Kazumi Kashiyama

Cosmological Fast Radio Bursts from Binary White Dwarf Mergers

Recently, Thornton et?al. reported the detection of four fast radio bursts (FRBs). The dispersion measures indicate that the sources of these FRBs are at cosmological distance. Given the large full sky event rate ~104 sky?1 day?1, the FRBs are a promising target for multi-messenger astronomy. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which are produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, emission duration and event rate, can be consistently explained in this scenario. As a result, we predict that some FRBs can accompany type Ia supernovae (SNe Ia) or X-ray debris disks. Simultaneous detection could test our scenario and probe the progenitors of SNe Ia, and moreover would provide a novel constraint on the cosmological parameters. We strongly encourage future SN and X-ray surveys that follow up FRBs.

A burst in a wind bubble and the impact on baryonic ejecta: high-energy gamma-ray flashes and afterglows from fast radio bursts and pulsar-driven supernova remnants

Tenuous wind bubbles, which are formed by the spin-down activity of central compact remnants, are relevant in some models of fast radio bursts (FRBs) and super-luminous supernovae. We study their high-energy signatures, focusing on the role of pair-enriched bubbles produced by young magnetars, rapidly-rotating neutron stars, and magnetized white dwarfs. (i) First, we study the nebular properties and the conditions allowing for escape of high-energy gamma-rays and radio waves, showing that their escape is possible for nebulae with ages of >10-100 yr. In the rapidly-rotating neutron star scenario, we find that radio emission from the quasi-steady nebula itself may be bright enough to be detected especially at sub-mm frequencies, which is relevant as a possible counterpart of pulsar-driven SNe and FRBs. (ii) Second, we consider the fate of bursting emission in the nebulae. We suggest that an impulsive burst may lead to a highly relativistic flow, which would interact with the nebula. If the shocked nebula is still relativistic, pre-existing non-thermal particles in the nebula can be significantly boosted by the forward shock, leading to short-duration (maybe millisecond or longer) high-energy gamma-ray flashes. Possible dissipation at the reverse shock may also lead to gamma-ray emission. (iii) After such flares, interactions with the baryonic ejecta may lead to afterglow emission with a duration of days to weeks. In the magnetar scenario, this burst-in-bubble model leads to the expectation that nearby (<10-100 Mpc) high-energy gamma-ray flashes may be detected by HAWC and CTA, and the subsequent afterglow emission may be seen by radio telescopes such as VLA. (iv) Finally, we discuss several implications specific to FRBs, including constraints on the emission regions and limits on soft gamma-ray counterparts.

Sub-PeV Neutrinos from TeV Unidentified Sources in the Galaxy

The IceCube collaboration discovery of 28 high-energy neutrinos over the energy range 30 TeV <~ E_nu <~ 1 PeV, a 4.3-sigma excess over expected backgrounds, represents the first high-confidence detection of cosmic neutrinos at these energies. In light of this discovery, we explore the possibility that some of the Sub-PeV cosmic neutrinos might originate in our Galaxys TeV unidentified (TeV UnID) sources. While typically resolved at TeV energies, these sources lack prominent radio or X-ray counterparts, and so have been considered promising sites for hadron acceleration within our Galaxy. Modeling the TeV UnID sources as Galactic hypernova remnants, we predict Sub-PeV neutrino fluxes and spectra consistent with their contributing a minority of n_nu <~ 2 of the observed events. This is consistent with our analysis of the spatial distribution of the Sub-PeV neutrinos and TeV UnID sources, which finds that a best-fit of one, and maximum of 3.8 (at 90%-confidence), of the ~16 non-atmospheric Sub-PeV neutrinos may originate in TeV UnID sources, with the remaining 75% to 95% of events being drawn from an isotropic background. If our scenario is correct, we expect excess Sub-PeV neutrinos to accumulate along the Galactic plane, within |l| <~ 30 deg of the Galactic center and in the Cygnus region, as observations by IceCube and other high-energy neutrino facilities go forward. Our scenario also has implications for radio, X-ray, and TeV observations of the TeV UnID sources.

Long-duration X-Ray Flash and X-Ray-rich Gamma-Ray Bursts from Low-mass Population III Stars

Recent numerical simulations suggest that Population III (Pop III) stars were born with masses not larger than ~100 M ☉ and typically ~40 M ☉. By self-consistently considering the jet generation and propagation in the envelope of these low-mass Pop III stars, we find that a Pop III blue supergiant star has the possibility of giving rise to a gamma-ray burst (GRB) even though it keeps a massive hydrogen envelope. We evaluate observational characteristics of Pop III GRBs and predict that Pop III GRBs have a duration of ~105 s in the observer frame and a peak luminosity of ~5 × 1050 erg s–1. Assuming that the E p-L p (or E p-E γ, iso) correlation holds for Pop III GRBs, we find that the spectrum peak energy falls at approximately a few keV (or ~100 keV) in the observer frame. We discuss the detectability of Pop III GRBs by future satellite missions such as EXIST and Lobster. If the E p-E γ, iso correlation holds, we have the possibility to detect Pop III GRBs at z ~ 9 as long-duration X-ray-rich GRBs by EXIST. Conversely, if the E p-L p correlation holds, we have the possibility to detect Pop III GRBs up to z ~ 19 as long-duration X-ray flashes by Lobster.

GAMMA-RAY AND HARD X-RAY EMISSION FROM PULSAR-AIDED SUPERNOVAE AS A PROBE OF PARTICLE ACCELERATION IN EMBRYONIC PULSAR WIND NEBULAE

It has been suggested that some classes of luminous supernovae (SNe) and gamma-ray bursts (GRBs) are driven by newborn magnetars. Fast-rotating proto-neutron stars have also been of interest as potential sources of gravitational waves (GWs). We show that for a range of rotation periods and magnetic fields, hard X rays and GeV gamma rays provide us with a promising probe of pulsar-aided SNe. It is observationally known that young pulsar wind nebulae (PWNe) in the Milky Way are very efficient lepton accelerators. We argue that, if embryonic PWNe satisfy similar conditions at early stages of SNe (in ~1-10 months after the explosion), external inverse-Compton emission via upscatterings of SN photons is naturally expected in the GeV range as well as broadband synchrotron emission. To fully take into account the Klein-Nishina effect and two-photon annihilation process that are important at early times, we perform detailed calculations including electromagnetic cascades. Our results suggest that hard X-ray telescopes such as NuSTAR can observe such early PWN emission by followup observations in months-to-years. GeV gamma rays may also be detected by Fermi for nearby SNe, which serve as counterparts of these GW sources. Detecting the signals will give us an interesting probe of particle acceleration at early times of PWNe, as well as clues to driving mechanisms of luminous SNe and GRBs. Since the Bethe-Heitler cross section is lower than the Thomson cross section, gamma rays would allow us to study subphotospheric dissipation. We encourage searches for high-energy emission from nearby SNe, especially Type Ibc SNe including super-luminous objects.

ULTRAFAST OUTFLOWS FROM BLACK HOLE MERGERS WITH A MINIDISK

Recently, the direct detection of gravitational waves from black hole (BH) mergers was announced by the Advanced LIGO Collaboration. Multi-messenger counterparts of stellar-mass BH mergers are of interest, and it had been suggested that a small disk or celestial body may be involved in the binary of two BHs. To test such possibilities, we consider the fate of a wind powered by an active mini-disk in a relatively short, super-Eddington accretion episode onto a BH with ~10-100 solar masses. We show that its thermal emission could be seen as a fast optical transient with the duration from hours to days. We also find that the coasting outflow forms external shocks due to interaction with the interstellar medium, whose synchrotron emission might be expected in the radio band on a time scale of years. Finally, we also discuss a possible jet component and the associated high-energy neutrino emission as well as ultra-high-energy cosmic-ray acceleration.

Gravitational wave background from Population III binary black holes consistent with cosmic reionization

The recent discovery of the gravitational wave source GW150914 has revealed a coalescing binary black hole (BBH) with masses of

Multi-Messenger Tests for Fast-Spinning Newborn Pulsars Embedded in Stripped-Envelope Supernovae

\sim 30~M_\odot

Direct collapse black hole formation via high-velocity collisions of protogalaxies

. Previous proposals for the origin of such a massive binary include Population III (PopIII) stars. PopIII stars are efficient producers of BBHs and of a gravitational wave background (GWB) in the

Testing the young neutron star scenario with persistent radio emission associated with FRB 121102

10-100