arXiv subjects

Ryo Yamazaki

Publications and source records attributed to Ryo Yamazaki.

At least 19 recordsLinked to original sources

arXiv2026

Revisiting early afterglows of gamma-ray bursts with finite-thickness ejecta: Implications from XRF 080330 and GRB 080710

We revisit the physical origin of the achromatic peaks and breaks observed several thousand seconds after the burst in the multi-wavelength afterglows of XRF 080330 and GRB 080710. Using a numerical afterglow model that consistently incorporates finite ejecta thickness and a generalized external density profile, we perform Bayesian inference to estimate model parameters describing these events. Our analysis shows that the gradual rise and achromatic temporal features in both events are more naturally explained by jet dynamical evolution with finite shell thickness rather than by off-axis viewing effects. The inferred initial radial width of the ejecta is of order $10^{13}$ cm for both bursts, implying a central engine activity timescale significantly longer than that suggested by the prompt gamma-ray duration alone. Taken together, these results demonstrate that early afterglow light curves are strongly influenced by transition dynamics when finite ejecta thickness is properly taken into account, thereby providing a physical link between the prompt and afterglow phases and highlighting limitations of simply applying the thin-shell approximation when interpreting early-time afterglows. Furthermore, Bayesian model comparison favors a generalized circumburst density profile over the canonical uniform or steady-wind models, suggesting that fixing the external density structure to idealized profiles a priori may obscure crucial information about the progenitor's pre-burst activity.

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arXiv2025

The electron temperature distribution and the high ionization just behind the shock in the Cygnus Loop

The physical processes behind astrophysical collisionless shocks, such as thermal relaxation and ionization after shock passage, remain poorly understood. To investigate these processes, we analyze the northeastern region of the Cygnus Loop with XMM-Newton. The electron temperature is found to increase towards the interior of the remnant ranging from 0.15-0.19 keV energy range within a spatial scale of 6 arcmin (or 1.27 pc at a distance of 725 pc) from the shock front. This can be explained well by a modified Sedov solution with radiative cooling. We also show that the ionization timescales determined from our spectroscopy are significantly larger than those estimated based on the electron density of the surrounding materials and the shock velocity. This excess can be qualitatively explained by a mixing of inner multiple plasma components with different ionization states due to turbulence.

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arXiv2025

Double-Peaked Optical Afterglow in GRB 110213A Inferring a Magnetized Thick Shell Ejecta

Gamma-ray bursts early afterglows are important tracers for determining the radial structure and magnetization of the ejecta. In this paper, we focus on GRB 110213A that shows double-peaked optical afterglow lightcurves and the shallow decay feature of the X-ray afterglow. We adopt a semi-analytic model for the dynamics of forward and reverse shocks generated through an interaction between an arbitrary magnetized ejecta with a finite thickness and a stratified circumstellar medium. Multiwavelength radiation from forward and reverse shocks seen from an arbitrary viewing angle is calculated under a thin-shell approximation. Our analysis with multimodal nested sampling methods for GRB 110213A suggests that the thick shell ejecta naturally explains the shallow decay feature of the X-ray afterglow. The combination of the reverse shock emission in the strongly magnetized jet and forward shock emission in the weakly magnetized circumstellar medium makes the double peak feature of the optical afterglows. The estimated low radiative efficiency in the prompt phase may be a consequence of the high magnetization of the jet in this case. A multi-messenger emission simulator based on the magnetic bullet afterglow model is publicly available as the open source Julia package "Magglow".

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arXiv2025

Mass Determination of Supermassive Black Holes Governing Evolution of Radio Emitters

Tidal disruption events (TDEs) involving supermassive black holes (SMBHs) often exhibit radio emission, yet its physical origin remains uncertain, especially in non-jetted cases. In this Letter, we formulate a general dynamical framework for a radio-emitting shell driven by disk winds and expanding through a power-law ambient medium under the influence of SMBH gravity. We derive and classify power-law-in-time solutions to the governing equations in the adiabatic regime. In particular, a universal $t^{2/3}$ scaling emerges naturally when gravitational energy dominates or is comparable to thermal energy, irrespective of the ambient density profile, whereas the classical Sedov-Taylor solution is recovered when gravity is negligible. Our analysis reveals that, in regimes where SMBH gravity governs the shell expansion, the SMBH mass can be inferred from radio observations of the shell. This approach is independent of and complementary to conventional mass estimators, with direct implications for interpreting radio-emitting TDEs and probing SMBH demographics. Our formalism further predicts that 10-100 GHz monitoring with existing and planned facilities can yield SMBH masses within months of disruption, providing a time-domain analogue to reverberation mapping.

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arXiv2025

Two-component jet model for the afterglow emission of GRB 201216C and GRB 221009A and implications for jet structure of very-high-energy gamma-ray bursts

In recent years, afterglow emission in the very-high-energy (VHE) band above 100 GeV has been clearly detected for at least five gamma-ray bursts (GRBs 180720B, 190114C, 190829A, 201216C and 221009A). For some of these VHE GRBs, we previously proposed a two-component jet model, consisting of two uniform jets with narrow and wide opening angles to explain their multiwavelength afterglows including VHE gamma rays. In this paper, we show that the VHE spectra and light curves of GRBs 201216C and 221009A can also be reasonably explained by our two-component jet model, based on two top-hat jets propagating into a constant-density circumburst medium. We find that for the five VHE GRBs, the collimation-corrected kinetic energies of the narrow and wide jets have typical values of 5*10^{49} erg and 5*10^{50} erg, respectively. We discuss the similarities and differences among the VHE GRBs, and the implications for the structure of their jets. In agreement with previous studies, the narrow jet of GRB 221009A has an atypically small opening angle, so that its intrinsic, collimation-corrected energy remains within a plausible range despite the unusually large isotropic-equivalent energy.

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arXiv2025

Measurement of the Forward Shock Velocities of the Supernova Remnant N132D Based on the Thermal X-ray Emission

Measuring shock velocities is crucial for understanding the energy transfer processes at the shock fronts of supernova remnants (SNRs), including acceleration of cosmic rays. Here we present shock velocity measurements on the SNR N132D, based on the thermal properties of the shock-heated interstellar medium. We apply a self-consistent model developed in our previous work to X-ray data from deep Chandra observations with an effective exposure of $\sim$ 900 ks. In our model, both temperature and ionization relaxation processes in post-shock plasmas are simultaneously calculated, so that we can trace back to the initial condition of the shock-heated plasma to constrain the shock velocity. We reveal that the shock velocity ranges from 800 to 1500 $\rm{km~s^{-1}}$ with moderate azimuthal dependence. Although our measurement is consistent with the velocity determined by independent proper motion measurements in the south rim regions, a large discrepancy between the two measurements (up to a factor of 4) is found in the north rim regions. This implies that a substantial amount of the kinetic energy has been transferred to the nonthermal component through highly efficient particle acceleration. Our results are qualitatively consistent with the $\gamma$-ray observations of this SNR.

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arXiv2025

Gravitational Wave Memory from Accelerating Relativistic Jets in Multiple Thick Shell Scenarios

Gravitational wave (GW) memory, a permanent distortion of the space-time metric, is anticipated during the acceleration of relativistic jets in gamma-ray bursts (GRBs). While the precise mechanism behind GRBs is not yet fully understood, detecting GW memory may contribute to clarifying their nature. In this paper, we consider various scenarios of GW memory emission, including both single and multiple shells with thin- and thick-shells. In particular, the memory spectrum for each scenario is compared with the sensitivity of next-generation detectors, namely DECIGO and ET-D. Physical properties spread over a broad-band region, emphasizing the importance of combined and wide-band observations. We also simulate GW memory based on nearby, realistic scenarios and demonstrate its detectability.

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arXiv2024

Is the low-energy tail of shock-accelerated protons responsible for over-ionized plasma in supernova remnants?

Over-ionized, recombining plasma is an emerging class of X-ray bright supernova remnants (SNRs). This unique thermal state where the ionization temperature ($T_{\rm z}$) is significantly higher than the electron temperature ($T_{\rm e}$) is not expected from the standard evolution model assuming a point explosion in a uniform interstellar medium, requiring a new scenario for the dynamical and thermal evolution. A recently proposed idea attributes the over-ionization state to additional ionization contribution from the low-energy tail of shock-accelerated protons. However, this new scenario has been left untested, especially from the atomic physics point of view. We report calculation results of the proton impact ionization rates of heavy-element ions in ejecta of SNRs. We conservatively estimate the requirement for accelerated protons, and find that their relative number density to thermal electrons needs to be higher than $5~(T_{\rm e}/{\rm 1~keV})\%$ in order to explain the observed over-ionization degree at $T_{\rm z}/T_{\rm e} \ge 2$ for K-shell emission. We conclude that the proton ionization scenario is not feasible because such a high abundance of accelerated protons is prohibited by the injection fraction from thermal to non-thermal energies, which is expected to be $\sim 1\%$ at largest.

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arXiv2023

Observational constraints on the maximum energies of accelerated particles in supernova remnants

Supernova remnants (SNRs) are thought to be the most plausible sources of Galactic cosmic rays. One of the principal questions is whether they are accelerating particles up to the maximum energy of Galactic cosmic rays ($\sim$PeV). In this paper, we summarize our recent studies on gamma-ray-emitting SNRs. We first evaluated the reliability of SNR age estimates to quantitatively discuss time dependence of their acceleration parameters. Then we systematically modeled their gamma-ray spectra to constrain the acceleration parameters. The current maximum energy estimates were found to be well below PeV for most sources. The basic time dependence of the maximum energy assuming the Sedov evolution ($\approx t^{-0.8\pm0.2}$) cannot be explained with the simplest acceleration condition (Bohm limit) and requires shock-ISM (interstellar medium) interaction. The inferred maximum energies during lifetime averaged over the sample can be expressed as $\lesssim 20$ TeV ($t_{{\rm M}}/\text{1 kyr})^{-0.8}$ with $t_{\rm M}$ being the age at the maximum, which reaches $\sim$PeV only if $t_{\rm M} \lesssim 10$ yr. The maximum energies during lifetime are suggested to have a variety of 1-2 orders of magnitude from object to object on the other hand. This variety will reflect the dependence on environments.

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arXiv2023

GRB 080710: A narrow, structured jet showing a late, achromatic peak in the optical and infrared afterglow?

We present a possible theoretical interpretation of the observed afterglow emission of long gamma-ray burst GRB 080710. While its prompt GRB emission properties are normal, the afterglow light curves in the optical and infrared bands are exceptional in two respects. One is that the observed light curves of different wavelengths have maximum at the same time, and that the achromatic peak time, $2.2\times10^3$ s after the burst trigger, is about an order of magnitude later than typical events. The other is that the observed flux before the peak increases more slowly than theoretically expected so far. Assuming that the angular distribution of the outflow energy is top-hat or Gaussian-shaped, we calculate the observed light curves of the synchrotron emission from the relativistic jets and explore the model parameters that explain the observed data. It is found that a narrowly collimated Gaussian-shaped jet with large isotropic-equivalent energy is the most plausible model for reproducing the observed afterglow behavior. Namely, an off-axis afterglow scenario to the achromatic peak is unlikely. The inferred values of the opening angle and the isotropic equivalent energy of the jet are possibly similar to those of GRB 221009A, but the jet of GRB 080710 has a much smaller efficiency of the prompt gamma-ray emission. Our results indicate a greater diversity of the GRB jet properties than previously thought.

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arXiv2023

On the influence of shock-cloud interactions on the nonthermal X-ray emission from the supernova remnant RCW 86

It is an open issue how the surrounding environment of supernova remnant shocks affect nonthermal X-rays from accelerated electrons, with or without interacting dense material. We have conducted spatially resolved X-ray spectroscopy of the shock-cloud interacting region of RCW 86 with XMM-Newton. It is found that bright soft X-ray filaments surround the dense cloud observed with 12CO and HI emission lines. These filaments are brighter in thermal X-ray emission, and fainter and possibly softer in synchrotron X-rays, compared to those without interaction. Our results show that the shock decelerates due to the interaction with clouds, which results in an enhancements of thermal X-ray emission. This could possibly also explain the softer X-ray synchrotron component, because it implies that those shocks that move through a low density environment, and therefore decelerate much less, can be more efficient accelerators. This is similar to SN 1006 and Tycho, and is in contrast to RX J1713.7-3946. This difference among remnants may be due to the clumpiness of dense material interacting with the shock, which should be examined with future observations.

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arXiv2023

Disk-wind-driven Expanding Radio-emitting Shell in Tidal Disruption Events

We study the evolution of a non-relativistically expanding thin shell in radio-emitting tidal disruption events (TDEs) based on a one-dimensional spherically symmetric model considering the effect of both a time-dependent mass loss rate of the disk wind and the ambient mass distribution. The analytical solutions are derived in two extreme limits: one is the approximate solution near the origin in the form of the Taylor series, and the other is the asymptotic solution in which the ambient matter is dominant far away from the origin. Our numerical solutions are confirmed to agree with the respective analytical solutions. We find that no simple power-law of time solution exists in early to middle times because the mass loss rate varies over time, affecting the shell dynamics. We also discuss the application of our model to the observed radio-emitting TDE, AT2019dsg.

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arXiv2023

Two-component jet model for multiwavelength afterglow emission of the extremely energetic burst GRB 221009A

Recently gamma-ray bursts (GRBs) have been detected at very high-energy (VHE) gamma-rays by imaging atmospheric Cherenkov telescopes, and a two-component jet model has often been invoked to explain multiwavelength data. In this work, multiwavelength afterglow emission from an extremely bright GRB, GRB 221009A, is examined. The isotropic-equivalent gamma-ray energy of this event is among the largest, which suggests that similarly to previous VHE GRBs, the jet opening angle is so small that the collimation-corrected gamma-ray energy is nominal. Afterglow emission from such a narrow jet decays too rapidly, especially if the jet propagates into uniform circumburst material. In the two-component jet model, another wide jet component with a smaller Lorentz factor dominates late-time afterglow emission, and we show that multiwavelength data of GRB 221009A can be explained by narrow and wide jets with opening angles similar to those employed for other VHE GRBs. We also discuss how model degeneracies can be disentangled with observations.

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arXiv2023

The Radio to GeV Afterglow of GRB 221009A

GRB 221009A ($z=0.151$) is one of the closest known long $γ$-ray bursts (GRBs). Its extreme brightness across all electromagnetic wavelengths provides an unprecedented opportunity to study a member of this still-mysterious class of transients in exquisite detail. We present multi-wavelength observations of this extraordinary event, spanning 15 orders of magnitude in photon energy from radio to $γ$-rays. We find that the data can be partially explained by a forward shock (FS) from a highly-collimated relativistic jet interacting with a low-density wind-like medium. Under this model, the jet's beaming-corrected kinetic energy ($E_K \sim 4\times10^{50}$ erg) is typical for the GRB population. The radio and mm data provide strong limiting constraints on the FS model, but require the presence of an additional emission component. From equipartition arguments, we find that the radio emission is likely produced by a small amount of mass ($\lesssim6\times10^{-7} M_\odot$) moving relativistically ($Γ\gtrsim9$) with a large kinetic energy ($\gtrsim10^{49}$ erg). However, the temporal evolution of this component does not follow prescriptions for synchrotron radiation from a single power-law distribution of electrons (e.g. in a reverse shock or two-component jet), or a thermal electron population, perhaps suggesting that one of the standard assumptions of afterglow theory is violated. GRB 221009A will likely remain detectable with radio telescopes for years to come, providing a valuable opportunity to track the full lifecycle of a powerful relativistic jet.

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arXiv2022

Synchrotron Self-Compton Emission in the Two-Component Jet Model for Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are intense bursts of high-energy photons (prompt emissions) caused by relativistic jets. After the emissions, multi-wavelength afterglows, from radio to very-high-energy (VHE) gamma-ray, last for more than a few days. In the past three years, the VHE gamma-ray photons from four GRBs (GRBs 180720B, 190114C, 190829A and 201216C) were detected by ground-based Imaging Atmospheric Cherenkov Telescopes, such as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the High Energy Stereoscopic System (H.E.S.S.). One of them, GRB 190829A, had some peculiar features of showing achromatic peaks in X-ray and optical bands at $1.4\times10^3$ s and being classified as low-luminosity GRBs. Previously, we proposed a two-component jet model, which has `narrow jet' with a small initial opening half-angle $θ_0=0.015$ rad and large bulk Lorentz factor $Γ_0=350$, and `wide jet' with $θ_0=0.1$ rad and $Γ_0=20$. The narrow jet explained the early X-ray and optical emissions and apparently small isotropic gamma-ray energy and peak energy in the off-axis viewing case. Furthermore, the late X-ray and radio (1.3 and 15.5 GHz) afterglows were emitted from the wide jet. Here, we calculate the VHE gamma-ray flux by the synchrotron self-Compton (SSC) emission. The multi-wavelength afterglows of GRB 190829A including the VHE gamma-ray emission are well explained by our two-component jet model. The afterglow emissions from our two-component jet are also consistent with the observational results of GRBs 180720B, 190114C and 201216C, when the jets are viewed on-axis. Furthermore, we discuss the detectability of off-axis orphan afterglows by the Cherenkov Telescope Array (CTA).

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arXiv2022

The Gamma-Ray Emission from the Supernova Remnant RX J1713.7-3946 Interacting with Two-phase Medium

We study the origin of gamma rays from the supernova remnant (SNR) RX J1713.7-3946. Using an analytical model, we calculate the distribution of cosmic rays (CRs) around the SNRs. Motivated by the results of previous studies, we assume that the SNR is interacting with two-phase interstellar medium (ISM), where dense clumps are surrounded by tenuous interclump medium. We also assume that only higher-energy protons (~> TeV) can penetrate the dense clumps. We find that pi^0-decay gamma rays produced by protons reproduce the observed gamma-ray spectrum peaked at ~TeV. On the other hand, it has recently been indicated that the observed ISM column density (N_p), the X-ray surface brightness (I_X), and the gamma-ray surface brightness (I_g) at grid points across the SNR form a plane in the three-dimensional (3D) space of (N_p, I_X, I_g). We find that the planar configuration is naturally reproduced if the ISM or the CR electron-to-proton ratio is not spherically uniform. We show that the shift of the observed data in the 3D space could be used to identify which of the quantities, the ISM density, the CR electron-to-proton ratio, or the magnetic field, varies in the azimuthal direction of the SNR.

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arXiv2022

X-ray Line Diagnostics of Ion Temperature at Cosmic-Ray Accelerating Collisionless Shocks

A novel collisionless shock jump condition is suggested by modeling the entropy production at the shock transition region. We also calculate downstream developments of the atomic ionization balance and the ion temperature relaxation in supernova remnants (SNRs). The injection process and subsequent acceleration of cosmic-rays (CRs) in the SNR shocks are closely related to the formation process of the collisionless shocks. The formation of the shock is caused by wave-particle interactions. Since the wave-particle interactions result in energy exchanges between electromagnetic fields and charged particles, the randomization of particles associated with the shock transition may occur with the rate given by the scalar product of the electric field and current. We find that order-of-magnitude estimates of the randomization with reasonable strength of the electromagnetic fields in the SNR constrain the amount of the CR nuclei and ion temperatures. The constrained amount of the CR nuclei can be sufficient to explain the Galactic CRs. The ion temperature becomes significantly lower than in the case of no CRs. To distinguish the case without CRs, we perform synthetic observations of atomic line emissions from the downstream region of the SNR RCW~86. Future observations by {\it XRISM} and {\it Athena} can distinguish whether the SNR shock accelerates the CRs or not from the ion temperatures.

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arXiv2022

Gamma-ray Diagnostics of r-process Nucleosynthesis in the Remnants of Galactic Binary Neutron-Star Mergers

We perform a full nuclear-network numerical calculation of the $r$-process nuclei in binary neutron-star mergers (NSMs), with the aim of estimating $γ$-ray emissions from the remnants of Galactic NSMs up to $10^6$ years old. The nucleosynthesis calculation of 4,070 nuclei is adopted to provide the elemental composition ratios of nuclei with an electron fraction $Y_{\rm e}$ between 0.10 and 0.45 . The decay processes of 3,237 unstable nuclei are simulated to extract the $γ$-ray spectra. As a result, the NSMs have different spectral color in $γ$-ray band from various other astronomical objects at less than $10^5$ years old. In addition, we propose a new line-diagnostic method for $Y_{\rm e}$ that uses the line ratios of either $^{137{\rm m}}$Ba/$^{85}$K or $^{243}$Am/$^{60{\rm m}}$Co, which become larger than unity for young and old $r$-process sites, respectively, with a low $Y_{\rm e}$ environment. From an estimation of the distance limit for $γ$-ray observations as a function of the age, the high sensitivity in the sub-MeV band, at approximately $10^{-9}$ photons s$^{-1}$ cm$^{-2}$ or $10^{-15}$ erg s$^{-1}$ cm$^{-2}$, is required to cover all the NSM remnants in our Galaxy if we assume that the population of NSMs by \citet{2019ApJ...880...23W}. A $γ$-ray survey with sensitivities of $10^{-8}$--$10^{-7}$ photons s$^{-1}$ cm$^{-2}$ or $10^{-14}$--$10^{-13}$ erg s$^{-1}$ cm$^{-2}$ in the 70--4000 keV band is expected to find emissions from at least one NSM remnant under the assumption of NSM rate of 30 Myr$^{-1}$. The feasibility of $γ$-ray missions to observe Galactic NSMs are also studied.

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