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One of the important and interesting problems in astrophysics and plasma physics is collimation of plasma jets. The collimation mechanism, which causes a plasma flow to propagate a long distance, has not been understood in detail. We have been investigating a model experiment to simulate astrophysical plasma jets with an external magnetic field [Nishio et al., EPJ. Web of Conferences 59, 15005 (2013)]. The experiment was performed by using Gekko XII HIPER laser system at Institute of Laser Engineering, Osaka University. We shot CH plane targets (3 mm × 3 mm × 10 μm) and observed rear-side plasma flows. A collimated plasma flow or plasma jet was generated by separating focal spots of laser beams. In this report, we measured plasma jet structure without an external magnetic field with shadowgraphy, and simultaneously measured the local parameters of the plasma jet, i.e., electron density, electron and ion temperatures, charge state, and drift velocity, with collective Thomson scattering.

A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors for the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA.

We have performed 2D relativistic hydrodynamic simulations of jets from a collapsar. The light curves and spectrum of the photospheric thermal radiation from ultrarelativistic gamma-ray burst (GRB) jets are calculated by the post process. At the head the jet has a dissipated region in which internal shocks can be seen and a freely expanding region follows. The freely expanding region shows a bullet like shape. The light curves continue about 100 s which corresponds to the jet injection duration. The light curves for θv ≤ 2° show quick rise and time variability for the first phase and low and steady luminosity phase follows. The light curves for θv ≥ 3° show slow rise and low luminosity. The spectrum below the peak energy is a power law and the index is 1 ~ 2.6 which is softer than that of single temperature plank distribution Some of them are close to the observed one, i.e., Band function.

Two-dimensional magnetohydrodynamic simulations are performed using the ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB jet, taking account of realistic equation of state, neutrino cooling and heating processes, magnetic fields, and gravitational force from the central black hole and self gravity. It is found that neutrino heating processes are not so efficient to launch a jet in this study. It is also found that a jet is launched mainly by B_\phi fields that are amplified by the winding-up effect. However, since the ratio of total energy relative to the rest mass energy in the jet is not so high as several hundred, we conclude that the jets seen in this study are not be a GRB jet. This result suggests that general relativistic effects, which are not included in this study, will be important to generate a GRB jet. Also, the accretion disk with magnetic fields may still play an important role to launch a GRB jet, although a simulation for much longer physical time (\sim 10-100 s) is required to confirm this effect. It is shown that considerable amount of 56Ni is synthesized in the accretion disk. Thus there will be a possibility for the accretion disk to supply sufficient amount of 56Ni required to explain the luminosity of a hypernova. Also, it is shown that neutron-rich matter due to electron captures with high entropy per baryon is ejected along the polar axis. Moreover, it is found that the electron fraction becomes larger than 0.5 around the polar axis near the black hole by \nu_e capture at the region. Thus there will be a possibility that r-process and r/p-process nucleosynthesis occur at these regions. Finally, much neutrons will be ejected from the jet, which suggests that signals from the neutron decays may be observed as the delayed bump of afterglow or gamma-rays.

We investigate the outflow propagation in the collapsar in the context of gamma-ray bursts (GRBs) with 2D relativistic hydrodynamic simulations. We vary the specific internal energy and bulk Lorentz factor of the injected out- flow from non-relativistic regime to relativistic one, fixing the power of the outflow to be 1051erg s−1. We observed the collimated outflow, when the Lorentz factor of the injected outflow is roughly greater than 2. To the contrary, when the velocity of the injected outflow is slower, the expanding outflow is observed. The transition from collimated jet to expanding outflow continuously occurs by decreasing the injected velocity. Different features of the dynamics of the outflows would cause the difference between the GRBs and similar phenomena, such as, X-ray flashes.

We investigate the dynamics of an injected outflow propagating in a progenitor in the context of the collapsar model for gamma-ray bursts (GRBs) through two dimensional axisymmetric relativistic hydrodynamic simulations. Initially, we locally inject an outflow near the center of a progenitor. We calculate 25 models, in total, by fixing its total input energy to be 10^{51} ergs s^{-1} and radius of the injected outflow to be $7\times 10^7$ cm while varying its bulk Lorentz factor, $\Gamma_{0} = 1.05\sim 5$, and its specific internal energy, $\epsilon_0/c^2 = 0.1\sim 30$. The injected outflow propagates in the progenitor and drives a large-scale outflow or jet. We find a smooth but dramatic transition from a collimated jet to an expanding outflow among calculated models. The maximum Lorentz factor is, on the other hand, sensitive to both of $\Gamma_0$ and $\epsilon_0$; roughly $\Gamma_{\rm max} \sim \Gamma_0 (1+\epsilon_0/c^2)$. Our finding will explain a smooth transition between the GRBs, X-ray rich GRBs (XRRs) and X-ray Flashes (XRFs) by the same model but with different $\epsilon_0$ values.

Two-dimensional hydrodynamic simulations are performed to investigate explosive nucleosynthesis in a collapsar using the model of MacFadyen and Woosley (1999). It is shown that 56Ni is not produced in the jet of the collapsar sufficiently to explain the observed amount of a hypernova when the duration of the explosion is \sim 10 sec, which is considered to be the typical timescale of explosion in the collapsar model. Even though a considerable amount of 56Ni is synthesized if all explosion energy is deposited initially, the opening angles of the jets become too wide to realize highly relativistic outflows and gamma-ray bursts in such a case. From these results, it is concluded that the origin of 56Ni in hypernovae associated with GRBs is not the explosive nucleosynthesis in the jet. We consider that the idea that the origin is the explosive nucleosynthesis in the accretion disk is more promising. We also show that the explosion becomes bi-polar naturally due to the effect of the deformed progenitor. This fact suggests that the 56Ni synthesized in the accretion disk and conveyed as outflows are blown along to the rotation axis, which will explain the line features of SN 1998bw and double peaked line features of SN 2003jd. Some fraction of the gamma-ray lines from 56Ni decays in the jet will appear without losing their energies because the jet becomes optically thin before a considerable amount of 56Ni decays as long as the jet is a relativistic flow. We show that abundance of nuclei whose mass number \sim 40 in the ejecta depends sensitively on the energy deposition rate. So it may be determined by observations of chemical composition in metal poor stars which model is the proper one as a model of a gamma-ray burst accompanied by a hypernova., Comment: 29 pages with 16 figures. ApJ, accepted

We investigate the dynamics and morphology of jets propagating into the interstellar medium using 2D relativistic hydrodynamic simulations. The calculations are performed assuming axisymmetric geometry and trace a long propagation of jets. The jets are assumed to be 'light' with the density ratio between the beam to the ambient gas much less than unity. We examine the mechanism for the appearance of vortices at the head of jets in the hot spot. Such vortices are known as a trigger of a deceleration phase which appears after a short phase in which one dimensional analysis is dominant. We find that an oblique shock at the boundary rim near the end of the beam strongly affects the flow structure in and around the hot spot. Weakly shocked gas passed through this oblique shock and becomes a trigger for the generation of vortices. We also find the parameter dependence of these effects for the propagation and dynamics of the jets. The jet with slower propagation velocity is weakly pinched, has large vortices and shows very complex structure at the head of the jets and extended synchrotron emissivity., 36 pages, 16 figures, accepted for publication in ApJ

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas. © 2012 IOP Publishing Ltd.

We show that, contrary to common belief, internal shocks can arise in an accelerating radiation-dominated jet if it is confined even weakly to a converging opening angle because the acceleration declines. The radiation-dominated internal shock (RDIS) enables a very efficient yet highly nonthermal emission by Fermi-like photon acceleration, keeping the electron-positron ($e^{\pm}$) pair photosphere and inertia up to a high Lorentz factor >1000. In gamma-ray bursts (GRBs), a weak confinement would persist beyond the progenitor star or surrounding matter because of the fast cocoon accompanying the breakout jet. The simplest model predicts few high-energy cosmic rays and neutrinos, and a correlation between the early afterglow and the GeV-TeV prompt emission. The central engine allows a less fine-tuned baryon load than previously thought, even including pure-leptonic unmagnetized outflows., 10 pages, 3 figures, final version to be published in Progress of Theoretical Physics

We have calculated the light curves and spectrum of the photospheric thermal radiation from gamma‐ray burst jets, using the results of the 2D relativistic hydrodynamic simulations. We find that the thermal radiation from the photosphere of the GRB jet is really ‘observable’ even for a cosmological distant GRB. The light curve is flare like for the first phases. The light curves for on‐axis observers continue and exhibit time variability due to the intenal structure of the jets in Lorentz factor and density. The spectrum consists of multicolor components and exhibits power law distribution, for each viewing angle.

We report two different types of backflow from jets by performing 2D special relativistic hydrodynamical simulations. One is anti-parallel and quasi-straight to the main jet (quasi-straight backflow), and the other is bent path of the backflow (bent backflow). We find that the former appears when the head advance speed is comparable to or higher than the local sound speed at the hotspot while the latter appears when the head advance speed is slower than the sound speed bat the hotspot. Bent backflow collides with the unshocked jet and laterally squeezes the jet. At the same time, a pair of new oblique shocks are formed at the tip of the jet and new bent fast backflows are generated via these oblique shocks. The hysteresis of backflow collisions is thus imprinted in the jet as a node and anti-node structure. This process also promotes broadening of the jet cross sectional area and it also causes a decrease in the head advance velocity. This hydrodynamic process may be tested by observations of compact young jets., Comment: 9 pages, 5 figures, accepted for publication in ApJL

Collapsars are fast-spinning, massive stars, whose core collapse liberates an energy, that can be channeled in the form of ultrarelativistic jets. These jets transport the energy from the collapsed core to large distances, where it is dissipated in the form of long-duration gamma-ray bursts. In this paper we study the dynamics of ultrarelativistic jets produced in collapsars. Also we extrapolate our results to infer the angular energy distribution of the produced outflows in the afterglow phase. Our main focus is to look for global energetical properties which can be imprinted by the different structure of different progenitor stars. Thus, we employ a number of pre-supernova, stellar models (with distinct masses and metallicities), and inject in all of them jets with fixed initial conditions. We assume that at the injection nozzle, the jet is mildly relativistic (Lorentz factor $\sim 5$), has a finite half-opening angle ($5^\circ$), and carries a power of $10^{51} $erg s$^{-1}$. These jets arrive intact to the stellar surface and break out of it. A large Lorentz factor region $\Gamma\simmore 100$ develops well before the jet reaches the surface of the star, in the unshocked part of the beam, located between the injection nozzle and the first recollimation shock. These high values of $\Gamma$ are possible because the finite opening angle of the jet allows for free expansion towards the radial direction. We find a strong correlation between the angular energy distribution of the jet, after its eruption from the progenitor surface, and the mass of the progenitors. The angular energy distribution of the jets from light progenitor models is steeper than that of the jets injected in more massive progenitor stars. This trend is also imprinted in the angular distribution of isotropic equivalent energy., Comment: 16 pages, 18 figures, accepted for publication in ApJ, high resolution figures are available at http://www.cfs.chiba-u.ac.jp/~mizuta/ms15.pdf

We investigate relativistic jet propagation in the collapsar and interstellar medium, using 2‐dimensional relativistic hydrodynamic simulations. The jet is assumed to be initially thermal dominated plasma and have half opening angle (5 degrees). Realistic progenitor models are adopted and the difference of the outflow properties by different progenitor models are studied. There is not significant difference, especially in the energy distribution in unit solid angle as a function of the angle between jet and line of site.

Two‐dimensional magnetohydrodynamic simulations are performed using the ZEUS‐2D code to investigate the dynamics of a collapsar that generates a GRB jet, taking account of realistic equation of state, neutrino cooling and heating processes, magnetic fields, and gravitational force from the central black hole and self gravity. It is found that neutrino heating processes are not so efficient to launch a jet in this study. It is also found that a jet is launched mainly by Bφ fields that are amplified by the winding‐up effect. However, since the ratio of total energy relative to the rest mass energy in the jet is not so high as several hundred, we conclude that the jets seen in this study are not be a GRB jet. This result suggests that general relativistic effects, which are not included in this study, will be important to generate a GRB jet.

Two‐dimensional hydrodynamic simulations are performed to investigate explosive nucleosynthesis in a collapsar using the model of MacFadyen and Woosley (1999). It is shown that 56Ni is not produced in the jet of the collapsar sufficiently to explain the observed amount of a hypernova when the duration of the explosion is ∼10 sec, which is considered to be the typical timescale of explosion in the collapsar model. Even though considerable amount of 56Ni is synthesized if all explosion energy is deposited initially, the opening angles of the jets become too wide to realize highly relativistic outflows and gamma‐ray bursts in such a case. Moreover, the synthesized 56Ni can not be used to brighten the supernova, since most of 56Ni exist in the jet component rather than in the supernova ejecta, and the jet becomes optically thin before considerable amount of 56Ni decays. From these results, it is concluded that the origin of 56Ni in hypernovae is not the explosive nucleosynthesis in the jet. We consider that t...

We explore the spectral and polarization properties of photospheric emissions from stratified jets in which multiple components, separated by a sharp velocity shear regions, are distributed in lateral direction. Propagation of thermal photons injected at high optical depth region are calculated until they escape from the photosphere. It is found that presence of the lateral structure within the jet leads to non-thermal feature of the spectra and significant polarization signal in the resulting emission. The deviation from thermal spectra as well as the polarization degree tends to be enhanced as the velocity gradient in the shear region increases. In particular, we show that emissions from multi-component jet can reproduce the typical observed spectra of gamma-ray bursts (GRBs) irrespective to the position of the observer when a velocity shear region is closely spaced in various lateral ($\theta$) positions. The degree of polarization associated in the emission is significant (> few%) at wide range of observer angles and can be higher than 30%., Comment: 21 pages, 12 figures, accepted for publication in ApJ

In this paper we present a numerical study of plasma jets produced by intense laser matter interactions. Through this study we hope to better understand astrophysical jets and their recent experimental simulations in the laboratory. We paid special attention to radiation cooling and the interaction of the jet with ambient gas. Four cases are presented in this paper; two of them deal with the propagation of jets in vacuum, while in the other two the propagation takes place in the ambient gas. Available experimental results are reproduced to good accuracy in the vacuum case. For jets in ambient gas, we find that the existence of the surrounding gas confines the jet into a narrow cylindrical shape so that both the density and temperature of the jet remain high enough for effective radiation cooling. As a result, a collimated plasma jet is formed in these cases. The dimensionless parameters characterizing the laboratory jets and protostellar jets have overlapping domains. We also discuss the cooling lengths for our model and compare them with the corresponding values in the astrophysical jets. A plasma jet in the ambient gas experiment is proposed which is within the reach of present day technology, and can be relevant to astrophysical phenomena., Comment: Accepted for publication in ApJ

We investigate the jet propagation and breakout from the stellar progenitor for gamma-ray burst (GRB) collapsars by performing two-dimensional relativistic hydrodynamic simulations and analytical modeling. We find that the jet opening angle is given by $\theta_j \sim 1/5 \Gamma_{0}$, and infer the initial Lorentz factor of the jet at the central engine, $\Gamma_0$, is a few for existing observations of $\theta_j$. The jet keeps the Lorentz factor low inside the star by converging cylindrically via collimation shocks under the cocoon pressure, and accelerates at jet breakout before the free expansion to a hollow-cone structure. In this new picture the GRB duration is determined by the sound crossing time of the cocoon, after which the opening angle widens, reducing the apparent luminosity. Some bursts violating the maximum opening angle $\theta_{j,\max}\sim 1/5 \sim 12^{\circ}$ imply the existence of a baryon-rich sheath or a long-acting jet. We can explain the slopes in both Amati and Yonetoku spectral relations using an off-centered photosphere model, if we make only one assumption that the total jet luminosity is proportional to the initial Lorentz factor of the jet. We also numerically calibrate the pre-breakout model (Bromberg et al.) for later use., Comment: 20 pages, 16 figures

We explore photospheric emissions from stratified two-component jets, wherein a highly relativistic spine outflow is surrounded by a wider and less relativistic sheath outflow. Thermal photons are injected in regions of high optical depth and propagated until they escape at the photosphere. Due to the presence of shear in velocity (Lorentz factor) at the boundary of the spine and sheath region, a fraction of the injected photons are accelerated via a Fermi-like acceleration mechanism such that a high energy power-law tail is formed in the resultant spectrum. We show, in particular, that if a velocity shear with a considerable variance in the bulk Lorentz factor is present, the high energy part of observed Gamma-ray Bursts (GRBs) photon spectrum can be explained by this photon acceleration mechanism. We also show that the accelerated photons may also account for the origin of the extra hard power-law component above the bump of the thermal-like peak seen in some peculiar bursts (e.g., GRB 090510, 090902B, 090926A). It is demonstrated that time-integrated spectra can also reproduce the low energy spectrum of GRBs consistently due to a multi-temperature effect when time evolution of the outflow is considered. Finally, we show that the empirical Ep-Lp relation can be explained by differences in the outflow properties of individual sources., Comment: 18 pages, 14 figures, accepted for publication in ApJ

In this study, the light curves and spectrum of the photospheric thermal radiation from ultrarelativistic gamma-ray burst (GRB) jets are calculated using two-dimensional relativistic hydrodynamic simulations of jets from a collapsar. As the jet advances, the density around the head of the jet decreases, and its Lorentz factor reaches as high as 200 at the photosphere and 400 inside the photosphere. For an on-axis observer, the photosphere appears concave due to the low density and high beaming factor of the jet. The luminosity varies because of the abrupt change in the position of the photosphere due to the internal structure of the jet. Comparing our results with GRB 090902B, the flux level of the thermal-like component is similar to our model, although the peak energy appears slightly higher (but still within a factor of two). From the comparison, we estimate that the bulk Lorentz factor of GRB 090902B is {Gamma} {approx} 2.4 x 10{sup 2}(r/10{sup 12} cm), where r is the radius of the photosphere. The spectrum for an on-axis observer is harder than that for an off-axis observer. There is a time lag of a few seconds for high energy bands in the light curve. This may be themore » reason for the delayed onset of GeV emission seen in GRB 080916C. The spectrum below the peak energy is a power law and the index is 2.3-2.6, which is softer than that of a single temperature Planck distribution but still harder than that of the typical value of the observed spectrum.« less

The highly relativistic jets from collapsars are studied in the contect of the gamma‐ray burst (GRB). The angular energy distribution of the jets after the eruption from the stellar surface is well fitted by the smoothly broken power law. The distribution of the jets from light progenitor models is steeper than that of the jets injected in more massive progenitor stars. The cocoon property of the jet in the progenitor decides this characteristics of the jet.