arXiv subjects

Noriyuki Katoh

Publications and source records attributed to Noriyuki Katoh.

7 recordsLinked to original sources

arXiv2022

Discovery of a Long-Duration Superflare on a Young Solar-Type Star EK Draconis with Nearly Similar Time Evolution for H$α$ and White-Light Emissions

Young solar-type stars are known to show frequent "superflares", which may severely influence the habitable worlds on young planets via intense radiations and coronal mass ejections. Here we report an optical spectroscopic and photometric observation of a long-duration superflare on the young solar-type star EK Draconis (50-120 Myr age) with the Seimei telescope and $Transiting$ $Exoplanet$ $Survey$ $Satellite$ ($TESS$). The flare energy 2.6$\times$10$^{34}$ erg and white-light flare duration 2.2 hr are much larger than those of the largest solar flares, and this is the largest superflare on a solar-type star ever detected by optical spectroscopy. The H$α$ emission profile shows no significant line asymmetry, meaning no signature of a filament eruption, unlike the only previous detection of a superflare on this star (Namekata et al. 2021, $Nat.Astron$). Also, it did not show significant line broadening, indicating that the non-thermal heating at the flare footpoints are not essential or that the footpoints are behind the limb. The time evolution and duration of the H$α$ flare are surprisingly almost the same as those of the white-light flare, which is different from general M-dwarf (super-)flares and solar flares. This unexpected time evolution may suggest that different radiation mechanisms than general solar flares are predominant, as follows: (1) radiation from (off-limb) flare loops, and (2) re-radiation via radiative backwarming, in both of which the cooling timescales of flare loops could determine the timescales of H$α$ and white light.

astro-ph.SR↗

arXiv2021

Probable detection of an eruptive filament from a superflare on a solar-type star

Solar flares are often accompanied by filament/prominence eruptions ($\sim10^{4}$ K and $\sim 10^{10-11}$ cm$^{-3}$), sometimes leading to coronal mass ejections (CMEs) that directly affect the Earth's environment. `Superflares' are found on some active solar-type (G-type main-sequence) stars, but the association of filament eruptions/CMEs has not been established. Here we show that our optical spectroscopic observation of the young solar-type star EK Draconis reveals the evidence for a stellar filament eruption associated with a superflare. This superflare emitted a radiated energy of $2.0\times10^{33}$ erg, and blue-shifted hydrogen absorption component with a large velocity of $-510$ km s$^{-1}$ was observed shortly after. The temporal changes in the spectra greatly resemble those of solar filament eruptions. Comparing this eruption with solar filament eruptions in terms of the length scale and velocity strongly suggests that a stellar CME occurred. The erupted filament mass of $1.1\times10^{18}$ g is 10 times larger than those of the largest solar CMEs. The massive filament eruption and an associated CME provide the opportunity to evaluate how they affect the environment of young exoplanets/young Earth and stellar mass/angular-momentum evolution.

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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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arXiv2020

Time-resolved spectroscopy and photometry of an M dwarf flare star YZ Canis Minoris with OISTER and TESS: Blue asymmetry in H$α$ line during the non-white light flare

In this paper, we present the results from spectroscopic and photometric observations of the M-type flare star YZ CMi in the framework of the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER) collaborations during the Transiting Exoplanet Survey Satellite (TESS) observation period. We detected 145 white-light flares from the TESS light curve and 4 H$α$ flares from the OISTER observations performed between 2019-01-16 and 2019-01-18. Among them, 3 H$α$ flares were associated with white-light flares. However, one of them did not show clear brightening in continuum; during this flare, the H$α$ line exhibited blue-asymmetry which has lasted for $\sim 60$ min. The line of sight velocity of the blue-shifted component is $-80$ - $-100$ km s$^{-1}$. This suggests that there can be upward flows of chromospheric cool plasma even without detectable red/NIR continuum brightening. By assuming that the blue-asymmetry in H$α$ line was caused by a prominence eruption on YZ CMi, we estimated the mass and kinetic energy of the upward-moving material to be $10^{16}$ - $10^{18}$ g and $10^{29.5}$ - $10^{31.5}$ erg, respectively. The estimated mass is comparable to expectations from the empirical relation between the flare X-ray energy and mass of upward-moving material for stellar flares and solar CMEs. In contrast, the estimated kinetic energy for the non-white-light flare on YZ CMi is roughly $2$ orders of magnitude smaller than that expected from the relation between flare X-ray energy and kinetic energy for solar CMEs. This could be understood by the difference in the velocity between CMEs and prominence eruptions.

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arXiv2020

Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER

We report multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with Seimei Telescope (6150--7930 Å), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700--7500 Å), NICER (Neutron Star Interior Composition Explorer; 0.2--12.0 keV), and collaborations of OISTER (Optical and Infrared Synergetic Telescopes for Education and Research) program. Twelve flares are detected in total which include ten H$α$, four X-ray, and four optical-continuum flares; one of them is a superflare with the total energy of $\sim$ 2.0$\times$10$^{33}$ erg. We found that (1) during the superflare, the H$α$ emission line full width at 1/8 maximum dramatically increases to 14 Å from 8 Å in the low-resolution spectra (R$\sim$ 2000) accompanied with the large white-light flares, (2) some weak H$α$/X-ray flares are not accompanied with white-light emissions, and (3) the non-flaring emissions show clear rotational modulations in X-ray and H$α$ intensity in the same phase. To understand these observational features, one-dimensional hydrodynamic flare simulations are performed by using the RADYN code. As a result of simulations, we found the simulated H$α$ line profiles with hard and high-energy non-thermal electron beams are consistent with that of the initial phase line profiles of the superflares, while those with more soft- and/or weak-energy beam are consistent with those in decay phases, indicating the changes in the energy fluxes injected to the lower atmosphere. Also, we found that the relation between optical continuum and H$α$ intensity is nonlinear, which can be one cause of the non-white-light flares. The flare energy budget exhibits diversity in the observations and models, and more observations of stellar flares are necessary for constraining the occurrence of various emission line phenomena in stellar flares.

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arXiv2020

Type Ia SN 2019ein: New Insights into the Similarities and diversities among High-Velocity SNe Ia

We present optical observations of type Ia supernova (SN) 2019ein, starting at 2 days after the estimated explosion date. The spectra and the light curves show that SN 2019ein belongs to the High-Velocity (HV) and Bload Line groups with relatively rapid decline in the light curves (Delta m15(B) = 1.36 +- 0.02 mag) and the short rise time (15.37 +- 0.55 days). The Si II 6355 velocity, associated with a photospheric component but not with a detached high-velocity feature, reached ~ 20,000 km s-1 at 12 days before the B-band maximum. The line velocity however decreased very rapidly and smoothly toward the maximum light, where it was ~ 13,000 km s-1 as relatively low among HV SNe. This indicates that the speed of the spectral evolution of HV SNe Ia is correlated not only to the velocity at the maximum light, but also to the light curve decline rate like the case for Normal-Velocity (NV) SNe Ia. Spectral synthesis modeling shows that the outermost layer at > 17,000 km s-1 is well described by the O-Ne-C burning layer extending to at least 25,000 km s-1, and there is no unburnt carbon below 30,000 km s-1; these properties are largely consistent with the delayed detonation scenario, and are shared with the prototypical HV SN 2002bo despite the large difference in Delta m15(B). This structure is strikingly different from that derived for the well-studied NV SN 2011fe. We suggest that the relation between the mass of 56Ni (or Delta m15) and the extent of the O-Ne-C burning layer provides an important constraint on the explosion mechanism(s) of HV and NV SNe.

astro-ph.HE↗

arXiv2018

X-ray, Optical, and Near-infrared Monitoring of the New X-ray Transient MAXI J1820+070 in the Low/hard State

We report X-ray, optical, and near-infrared monitoring of the new X-ray transient MAXI J1820$+$070 discovered with MAXI on 2018 March 11. Its X-ray intensity reached $\sim 2$ Crab in 2--20 keV at the end of March, and then gradually decreased until the middle of June. In this period, the X-ray spectrum was described by Comptonization of the disk emission, with a photon index of $\sim$1.5 and an electron temperature of $\sim$50 keV, which is consistent with a black hole X-ray binary in the low/hard state. The electron temperature and the photon index were slightly decreased and increased with increasing flux, respectively. The source showed significant X-ray flux variation on timescales of seconds. This short-term variation was found to be associated with changes in the spectral shape, and the photon index became slightly harder at higher fluxes. This suggests that the variation was produced by a change in the properties of the hot electron cloud responsible for the strong Comptonization. Modeling a multi-wavelength SED around the X-ray flux peak at the end of March, covering the near-infrared to X-ray bands, we found that the optical and near-infrared fluxes were likely contributed substantially by the jet emission. Before this outburst, the source was never detected in the X-ray band with MAXI (with a 3$σ$ upper limit of $\sim$0.2 mCrab in 4--10 keV, obtained from the 7-year data in 2009--2016), whereas weak optical and infrared activity was found at their flux levels $\sim$3 orders of magnitude lower than the peak fluxes in the outburst.

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