arXiv subjects

Mitsuru Kokubo

Publications and source records attributed to Mitsuru Kokubo.

At least 37 records · Page 2Linked to original sources

arXiv2022

H1821+643: The most X-ray and infrared luminous AGN in the Swift/BAT survey in the process of rapid stellar and supermassive black hole mass assembly

H1821+643 is the most X-ray luminous non-beamed AGN of $L_\mathrm{14-150 keV}= 5.2\times 10^{45}$ erg s$^{-1}$ in the Swift/BAT ultra-hard X-ray survey and it is also a hyper-luminous infrared (IR) galaxy $L_\mathrm{IR} = 10^{13.2} L_\odot$ residing in the center of a massive galaxy cluster, which is a unique environment achieving the rapid mass assembly of black holes (BH) and host galaxies in the local universe. We decompose the X-ray to IR spectral energy distribution (SED) into the AGN and starburst component using the SED fitting tool CIGALE-2022.0 and show that H1821+643 consumes a large amount of cold gas ($\dot{M}_\mathrm{con}$) with star-formation rate of $\log ( \mathrm{SFR}/M_{\odot}~\mathrm{yr}^{-1}) = 3.01 \pm 0.04$ and BH accretion rate of $\log (\dot{M}_\mathrm{BH}/M_{\odot}~\mathrm{yr}^{-1}) = 1.20 \pm 0.05$. This high $\dot{M}_\mathrm{con}$ is larger than the cooling rate ($\dot{M}_\mathrm{cool}$) of the intra-cluster medium (ICM), $\dot{M}_\mathrm{con}/\dot{M}_\mathrm{cool} \gtrsim 1$, which is one to two order magnitude higher than the typical value of other systems, indicating that H1821 provides the unique and extreme environment of rapid gas consumption. We also show that H1821+643 has an efficient cooling path achieving from $10^7$ K to $10^2$ K thanks to [OIII] 63 $μ\mathrm{m}$, which is a main coolant in low temperature range ($10^4$ K to $10^2$ K) with a cooling rate of $\dot{M}_{\mathrm{cool}}=3.2\times 10^5\ M_{\odot}\mathrm{~yr^{-1}}$, and the star-forming region extends over 40 kpc scale.

astro-ph.GA↗

arXiv2022

Measurement of AGN dust extinction based on the near-infrared flux variability of WISE data

We present the measurement of the line-of-sight extinction of the dusty torus for a large number of obscured active galactic nuclei (AGNs) based on the reddening of the colour of the variable flux component in near-infrared (NIR) wavelengths. We collected long-term monitoring data by $\textit{Wide-field Infrared Survey Explorer (WISE)}$ for 513 local AGNs catalogued by the $\mathit{Swift/}$BAT AGN Spectroscopic Survey (BASS) and found that the multi-epoch NIR flux data in two different bands (WISE $W1$ and $W2$) are tightly correlated for more than 90% of the targets. The flux variation gradient (FVG) in the $W1$ and $W2$ bands was derived by applying linear regression analysis, and we reported that those for unobscured AGNs fall in a relatively narrow range, whereas those for obscured AGNs are distributed in a redder and broader range. The AGN's line-of-sight dust extinction ($A_V$) is calculated using the amount of the reddening in the FVG and is compared with the neutral hydrogen column density ($N_{\rm{}H}$) of the BASS catalogue. We found that the $N_{\rm{}H}/A_V$ ratios of obscured AGNs are greater than those of the Galactic diffuse interstellar medium (ISM) and are distributed with a large scatter by at most two orders of magnitude. Furthermore, we found that the lower envelope of the $N_{\rm{}H}/A_V$ of obscured AGNs is comparable to the Galactic diffuse ISM. These properties of the $N_{\rm{}H}/A_V$ can be explained by increase in the $N_{\rm{}H}$ attributed to the dust-free gas clouds covering the line of sight in the broad-line region.

astro-ph.GA↗

arXiv2022

Multiple giant eruptions and X-ray emission in the recoiling AGN/LBV candidate SDSS1133

We present a comprehensive analysis of 20 years worth of multi-color photometric light curves, multi-epoch optical spectra, and X-ray data of an off-nuclear variable object SDSS1133 in Mrk 177 at $z=0.0079$. The UV-optical light curves reveal that SDSS1133 experienced four outbursts in 2001, 2014, 2019, and 2021. The persistent UV-optical luminosity in the non-outbursting state is $\sim 10^{41}$ erg/s with small-scale flux variations, and peak luminosities during the outbursts reach $\sim 10^{42}$ erg/s. The optical spectra exhibit enduring broad hydrogen Balmer P-Cygni profiles with the absorption minimum at $\sim -2,000$ km/s, indicating the presence of fast moving ejecta. {\it Chandra} detected weak X-ray emission at a $0.3-10$ keV luminosity of $L_{X} = 4 \times 10^{38}$ erg/s after the 2019 outburst. These lines of evidence suggests that SDSS1133 is an extreme luminous blue variable (LBV) star experiencing multiple giant eruptions with interactions of the ejected shell with different shells and/or circumstellar medium (CSM), and disfavors the recoiling Active Galactic Nuclei (AGN) scenario suggested in the literature. We suggest that pulsational pair-instability may provide a viable explanation for the multiple energetic eruptions in SDSS1133. If the current activity of SDSS1133 is a precursor of a supernova explosion, we may be able to observe a few additional giant eruptions and then the terminal supernova explosion or collapse to a massive black hole in future observations.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2021

Third Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The paper presents the third data release of Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), a wide-field multi-band imaging survey with the Subaru 8.2m telescope. HSC-SSP has three survey layers (Wide, Deep, and UltraDeep) with different area coverages and depths, designed to address a wide array of astrophysical questions. This third release from HSC-SSP includes data from 278 nights of observing time and covers about 670 square degrees in all five broad-band filters at the full depth ($\sim26$~mag at $5σ$) in the Wide layer. If we include partially observed area, the release covers 1,470 square degrees. The Deep and UltraDeep layers have $\sim80\%$ of the originally planned integration times, and are considered done, as we have slightly changed the observing strategy in order to compensate for various time losses. There are a number of updates in the image processing pipeline. Of particular importance is the change in the sky subtraction algorithm; we subtract the sky on small scales before the detection and measurement stages, which has significantly reduced false detections. Thanks to this and other updates, the overall quality of the processed data has improved since the previous release. However, there are limitations in the data (for example, the pipeline is not optimized for crowded fields), and we encourage the user to check the quality assurance plots as well as a list of known issues before exploiting the data. The data release website is https://hsc-release.mtk.nao.ac.jp/.

astro-ph.IM↗

arXiv2021

Reverberation Mapping of Two Luminous Quasars: the Broad-line Region Structure and Black Hole Mass

We report the results of a multi-year spectroscopic and photometric monitoring campaign of two luminous quasars, PG~0923+201 and PG~1001+291, both located at the high-luminosity end of the broad-line region (BLR) size-luminosity relation with optical luminosities above $10^{45}~{\rm erg~s^{-1}}$. PG~0923+201 is for the first time monitored, and PG~1001+291 was previously monitored but our campaign has a much longer temporal baseline. We detect time lags of variations of the broad H$β$, H$γ$, Fe {\sc ii} lines with respect to those of the 5100~Å continuum. The velocity-resolved delay map of H$β$ in PG~0923+201 indicates a complicated structure with a mix of Keplerian disk-like motion and outflow, and the map of H$β$ in PG~1001+291 shows a signature of Keplerian disk-like motion. Assuming a virial factor of $f_{\rm BLR}=1$ and FWHM line widths, we measure the black hole mass to be $118_{-16}^{+11}\times 10^7 M_{\odot}$ for PG~0923+201 and $3.33_{-0.54}^{+0.62}\times 10^7 M_{\odot}$ for PG~1001+291. Their respective accretion rates are estimated to be $0.21_{-0.07}^{+0.06} \times L_{\rm Edd}\,c^{-2}$ and $679_{-227}^{+259}\times L_{\rm Edd}\,c^{-2}$, indicating that PG~0923+201 is a sub-Eddington accretor and PG~1001+291 is a super-Eddington accretor. While the H$β$ time lag of PG~0923+201 agrees with the size-luminosity relation, the time lag of PG~1001+291 shows a significant deviation, confirming that in high-luminosity AGN the BLR size depends on both luminosity and Eddington ratio. Black hole mass estimates from single AGN spectra will be over-estimated at high luminosities and redshifts if this effect is not taken into account.

astro-ph.GA↗

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.

astro-ph.EP↗

arXiv2020

Constraints on the rate of supernovae lasting for more than a year from Subaru/Hyper Suprime-Cam

Some supernovae such as pair-instability supernovae are predicted to have the duration of more than a year in the observer frame. To constrain the rates of supernovae lasting for more than a year, we conducted a long-term deep transient survey using Hyper Suprime-Cam (HSC) on the 8.2m Subaru telescope. HSC is a wide-field (a 1.75 deg2 field-of-view) camera and it can efficiently conduct transient surveys. We observed the same 1.75 deg2 field repeatedly using the g, r, i, and z band filters with the typical depth of 26 mag for 4 seasons (from late 2016 to early 2020). Using these data, we searched for transients lasting for more than a year. Two supernovae were detected in 2 continuous seasons, one supernova was detected in 3 continuous seasons, but no transients lasted for all 4 seasons searched. The discovery rate of supernovae lasting for more than a year with the typical limiting magnitudes of 26 mag is constrained to be 1.4^{+1.3}_{-0.7}(stat.)^{+0.2}_{-0.3}(sys.) events deg-2 yr-1. All the long-lasting supernovae we found are likely Type IIn supernovae and our results indicate that about 40% of Type IIn supernovae have long-lasting light curves. No plausible pair-instability supernova candidates lasting for more than a year are discovered. By comparing the survey results and survey simulations, we constrain the luminous pair-instability supernova rate up to z ~ 3 should be of the order of 100 Gpc-3 yr-1 at most, which is 0.01 - 0.1 per cent of the core-collapse supernova rate.

astro-ph.HE↗

arXiv2020

Tracing the co-evolution path of super massive black holes and spheroids with AKARI-selected ultra-luminous IR galaxies at intermediate redshifts

We present the stellar population and ionized-gas outflow properties of ultra-luminous IR galaxies (ULIRGs) at $z=$ 0.1-1.0, which are selected from AKARI FIR all-sky survey. We construct a catalog of 1077 ULIRGs to examine feedback effect after major mergers. 202 out of the 1077 ULIRGs are spectroscopically identified by SDSS and Subaru/FOCAS observations. Thanks to deeper depth and higher resolution of AKARI compared to the previous IRAS survey, and reliable identification from WISE MIR pointing, the sample is unique in identifying optically-faint (i$\sim$20) IR-bright galaxies, which could be missed in previous surveys. A self-consistent spectrum-SED decomposition method, which constrains stellar population properties in SED modeling based on spectral fitting results, has been employed for 149 ULIRGs whose optical continua are dominated by host galaxies. They are massive galaxies ($M_{\rm star}\sim10^{11}$-$10^{12}$ M$_{\odot}$), associated with intense star formation activities (SFR $\sim$ 200-2000 M$_{\odot}$ yr$^{-1}$). The sample covers a range of AGN bolometric luminosity of $10^{10}$-$10^{13}$ L$_{\odot}$, and the outflow velocity measured from [OIII] 5007A line shows a correlation with AGN luminosity. Eight galaxies show extremely fast outflows with velocity up to 1500-2000 km s$^{-1}$. However, the co-existence of vigorous starbursts and strong outflows suggests the star formation has not been quenched during the ULIRG phase. By deriving stellar mass and mass fraction of young stellar population, we find no significant discrepancies between stellar properties of ULIRGs with weak and powerful AGNs. The results are not consistent with the merger-induced evolutionary scenario, which predicts that SF-dominated ULIRGs show smaller stellar mass and younger stellar populations compared to AGN-dominated ULIRGs.

astro-ph.GA↗

arXiv2020

Dust Reverberation Mapping of Type 2 AGN NGC 2110 Realized with X-ray and 3-5 $μ$m IR monitoring

The dust reverberation mapping is one of powerful methods to investigate the structure of the dusty tori in AGNs, and it has been performed on more than a hundred type 1 AGNs. However, no clear results have been reported on type 2 AGNs because their strong optical-UV extinction completely hides their accretion disc emission. Here we focus on an X-ray-bright type 2 AGN, NGC 2110, and utilize 2-20 keV X-ray variation monitored by MAXI to trace disc emission, instead of optical-UV variation. Comparing it with light curves in the WISE infrared (IR) W1 band ($λ= 3.4$ $μ$m) and W2 band ($λ= 4.6$ $μ$m) with cross-correlation analyses, we found candidates of the dust reverberation time lag at $\sim60$ days, $\sim130$ days, and $\sim1250$ days between the X-ray flux variation and those of the IR bands. By examining the best-fitting X-ray and IR light curves with the derived time lags, we found that the time lag of $\sim130$ days is most favoured. With this time lag, the relation between the time lag and luminosity of NGC 2110 is consistent with those in type 1 AGNs, suggesting that the dust reverberation in NGC 2110 mainly originates in hot dust in the torus innermost region, the same as in type 1 AGNs. As demonstrated by the present study, X-ray and IR simultaneous monitoring can be a promising tool to perform the dust reverberation mapping on type 2 AGNs.

astro-ph.GA↗

arXiv2020

Properties of AGN Multiband Optical Variability in the HSC SSP Transient Survey

We study variability of active galactic nuclei (AGNs) by using the deep optical multiband photometry data obtained from the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) survey in the COSMOS field. The images analyzed here were taken with 8, 10, 13, and 15 epochs over three years in the $g$, $r$, $i$, and $z$ bands, respectively. We identified 491 robust variable AGN candidates, down to $i=25$ mag and with redshift up to $4.26$. Ninety percent of the variability-selected AGNs are individually identified with the X-ray sources detected in the Chandra COSMOS Legacy survey. We investigate their properties in variability by using structure function analysis and find that the structure function for low-luminosity AGNs ($L_{\mathrm{bol}}\lesssim10^{45}$ erg s$^{-1}$) shows a positive correlation with luminosity, which is the opposite trend for the luminous quasars. This trend is likely to be caused by larger contribution of the host galaxy light for lower-luminosity AGNs. Using the model templates of galaxy spectra, we evaluate the amount of host galaxy contribution to the structure function analysis and find that dominance of the young stellar population is needed to explain the observed luminosity dependence. This suggests that low-luminosity AGNs at $0.8\lesssim z\lesssim1.8$ are predominantly hosted in star-forming galaxies. The X-ray stacking analysis reveals the significant emission from the individually X-ray undetected AGNs in our variability-selected sample. The stacked samples show very large hardness ratios in their stacked X-ray spectrum, which suggests that these optically variable sources have large soft X-ray absorption by dust-free gas.

astro-ph.GA↗

arXiv2020

Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey

Rapidly evolving transients form a new class of transients which show shorter timescales of the light curves than those of typical core-collapse and thermonuclear supernovae. We performed a systematic search for rapidly evolving transients using the deep data taken with the Hyper Suprime-Cam Subaru Strategic Program Transient Survey. By measuring the timescales of the light curves of 1824 transients, we identified 5 rapidly evolving transients. Our samples are found in a wide range of redshifts (0.3 $\le$ z $\le$ 1.5) and peak absolute magnitudes ($-$17 $\ge$ $M_i$ $\ge$ $-$20). The properties of the light curves are similar to those of the previously discovered rapidly evolving transients. They show a relatively blue spectral energy distribution, with the best-fit blackbody of 8,000 - 18,000 K. We show that some of the transients require power sources other than the radioactive decays of $^{56}$Ni because of their high peak luminosities and short timescales. The host galaxies of all the samples are star-forming galaxies, suggesting a massive star origin for the rapidly evolving transients. The event rate is roughly estimated to be $\sim$4,000 events yr$^{-1}$ Gpc$^{-3}$, which is about 1 $\%$ of core-collapse supernovae.

astro-ph.HE↗

arXiv2020

Dust Destruction by Charging: A Possible Origin of Grey Extinction Curves of Active Galactic Nuclei

Observed extinction curves of active galactic nuclei (AGNs) are significantly different from those observed in the Milky Way. The observations require preferential removal of small grains at the AGN environment; however, the physics for this remains unclear. In this paper, we propose that dust destruction by charging, or Coulomb explosion, may be responsible for AGN extinction curves. Harsh AGN radiation makes a dust grain highly charged through photoelectric emission, and grain fission via the Coulomb explosion occurs when the electrostatic tensile stress of a charge grain exceeds its tensile strength. We show that the Coulomb explosion can preferentially remove both small silicate and graphite grains and successfully reproduce both flat extinction curves and the absence of 2175Å~bump.

astro-ph.GA↗

arXiv2019

Rapid Luminosity Decline and Subsequent Reformation of the Innermost Dust Distribution in the Changing-look AGN Mrk 590

We examine the long-term optical/near-infrared (NIR) flux variability of a "changing-look" active galactic nucleus (AGN) Mrk 590 between 1998 and 2007. Multi-band multi-epoch optical/NIR photometry data from the SDSS Stripe 82 database and the Multicolor Active Galactic Nuclei Monitoring (MAGNUM) project reveal that Mrk 590 experienced a sudden luminosity decrease during the period from 2000 to 2001. Detection of dust reverberation lag signals between $V$- and $K$-band light curves obtained by the MAGNUM project during the faint state in $2003-2007$ suggests that the dust torus innermost radius $R_\text{dust}$ of Mrk 590 had become very small [$R_\text{dust} \simeq 32$~ light-days (lt-days)] by the year 2004 according to the aforementioned significant decrease in AGN luminosity. The $R_\text{dust}$ in the faint state is comparable to the H$β$ broad line region (BLR) radius of $R_{\text{H}β, \text{BLR}} \simeq 26$ lt-days measured by previous reverberation mapping observations during the bright state of Mrk 590 in $1990-1996$. These observations indicate that the innermost radius of the dust torus in Mrk 590 decreased rapidly after the AGN ultraviolet-optical luminosity drop, and that the replenishment time scale of the innermost dust distribution is less than 4 years, which is much shorter than the free fall time scale of BLR gas or dust clouds. We suggest that rapid replenishment of the innermost dust distribution can be accomplished either by new dust formation in radiatively-cooled BLR gas clouds or by new dust formation in the disk atmosphere and subsequent vertical wind from the dusty disk as a result of radiation pressure.

astro-ph.GA↗

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

Discovery of a strong ionized-gas outflow in an AKARI-selected Ultra-luminous Infrared Galaxy at z = 0.5

In order to construct a sample of ultra-luminous infrared galaxies (ULIRGs, with infrared luminosity, $L_{\rm IR} > 10^{12}$ L$_{\odot}$) at 0.5 < z < 1, we are conducting an optical follow-up program for bright 90-$μ$m FIR sources with a faint optical (i < 20) counterpart selected in the AKARI Far-Infrared Surveyor (FIS) Bright Source catalog (Ver.2). AKARI-FIS-V2 J0916248+073034, identified as a ULIRG at z = 0.49 in the spectroscopic follow-up observation, indicates signatures of an extremely strong outflow in its emission line profiles. Its [OIII] 5007Å emission line shows FWHM of 1830 km s$^{-1}$ and velocity shift of -770 km s$^{-1}$ in relative to the stellar absorption lines. Furthermore, low-ionization [OII] 3726Å 3729Å doublet also shows large FWHM of 910 km s$^{-1}$ and velocity shift of -380 km s$^{-1}$. After the removal of an unresolved nuclear component, the long-slit spectroscopy 2D image possibly shows that the outflow extends to radius of 4 kpc. The mass outflow and energy ejection rates are estimated to be 500 M$_{\odot}$ yr$^{-1}$ and $4\times10^{44}$ erg s$^{-1}$, respectively, which imply that the outflow is among the most powerful ones observed in ULIRGs and QSOs at 0.3 < z < 1.6. The co-existence of the strong outflow and intense star formation (star formation rate of 990 M$_{\odot}$ yr$^{-1}$) indicates that the feedback of the strong outflow has not severely affect the star-forming region of the galaxy.

astro-ph.GA↗

arXiv2019

A Long-Duration Luminous Type IIn Supernova KISS15s: Strong Recombination Lines from the Inhomogeneous Ejecta-CSM Interaction Region and Hot Dust Emission from Newly Formed Dust

We report the discovery of an SN1988Z-like type IIn supernova KISS15s found in a low-mass star-forming galaxy at redshift z=0.038 during the course of the Kiso Supernova Survey (KISS). KISS15s shows long-duration optical continuum and emission line light curves, indicating that KISS15s is powered by a continuous interaction between the expanding ejecta and dense circumstellar medium (CSM). The H$α$ emission line profile can be decomposed into four Gaussians of narrow, intermediate, blue-shifted intermediate, and broad velocity width components, with a full width at half maximum of $\lesssim 100$, $\sim 2,000$, and $\sim 14,000$ km s${}^{-1}$ for the narrow, intermediate, and broad components, respectively. The presence of the blue-shifted intermediate component, of which the line-of-sight velocity relative to the systemic velocity is about $-5,000$ km s${}^{-1}$, suggests that the ejecta-CSM interaction region has an inhomogeneous morphology and anisotropic expansion velocity. We found that KISS15s shows increasing infrared continuum emission, which can be interpreted as hot dust thermal emission of $T \sim 1,200$ K from newly formed dust in a cool, dense shell in the ejecta-CSM interaction region. The progenitor mass-loss rate, inferred from bolometric luminosity, is $\dot{M} \sim 0.4 M_{\odot} \text{yr}^{-1} (v_{w}/40 \text{km}~\text{s}^{-1})$, where $v_{w}$ is the progenitor's stellar wind velocity. This implies that the progenitor of KISS15s was a red supergiant star or a luminous blue variable that had experienced a large mass-loss in the centuries before the explosion.

astro-ph.HE↗