arXiv subjects

Shiho Kobayashi

Publications and source records attributed to Shiho Kobayashi.

69 records · Page 4Linked to original sources

arXiv2003

A Characteristic Dense Environment or Wind Signature in Prompt GRB Afterglows

We discuss the effects of synchrotron self-absorption in the prompt emission from the reverse shock of GRB afterglows occurring in a dense environment, such as the wind of a massive stellar progenitor or a dense ISM in early galaxies. We point out that, when synchrotron losses dominate over inverse Compton losses, the higher self-absorption frequency in a dense environment implies a bump in the reverse shock emission spectrum, which can result in a more complex optical/IR light curve than previously thought. This bump is prominent especially if the burst ejecta is highly magnetized. In the opposite case of low magnetization, inverse Compton losses lead to a prompt X-ray flare. These effects give a possible new diagnostic for the magnetic energy density in the fireball, and for the presence of a dense environment.

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arXiv2003

Early Optical Afterglows from Wind-Type Gamma-Ray Bursts

We study prompt optical emission from reverse shocks in the wind-type gamma-ray bursts. The emission is evaluated in both the thick and thin shell regimes. We discuss the angular time delay effect and the post-shock evolution of the fireball ejecta, which determine the decay index of the prompt optical emission and the duration of the radio flare. We discuss distinct emission signatures of the wind environment compared with the constant interstellar medium environment. We also present two recipes for directly constraining the initial or early time Lorentz factor of the fireball using the reverse and forward shock optical afterglow data for the wind case.

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arXiv2003

Gamma-ray burst early optical afterglows: implications for the initial Lorentz factor and the central engine

Early optical afterglows have been observed from GRB 990123, GRB 021004, and GRB 021211, which reveal rich emission features attributed to reverse shocks. It is expected that Swift will discover many more early afterglows. Here we investigate in a unified manner both the forward and the reverse external shock emission components, and introduce a straightforward recipe for directly constraining the initial Lorentz factor of the fireball using early optical afterglow data. The scheme is largely independent of the shock microphysics. We identify two types of combinations of the reverse and forward shock emission, and explore their parameter regimes. We also discuss a possible diagnostic for magnetized ejecta. There is evidence that the central engine of GRB 990123 is strongly magnetized.

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arXiv2003

Gravitational Radiation from Gamma-Ray Burst Progenitors

We study gravitational radiation from various proposed gamma-ray burst (GRB) progenitor models, in particular compact mergers and massive stellar collapses. These models have in common a high angular rotation rate, and the final stage involves a rotating black hole and accretion disk system. We consider the in-spiral, merger and ringing phases, and for massive collapses we consider the possible effects of asymmetric collapse and break-up, as well bar-mode instabilities in the disks. We calculate the strain and frequency of the gravitational waves expected from various progenitors, at distances based on occurrence rate estimates. Based on simplifying assumptions, we give estimates of the probability of detection of gravitational waves by the advanced LIGO system from the different GRB scenarios.

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arXiv2003

Polarized Gravitational Waves from Gamma-Ray Bursts

Significant gravitational wave emission is expected from gamma-ray bursts arising from compact stellar mergers, and possibly also from bursts associated with fast-rotating massive stellar core collapses. These models have in common a high angular rotation rate, and observations provide evidence for jet collimation of the photon emission, with properties depending on the polar angle, which may also be of relevance for X-ray flashes. Here we consider the gravitational wave emission and its polarization as a function of angle which is expected from such sources. We discuss possible correlations between the burst photon luminosity, or the delay between gravitational wave bursts and X-ray flashes, and the polarization degree of the gravitational waves.

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arXiv2002

GRB 021004: Reverse Shock Emission

We show that the re-brightening in the GRB 021004 optical afterglow light curve around ~ 0.1 day can be explained within the framework of the standard fireball model. The superposition of a forward and reverse shock emissions result in the peculiar behavior. With the standard values of parameters obtained in other afterglow observations, we can construct an example case in which theoretical estimates reasonably fit the broadband observations. Therefore, the peculiarity of the light curve might be a common feature in early gamma-ray burst afterglows.

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arXiv2002

Luminosity and Variability of Collimated Gamma-ray Bursts

Within the framework of the internal shock model, we study the luminosity and the variability in gamma-ray bursts from collimated fireballs. In particular we pay attention to the role of the photosphere due to $e^\pm$ pairs produced by internal shock synchrotron photons. It is shown that the observed Cepheid-like relationship between the luminosity and the variability can be interpreted as a correlation between the opening angle of the fireball jet and the mass included at the explosion with a standard energy output. We also show that such a correlation can be a natural consequence of the collapsar model. Using a multiple-shell model, we numerically calculate the temporal profiles of gamma-ray bursts. Collimated jets, in which the typical Lorentz factors are higher than in wide jets, can produce more variable temporal profiles due to smaller angular spreading time scales at the photosphere radius. Our simulations quantitatively reproduce the observed correlation.

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arXiv2000

Ultra Efficient Internal Shocks

Gamma-ray bursts are believed to originate from internal shocks which arise in an irregular relativistic wind. The process has been thought to be inefficient, converting only a few percent of the kinetic energy into gamma-rays. We define ultra efficient internal shocks as those in which the fraction of emitted energy is larger than the fraction of energy given to the radiating electrons at each collision. We show that such a scenario is possible and even plausible. In our model, colliding shells which do not emit all their internal energy are reflected from each other and it causes subsequent collisions, allowing more energy to be emitted. As an example, we obtain about 60% overall efficiency even if the fraction of energy that goes to electrons is $ε_e=0.1$ provided that the shells' Lorentz factor varies between 10 and $10^4$. The numerical temporal profile reflects well the activity of the source which ejects the shells, though numerous collisions take place in this model.

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arXiv2000

Light Curves of GRB Optical Flashes

The standard model of GRB afterglows assumes that relativistically expanding material is decelerating due to interaction with the surrounding medium. The afterglows are well described by the synchrotron radiation from a forward shock, while the strong optical flash associated with GRB 990123 can be attributed to the emission from a reverse shock. We give a detailed study on the reverse shock emission. The full light curves are calculated for a long and a short GRB cases. We discuss the lack of the prompt optical detections by ROTSE for GRB 981121 and GRB 981223.

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arXiv1999

Optical Flashes and Radio Flares in GRBs afterglow: Numerical Study

The strong optical flash observed by ROTSE, as well as the radio flare associated with GRB 990123 can be attributed to the emission of the fireball ejecta, initially heated by the reverse shock. We numerically study the evolution of an adiabatic relativistic fireball interacting with an ambient uniform medium, both in the initial energy transfer stage and in its late evolution. It is shown that the Blandford-McKee solution adequately describes the evolution of the shocked shell quite early on and for as long as the fireball material has relativistic temperatures. In the case where the reverse shock is only mildly relativistic the shocked shell becomes cold almost immediately and the evolution deviates from the Blandford-McKee solution. We derive analytical expressions for the ejecta evolution in its cold regime. This solution gives a good approximation to the numerical results. We estimate the radiation from the fireball ejecta using the numerical hydrodynamic evolution in both cases: cold and hot shells. Surprisingly, we find that both evolutions give rather similar light curves, decaying approximately as $T^{-2}$ in the optical and peaking at about one day in the radio, even though the hydrodynamics is different.

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arXiv1998

Hydrodynamics of a Relativistic Fireball: the Complete Evolution

We study numerically the evolution of an adiabatic relativistic fireball expanding into a cold uniform medium. We follow the stages of initial free expansion and acceleration, coasting and then deceleration and slowing down to a non-relativistic velocity. We compare the numerical results with simplified analytical estimates. We show that the relativistic self similar Blandford-McKee solution describes well the relativistic deceleration epoch. It is an excellent approximation throughout the relativistic deceleration stage, down to $γ\sim 5$, and a reasonable approximation even down to $γ\sim 2$ though the solution is rigorous only for $γ\gg 1$. We examine the transition into the Blandford-McKee solution, and the transition from the solution to the non-relativistic self similar Sedov-Taylor solution. These simulations demonstrate the attractive nature of the Blandford-McKee solution and its stability to radial perturbations.

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arXiv1997

Can internal shocks produce the variability in GRBs?

We discuss the possibility that gamma-ray bursts result from internal shocks in an ultra-relativistic matter. Using a simple model we calculate the temporal structure and we estimate the efficiency of this process. In this model the ultra-relativistic matter flow is represented by a succession of shells with random values of the Lorentz factor. We calculate the shocks that take place between those shells and we estimate the resulting emission. Internal shocks can produce the highly variable temporal structure observed in most of the bursts provided that the source emitting the relativistic flow is highly variable. The observed peaks are in almost one to one correlation to the activity of the emitting source. A large fraction of the kinetic energy is converted to radiation. The most efficient case is when an inner engine produces shells with comparable energy but very different Lorentz factors. It also gives the most preferable temporal structure.

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arXiv1996

Re-examination of the Hubble Constant from the Sunyaev-Zel'dovich Effect: Implication for Cosmological Parameters

We performed a systematic and comprehensive estimate of the Hubble constant $H_0$ via the Sunyaev-Zel'dovich temperature decrement and the X-ray measurements for clusters up to the redshift $z=0.541$, with particular attention to its dependence on the density parameter $Ω_0$, and the cosmological constant $λ_0$. The resulting values of $H_0$ are largely consistent with that derived from the Cepheid distance for nearby galaxies. We discuss how the angular diameter distance $d_A$ vs. $z$ diagram from the accurate SZ measurements of high-$z$ clusters can distinguish the values of $Ω_0$ and $λ_0$.

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arXiv1995

Anderson Localization in the Sky and Cosmological Magnetic Field

We discuss the Anderson localization of electromagnetic fields in the fluctuating plasma induced by the gravitational density perturbation before the recombination time of the Universe. Randomly distributed localized coherent electromagnetic fields emerge in the thermal equilibrium before the recombination time. We argue that the localized coherent electric fields eventually produce cosmological magnetic fields after the decoupling time.

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arXiv1994

Angular two-point correlation functions for cosmological gamma-ray burst model

We compute the angular two-point correlation functions of the gamma-ray bursts at cosmological distances. Since the gamma-ray burst emission mechanism is not yet established, we simply assume that the gamma-ray burst sources are associated with high-redshift galaxies in some way. Then on the basis of several simple models for the evolution of galaxy spatial correlations, we calculate the amplitude of angular two-point correlation functions on scales appropriate for the Compton Gamma Ray Observatory data. we find that in most cases the predicted correlations are difficult to detect with the current data rate and the angular resolution, but models in which the bursts preferentially occur at relatively low redshift ($z \simlt 0.5$) predict correlation amplitudes on $θ\sim 5^\circ$ which will be marginally detectable with the Gamma Ray Observatory data in several years. If future observations detect a signal of angular correlations, it will imply useful information on the correlation of galaxies at high redshifts provided that the gamma-ray bursts are cosmological.

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