arXiv subjects

Toshikazu Shigeyama

Publications and source records attributed to Toshikazu Shigeyama.

At least 55 records · Page 3Linked to original sources

arXiv2019

An optical search for transients lasting a few seconds

Using a prototype of the Tomo-e Gozen wide-field CMOS mosaic camera, we acquire wide-field optical images at a cadence of 2 Hz and search them for transient sources of duration 1.5 to 11.5 seconds. Over the course of eight nights, our survey encompasses the equivalent of roughly two days on one square degree, to a fluence equivalent to a limiting magnitude about $V = 15.6$ in a 1-second exposure. After examining by eye the candidates identified by a software pipeline, we find no sources which meet all our criteria. We compute upper limits to the rate of optical transients consistent with our survey, and compare those to the rates expected and observed for representative sources of ephemeral optical light.

astro-ph.IM↗

arXiv2019

New constraint on the atmosphere of (50000) Quaoar from a stellar occultation

We report observations of a stellar occultation by the classical Kuiper belt object (50000) Quaoar occurred on 28 June 2019. A single-chord high-cadence (2 Hz) photometry dataset was obtained with the Tomo-e Gozen CMOS camera mounted on the 1.05 m Schmidt telescope at Kiso Observatory. The obtained ingress and egress data do not show any indication of atmospheric refraction and allow to set new $1σ$ and $3σ$ upper limits of 6 and 16 nbar, respectively, for the surface pressure of a pure methane atmosphere. These upper limits are lower than the saturation vapor pressure of methane at Quaoar's expected mean surface temperature ($T \sim 44$ K) and imply the absence of a $\sim$10 nbar-level global atmosphere formed by methane ice on Quaoar's surface.

astro-ph.EP↗

arXiv2019

The optically thick rotating magnetic wind from a massive white dwarf merger product

WD J005311 is a newly identified white dwarf (WD) in a mid-infrared nebula. The spectroscopic observation indicates the existence of a neon-enriched carbon/oxygen wind with a terminal velocity of $v_{\infty,\rm obs}\sim 16,000\,\rm km\,s^{-1}$ and a mass loss rate of $\dot M_{\rm obs}\sim 3.5\times 10^{-6}\,M_\odot$ yr$^{-1}$. Here we consistently explain the properties of WD J005311 using a newly constructed wind solution, where the optically thick outflow is launched from the carbon burning shell on an oxygen-neon core and accelerated by the rotating magnetic field to become supersonic and unbound well below the photosphere. Our model implies that WD J005311 has a mass of $M_* \sim 1.1\mbox{-}1.3\,M_\odot$, a magnetic field of $B_* \sim (2\mbox{-}5)\times 10^7\,\rm G$, and a spin angular frequency of $Ω\sim 0.2\mbox{-}0.5 \,\rm s^{-1}$. The large magnetic field and fast spin support the carbon-oxygen WD merger origin. WD J005311 will neither explode as a type Ia supernova nor collapse into a neutron star. If the wind continues to blow another few kyr, WD J005311 will spin down significantly and join to the known sequence of slowly-rotating magnetic WDs. Otherwise it may appear as a fast-spinning magnetic WD and could be a new high energy source.

astro-ph.SR↗

arXiv2019

Type IIn Supernova Light Curves Powered by Forward and Reverse Shocks

We present a bolometric light curve model of Type IIn supernovae powered by supernova ejecta colliding with a circumstellar medium. We estimate the conversion efficiency of the ejecta's kinetic energy to radiation at the reverse and forward shocks and find that a large density contrast makes a difference in the efficiency. The emission from the reverse shock can maintain high efficiency for a long time, and becomes important at the late phase of the light curve. We first construct a semi-analytical model that is applicable to the late phase of the light curve when the diffusion time of photons in the shocked region becomes negligible. We further develop radiation transfer simulations that incorporate these physical processes into the light curve. The numerical calculations predict light curves at early phases, which are testable by present and future short-cadence surveys. We compare our model with the bolometric light curve constructed from observations for a type IIn supernova 2005ip. Due to the reduced efficiency at the forward shock, we find from our model that the mass-loss rate of the progenitor star was $\approx 1\times 10^{-2}\ {\rm M_\odot \ yr^{-1}}$ for a wind velocity of $100\ {\rm km \ s^{-1}}$, an order of magnitude higher compared to previous work that used simple assumptions of the efficiency. This highlights the importance of taking these two components into account when extracting the physical parameters from observations.

astro-ph.HE↗

arXiv2018

Radial Velocity Survey of Nearby OB Stars

We report on the current status of the radial velocity monitoring of nearby OB stars to look for binaries with small mass ratios. The combined data of radial velocities using the domestic 1-2 m-class telescopes seems to confirm the variations of radial velocities in a few weeks for four out of ten target single-lined spectroscopic binaries. More data are needed to estimate the exact periods and mass distributions.

astro-ph.SR↗

arXiv2018

Relativistic supernova ejecta colliding with a circumstellar medium: an application to the low-luminosity GRB 171205A

We perform multi-wavelength light curve modeling of the recently discovered low-luminosity gamma-ray burst (GRB) 171205A. The emission model is based on the relativistic ejecta-circumstellar medium (CSM) interaction scenario. The collision of freely expanding spherical ejecta traveling at mildly relativistic velocities with the CSM produces the reverse and forward shocks, which dissipate a part of the kinetic energy of the mildly relativistic ejecta. We show that the early gamma-ray emission followed by an X-ray tail can be well explained by the radiation diffusing out from the shocked gas. Mildly relativistic ejecta with a kinetic energy of $5\times10^{50}$ erg and a wind-like CSM with a mass-loss rate of a few $10^{-4}\ M_\odot$ yr$^{-1}$ for a wind velocity of $10^3$ km s$^{-1}$, which extends up to $\sim 3\times 10^{13}$ cm, are required to account for the gamma-ray luminosity and duration of GRB 171205A. We also calculate the photospheric and non-thermal emission after the optically thick stage, which can fit the late-time X-ray, optical, and radio light curves. Our results suggest that the relativistic ejecta-CSM interaction can be a potential power source for low-luminosity GRBs and other X-ray bright transients.

astro-ph.HE↗

arXiv2018

Surface Radioactivity or Interactions? Multiple Origins of Early-excess Type Ia Supernovae and Associated Subclasses

Early-phase Type Ia supernovae (SNe Ia), especially those with luminosity enhancement within the first few days of explosions ("early-excess SNe Ia"), play an irreplaceable role in addressing the long-standing progenitor and explosion issue of SNe Ia. In this paper, we systematically investigate 11 early-excess SNe Ia from subluminous to luminous subclasses. Eight of them are selected from 23 SNe Ia with extremely early-phase optical light curves ("golden" early-phase SNe Ia), and three of them are selected from 40 SNe Ia (including 14 golden samples) with early-phase UV/NUV light curves. We found that previously discovered early-excess SNe Ia show a clear preference for specific SN Ia subclasses. In particular, the early-excess feature shown in all six luminous (91T- and 99aa-like) SNe Ia is in conflict with the viewing angle dependence predicted by the companion-ejecta interaction scenario. Instead, such a high early-excess fraction is likely related to the explosion physics of luminous SNe Ia; i.e. a more efficient detonation happening in the progenitor of luminous SNe Ia may consequently account for the early-excess feature powered by the radiation from a $^{56}$Ni-abundant outer layer. The diversity of early-excess features shown in different SN Ia subclasses suggests multiple origins of the discovered early-excess SNe Ia, challenging their applicability as a robust progenitor indicator. Further understanding of the early-excess diversity relies not only on multiband photometry and prompt-response spectroscopy of individual early-excess SNe Ia but also on investigations of the general early-phase light-curve behavior of each SN Ia subclass, which can be realized through ongoing/forthcoming transient survey projects in the near future.

astro-ph.HE↗

arXiv2018

Luminosity Function of Faint Sporadic Meteors measured with a Wide-Field CMOS mosaic camera Tomo-e PM

Imaging observations of faint meteors were carried out on April 11 and 14, 2016 with a wide-field CMOS mosaic camera, Tomo-e PM, mounted on the 105-cm Schmidt telescope at Kiso Observatory, the University of Tokyo. Tomo-e PM, which is a prototype model of Tomo-e Gozen, can monitor a sky of ${\sim}1.98\,\mathrm{deg^2}$ at 2\,Hz. The numbers of detected meteors are 1514 and 706 on April 11 and 14, respectively. The detected meteors are attributed to sporadic meteors. Their absolute magnitudes range from $+4$ to $+10\,\mathrm{mag}$ in the $V$-band, corresponding to about $8.3{\times}10^{-2}$ to $3.3{\times}10^{-4}\,\mathrm{g}$ in mass. The present magnitude distributions we obtained are well explained by a single power-law luminosity function with a slope parameter $r = 3.1{\pm}0.4$ and a meteor rate $\log_{10}N_0 = -5.5{\pm}0.5$. The results demonstrate a high performance of telescopic observations with a wide-field video camera to constrain the luminosity function of faint meteors. The performance of Tomo-e Gozen is about two times higher than that of Tomo-e PM. A survey with Tomo-e Gozen will provide a more robust measurement of the luminosity function.

astro-ph.EP↗

arXiv2018

Repulsion of fallback matter due to central energy source in supernova

The flow of fallback matter being shocked and repelled back by an energy deposition from a central object is discussed by using newly found self-similar solutions. We show that there exists a maximum mass accretion rate if the adiabatic index of the flow is less than or equal to 4/3. Otherwise we can find a solution with an arbitrarily large accretion rate by appropriately shrinking the energy deposition region. Applying the self-similar solution to supernova fallback, we discuss how the fate of newborn pulsars or magnetars depends on the fallback accretion and their spin-down power. Combining the condition for the fallback accretion to bury the surface magnetic field into the crust, we argue that supernova fallback with a rate of dM_{fb}/dt ~ 10^{-(4-6)} M_sun /s could be the main origin of the diversity of Galactic young neutron stars, i.e., rotation-powered pulsars, magnetars, and central compact objects.

astro-ph.SR↗

arXiv2018

Free Neutron Ejection From Shock Breakout in Binary Neutron Star Mergers

Merging neutron stars generate shock waves that disintegrate heavy nuclei into nucleons especially in the outer envelope. It is expected that some of such neutrons having avoided capturing positrons remain as free neutrons even after disappearance of electron-positron pairs. To investigate how many free neutrons can be ejected from merging neutron stars, we performed special-relativistic Lagrangian hydrodynamics computation with simplified models of this phenomenon in which a spherically symmetric shock wave propagates in the hydrostatic envelope and emerge from the surface. We systematically study a wide parameter space of the size of the merging neutron stars and the energy involved in the shock waves. As a result, it is found that the mass of remaining free neutron is $10^{-7}$ to $10^{-6}\ M_{\odot}$, which is smaller than the previously expected by more than two orders of magnitude. There is a preferred energy of the order of $10^{48}$ erg that yields the maximum amount of free neutrons for large size of the envelope. We briefly discuss the emission from the free neutron layer and estimate the luminosity in the optical band to be about $7\times 10^{41}$ erg s $^{-1}(M_{\rm n}/10^{-6}\ M_\odot)$ in $\sim 30$ min after the merger.

astro-ph.HE↗

arXiv2018

Type Ia Supernovae in the First Few Days: Signatures of Helium Detonation versus Interaction

The mechanism for the early-phase blue and excessive emission within the first few days, reported so far for a few type Ia supernovae (SNe Ia), has been suggested to be interaction of the SN ejecta either with a non-degenerate companion star or circumstellar media (CSM). Recently, another mechanism has been suggested within the context of the He detonation-triggered SN scenario (i.e., double detonation scenario or He-ignited violent merger), in which the radioactive isotopes in the outermost layer of the SN ejecta produce the early emission. In this paper, we investigate properties of the early-phase excessive emission predicted by these different scenarios. The He detonation scenario shows different behaviors in the early flash than the companion/CSM interaction scenarios. Especially clear diagnostics is provided once the behaviors in the UV and in the optical are combined. The spectra synthesized for the He detonation scenario are characterized by the absorptions due to the He detonation products, which especially develop in the decay phase. We further expect a relation between the properties of the early-phase flash and those of the maximum SN emission, in a way the brighter and slower initial flash is accompanied by a more substantial effect of the additional absorptions (and reddening). This relation, however, should be considered together with the maximum luminosity of the SN, since the larger luminosity suppresses the effect of the additional absorption. With these expected features, we address the possible origins of the observed excessive early-phase emission for a few SNe.

astro-ph.HE↗

arXiv2018

Evidence for GeV Cosmic Rays from White Dwarfs in the Local Cosmic Ray Spectra and in the Gamma-ray Emissivity of the Inner Galaxy

Recent observations found that electrons are accelerated to $\sim$10 GeV and emit synchrotron hard X-rays in two magnetic white dwarfs (WDs), also known as cataclysmic variables (CVs). In nova outbursts of WDs, multi-GeV gamma-rays were detected inferring that protons are accelerated to 100 GeV or higher. In recent optical surveys, the WD density is found to be higher near the Sun than in the Galactic disk by a factor $\sim$2.5. The cosmic rays (CR) produced by local CVs and novae will accumulate in the local bubble for $10^6$ - $10^7$ yrs. On these findings, we search for CRs from historic CVs and novae in the observed CR spectra. We model the CR spectra at the heliopause as sums of Galactic and local components based on observational data as much as possible. The initial Galactic CR electron and proton spectra are deduced from the gamma-ray emissivity, the local electron spectrum from the hard X-ray spectra at the CVs, and the local proton spectrum inferred by gamma-ray spectrum at novae. These spectral shapes are then expressed in a simple set of polynomial functions of CR energy and regressively fitted until the high-energy ($>$100 GeV) CR spectra near Earth and the Voyager-1 spectra at the heliopause are reproduced. We then extend the modeling to nuclear CR spectra and find that one spectral shape fits all local nuclear CRs and the apparent hardening of the nuclear CR spectra is caused by the roll-down of local nuclear spectra around 100 - 200 GeV. All local CR spectra populate in a limited energy band below 100 - 200 GeV and enhance gamma-ray emissivity below $\sim$10 GeV. Such an enhancement is observed in the inner Galaxy, suggesting the CR fluxes from CVs and novae are substantially higher there.

astro-ph.HE↗

arXiv2018

X-ray Light Curve and Spectra of Shock Breakout in a Wind

We investigate the properties of X-ray emission from shock breakout of a supernova in a stellar wind. We consider a simple model describing aspherical explosions, in which the shock front with an ellipsoidal shape propagates into the dense circumstellar matter. For this model, both X-ray light curves and spectra are simultaneously calculated using a Monte Carlo method. We show that the shock breakout occurs simultaneously in all directions in a steady and spherically symmetric wind. As a result, even for the aspherical explosion, the rise and decay timescales of the light curve do not significantly depend on the viewing angles. This fact suggests that the light curve of the shock breakout may be used as a probe of the wind mass loss rate. We compare our results with the observed spectrum and light curve of XRO 080109/SN 2008D. The observation can be reproduced by an explosion with a shock velocity of 60% of the speed of light and a circumstellar matter with a mass loss rate of 5.e-4 Msun/yr.

astro-ph.HE↗

arXiv2017

Enrichment in r-process elements from multiple distinct events in the early Draco dwarf spheroidal galaxy

The stellar record of elemental abundances in satellite galaxies is important to identify the origin of r-process because such a small stellar system could have hosted a single r-process event, which would distinguish member stars that are formed before and after the event through the evidence of a considerable difference in the abundances of r-process elements, as found in the ultra-faint dwarf galaxy Reticulum II (Ret II). However, the limited mass of these systems prevents us from collecting information from a sufficient number of stars in individual satellites. Hence, it remains unclear whether the discovery of a remarkable r-process enrichment event in Ret II explains the nature of r-process abundances or is an exception. We perform high-resolution spectroscopic measurements of r-process abundances for twelve metal-poor stars in the Draco dwarf galaxy in the metallicity range of -2.5<[Fe/H]<-2. We found that these stars are separated into two groups with r-process abundances differing by one order of magnitude. A group of stars with high abundances of r-process elements was formed by a single r-process event that corresponds to the event evidenced in Ret II. On the other hand, the low r-process abundance group was formed by another sporadic enrichment channel producing a far fewer r-process elements, which is clearly identified for the first time. Accordingly, we identified two populations of stars with different r-process abundances, which are built by two r-process events that enriched gases at levels that differ by more than one order of magnitude.

astro-ph.SR↗

arXiv2017

A hybrid type Ia supernova with an early flash triggered by helium-shell detonation

Type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of carbon-oxygen white dwarfs. Though the uniformity of their light curves makes them powerful cosmological distance indicators, long-standing issues remain regarding their progenitors and explosion mechanisms. Recent detection of the early ultraviolet pulse of a peculiar subluminous SN Ia has been claimed as new evidence for the companion-ejecta interaction through the single-degenerate channel. Here, we report the discovery of a prominent but red optical flash at $\sim$ 0.5 days after the explosion of a SN Ia which shows hybrid features of different SN Ia sub-classes: a light curve typical of normal-brightness SNe Ia, but with strong titanium absorptions, commonly seen in the spectra of subluminous ones. We argue that the early flash of such a hybrid SN Ia is different from predictions of previously suggested scenarios such as the companion-ejecta interaction. Instead it can be naturally explained by a SN explosion triggered by a detonation of a thin helium shell either on a near-Chandrasekhar-mass white dwarf ($\gtrsim$ 1.3 M$_{\odot}$) with low-yield $^{56}$Ni or on a sub-Chandrasekhar-mass white dwarf ($\sim$ 1.0 M$_{\odot}$) merging with a less massive white dwarf. This finding provides compelling evidence that one branch of the previously proposed explosion models, the helium-ignition scenario, does exist in nature, and such a scenario may account for explosions of white dwarfs in a wider mass range in contrast to what was previously supposed.

astro-ph.HE↗

arXiv2017

R-process Element Cosmic Rays from Neutron Star Mergers

Neutron star mergers (NSMs) are one of the most plausible sources of r-process elements in the universe. Therefore NSMs can also be a major source of ultra-heavy elements in cosmic rays. In this paper, we first estimate the contribution of r-process elements synthesized in NSMs to the ultra-heavy element cosmic rays (UHCRs) by calculating transport equations that take into account energy loss processes and spallations. We show that the flux of UHCRs accelerated by NSMs themselves fluctuates by many orders of magnitude on the timescale of several million years and can overwhelm UHCRs accelerated by supernova remnants (SNRs) after an NSM takes place within a few kilo-parsec from the solar system. Experiments with very long exposure times using meteorites as UHCR detectors can detect this fluctuation. As a consequence, we show that if NSMs are the primary source of UHCRs, future experiments using meteorites have a possibility to reveal the event history of NSMs in the solar vicinity. We also describe a possible difference in the abundance pattern and energy spectrum of UHCRs between NSM and SNR accelerations.

astro-ph.HE↗

arXiv2016

Hydrodynamical interaction of mildly relativistic ejecta with an ambient medium

Hydrodynamical interaction of spherical ejecta freely expanding at mildly relativistic speeds into an ambient cold medium is studied in semi-analytical and numerical ways to investigate how ejecta produced in energetic stellar explosions dissipate their kinetic energy through the interaction with the surrounding medium. We especially focus on the case in which the circumstellar medium is well represented by a steady wind at a constant mass-loss rate having been ejected from the stellar surface prior to the explosion. As a result of the hydrodynamical interaction, the ejecta and circumstellar medium are swept by the reverse and forward shocks, leading to the formation of a geometrically thin shell. We present a semi-analytical model describing the dynamical evolution of the shell and compare the results with numerical simulations. The shell can give rise to bright emission as it gradually becomes transparent to photons. while it is optically thick. We develop an emission model for the expected emission from the optically thick shell, in which photons in the shell gradually diffuse out to the interstellar space. Then, we investigate the possibility that radiation powered by the hydrodynamical interaction is the origin of an underluminous class of gamma-ray bursts.

astro-ph.HE↗

arXiv2016

Contribution of Neutron Star Mergers to the R-process Chemical Evolution in the Hierarchical Galaxy Formation

The main astronomical source of r-process elements has not yet been identified. One plausible site is neutron star mergers (NSMs), but from perspective of the Galactic chemical evolution, it has been pointed out that NSMs cannot reproduce the observed r-process abundance distribution of metal-poor stars at [Fe/H] $< -3$. Recently, Tsujimoto & Shigeyama (2014) pointed out that NSM ejecta can spread into much larger volume than ejecta from a supernova. We re-examine the enrichment of r-process elements by NSMs considering this difference in propagation using the chemical evolution model under the hierarchical galaxy formation. The observed r-process enhanced stars around [Fe/H] $\sim -3$ are reproduced if the star formation efficiency is lower for low-mass galaxies under a realistic delay time distribution for NSMs. We show that a significant fraction of NSM ejecta escape from its host proto-galaxy to pollute intergalactic matter and other proto-galaxies. The propagation of r-process elements over proto-galaxies changes the abundance distribution at [Fe/H] $< -3$ and obtains distribution compatible with observations of the Milky Way halo stars. In particular, the pre-enrichment of intergalactic medium explains the observed scarcity of EMP stars without Ba and abundance distribution of r-process elements at [Fe/H] $\lesssim -3.5$.

astro-ph.GA↗