of optical and gigaelectronvolt-energy $γ$-ray emission from GRB 180720B during the first few hundred seconds, which is explained by synchrotron and inverse-Compton emission from the reverse shock propagating into the ejecta, implying a low-magnetization ejecta. Our optical measurements show a clear transition from the reverse shock to the forward shock driven into the circumburst medium, accompanied by a 90-degree change in the mean polarization angle and fluctuations in the polarization degree and angle. This indicates turbulence with large-scale toroidal and radially-stretched magnetic field structures in the reverse and forward shocks, respectively, which tightly couple to the physics of relativistic shocks and GRB jets -- launching, composition, dissipation and particle acceleration.

astro-ph.HE↗

arXiv2023

LST-1 observations of an enormous flare of BL Lacertae in 2021

The first prototype of LST (LST-1) for the Cherenkov Telescope Array has been in commissioning phase since 2018 and already started scientific observations with the low energy threshold around a few tens of GeV. In 2021, LST-1 observed BL Lac following the alerts based on multi-wavelength observations and detected prominent gamma-ray flares. In addition to the daily flux variability, LST-1 also detected sub-hour-scale intra-night variability reaching 3-4 times higher than the gamma-ray flux from the Crab Nebula above 100 GeV. In this proceeding, we will report the analysis results of LST-1 observations of BL Lac in 2021, especially focusing on flux variability.

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

Dynamical Energy Dissipation of Relativistic Magnetic Bullets

To demonstrate the magnetic energy dissipation via relativistic shocks, we carry out spherically symmetrical one-dimensional special relativistic magneto-hydrodynamic simulations of highly magnetised outflows with an adaptive mesh refinement method. We first investigate the detail of the dynamical energy dissipation via interaction between a single ejecta and an external medium. The energy dissipation timescales, which affect the early behaviour of the afterglow emission in gamma-ray bursts, are estimated for a wide range of magnetisation. In addition, we demonstrate the internal shock dissipation in multiple interactions between magnetically dominated relativistic ejecta and kinetically dominated non-relativistic winds. Our numerical results show that almost 10% of the magnetic energy in the ejecta can be converted into the thermal energy of the relativistic and low-magnetised outflows via shocks in the rarefaction waves or the winds. Such hot and less magnetised outflows are relevant for observed non-thermal emissions in blazars or gamma-ray bursts.

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.

astro-ph.HE↗

arXiv2022

GRB Prompt Emission with Anisotropic Electron Distribution

The typical spectrum of the prompt emission of gamma-ray bursts (GRBs) indicates that the electron cooling is suppressed in spite of the strong magnetic field in the standard synchrotron model. Recent Particle-in-Cell simulations show that the particle acceleration by magnetic reconnection in a magnetically dominated plasma can lead to small pitch angles especially in low-energy region. Such a small pitch angle prevents electrons from cooling via synchrotron radiation. In this paper, taking into account the effects of the synchrotron cooling and the adiabatic cooling, we numerically calculate the synchrotron spectra with anisotropic electron distributions. If we require a Poynting flux larger than $10^{50}~\mbox{erg}~\mbox{s}^{-1}$ as the model is motivated by magnetic reconnection, the bulk Lorentz factor of $\sim 1000$ and the electron minimum Lorentz factor of $γ_{\rm min}\sim 10^4$ are required to reproduce the typical GRB spectrum.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2021

Monte Carlo Study of Electron and Positron Cosmic-Ray Propagation with the CALET Spectrum

Focusing on the electron and positron spectrum measured with CALET, which shows characteristic structures, we calculate flux contributions of cosmic rays escaped from supernova remnants, which were randomly born. We adopt a Monte Carlo method to take into account the stochastic property of births of nearby sources. We find that without a complicated energy dependence of the diffusion coefficient, simple power-law diffusion coefficients can produce spectra similar to the CALET spectrum even with a dispersion in the injection index. The positron component measured with AMS-02 is consistent with a bump-like structure around 300 GeV in the CALET spectrum. One to three nearby supernovae can contribute up to a few tens of percent of the CALET flux at 2--4 TeV, ten or more unknown and distant ($\gtrsim 500$ pc) supernovae account for the remaining several tens of percent of the flux. The CALET spectrum, showing a sharp drop at $\sim 1$ TeV, allows for a contribution of cosmic rays from an extraordinary event which occured $\sim 400$ kyr ago. This type of event releases electrons/positrons with a total energy more than 10 times the average energy for usual supernovae, and its occurrence rate is lower than $1/300$ of the usual supernova rate.

astro-ph.HE↗

arXiv2021

Enhanced X-ray Emission Coinciding with Giant Radio Pulses from the Crab Pulsar

Giant radio pulses (GRPs) are sporadic bursts emitted by some pulsars, lasting a few microseconds. GRPs are hundreds to thousands of times brighter than regular pulses from these sources. The only GRP-associated emission outside radio wavelengths is from the Crab Pulsar, where optical emission is enhanced by a few percent during GRPs. We observed the Crab Pulsar simultaneously at X-ray and radio wavelengths, finding enhancement of the X-ray emission by $3.8\pm0.7\%$ (a 5.4$σ$ detection) coinciding with GRPs. This implies that the total emitted energy from GRPs is tens to hundreds of times higher than previously known. We discuss the implications for the pulsar emission mechanism and extragalactic fast radio bursts.

astro-ph.HE↗

arXiv2020

Probing Particle Acceleration through Broadband Early Afterglow Emission of MAGIC Gamma-Ray Burst GRB 190114C

Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) detected the gamma-ray afterglow of GRB 190114C, which can constrain microscopic parameters of the shock-heated plasma emitting non-thermal emission. Focusing on the early afterglow of this event, we numerically simulate the spectrum and multi-wavelength light curves with constant and wind-like circumstellar medium using a time-dependent code. Our results show that the electron acceleration timescale at the highest energies is likely shorter than 20 times the gyroperiod to reproduce the GeV gamma-ray flux and its spectral index reported by {\it Fermi}. This gives an interesting constraint on the acceleration efficiency for Weibel-mediated shocks. We also constrain the number fraction of non-thermal electrons $f_{\rm e}$, and the temperature of the thermal electrons. The early optical emission can be explained by the thermal synchrotron emission with $f_{\rm e} \lesssim 0.01$. On the other hand, the X-ray light curves restrict efficient energy transfer from protons to the thermal electrons, and $f_{\rm e}\sim1$ is required if the energy fraction of the thermal electrons is larger than $\sim10$\%. The parameter constraints obtained in this work give important clues to probing plasma physics with relativistic shocks.

astro-ph.HE↗

arXiv2020

Physical origin of GeV emission in the early phase of GRB 170405A: Clues from emission onsets with multi-wavelength observations

The origin of GeV emission from the early epoch of gamma-ray bursts (GRBs) is a widely discussed issue. The long gamma-ray burst GRB 170405A, observed by the Fermi Gamma-ray Space Telescope showed high-energy emission delayed by $\sim$20 s with respect to the X-ray emission, followed by temporally fading gamma-ray emission lasting for $\sim$1,000 s, as commonly observed in high-energy GRBs. In addition, a high-energy spectral cutoff at $\sim$50 MeV was detected in the prompt emission phase. If this feature is caused by pair-production opacity, the bulk Lorentz factor of the GRB ejecta can be estimated to be $Γ_{\rm bulk}$ = 170-420. Simultaneously with Fermi, GRB 170405A was observed by Swift/Burst Alert Telescope (BAT) and X-ray telescope (XRT), and a clear optical onset was detected $\sim$200 s after the burst by Swift/UltraViolet Optical Telescope (UVOT). By coupling the deceleration time to the derived bulk Lorentz factor, the deceleration time was found to correspond to the delayed onset in the optical band. While the delayed onset in the optical band is evidence that this emission had an external shock origin, the temporally extended emission in the GeV band before the optical onset is hard to reconcile with the standard synchrotron emission from the same external shock. This may imply that the common feature of GeV emission with a power-law decay does not necessarily have the same origin of the optical afterglow in all Fermi/LAT GRBs, particularly in their early epoch. Another emission mechanism to explain the GeV emission in GRB 170405A can be required such as an internal-shock or inverse Compton emission.

astro-ph.HE↗

arXiv2020

Synchrotron Gamma-Ray Emission Model of the Giant Outburst of Quasar 3C 279 in 2015 June: Fast Reconnection or Stochastic Acceleration with Electromagnetic Cascade?

We test the synchrotron emission scenario for the very bright gamma-ray flare of blazar 3C 279 observed in 2015 June using time-dependent numerical simulations. A bulk Lorentz factor as high as 100 can bring the synchrotron maximum energy above the GeV energy range. We find two possible solutions for the X-ray to gamma-ray spectrum. One is a prompt electron injection model with a hard power-law index as magnetic reconnection models suggest. A too strong magnetic field yields a too bright synchrotron X-ray flux due to secondary electron--positron pairs. Even in the prompt electron injection model, the Poynting flux luminosity is at most comparable to the gamma-ray or electron luminosity. Another model is the stochastic acceleration model, which leads to a very unique picture accompanying the electromagnetic cascade and re-acceleration of the secondary electron--positron pairs. In this model, the energy budget of the magnetic field is very low compared to gamma rays and electrons.

astro-ph.HE↗

arXiv2019

Closure relations of Gamma Ray Bursts in high energy emission

The synchrotron external shock model predicts the evolution of the spectral ($β$) and temporal ($α$) indices during the gamma-ray burst (GRB) afterglow for different environmental density profiles, electron spectral indices, electron cooling regimes, and regions of the spectrum. We study the relationship between $α$ and $β$, the so-called "closure relations" with GRBs detected by \textit{Fermi} Large Area Telescope (\textit{Fermi}-LAT) from 2008 August to 2018 August. The spectral and temporal indices for the > 100 MeV emission from the \textit{Fermi}-LAT as determined in the Second Fermi-LAT Gamma-ray Burst Catalog (2FLGC) are used in this work. We select GRBs whose spectral and temporal indices are well constrained (58 long-duration GRBs and 1 short-duration GRBs) and classify each GRB into the best-matched relation. As a result, we found that a number of GRBs require a very small fraction of the total energy density contained in the magnetic field ($ε_{B}$ $\lesssim$ 10$^{-7}$). The estimated mean and standard deviation of electron spectral index $\mathit{p}$ are 2.40 and 0.44, respectively. The GRBs satisfying a closure relation of the slow cooling tend to have a softer $\mathit{p}$ value compared to those of the fast cooling. Moreover, the Kolmogorov--Smirnov test of the two $\mathit{p}$ distributions from the fast and slow coolings rejects a hypothesis that the two distributions are drawn from the single reference distribution with a significance of 3.2 $σ$. Lastly, the uniform density medium is preferred over the medium that decreases like the inverse of distance squared for long-duration GRBs.

astro-ph.HE↗

arXiv2019

MAGIC eyes to the extreme: testing the blazar emission models on EHBLs

Extreme high-energy peaked BL Lac objects (EHBLs) are blazars whose synchrotron emission peaks at exceptionally high energies, above few keV, in the hard X-ray regime. So far, only a handful of those objects has been detected at very high energy (VHE, E > 100 GeV) gamma rays by Imaging Atmospheric Cherenkov Telescopes. Very remarkably, VHE observations of some of these blazars (like 1ES 0229+200) have provided evidence of a VHE gamma-ray emission extending to several TeV, which is difficult to explain with standard, one-zone synchrotron self-Compton models usually applied to BL Lac objects. The MAGIC collaboration coordinated a multi-year, multi-wavelength observational campaign on ten targets. The MAGIC telescopes detected VHE gamma rays from four EHBLs. In this paper we focus on the source 1ES 1426+426 and its X-ray and VHE gamma-ray properties. The results of different models (synchrotron self-Compton, spine-layer, hadronic) reproducing the broadband spectral energy distribution are also presented.

astro-ph.HE↗

arXiv2019

Long gamma-ray burst rate at very high redshift

Future missions for long gammma-ray burst (GRB) observations at high redshift such as HiZ-GUNDAM and THESEUS will provide clue to the star formation history in our universe. In this paper focusing on high redshift (z>8) GRBs, we calculate the detection rate of long GRBs by future observations, considering both Population (Pop) I&II stars and Pop III stars as GRB progenitors. For the Pop I&II star formation rate (SFR), we adopt an up-to-date model of high-redshift SFR based on the halo mass function and dark matter accretion rate obtained from cosmological simulations. We show that the Pop I&II GRB rate steeply decreases with redshift. This would rather enable us to detect the different type of GRBs, Pop III GRBs, at very high redshift. If 10% or more Pop III stars die as an ultra-long GRB, the future missions would detect such GRBs in one year in spite of their low fluence. More luminous GRBs are expected from massive compact Pop III stars produced via the binary merger. In our conventional case, the detection rate of such luminous GRBs is 3-20 /yr (z>8). Those future observations contribute to revealing of the Pop III star formation history.

astro-ph.HE↗

arXiv2019

Particle Energy Diffusion in Linear Magnetohydrodynamic Waves

In high-energy astronomical phenomena, the stochastic particle acceleration by turbulences is one of the promising processes to generate non-thermal particles. In this paper, we investigate the energy-diffusion efficiency of relativistic particles in a temporally evolving wave ensemble that consists of a single mode (Alfvén, fast or slow) of linear magnetohydrodynamic waves. In addition to the gyroresonance with waves, the transit-time damping (TTD) also contributes to the energy-diffusion for fast and slow-mode waves. While the resonance condition with the TTD has been considered to be fulfilled by a very small fraction of particles, our simulations show that a significant fraction of particles are in the TTD resonance owing to the resonance broadening by the mirror force, which non-resonantly diffuses the pitch angle of particles. When the cutoff scale in the turbulence spectrum is smaller than the Larmor radius of a particle, the gyroresonance is the main acceleration mechanism for all the three wave modes. For the fast-mode, the coexistence of the gyroresonance and TTD resonance leads to anomalous energy-diffusion. For a particle with its Larmor radius smaller than the cutoff scale, the gyroresonance is negligible, and the TTD becomes the dominant mechanism to diffuse its energy. The energy-diffusion by the TTD-only resonance with fast-mode waves agrees with the hard-sphere-like acceleration suggested in some high-energy astronomical phenomena.

astro-ph.HE↗

arXiv2018

Outflow and Emission Model of Pulsar Wind Nebulae with the Back-reaction of Particle Diffusion

We present a new pulsar wind nebula (PWN) model solving both advection and diffusion of non-thermal particles in a self-consistent way to satisfy the momentum and energy conservation laws. Assuming spherically symmetric (1--D) steady outflow, we calculate the emission spectrum integrating over the entire nebula and the radial profile of the surface brightness. We find that the back reaction of the particle diffusion modifies the flow profile. The photon spectrum and the surface brightness profile are different from the model calculations without the back reaction of the particle diffusion. Our model is applied to the two well-studied PWNe 3C 58 and G21.5-0.9. By fitting the spectra of these PWNe, we determine the parameter sets and calculate the radial profiles of X-ray surface brightness. For both the objects, obtained profiles of X-ray surface brightness and photon index are well consistent with observations. Our model suggests that particles escaped from the nebula significantly contribute to the gamma-ray flux. A $γ$-ray halo larger than the radio nebula is predicted in our model.

astro-ph.HE↗

arXiv2018

Late Engine Activity of GRB 161017A Revealed by Early Optical Observations

The long gamma-ray burst GRB 161017A was detected by Fermi and Swift, and its afterglow was observed by the MITSuME 50-cm optical telescope promptly about 50 s after the burst. Early optical observations revealed that the optical lightcurve exhibits a plateau and rebrightening in the early afterglow phase about 500 and 5000 s after the trigger, respectively. By investigating the behavior of the spectral and temporal flux variation, it was found that the plateau and re-brightening cannot be explained in the context of the simple standard afterglow model. These observational features can be explained with two independent refreshed shocks, which indicate the long-acting central engine. We evaluated the physical parameters of the subsequent shells, and we then determined the kinetic energy ratio of the two colliding shells to the leading shell to be roughly 1 and 8, respectively. In addition, two prominent X-ray flares about 200 s after the trigger may be signatures of delayed ejections of the energetic jets responsible for the refreshed shocks. Such late activity of the central engine and X-ray flares play a crucial role in understanding the mechanisms for jet formation and photon emission.

astro-ph.HE↗