arXiv subjects

Ko Arimatsu

Publications and source records attributed to Ko Arimatsu.

At least 37 records · Page 2Linked to original sources

arXiv2022

Detection of an extremely large impact flash on Jupiter by high-cadence multiwavelength observations

We report the detection of an optical impact flash on Jupiter on 15 October 2021 by a dedicated telescope, Planetary ObservatioN Camera for Optical Transient Surveys (PONCOTS), for the first time. Our temporally resolved three-band observations of the flash allowed investigations of its optical energy without the need for approximations on the impact brightness temperature. The kinetic energy of the impactor was equivalent to approximately two megatons of TNT, an order of magnitude greater than that of previously detected flashes on Jupiter and comparable with the Tunguska impact on Earth in 1908. This detection indicates that Tunguska-like impact events on Jupiter occur approximately once per year, two-three orders of magnitude more frequent than terrestrial impacts. The observed flash displayed a single-temperature blackbody spectrum with an effective temperature of approximately 8300 K without clear temporal variation, possibly representing common radiative features of terrestrial Tunguska-class superbolides.

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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

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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arXiv2021

Detectability of optical transients with timescales of sub-seconds

We search for optical transient sources with durations of $\sim 0.1$ to $\sim 1.3$ s using a dataset obtained in the Organized Autotelescopes for Serendipitous Event Survey (OASES) observation campaign. Since the OASES observations were carried out using two independent wide-field and high-cadence observation systems monitored the same field simultaneously, the obtained dataset provides a unique opportunity to develop a robust detection method for sub-second optical transients. In the dataset of a selected field around the ecliptic and the Galactic plane, we find no astronomical event candidate that satisfies our detection criteria. From the non-detection result, we derive an upper limit on the event rate of sub-second transients around the ecliptic and the Galactic plane for the first time, obtaining $\sim 0.090$ and $\sim 0.38~{\rm hr^{-1}~deg^{-2}}$ for $m = 12$ and 13 Vmag, respectively. In addition, future prospects of the sub-second scale transient event surveys are discussed.

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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.

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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.

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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.

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arXiv2017

Cold Molecular Gas Along the Merger Sequence in Local Luminous Infrared Galaxies

We present an initial result from the 12CO (J=1-0) survey of 79 galaxies in 62 local luminous and ultra-luminous infrared galaxy (LIRG and ULIRG) systems obtained using the 45 m telescope at the Nobeyama Radio Observatory. This is the systematic 12CO (J=1-0) survey of the Great Observatories All-sky LIRGs Survey (GOALS) sample. The molecular gas mass of the sample ranges 2.2 x 10^8 - 7.0 x 10^9 Msun within the central several kiloparsecs subtending 15" beam. A method to estimate a size of a CO gas distribution is introduced, which is combined with the total CO flux in the literature. The method is applied to a part of our sample and we find that the median CO radius is 1-4 kpc. From the early stage to the late stage of mergers, we find that the CO size decreases while the median value of the molecular gas mass in the central several kpc region is constant. Our results statistically support a scenario where molecular gas inflows towards the central region from the outer disk, to replenish gas consumed by starburst, and that such a process is common in merging LIRGs.

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arXiv2017

Organized Autotelescopes for Serendipitous Event Survey (OASES): design and performance

Organized Autotelescopes for Serendipitous Event Survey (OASES) is an optical observation project that aims to detect and investigate stellar occultation events by kilometer-sized trans-Neptunian objects (TNOs). In this project, multiple low-cost observation systems for wide-field and high-speed photometry were developed in order to detect rare and short-timescale stellar occultation events. The observation system consists of commercial off-the-shelf $0.28 \ {\rm m}$ aperture $f/1.58$ optics providing a $2.3 \times 1.8$ square-degree field of view. A commercial CMOS camera is coupled to the optics to obtain full-frame imaging with a frame rate greater than $10 \ {\rm Hz}$. As of September 2016, this project exploits two observation systems, which are installed on Miyako Island, Okinawa, Japan. Recent improvements in CMOS technology in terms of high-speed imaging and low readout noise mean that the observation systems are capable of monitoring $\sim 2000$ stars in the Galactic plane simultaneously with magnitudes down to ${\rm V} \sim 13.0$, providing $\sim 20\%$ photometric precision in light curves with a sampling cadence of $15.4 \ {\rm Hz}$. This number of monitored stars is larger than for any other existing instruments for coordinated occultation surveys. In addition, a precise time synchronization method needed for simultaneous occultation detection is developed using faint meteors. The two OASES observation systems are executing coordinated monitoring observations of a dense stellar field in order to detect occultations by kilometer-sized TNOs for the first time.

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arXiv2016

AKARI far-infrared maps of the zodiacal dust bands

Zodiacal emission is thermal emission from interplanetary dust. Its contribution to the sky brightness is non-negligible in the region near the ecliptic plane, even in the far-infrared (far-IR) wavelength regime. We analyse zodiacal emission observed by the AKARI far-IR all-sky survey, which covers 97% of the entire sky at arcminute-scale resolution in four photometric bands, with central wavelengths of 65, 90, 140, and 160 $μ$m. AKARI detected small-scale structures in the zodiacal dust cloud, including the asteroidal dust bands and the circumsolar ring, at far-IR wavelengths. Although the smooth component of the zodiacal emission structure in the far-IR sky can be reproduced well by models based on existing far-IR observations, previous zodiacal emission models have discrepancies in the small-scale structures compared with observations. We investigate the geometry of the small-scale dust-band structures in the AKARI far-IR all-sky maps and construct template maps of the asteroidal dust bands and the circumsolar ring components based on the AKARI far-IR maps. In the maps, $\pm 1.4°$, $\pm 2.1°$ and $\pm 10°$ asteroidal dust-band structures are detected in the 65 $μ$m and 90 $μ$m bands. A possible $\pm 17°$ band may also have been detected. No evident dust-band structures are identified in either the 140 $μ$m or the 160 $μ$m bands. By subtracting the dust-band templates constructed in this paper, we can achieve a similar level of flux calibration of the AKARI far-IR all-sky maps in the $|β| < 40°$ region to that in the region for $|β| > 40°$.

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arXiv2015

The $AKARI$ Far-Infrared All-Sky Survey Maps

We present a far-infrared all-sky atlas from a sensitive all-sky survey using the Japanese $AKARI$ satellite. The survey covers $> 99$% of the sky in four photometric bands centred at 65 $μ$m, 90 $μ$m, 140 $μ$m, and 160 $μ$m with spatial resolutions ranging from 1 to 1.5 arcmin. These data provide crucial information for the investigation and characterisation of the properties of dusty material in the Interstellar Medium (ISM), since significant portion of its energy is emitted between $\sim$50 and 200 $μ$m. The large-scale distribution of interstellar clouds, their thermal dust temperatures and column densities, can be investigated with the improved spatial resolution compared to earlier all-sky survey observations. In addition to the point source distribution, the large-scale distribution of ISM cirrus emission, and its filamentary structure, are well traced. We have made the first public release of the full-sky data to provide a legacy data set for use by the astronomical community.

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arXiv2015

Calibration of the AKARI Far-infrared All Sky Survey Maps

We present an initial analysis of the properties of the all-sky image obtained by the Far-Infrared Surveyor (FIS) onboard the AKARI satellite, at 65~$μ$m (N60), 90~$μ$m (WIDE-S), 140~$μ$m (WIDE-L),and 160~$μ$m (N160). Absolute flux calibration was determined by comparing the data with the COBE/DIRBE data sets, and the intensity range was as wide as from a few MJy~sr$^{-1}$ to $>$1~GJy~sr$^{-1}$. The uncertainties are considered to be the standard deviations with respect to the DIRBE data, and they are less than 10\% for intensities above 10, 3, 25, and 26~MJy~sr$^{-1}$ at the N60, WIDE-S, WIDE-L, and N160 bands, respectively. The characteristics of point sources in the image were also determined by stacking maps centred on photometric standard stars. The full width at half maxima of the point spread functions (PSFs) were 63$"$, 78$"$, and 88$"$ at the N60, WIDE-S, and WIDE-L bands, respectively. The PSF at the N160 band was not obtained due to the sensitivity, but it is thought to be the same as that of the WIDE-L one.

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arXiv2014

Kiso Supernova Survey (KISS): Survey Strategy

The Kiso Supernova Survey (KISS) is a high-cadence optical wide-field supernova (SN) survey. The primary goal of the survey is to catch the very early light of a SN, during the shock breakout phase. Detection of SN shock breakouts combined with multi-band photometry obtained with other facilities would provide detailed physical information on the progenitor stars of SNe. The survey is performed using a 2.2x2.2 deg field-of-view instrument on the 1.05-m Kiso Schmidt telescope, the Kiso Wide Field Camera (KWFC). We take a three-minute exposure in g-band once every hour in our survey, reaching magnitude g~20-21. About 100 nights of telescope time per year have been spent on the survey since April 2012. The number of the shock breakout detections is estimated to be of order of 1 during our 3-year project. This paper summarizes the KISS project including the KWFC observing setup, the survey strategy, the data reduction system, and CBET-reported SNe discovered so far by KISS.

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arXiv2014

Point source calibration of the AKARI/FIS all-sky survey maps for staking analysis

Investigations of the point spread functions (PSFs) and flux calibrations for stacking analysis have been performed with the far-infrared (wavelengths range of 60 to 140 um all-sky maps taken by the Far-Infrared Surveyor (FIS) onboard the AKARI satellite. The PSFs are investigated by stacking the maps at the positions of standard stars with their fluxes of 0.02 -10 Jy. The derived full widths at the half maximum (FWHMs) of the PSFs are ~ 60 arcsec at 65 and 90 um and ~ 90 arcsec at 140 um, which are much smaller than that of the previous all-sky maps obtained with IRAS (~ 6 arcmin). Any flux dependence in the PSFs is not seen on the investigated flux range. By performing the flux calibrations, we found that absolute photometry for faint sources can be carried out with constant calibration factors, which range from 0.6 to 0.8. After applying the calibration factors, the photometric accuracies for the stacked sources in the 65, 90, and 140 um bands are 9, 3, and 21 %, respectively, even below the detection limits of the survey. Any systematic dependence between the observed flux and model flux is not found. These results indicate that the FIS map is a useful dataset for the stacking analyses of faint sources at far-infrared wavelengths.

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arXiv2013

Interstellar dust properties of M51 from AKARI mid-infrared images

Using mid-infrared (MIR) images of four photometric bands of the Infrared Camera (IRC) onboard the AKARI satellite, S7 (7 um), S11 (11 um), L15 (15 um), and L24 (24 um), we investigate the interstellar dust properties of the nearby pair of galaxies M51 with respect to its spiral arm structure. The arm and interarm regions being defined based on a spatially filtered stellar component model image, we measure the arm-to-interarm contrast for each band. The contrast is lowest in the S11 image, which is interpreted as that among the four AKARI MIR bands the S11 image best correlates with the spatial distribution of dust grains including colder components, while the L24 image with the highest contrast traces warmer dust heated by star forming activities. The surface brightness ratio between the bands, i.e. color, is measured over the disk of the main galaxy, M51a, at 300 pc resolution. We find that the distribution of S7/S11 is smooth and well traces the global spiral arm pattern while L15/S11 and L24/S11 peak at individual HII regions. This result indicates that the ionization state of PAHs is related to the spiral structure. Comparison with observational data and dust models also supports the importance of the variation in the PAH ionization state within the M51a disk. However, the mechanism driving this variation is not yet clear from currently available data sets. Another suggestion from the comparison with the models is that the PAH fraction to the total dust mass is higher than previously estimated.

astro-ph.CO↗

arXiv2012

AKARI Far-Infrared All-Sky Survey Maps

Far-infrared observations provide crucial data for the investigation and characterisation of the properties of dusty material in the Interstellar Medium (ISM), since most of its energy is emitted between ~100 and 200 um. We present the first all-sky image from a sensitive all-sky survey using the Japanese AKARI satellite, in the wavelength range 50 -- 180 um. Covering >99% of the sky in four photometric bands with four filters centred at 65 um, 90 um, 140 um, and 160 um wavelengths, this achieved spatial resolutions from 1 to 2 arcmin and a detection limit of <10 MJy sr-1, with absolute and relative photometric accuracies of <20%. All-sky images of the Galactic dust continuum emission enable astronomers to map the large-scale distribution of the diffuse ISM cirrus, to study its thermal dust temperature, emissivity and column density, and to measure the interaction of the Galactic radiation field and embedded objects with the surrounding ISM. In addition to the point source population of stars, protostars, star-forming regions, and galaxies, the high Galactic latitude sky is shown to be covered with a diffuse filamentary-web of dusty emission that traces the potential sites of high latitude star formation. We show that the temperature of dust particles in thermal equilibrium with the ambient interstellar radiation field can be estimated by using 90 um, 140 um, and 160 um data. The FIR AKARI full-sky maps provide a rich new data set within which astronomers can investigate the distribution of interstellar matter throughout our Galaxy, and beyond.

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arXiv2012

A Search for Infrared Emission from Core-Collapse Supernovae at the Transitional Phase

Most of the observational studies of supernova (SN) explosions are limited to early phases (< a few yr after the explosion) of extragalactic SNe and observations of SN remnants (> 100 yr) in our Galaxy or very nearby galaxies. SNe at the epoch between these two, which we call "transitional" phase, have not been explored in detail except for several extragalactic SNe including SN 1987A in the Large Magellanic Cloud. We present theoretical predictions for the infrared (IR) dust emissions by several mechanisms; emission from dust formed in the SN ejecta, light echo by circumstellar and interstellar dust, and emission from shocked circumstellar dust. We search for IR emission from 6 core-collapse SNe at the transitional phase in the nearby galaxies NGC 1313, NGC 6946, and M101 by using the data taken with the AKARI satellite and Spitzer. Among 6 targets, we detect the emission from SN 1978K in NGC 1313. SN 1978K is associated with 1.3 x 10^{-3} Msun of silicate dust. We show that, among several mechanisms, the shocked circumstellar dust is the most probable emission source to explain the IR emission observed for CSM-rich SN 1978K. IR emission from the other 5 objects is not detected. Our current observations are sensitive to IR luminosity of > 10^{38} erg s^{-1}, and the non-detection of SN 1962M excludes the existence of the shocked circumstellar dust for a high gas mass-loss rate of sim 10^{-4} Msun yr^{-1}. Observations of SNe at the transitional phase with future IR satellites will fill the gap of IR observations of SNe with the age of 10-100 years, and give a new opportunity to study the circumstellar and interstellar environments of the progenitor, and possibly dust formation in SNe.

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arXiv2011

Characterization and Improvement of the Image Quality of the Data Taken with the Infrared Camera (IRC) Mid-Infrared Channels onboard AKARI

Mid-infrared images frequently suffer artifacts and extended point spread functions (PSFs). We investigate the characteristics of the artifacts and the PSFs in images obtained with the Infrared Camera (IRC) onboard AKARI at four mid-infrared bands of the S7 (7μm), S11 (11μm), L15 (15μm), and L24 (24 μm). Removal of the artifacts significantly improves the reliability of the ref- erence data for flat-fielding at the L15 and L24 bands. A set of models of the IRC PSFs is also constructed from on-orbit data. These PSFs have extended components that come from diffraction and scattering within the detector arrays. We estimate the aperture correction factors for point sources and the surface brightness correction factors for diffuse sources. We conclude that the surface brightness correction factors range from 0.95 to 0.8, taking account of the extended component of the PSFs. To correct for the extended PSF effects for the study of faint structures, we also develop an image reconstruction method, which consists of the deconvolution with the PSF and the convolution with an appropriate Gaussian. The appropriate removal of the artifacts, improved flat-fielding, and image reconstruction with the extended PSFs enable us to investigate de- tailed structures of extended sources in IRC mid-infrared images.

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