arXiv subjects

Kunihito Ioka

Publications and source records attributed to Kunihito Ioka.

At least 37 records · Page 2Linked to original sources

arXiv2024

Parametric decay instability of circularly polarized Alfvén wave in magnetically dominated plasma

We investigate parametric decay instability (PDI) of circularly polarized Alfvén wave into daughter acoustic wave and backward Alfvén wave in magnetically-dominated plasma, in which the magnetization parameter $σ$ (energy density ratio of background magnetic field to matter) exceeds unity. We analyze relativistic magnetohydrodynamics (MHD), focusing on wave frequencies sufficiently lower than the plasma and cyclotron frequencies. We derive analytical formulae for the dispersion relation and growth rate of the instability as a function of the magnetization $σ$, wave amplitude $η$, and plasma temperature $θ$. We find that PDI persists even in high magnetization $σ$, albeit with a decreased growth rate up to $σ\to\infty$. Our formulae are useful for estimating the decay of Alfvén wave into acoustic wave and heat in high magnetization $σ$ plasma, which is a ubiquitous phenomenon such as in pulsars, magnetars, and fast radio bursts.

astro-ph.HE↗

arXiv2024

Collective Thomson scattering in magnetized electron and positron pair plasma and the application to induced Compton scattering

We consider collective Thomson scattering of an incident X-mode wave (with the electric vector perpendicular to the background magnetic field) in magnetized electron and positron pair plasma. The collective effects do not exactly cancel out in contrast to the non-magnetized case. Still, the cross-section is comparable to the non-collective one, with the same suppression by the square of the cyclotron frequency in a strong magnetic field. The comparable cross-section holds even though the net current is nearly zero from the drift motion of electrons and positrons. The plasma response does not also affect the cross-section so much. The spectrum of the scattered wave in finite temperature plasma peaks at cyclotron overtones. Based on these results, we also estimate induced Compton scattering in strongly magnetized pair plasma. Implications for pulsars and fast radio bursts are discussed.

astro-ph.HE↗

arXiv2024

Late Engine Activity in Neutron Star Mergers and Its Cocoon: An Alternative Scenario for the Blue Kilonova

Follow-up observations of short gamma-ray bursts (sGRBs) have continuously unveiled late extended/plateau emissions, attributed to jet launch due to late engine activity, the nature of which remains enigmatic. Observations of GW170817 confirmed that sGRBs are linked to neutron star (NS) mergers, and discovered a kilonova (KN) transient. Nevertheless, the origin of the early "blue" KN in GW170817 remains unclear. Here, we investigate the propagation of late jets in the merger ejecta. By analytically modeling jet dynamics, we determine the properties of the jet heated cocoon, and estimate its cooling emission. Our results reveal that late jets generate significantly brighter cocoons compared to prompt jets, primarily due to reduced energy loss by adiabatic cooling. Notably, with typical late jets, emission from the cocoon trapped inside the ejecta can reproduce the blue KN emission. We estimate that the forthcoming Einstein Probe mission will detect the early cocoon emission with a rate of $\sim 2.1_{-1.6}^{+3.2}$ yr$^{-1}$, and that optical/UV follow-ups in the LIGO-VIRGO-KAGRA O5 run will be able to detect $\sim 1.0_{-0.7}^{+1.5}$ cocoon emission events. As an electromagnetic counterpart, this emission provides an independent tool to probe NS mergers in the Universe, complementing insights from sGRBs and gravitational waves.

astro-ph.HE↗

arXiv2023

Outflow energy and black-hole spin evolution in collapsar scenarios

We explore the collapsar scenario for long gamma-ray bursts by performing axisymmetric neutrino-radiation magnetohydrodynamics simulations in full general relativity for the first time. In this paper, we pay particular attention to the outflow energy and the evolution of the black-hole spin. We show that for a strong magnetic field with an aligned field configuration initially given, a jet is launched by magnetohydrodynamical effects before the formation of a disk and a torus, and after the jet launch, the matter accretion onto the black hole is halted by the strong magnetic pressure, leading to the spin-down of the black hole due to the Blandford-Znajek mechanism. The spin-down timescale depends strongly on the magnetic-field strength initially given because the magnetic-field strength on the black-hole horizon, which is determined by the mass infall rate at the jet launch, depends strongly on the initial condition, although the total jet-outflow energy appears to be huge $>10^{53}$ erg depending only weakly on the initial field strength and configuration. For the models in which the magnetic-field configuration is not suitable for quick jet launch, a torus is formed and after a long-term magnetic-field amplification, a jet can be launched. For this case, the matter accretion onto the black hole continues even after the jet launch and black-hole spin-down is not found. We also find that the jet launch is often accompanied with the powerful explosion of the entire star with the explosion energy of order $10^{52}$ erg by magnetohydrodynamical effects. We discuss an issue of the overproduced energy for the early-jet-launch models.

astro-ph.HE↗

arXiv2023

Cocoon cooling emission in neutron star mergers

In the gravitational wave event GW170817, there was a $\sim 10$ hours gap before electromagnetic (EM) observations, without detection of the cocoon. The cocoon is heated by a \textit{short} gamma-ray burst (\textit{s}GRB) jet propagating through the ejecta of a Neutron Star (NS) merger, and a part of the cocoon escapes the ejecta with an opening angle of $20^{\circ}$--$30^{\circ}$. Here we model the cocoon and calculate its EM emission. Our 2D hydrodynamic simulations suggest that the density and energy distributions, after entering homologous expansion, are well-fitted with power-law functions, in each of the relativistic and non-relativistic parts of the escaped cocoon. Modeling these features, we calculate the cooling emission analytically. We find that the cocoon outshines the r-process kilonova/macronova at early times (10--10$^{3}$ s), peaking at UV bands. The relativistic velocity of the cocoon's photosphere is measurable with instruments such as Swift, ULTRASAT and LSST. We also imply that energetic cocoons, including failed jets, might be detected as X-ray flashes. Our model clarifies the physics and parameter dependence, covering a wide variety of central engines and ejecta of NS mergers and \textit{s}GRBs in the multi-messenger era.

astro-ph.HE↗

arXiv2023

Off-axis MeV and very-high-energy gamma-ray emissions from structured gamma-ray burst jets

Very-high-energy (VHE) photons around TeV energies from a gamma-ray burst (GRB) jet will play an essential role in the multi-messenger era, with a fair fraction of the events being observed off-axis to the jet. We show that different energy photons (MeV and TeV photons in particular) arrive from different emission zones for off-axis observers even if the emission radius is the same. The location of the emission region depends on the jet structure of the surface brightness, and the structures are generally different at different energies, mainly due to the attenuation of VHE photons by electron-positron pair creation. This off-axis zone-shift effect does not justify the usual one-zone approximation and also produces a time-delay of VHE photons comparable to the GRB duration, which is crucial for future VHE observations, such as by the Cherenkov Telescope Array.

astro-ph.HE↗

arXiv2023

External Inverse-Compton and Proton Synchrotron Emission from the Reverse Shock as the Origin of VHE Gamma-Rays from the Hyper-Bright GRB 221009A

The detection of the hyper-bright gamma-ray burst (GRB) 221009A enables us to explore the nature of GRB emission and the origin of very-high-energy (VHE) gamma-rays. We analyze the ${\it Fermi}$-LAT data and investigate GeV-TeV emission in the framework of the external reverse shock model. We show that early $\sim1-10$ GeV emission can be explained by the external inverse-Compton mechanism via upscattering MeV gamma-rays by electrons accelerated at the reverse shock, in addition to the synchrotron self-Compton component. The predicted early optical flux could have been brighter than the naked-eye GRB 080319B. We also show that proton synchrotron emission from accelerated ultra-high-energy cosmic rays (UHECRs) is detectable, and could potentially explain $\gtrsim \rm TeV$ photons detected by LHAASO or UHECR acceleration can be constrained. Our model suggests that the detection of $\mathcal{O}(10\rm~TeV)$ photons with energy up to $\sim18$ TeV is possible for reasonable models of the extragalactic background light without invoking new physics, and predicts anti-correlations between MeV photons and TeV photons, which can be tested with the LHAASO data.

astro-ph.HE↗

arXiv2023

Expanding Fireball in Magnetar Bursts and Fast Radio Bursts

A fireball of radiation plasma created near the surface of a neutron star (NS) expands under its own pressure along magnetic field lines, and produces photon emission and relativistic matter outflow. We comprehensively classify the expanding fireball evolution into five cases and obtain the photospheric luminosity and the kinetic energy of the outflow, taking into account key processes; lateral diffusion of photons escaping from a magnetic flux tube, effects of strong magnetic field, baryon loading from the NS surface, and radiative acceleration via cyclotron resonant scattering, some of which have not been considered in the context of gamma-ray bursts. Applying our model to magnetar bursts with fast radio bursts (FRBs), in particular the X-ray short bursts from SGR 1935+2154 associated with the Galactic FRB 20200428A, we show that the burst radiation can accelerate the outflow to high Lorentz factor with sufficient energy to power FRBs.

astro-ph.HE↗

arXiv2022

Cocoon breakout and escape from the ejecta of neutron star mergers

The cocoon is an inevitable product of a jet propagating through ambient matter, and takes a fair fraction of the jet energy. In short gamma-ray bursts, the ambient matter is the ejecta from the merger of neutron stars, expanding with a high velocity $\sim 0.2 c$, in contrast to the static stellar envelope in collapsars. Using 2D relativistic hydrodynamic simulations with the ejecta density profile as $ρ\propto r^{-2}$, we find that the expansion makes a big difference; only 0.5--5\% of the cocoon mass escapes from (faster than) the ejecta, with an opening angle $20^{\circ}$--$30^{\circ}$, while it is $\sim 100\%$ and spherical in collapsars. We also analytically obtain the shares of mass and energies for the escaped and trapped cocoons. Because the mass of the escaped cocoon is small and the trapped cocoon is concealed by the ejecta and the escaped cocoon, we conclude that it is unlikely that the cocoon emission was observed as a counterpart to the gravitational wave event GW170817.

astro-ph.HE↗

arXiv2022

Probing particle acceleration at trans-relativistic shocks with off-axis gamma-ray burst afterglows

Particle acceleration is expected to be different between relativistic and non-relativistic collisionless shocks. We show that electromagnetic counterparts to gravitational waves (GWs), gamma-ray burst (GRB) afterglows, are ideal targets for observing trans-relativistic evolution of accelerated electron distribution because the GWs spot nearby GRBs with off-axis jets, otherwise missed in gamma-ray observations. We find that the relativistic spectral slope begins to change steeply near the peak time of the light curve and approaches the non-relativistic limit in about 10 times the peak time. The trans-relativistic evolution of the afterglow synchrotron spectrum is consistent with GRB 170817A observations within errors, and will be measurable in similar but more distant events at a GW horizon $\sim 200$ Mpc in a denser environment. We roughly estimate that such events represent a fraction of 10-50 per cent of the GRB 170817A-like off-axis short GRBs. We also find that the spectral evolution does not depend on the jet structure if their light curves are similar to each other.

astro-ph.HE↗

arXiv2022

Probing cosmology and gastrophysics with fast radio bursts: Cross-correlations of dark matter haloes and cosmic dispersion measures

For future surveys of fast radio bursts (FRBs), we clarify information available from cosmic dispersion measures (DMs) through cross-correlation analyses of foreground dark matter haloes (hosting galaxies and galaxy clusters) with their known redshifts. With a halo-model approach, we predict that the cross-correlation with cluster-sized haloes is less affected by the details of gastrophysics, providing robust cosmological information. For less massive haloes, the cross-correlation at angular scales of $<10\, \mathrm{arcmin}$ is sensitive to gas expelled from the halo centre due to galactic feedback. Assuming $20000$ FRBs over $20000 \, {\rm deg}^2$ with a localisation error being 3 arcmin, we expect that the cross-correlation signal at halo masses of $10^{12}-10^{14}\, M_\odot$ can be measured with a level of $\sim 1\%$ precision in a redshift range of $0<z<1$. Such precise measurements enable one to put a 1.5\% level constraint on $σ_8\, (Ω_\mathrm{M}/0.3)^{0.5}$ and a 3\% level constraint on $(Ω_\mathrm{b}/0.049)(h/0.67)(f_\mathrm{e}/0.95)$ ($σ_8$, $Ω_\mathrm{M}$, $Ω_\mathrm{b}$, $h$ and $f_\mathrm{e}$ are the linear mass variance smoothed at $8\, h^{-1}\mathrm{Mpc}$, mean mass density, mean baryon density, the present-day Hubble parameter and fraction of free electrons in cosmic baryons today), whereas the gas-to-halo mass relation in galaxies and clusters can be constrained with a level of $10\%-20\%$. Furthermore the cross-correlation analyses can break the degeneracy among $Ω_\mathrm{b}$, $h$ and $f_\mathrm{e}$, inherent in the DM-redshift relation. Our proposal opens new possibilities for FRB cosmology, while it requires extensive galaxy redshift catalogues and further improvement of the halo model.

astro-ph.CO↗

arXiv2021

Fallback Accretion Halted by R-process Heating in Neutron Star Mergers and Gamma-Ray Bursts

The gravitational wave event GW170817 with a macronova/kilonova shows that a merger of two neutron stars ejects matter with radioactivity including $r$-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB), such as extended and plateau emissions. We investigate the fallback accretion with the $r$-process heating by performing one-dimensional hydrodynamic simulations and developing a semi-analytical model. We show that the usual fallback rate $dM/dt \propto t^{-5/3}$ is halted by the heating because pressure gradients accelerate ejecta beyond an escape velocity. The suppression is steeper than Chevalier's power-law model through Bondi accretion within a turn-around radius. The characteristic halting timescale is $\sim 10^5$--$10^9$ sec for the GW170817-like $r$-process heating, which is long enough to continue the long-lasting emission of sGRBs. The halting timescale is sensitive to the uncertainty of the $r$-process. Future observation of fallback halting could constrain the $r$-process heating on the year scale.

astro-ph.HE↗

arXiv2021

Fallback Accretion Model for the Years-to-Decades X-ray Counterpart to GW170817

A new component was reported in the X-ray counterpart to the binary neutron-star merger and gravitational wave event GW170817, exceeding the afterglow emission from an off-axis structured jet. The afterglow emission from the kilonova/macronova ejecta may explain the X-ray excess but exceeds the radio observations if the spectrum is the same. We propose a fallback accretion model that a part of ejecta from the neutron star merger falls back and forms a disk around the central compact object. In the super-Eddington accretion phase, the X-ray luminosity stays near the Eddington limit of a few solar masses and the radio is weak, as observed. This will be followed by a power law decay. The duration of the constant luminosity phase conveys the initial fallback timescale $t_0$ in the past. The current multi-year duration requires $t_0 > 3$--$30$ sec, suggesting that the disk wind rather than the dynamical ejecta falls back after the jet launch. Future observations in the next decades will probe the timescale of $t_0 \sim 10$--$10^4$ sec, around the time of extended emission in short gamma-ray bursts. The fallback accretion has not been halted by the $r$-process heating, implying that fission is weak on the year scale. We predict that the X-ray counterpart will disappear in a few decades due to the $r$-process halting or the depletion of fallback matter.

astro-ph.HE↗

arXiv2021

Binary comb models for FRB 121102

The first repeating fast radio burst source, FRB 121102, is observed to emit bursts periodically. We show that FRB 121102 can be interpreted as an interacting neutron star binary system with an orbital period of 159 days. We develop a binary comb model by introducing an eccentricity in the orbit. Besides the original funnel mode of the binary comb model, which was applied to FRB 180916.J0158+65 by Ioka and Zhang 2020, we also identify two new modes of the binary comb model, i.e. the tau-crossing mode and the inverse funnel mode, and apply them to interpret FRB 121102. These new developments expand the applicable parameter space, allowing the companion star to be a massive star, a massive black hole, or a supermassive black hole, with the latter two having larger parameter spaces. These models are also consistent with other observations, such as the persistent bright radio counterpart associated with the source. We also argue that the observed frequency dependence of the active window does not disfavor the binary comb model, in contrast to recent claims, and propose two possible scenarios to interpret the data.

astro-ph.HE↗

arXiv2021

Gravitational Wave Physics and Astronomy in the nascent era

The detections of gravitational waves (GW) by LIGO/Virgo collaborations provide various possibilities to physics and astronomy. We are quite sure that GW observations will develop a lot both in precision and in number owing to the continuous works for the improvement of detectors, including the expectation to the newly joined detector, KAGRA, and the planned detector, LIGO-India. In this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics.

gr-qc↗

arXiv2021

Neutron conversion-diffusion: a new model for structured short gamma-ray burst jets compatible with GRB 170817

We present a generic theoretical model for the structuring of a relativistic jet propagating through the ejecta of a binary neutron star merger event, introducing the effects of the neutron conversion-diffusion, which provides a baryon flux propagating transversely from the ejecta towards the jet axis. This results naturally in an increased baryon load structure of the outer jet with the approximate isotropic energy distribution $E_{iso}(θ) \propto θ^{-4}$, which is compatible with the first gravitational wave and short gamma-ray burst event GW170817/GRB 170817A observed at an off-axis angle of the jet.

astro-ph.HE↗

arXiv2021

Statistical modelling of the cosmological dispersion measure

We have investigated the basic statistics of the cosmological dispersion measure (DM) -- such as its mean, variance, probability distribution, angular power spectrum and correlation function -- using the state-of-the-art hydrodynamic simulations, IllustrisTNG300, for the fast radio burst (FRB) cosmology. To model the DM statistics, we first measured the free-electron abundance and the power spectrum of its spatial fluctuations. The free-electron power spectrum turns out to be consistent with the dark matter power spectrum at large scales, but it is strongly damped at small scales ($\lesssim 1$Mpc) owing to the stellar and active galactic nucleus feedback. The free-electron power spectrum is well modelled using a scale-dependent bias factor (the ratio of its fluctuation amplitude to that of the dark matter). We provide analytical fitting functions for the free-electron abundance and its bias factor. We next constructed mock sky maps of the DM by performing standard ray-tracing simulations with the TNG300 data. The DM statistics are calculated analytically from the fitting functions of the free-electron distribution, which agree well with the simulation results measured from the mock maps. We have also obtained the probability distribution of source redshift for a given DM, which helps in identifying the host galaxies of FRBs from the measured DMs. The angular two-point correlation function of the DM is described by a simple power law, $ξ(θ) \approx 2400 (θ/{\rm deg})^{-1} \, {\rm pc}^2 \, {\rm cm}^{-6}$, which we anticipate will be confirmed by future observations when thousands of FRBs are available.

astro-ph.CO↗

arXiv2021

Diverse Jet Structures Consistent with the Off-axis Afterglow of GRB 170817A

The jet structure of short gamma-ray bursts (GRBs) has been controversial after the detection of GRB 170817A as the electromagnetic counterparts to the gravitational wave event GW170817. Different authors use different jet structures for calculating the afterglow light curves. We formulated a method to inversely reconstruct a jet structure from a given off-axis GRB afterglow, without assuming any functional form of the structure. By systematically applying our inversion method, we find that more diverse jet structures are consistent with the observed afterglow of GRB 170817A within errors: such as hollow-cone, spindle, Gaussian, and power-law jet structures. In addition, the total energy of the reconstructed jet is arbitrary, proportional to the ambient density $n_0$, with keeping the same jet shape if the parameters satisfy the degeneracy combination $n_0 \varepsilon_\mathrm{B}^{(p+1)/(p+5)} \varepsilon_\mathrm{e}^{4(p-1)/(p+5)} = \mathrm{const.}$. Observational accuracy less than $\sim 6$ per cent is necessary to distinguish the different shapes, while the degeneracy of the energy scaling would be broken by observing the spectral breaks and viewing angle. Future events in denser environment with brighter afterglows and observable spectral breaks are ideal for our inversion method to pin down the jet structure, providing the key to the jet formation and propagation.

astro-ph.HE↗