arXiv subjects

Toshikazu Shigeyama

Publications and source records attributed to Toshikazu Shigeyama.

At least 37 records · Page 2Linked to original sources

arXiv2022

MUSSES2020J: The Earliest Discovery of a Fast Blue Ultraluminous Transient at Redshift 1.063

In this Letter, we report the discovery of an ultraluminous fast-evolving transient in rest-frame UV wavelengths, MUSSES2020J, soon after its occurrence by using the Hyper Suprime-Cam (HSC) mounted on the 8.2 m Subaru telescope. The rise time of about 5 days with an extremely high UV peak luminosity shares similarities to a handful of fast blue optical transients whose peak luminosities are comparable with the most luminous supernovae while their timescales are significantly shorter (hereafter "fast blue ultraluminous transient," FBUT). In addition, MUSSES2020J is located near the center of a normal low-mass galaxy at a redshift of 1.063, suggesting a possible connection between the energy source of MUSSES2020J and the central part of the host galaxy. Possible physical mechanisms powering this extreme transient such as a wind-driven tidal disruption event and an interaction between supernova and circumstellar material are qualitatively discussed based on the first multiband early-phase light curve of FBUTs, although whether the scenarios can quantitatively explain the early photometric behavior of MUSSES2020J requires systematical theoretical investigations. Thanks to the ultrahigh luminosity in UV and blue optical wavelengths of these extreme transients, a promising number of FBUTs from the local to the high-z universe can be discovered through deep wide-field optical surveys in the near future.

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arXiv2022

Stationary accretion flow with nuclear burning

We present a series of numerical solutions of spherically symmetric stationary flows with nuclear burning accreted by a neutron star (or black hole). We consider the accretion of matter composed of carbon and oxygen, which mimics the flow after a neutron star is engulfed by a CO star or CO core of a massive star. It is found that there are two types of transonic solutions depending on the accretion rate. The flow with a small accretion rate reaches the center (or the surface of the central object) at supersonic speeds. The other type with a large accretion rate has another sonic point inside the transonic point and the flow truncates at the sonic point. The critical accretion rate dividing these two types is derived as a function of the mass of the central object and the specific enthalpy in the ambient matter. We discuss implications from the solutions for a new mechanism of super-Chandrasekhar type Ia supernovae and type Icn supernovae.

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arXiv2022

Eruption of the Envelope of Massive Stars by Energy Injection with Finite Duration

A significant fraction of supernovae show signatures of dense circumstellar material (CSM). While multiple scenarios for creating a dense CSM exist, mass eruption due to injection of energy at the base of the outer envelope is a likely possibility. We carry out radiation hydrodynamical simulations of eruptive mass loss from a typical red supergiant progenitor with initial mass of $15\ M_\odot$, for the first time focusing on the timescale of the injection as well as energy. We find that not only sufficient injection energy but also sufficient rate of energy injection per unit time, $L_{\rm{min}} \sim 8\times 10^{40}$ erg s$^{-1}$ in this particular model, is required for eruption of unbound CSM. This result suggests that the energy injection rate needs to be greater than the binding energy of the envelope divided by the dynamical timescale for the eruption. The density profile of the resulting CSM, whose shape was analytically and numerically predicted in the limit of instantaneous energy injection, similarly holds for a finite injection timescale. We discuss our findings in the framework of proposed mass outburst scenarios, specifically wave-driven outbursts and common envelope ejection.

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arXiv2022

CHIPS: Complete History of Interaction-Powered Supernovae

We present the public release of the Complete History of Interaction-Powered Supernovae (CHIPS) code, suited to model a variety of transients that arise from interaction with a dense circumstellar medium (CSM). Contrary to existing modellings which mostly attach the CSM by hand, CHIPS self-consistently simulates both the creation of the CSM from mass eruption of massive stars prior to core-collapse, and the subsequent supernova light curve. We demonstrate the performance of CHIPS by presenting examples of the density profiles of the CSM and the light curves. We show that the gross light curve properties of putative interaction-powered transients, such as Type IIn supernovae, rapidly evolving transients and recently discovered fast blue optical transients, can be comprehensively explained with the output of CHIPS.

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arXiv2021

Discovery of the Fastest Early Optical Emission from Overluminous SN Ia 2020hvf: A Thermonuclear Explosion within a Dense Circumstellar Environment

In this Letter we report a discovery of a prominent flash of a peculiar overluminous Type Ia supernova, SN 2020hvf, in about 5 hours of the supernova explosion by the first wide-field mosaic CMOS sensor imager, the Tomo-e Gozen Camera. The fast evolution of the early flash was captured by intensive intranight observations via the Tomo-e Gozen high-cadence survey. Numerical simulations show that such a prominent and fast early emission is most likely generated from an interaction between $0.01~M_{\odot}$ circumstellar material (CSM) extending to a distance of $\sim$$10^{13}~\text{cm}$ and supernova ejecta soon after the explosion, indicating a confined dense CSM formation at the final evolution stage of the progenitor of SN 2020hvf. Based on the CSM-ejecta interaction-induced early flash, the overluminous light curve, and the high ejecta velocity of SN 2020hvf, we suggest that the SN 2020hvf may originate from a thermonuclear explosion of a super-Chandrasekhar-mass white dwarf ("super-$M\rm_{Ch}$ WD"). Systematical investigations on explosion mechanisms and hydrodynamic simulations of the super-$M\rm_{Ch}$ WD explosion are required to further test the suggested scenario and understand the progenitor of this peculiar supernova.

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arXiv2021

AT2018lqh: Black Hole Born from a Rotating Star?

Recently an intriguing transient AT 2018lqh, with only a day-scale duration and a high luminosity of $7\times 10^{42}\ {\rm erg\ s^{-1}}$, has been discovered. While several possibilities are raised on its origin, the nature of this transient is yet to be unveiled. We propose that a black hole (BH) with $\sim 30\, M_\odot$ forming from a rotating blue supergiant can generate a transient like AT 2018lqh. We find that this scenario can consistently explain the optical/UV emission and the tentative late-time X-ray detection, as well as the radio upper limits. If super-Eddington accretion onto the nascent BH powers the X-ray emission, continued X-ray observations may be able to test the presence of an accretion disk around the BH.

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arXiv2021

A necessary condition for supernova fallback invading newborn neutron-star magnetosphere

We numerically investigate the dynamics of a supernova fallback accretion confronting with a relativistic wind from a newborn neutron star (NS). The time evolution of the accretion shock in the radial direction is basically characterized by the encounter radius of the flow $r_\mathrm{enc}$ and a dimensionless parameter $ζ\equiv L/\dot M_\mathrm{fb}c^2$, where $L$ is the NS wind luminosity and $\dot M_\mathrm{fb}$ is the fallback mass accretion rate. We find that the critical condition for the fallback matter to reach near the NS surface can be simply described as $ζ< ζ_\mathrm{min} \equiv GM_*/c^2r_\mathrm{enc}$ or $r_\mathrm{enc}L/G M_* \dot M_\mathrm{fb} < 1$ independent of the wind Lorentz factor, where $M_*$ is the NS mass. With combining the condition for the fallback matter to bury the surface magnetic field under the NS crust, we discuss the possibility that the trifurcation of NSs into rotation-powered pulsars, central compact objects (CCOs), and magnetars can be induced by supernova fallback.

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arXiv2021

J-GEM optical and near-infrared follow-up of gravitational wave events during LIGO's and Virgo's third observing run

The Laser Interferometer Gravitational-wave Observatory Scientific Collaboration and Virgo Collaboration (LVC) sent out 56 gravitational-wave (GW) notices during the third observing run (O3). Japanese collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM) performed optical and near-infrared observations to identify and observe an electromagnetic (EM) counterpart. We constructed web-based system which enabled us to obtain and share information of candidate host galaxies for the counterpart, and status of our observations. Candidate host galaxies were selected from the GLADE catalog with a weight based on the three-dimensional GW localization map provided by LVC. We conducted galaxy-targeted and wide-field blind surveys, real-time data analysis, and visual inspection of observed galaxies. We performed galaxy-targeted follow-ups to 23 GW events during O3, and the maximum probability covered by our observations reached to 9.8%. Among them, we successfully started observations for 10 GW events within 0.5 days after the detection. This result demonstrates that our follow-up observation has a potential to constrain EM radiation models for a merger of binary neutron stars at a distance of up to $\sim$100~Mpc with a probability area of $\leq$ 500~deg$^2$.

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arXiv2021

An Analytical Density Profile of Dense Circumstellar Medium in Type II Supernovae

Observations of Type II supernovae imply that a large fraction of its progenitors experience enhanced mass loss years to decades before core collapse, creating a dense circumstellar medium (CSM). Assuming that the CSM is produced by a single mass eruption event, we analytically model the density profile of the resulting CSM. We find that a double power-law profile, where the inner (outer) power-law index has a characteristic value of -1.5 (-10 to -12), gives a good fit to the CSM profile obtained using radiation hydrodynamical simulations. With our profile the CSM is well described by just two parameters, the transition radius $r_*$ and density at $r=r_*$ (alternatively $r_*$ and the total CSM mass). We encourage future studies to include this profile, if possible, when modelling emission from interaction-powered transients.

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arXiv2021

Multi-wavelength View of the Type IIn Zoo: Optical to X-ray Emission Model of Interaction-Powered Supernovae

Transients powered by interaction with the circumstellar medium (CSM) are often observed in wavelengths other than optical, and multi-wavelength modelling can be important when inferring the properties of the explosion and CSM, or for distinguishing from other powering mechanisms. We develop a model calculating time dependent emission spectrum of interaction-powered transients. We solve energy equations of electron-proton plasma in the shocked SN ejecta and CSM and a radiation transfer equation out to the outer edge of the CSM, incorporating the collisional relaxation and the comptonization of the bremsstrahlung radiation. We compare our model to observations of Type IIn supernovae covering frequency ranges from optical to X-rays. For SN 2010jl the observed optical and X-ray light curves can be consistently explained if clumpy or asymmetric structure in the CSM is assumed, in agreement with previous studies. For SN 2014C our model successfully reproduces the X-ray bremsstrahlung component and the emergence of H$α$ emission at 400 days after explosion. Finally we find a parameter space where the supernova is extremely X-ray bright, reaching $10^{43}$-$10^{44}\ {\rm erg\ s^{-1}}$ for up to $100$ days. Such X-ray transients are likely detectable with all-sky surveys by e.g. eROSITA.

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arXiv2021

First Star Survivors as Metal-Rich Halo Stars that Experienced Supernova Explosions in Binary Systems

The search for the first stars formed from metal-free gas in the universe is one of the key issues in astronomy because it relates to many fields, such as the formation of stars and galaxies, the evolution of the universe, and the origin of elements. It is not still clear if metal-free first stars can be found in the present universe. These first stars are thought to exist among extremely metal-poor stars in the halo of our Galaxy. Here we propose a new scenario for the formation of low-mass first stars that have survived until today and observational counterparts in our Galaxy. The first stars in binary systems, consisting of massive- and low-mass stars, are examined using stellar evolution models, simulations of supernova ejecta colliding with low-mass companions, and comparisons with observed data. These first star survivors will be observed as metal-rich halo stars in our Galaxy. We may have identified a candidate star in the observational database where elemental abundances and kinematic data are available. Our models also account for the existence of several solar-metallicity stars in the literature having space velocities equivalent to the halo population. The proposed scenario demands a new channel of star formation in the early universe and is a supplementary scenario for the origin of the known metal-poor stars.

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arXiv2021

Comparison between the first and second mass eruptions from progenitors of Type IIn supernovae

Some massive stars experience episodic and intense mass loss phases with fluctuations in the luminosity. Ejected material forms circumstellar matter around the star, and the subsequent core collapse results in a Type IIn supernova that is characterized by interaction between supernova ejecta and circumstellar matter. The energy source that triggers these mass eruptions and dynamics of the outflow have not been clearly explained. Moreover, the mass eruption itself can alter the density structure of the envelope and affect the dynamics of the subsequent mass eruption if these events are repeated. A large amount of observational evidence suggests multiple mass eruptions prior to core collapse. We investigate the density structure of the envelope altered by the first mass eruption and the nature of the subsequent second mass eruption event in comparison with the first event. We deposited extra energy at the bottom of the hydrogen envelope of 15$M_\odot$ stars twice and calculated the time evolution by radiation hydrodynamical simulation code. We did not deal with the origin of the energy source, but focused on the dynamics of repeated mass eruptions from a single massive star. There are significant differences between the first and second mass eruptions in terms of the luminosity and the color. The second eruption leads to a redder burst event in which the associated brightening phase lasts longer than the first. The amount of ejected matter is different even with the same deposited energy in the first and second event, but the difference depends on the density structure of the star. Upcoming high cadence and deep transient surveys will provide us a lot of pre-supernova activities, and some of which might show multi-peaked light curves. These should be interpreted taking the effect of density structure altered by the preceding outburst events into consideration.

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arXiv2021

Relationship between Radar Cross Section and Optical Magnitude based on Radar and Optical Simultaneous Observations of Faint Meteors

Radar and optical simultaneous observations of meteors are important to understand the size distribution of the interplanetary dust. However, faint meteors detected by high power large aperture radar observations, which are typically as faint as 10 mag. in optical, have not been detected until recently in optical observations, mainly due to insufficient sensitivity of the optical observations. In this paper, two radar and optical simultaneous observations were organized. The first observation was carried out in 2009 to 2010 using Middle and Upper Atmosphere Radar (MU radar) and an image-intensified CCD camera. The second observation was carried out in 2018 using the MU radar and a mosaic CMOS camera, Tomo-e Gozen, mounted on the 1.05-m Kiso Schmidt Telescope. In total, 331 simultaneous meteors were detected. The relationship between radar cross sections and optical V-band magnitudes was well approximated by a linear function. A transformation function from the radar cross section to the V-band magnitude was derived for sporadic meteors. The transformation function was applied to about 150,000 meteors detected by the MU radar in 2009--2015, large part of which are sporadic, and a luminosity function was derived in the magnitude range of $-1.5$ to $9.5$ mag. The luminosity function was well approximated by a single power-law function with the population index of $r = 3.52{\pm}0.12$. The present observation indicates that the MU radar has capability to detect interplanetary dust of $10^{-5}$ to $10^{0}$ g in mass as meteors.

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arXiv2020

Intermediate Luminosity Red Transients by Black Holes Born from Erupting Massive Stars

We consider black hole formation in failed supernovae when a dense circumstellar medium (CSM) is present around the massive star progenitor. By utilizing radiation hydrodynamical simulations, we calculate the mass ejection of blue supergiants and Wolf-Rayet stars in the collapsing phase and the radiative shock occurring between the ejecta and the ambient CSM. We find that the resultant emission is redder and dimmer than normal supernovae (bolometric luminosity of $\sim 10^{40}-10^{41}\ {\rm erg\ s^{-1}}$, effective temperature of $\sim 5\times 10^3$ K, and timescale of 10-100 days) and shows a characteristic power-law decay, which may comprise a fraction of intermediate luminosity red transients (ILRTs) including AT 2017be. In addition to searching for the progenitor star in the archival data, we encourage X-ray follow-up observations of such ILRTs $\sim$ 1-10 yr after the collapse, targeting the fallback accretion disk.

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arXiv2020

Radiation hydrodynamical simulations of eruptive mass Loss from progenitors of Type Ibn/IIn supernovae

Observations suggest that some massive stars experience violent and eruptive mass loss associated with significant brightening that cannot be explained by hydrostatic stellar models. This event seemingly forms dense circumstellar matter (CSM). The mechanism of eruptive mass loss has not been fully explained. We focus on the fact that the timescale of nuclear burning gets shorter than the dynamical timescale of the envelope a few years before core collapse for some massive stars. To reveal the properties of the eruptive mass loss, we investigate its relation to the energy injection at the bottom of the envelope supplied by nuclear burning taking place inside the core. In this study, we do not specify the actual mechanism for transporting energy from the site of nuclear burning to the bottom of the envelope. Instead, we parameterize the amount of injected energy and the injection time and try to extract information on these parameters from comparisons with observations. We carried out 1-D radiation hydrodynamical simulations for progenitors of red, yellow, and blue supergiants, and Wolf-Rayet stars. We calculated the evolution of the progenitors with a public stellar evolution code. We obtain the light curve associated with the eruption, the amount of ejected mass, and the CSM distribution at the time of core-collapse. The energy injection at the bottom of the envelope of a massive star within a period shorter than the dynamical timescale of the envelope could reproduce some observed optical outbursts prior to the core-collapse and form the CSM, which can power an interaction supernova (SN) classified as type IIn.

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arXiv2020

The HSC-SSP Transient Survey: Implications from Early Photometry and Rise Time of Normal Type Ia Supernovae

With a booming number of Type Ia supernovae (SNe Ia) discovered within a few days of their explosions, a fraction of SNe Ia that show luminosity excess in the early phase (early-excess SNe Ia) have been confirmed. In this article, we report early-phase observations of seven photometrically normal SNe Ia (six early detections and one deep non-detection limit) at the COSMOS field through a half-year transient survey as a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP). In particular, a blue light-curve excess was discovered for HSC17bmhk, a normal SN Ia with rise time longer than 18.8 days, during the first four days after the discovery. The blue early excess in optical wavelength can be explained not only by interactions with a non-degenerate companion or surrounding dense circumstellar matter but also radiation powered by radioactive decays of $^{56}$Ni at the surface of the SN ejecta. Given the growing evidence of the early-excess discoveries in normal SNe Ia that have longer rise times than the average and a similarity in the nature of the blue excess to a luminous SN Ia subclass, we infer that early excess discovered in HSC17bmhk and other normal SNe Ia are most likely attributed to radioactive $^{56}$Ni decay at the surface of the SN ejecta. In order to successfully identify normal SNe Ia with early excess similar to that of HSC17bmhk, early UV photometries or high-cadence blue-band surveys are necessary.

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arXiv2020

Fast Luminous Blue Transients in the Reionization Era and Beyond

To determine the epoch of reionization precisely and to reveal the property of inhomogeneous reionization are some of the most important topics of modern cosmology. Existing methods to investigate reionization which use cosmic microwave background, Ly$α$ emitters, quasars, or gamma ray bursts, have difficulties in terms of accuracy or event rate. We propose that recently discovered fast luminous blue transients (FLBTs) have a potential as a novel probe of reionization. We study the detectability of FLBTs at the epoch of reionization with upcoming WFIRST Wide-Field Instruments (WFI), using a star formation rate derived from galaxy observations and an event rate of FLBTs proportional to the star formation rate. We find that if FLBTs occur at a rate of 1\% of the core-collapse supernova rate, 2 (0.3) FLBTs per year per deg$^2$ at $z>6$ ($z>8$) can be detected by a survey with a limiting magnitude of 26.5 mag in the near-infrared band and a cadence of 10 days. We conclude that the WFIRST supernova deep survey can detect $\sim20$ FLBTs at the epoch of reionization in the near future.

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arXiv2019

A numerical light curve model for interaction-powered supernovae

We construct a numerical light curve model for interaction-powered supernovae that arise from an interaction between the ejecta and the circumstellar matter (CSM). In order to resolve the shocked region of an interaction-powered supernova, we solve the fluid equations and radiative transfer equation assuming the steady states in the rest frames of the reverse and forward shocks at each time step. Then we numerically solve the radiative transfer equation and the energy equation in the CSM with the thus obtained radiative flux from the forward shock as a radiation source. We also compare results of our models with observational data of two supernovae 2005kj and 2005ip classified as type IIn and discuss the validity of our assumptions. We conclude that our model can predict physical parameters associated with supernova ejecta and the CSM from the observed features of the light curve as long as the CSM is sufficiently dense. Furthermore, we found that the absorption of radiation in the CSM is an important factor to calculate the luminosity.

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