Coherent Radiation in Gamma-Ray Bursts and Relativistic Collisionless Shocks

We suggest that coherent radiation may occur in relativistic collisionless shocks via two-stream Weibel instabilities. The coherence amplifies the radiation power by many orders [$\sim 10^{12}$ in Gamma-Ray Bursts (GRBs)] and particles cool very fast before being randomized. We imply (1) GRBs accompany strong infrared emission, (2) protons efficiently transfer energy to electrons and (3) prompt GRBs might be the upscattered coherent radiation.Comment: 6 pages, accepted for publication in Progress of Theoretical Physic

Cosmic Dispersion Measure from Gamma-Ray Burst Afterglows: Probing the Reionization History and the Burst Environment

We show a possible way to measure the column density of free electrons along the light path, the so-called Dispersion Measure (DM), from the early $[\sim 415 (\nu/1 {\rm GHz})^{-2} ({\rm DM}/10^{5} {\rm pc} {\rm cm}^{-3}) {\rm s}]$ radio afterglows of the gamma-ray bursts. We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time $\sim 10^{3}(\nu/160 {\rm MHz})^{-2}$ s to measure the intergalactic DM (\simg 6000 pc cm$^{-3}$ at redshift $z>6$) up to $z\sim 30$, from which we can determine the reionization history of the universe and identify the missing warm-hot baryons. At low $z$, DM in the host galaxy may reach $\sim 10^{5}$ pc cm$^{-3}$ depending on the burst environment, which may be probed by the current detectors. Free-free absorption and diffractive scattering may also affect the radio emission in a high density.Comment: 4 pages, 3 figures, emulateapj.cls used, accepted for publication in ApJ

Very High Lorentz Factor Fireballs and Gamma-Ray Burst Spectra

Collisionless entrainment of the surrounding matter imports the relativistic baryon component in the Gamma-Ray Burst (GRB) fireball frame. We show that half the fireball energy can be transferred from radiation to the comoving hot motions of baryons under the photosphere. The yet baryon-poor fireball can reexpand to a very high Lorentz factor (VHLF) \Gamma ~ 10^3-10^6 by its own relativistic collisionless pressure beyond the photosphere (so-called collisionless bulk acceleration), leading to internal and external shocks. A simple synchrotron emission from the VHLF internal shocks produces (i) the extra power-law spectral component with variability observed in the Fermi GeV bursts, up to the TeV range for the future Cherenkov Telescope Array (CTA), (ii) the GeV onset delay with a weak luminosity dependence t_{delay} ~ L^{-1/5}, and (iii) the spectral break of GRB 090926 by the synchrotron cooling break or the maximum synchrotron cutoff limited by the dynamical time, not by the e+- creation cutoff. The relativistic baryon component could also heat the photospheric thermal photons into the main GRB Band spectrum via pp, p\gamma (Bethe-Heitler and photomeson), and Coulomb thermalization processes. In this hot photosphere-internal-external shock model, we can predict the anticorrelation of ~TeV neutrinos and GeV gamma-rays, which may be detectable using IceCube. The spectral peak and luminosity (Yonetoku) relation is also reproduced if the progenitor stars are nearly identical. We also discuss the steep/shallow decay of early X-ray afterglows and short GRBs.Comment: 21 pages, 6 figures, final version to be published in Progress of Theoretical Physic

Can an Off-axis Gamma-Ray Burst Jet in GW170817 Explain All the Electromagnetic Counterparts?

Gravitational waves from a merger of two neutron stars (NSs) were discovered for the first time in GW170817, together with diverse electromagnetic (EM) counterparts. To make constraints on a relativistic jet from the NS merger, we calculate the EM signals in (1) the short gamma-ray burst sGRB 170817A from an off-axis jet, (2) the optical-infrared macronova (or kilonova), especially the blue macronova, from a jet-powered cocoon, and (3) the X-ray and radio afterglows from the interaction between the jet and interstellar medium. We find that a typical sGRB jet is consistent with these observations, and there is a parameter space to explain all the observations in a unified fashion with an isotropic energy $\sim 10^{51}$-$10^{52}$ erg, opening angle $\sim 20^{\circ}$, and viewing angle $\sim 30^{\circ}$. The off-axis emission is less de-beamed than the point-source case because the viewing angle is comparable to the opening angle. We also analytically show that the jet energy accelerates a fair fraction of the merger ejecta to a sub-relativistic velocity $\sim 0.3$-$0.4$c as a cocoon in a wide parameter range. The ambient density might be low $\sim 10^{-3}$-$10^{-6}$ cm$^{-3}$, which can be tested by future observations of radio flares and X-ray remnants.Comment: 25 pages, 6 figures, accepted for publication in PTE

Can Gamma-Ray Burst Jets Break Out the First Stars?

We show that a relativistic gamma-ray burst (GRB) jet can potentially pierce the envelope of very massive first generation star (Population III; Pop III) by using the stellar density profile to estimate both the jet luminosity (via accretion) and its penetrability. The jet breakout is possible even if the Pop III star has a supergiant hydrogen envelope without mass loss, thanks to the long-lived powerful accretion of the envelope itself. While the Pop III GRB is estimated to be energetic, E_{gamma,iso} ~ 10^{55} erg, the supergiant envelope hides the initial bright phase into the cocoon component, leading to a GRB with a long duration ~ 1000(1+z) sec and an ordinary isotropic luminosity ~ 10^{52} erg s^{-1} (~ 10^{-9} erg cm^{-2} s^{-1} at redshift z ~ 20). The neutrino-annihilation is not effective for Pop III GRBs because of a low central temperature, while the magnetic mechanism is viable. We also derive analytic estimates of the breakout conditions, which are applicable to various progenitor models. The GRB luminosity and duration are found to be very sensitive to the core and envelope mass, providing possible probes of the first luminous objects at the end of the high redshift dark ages.Comment: 7 pages, 2 figures; accepted for publication in Ap