arXiv subjects

Hirotaka Ito

Publications and source records attributed to Hirotaka Ito.

21 records · Page 2Linked to original sources

arXiv2013

New class of Very High Energy gamma-ray emitter: radio-dark mini-shells surrounding AGN jets

We explore non-thermal emission from a shocked interstellar medium, which is identified as an expanding shell, driven by a relativistic jet in active galactic nuclei (AGNs). In this work, we particularly focus on parsec-scale size mini-shells surrounding mini radio lobes. From radio to X-ray band, the mini radio lobe emission dominates the faint emission from the mini-shell. On the other hand, we find that inverse-Compton (IC) emission from the shell can overwhelm the associated lobe emission at very high energy (VHE; E>100 GeV) gamma-ray range, because energy densities of synchrotron photons from the lobe and/or soft photons from the AGN nucleus are large and IC scattering effectively works. The predicted IC emission from nearby mini-shells can be detected with the Cherencov Telescope Array (CTA) and they are potentially a new class of VHE gamma-ray emitters.

astro-ph.HE↗

arXiv2011

Jet propagations, breakouts and photospheric emissions in collapsing massive progenitors of long duration gamma ray bursts

We investigate by two-dimensional axisymmetric relativistic hydrodynamical simulations (1) jet propagations through an envelope of a rapidly rotating and collapsing massive star, which is supposed to be a progenitor of long duration gamma ray bursts (GRBs), (2) breakouts and subsequent expansions into stellar winds and (3) accompanying photospheric emissions. We find that if the envelope rotates uniformly almost at the mass shedding limit, its outer part stops contracting eventually when the centrifugal force becomes large enough. Then another shock wave is formed, propagates outwards and breaks out of the envelope into the stellar wind. Which breaks out earlier, the jet or the centrifugal bounce-induced shock, depends on the timing of jet injection. If the shock breakout occurs earlier owing to a later injection, the jet propagation and subsequent photospheric emissions are affected substantially. We pay particular attention to observational consequences of the difference in the timing of jet injection. We calculate optical depths to find the location of photospheres, extracting densities and temperatures at appropriate retarded times from the hydrodynamical data. We show that the luminosity and observed temperature of the photospheric emissions are both much lower than those reported in previous studies. Although luminosities are still high enough for GRBs, the observed temperature are lower than the energy at the spectral peak expected by the Yonetoku-relation. This may imply that energy exchanges between photons and matter are terminated deeper inside or some non-thermal processes are operating to boost photon energies.

astro-ph.HE↗

arXiv2005

Effects of Magnetic Fields on Proto-Neutron Star Winds

We discuss effects of magnetic fields on proto-neutron star winds by performing numerical simulation. We assume that the atmosphere of proto-neutron star has a homogenous magnetic field (ranging from ~10^{12} G to ~10^{15} G) perpendicular to the radial direction and examine the dependence of the three key quantities (dynamical time scale, electron fraction, and entropy per baryon) for the successful r-process on the magnetic field strength. Our results show that even with a magneter-class field strength, ~10^{15} G, the feature of the wind dynamics varies only little from that of non-magnetic winds, and that the condition for successful r-process is not realized.

astro-ph↗