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This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/., Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (greater than two seconds) duration are produced by the core-collapse of massive stars1, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars2. A third class of events with hybrid high-energy properties was identified3, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions4,5,6, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented7,8. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 1042 erg per second) kilonova possibly formed in the ejecta of a compact object merger. © 2022, The Author(s)., This work was supported by the European Research Council through the Consolidator grant BHianca (grant agreement ID 101002761) and by the National Science Foundation (under award number 12850). B.O’C. was partially supported by the National Aeronautics and Space Administration (NASA) through grants NNX16AB66G, NNX17AB18G and 80NSSC20K0389. N.K. was financially supported by Grants-in-Aid for Scientific Research 17H06362 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was partially supported by the Optical and Near-Infrared Astronomy Inter-University Cooperation Program of the MEXT of Japan, and the joint research programme of the Institute for Cosmic Ray Research (ICRR), the University of Tokyo. Research at Perimeter Institute is supported in part by the Government of Canada through the Department of Innovation, Science and Economic Development and by the Province of Ontario through the Ministry of Colleges and Universities. The development of afterglow models used in this work was partially supported by the European Union Horizon 2020 programme under the AHEAD2020 project (grant agreement number 871158). C.L.F., E.A.C. and R.W. are supported by the US Department of Energy through the Los Alamos National Laboratory (LANL), operated by Triad National Security, LLC, for the National Nuclear Security Administration of US Department of Energy (contract no. 89233218CNA000001). This work includes observations obtained at the international Gemini Observatory (principal investigator (PI): B.O’C.; GS-2022A-Q-141), a programme of NSF’s NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF on behalf of the Gemini Observatory partnership: the NSF (USA), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovacíon (Argentina), Ministério da Cieˆncia, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). The HST data (ObsID: 16846; PI: E.T.) used in this work was obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. This research is partially based on observations under our ToO proposal number DOT-2021-C2-P71 (PI: R.G.) and proposal number DOT-2021-C2-P54 (PI: S.B.P.) obtained at the 3.6-m Devasthal Optical Telescope (DOT), which is a National Facility run and managed by Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous Institute under the Department of Science and Technology, Government of India. R.G. and S.B.P. acknowledge all the observing and support staff of the 3.6-m DOT and 1.3-m DFOT for maintaining and running the observational facilities at Devasthal Nainital. R.G. is especially thankful to B. Kumar, A. K. Ror and M. Sarkar for the observations with the 3.6-m DOT and 1.3-m DFOT under the approved observing proposals: DOT-2021-C2-P71 (PI: R.G.), DFOT-2021B-P29 (PI: R.G.), and DOT-2021-C2-P54 (PI: S.B.P.). R.G., A.A., K.M. and S.B.P. acknowledge the BRICS grant (DST/IMRCD/BRICS/PilotCall1/ProFCheap/2017(G)) for financial support. R.G. and S.B.P. also acknowledge the financial support of ISRO under the AstroSat archival data utilization programme (DS_2B-13013(2)/1/2021-Sec.2). A.A. acknowledges funds and assistance provided by the Council of Scientific & Industrial Research (CSIR), India with file no. 09/948(0003)/2020-EMR-I. M.D.C.-G. acknowledges support from the Ramón y Cajal Fellowship RYC2019-026465-I. Y.-D.H. acknowledges support under the additional funding from RYC2019-026465-I. A.J.C.-T. acknowledges support from the Spanish Ministry project PID2020-118491GB-I00, Junta de Andalucia project P20_01068 and the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). Based on observations collected at the Centro Astronómico Hispano-Alemán (CAHA) at Calar Alto (ToO Program 21B-2.2-028: PI: A.J.C.-T.), operated jointly by Junta de Andalucía and Consejo Superior de Investigaciones Científicas (IAA-CSIC). These results made use of the Lowell Discovery Telescope (LDT) at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the LDT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. This research used resources provided by the LANL Institutional Computing Program. Figure 1 was created with the help of the NOIRLab/IPAC/ESA/STScI/CfA FITS Liberator. We made use of IRAF, which is distributed by the NSF NOIRLab. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

We present comprehensive spectroscopic and photometric analyses of the intermediate luminosity type lax supernova (SN lax) 2019muj based on multi-band datasets observed through the framework of the OISTER target-of-opportunity program. SN 2019muj exhibits almost identical characteristics to the subluminous SNe lax 2008ha and 2010ae in terms of the observed spectral features and the light curve evolution at the early phase, except for the peak luminosity. The long-term observations unveil late-time flattening light curves as seen in luminous SN lax 2014dt. This can be explained by the existence of an inner dense and optically thick component possibly associated with a bound white dwarf remnant left behind after the explosion. We demonstrate that the weak deflagration model with a wide range of explosion parameters can reproduce the late-phase light curves of other SNe lax. Therefore, we conclude that a common explosion mechanism operates for different subclass SNe lax., DST-JSPS grant [DST/INT/JSPS/P-281/2018]; Optical and Near-infrared Astronomy Inter-University Cooperation Program; JSPS KAKENHI [JP19K23461, 21K13959, JP20H00174, JP20H04737, JP18H04585, JP18H05223, JP17H02864, JP17K14253]; Turkish Scientific and Technical Research Council (TUBITAK-2211C); Turkish Scientific and Technical Research Council (TUBITAK-2214A); [19B-N-CN02]; [19B-N-CT01], We are grateful to the staff at the Seimei and Subaru telescopes for their support. The spectral data using the Seimei telescope were taken under the programs 19B-N-CN02, 19B-N-CT01. The latephase observation was performed with the Subaru telescope under S19B-055. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawaii. We thank the support staff at IAO and CREST who enabled the 2m HCT observations. The IAO is operated by the Indian Institute of Astrophysics, Bangalore, India. D.K.S. and G.C.A. acknowledge partial support through DSTJSPS grant DST/INT/JSPS/P-281/2018. The authors also thank T. J. Moriya and M. Tanaka for insightful comments. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with theNational Aeronautics and Space Administration. The spectral data of comparison SNe were downloaded from the SUSPECT7 (Richardson et al. 2001) and WISeREP8 (Yaron & Gal-Yam 2012) databases. This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory's High Energy Astrophysics Division. This work is supported by the Optical and Near-infrared Astronomy Inter-University Cooperation Program. The authors declare that there is no conflict of interests. M.K. acknowledges support by JSPS KAKENHI Grants (JP19K23461, 21K13959). K.M. acknowledges support by JSPS KAKENHI Grants (JP20H00174, JP20H04737, JP18H04585, JP18H05223, JP17H02864). M.Y. is partly supported by JSPS KAKENHI Grant (JP17K14253). U.B. acknowledges the support provided by the Turkish Scientific and Technical Research Council (TUBITAK-2211C and 2214A).

We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period ($P_{\rm orb}$) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-$P_{\rm orb}$ SU UMa-type dwarf novae. The estimated mass ratio, ($q=M_2/M_1 = 0.278(1)$), is almost identical to the upper limit of tidal instability by the 3:1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3:1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-$P_{\rm orb}$ WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-$q$ systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-$q$ systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass $M_2$ of around 0.18 M$_{\odot}$, smaller than expected for the orbital period and the secondary's spectral type. We also see indications of enhanced sodium abundance in the secondary's spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system's $P_{\rm orb}$, which may account for the occurrence of a superoutburst at this relatively long period., Comment: 17 pages, 14 figures, accepted for publication in PASJ

Identification of optical counterparts of gravitational waves (GWs) is one of the most exciting topics in astronomy. Since a typical sky map error region of the LIGO/Virgo is much larger than the field-of-view of optical telescopes, it is important to search and rapidly identify optical counterparts through follow-up observation by optical telescopes. The method of rapid and accurate transient detection in huge set of observed images is important. Motivated by this, we are developing transient detection method with convolutional neural network (CNN). We constructed CNN-based classifier designed to separate a transient image, an image including a transient source, and non-transient image. The input data is a pair of an observed image and a reference image. Here we adopt an image taken by MITSuME 50 cm telescope as observed and Pan-STARRS image as reference. We trained it with more than 10,000 images of 77 background galaxies within 200 Mpc. The training data with artificially transient images is made by adding an artificial point source into an observed image with various positions and luminosities. We tested the performance of the classifier with test data and found that the classification accuracy is more than 90%. Furthermore, we are developing a high-speed image reduction pipeline with GPU (Graphics Processing Unit) for real-time analysis of observed images. To accelerate image reduction, the pipeline uses CuPy (a python library for numerical calculation on the GPU) and minimize fits I/O. We found that the reduction speed of the pipeline achieves 30 times faster than IRAF for 240 set of 1024 x 1024 pixel images. In this talk, we will introduce the current status of the development of the transient detection method and the GPU-accelerated image reduction pipeline. We will also introduce our plan of installation of them into the systems of MITSuME 50cm telescopes in Akeno and Okayama which have performed optical follow-up observations of gamma-ray bursts, gravitational waves and X-ray binaries.

The ongoing Zwicky Transient Facility (ZTF) survey is generating a massive alert rate from a variety of optical transients and variable stars, which are being filtered down to subsets meeting user-specified criteria by broker systems such as ANTARES. In a beta implementation of the algorithm of Soraisam et al. (2020) on ANTARES, we flagged AT 2020iko from the ZTF real-time alert stream as an anomalous source. This source is located close to a red extended SDSS source. In the first few epochs of detection, it exhibited a V-shaped brightness profile, preceded by non-detections both in ZTF and in ASASSN extending to 2014. Its full light curve shows a precursor event, followed by a main superoutburst and at least two rebrightenings. A low-resolution spectrum of this source points to a dwarf nova (DN) nature. Although some of the features of AT 2020iko indicate an SU UMa-type DN, its large amplitude, presence of rebrightenings, and inferred supercycle period of > 6 yr are in favor of AT 2020iko being a new WZ Sge-type dwarf nova candidate, a subset of rare DNe consisting of extreme mass-ratio (< 0.1) binaries with orbital period around the period minimum. AT 2020iko's precursor event brightened by 6.5 mag, while its decay spanned 3-5 mag. We speculate this superoutburst is associated with a less expanded accretion disk than in typical superoutbursts in WZ Sge systems, with the large depth of the precursor decay implying an extremely small mass-ratio. To the best of our knowledge, such a precursor event has not been recorded for any DN. This result serves to demonstrate the efficacy of our real-time anomaly search algorithm., 11 pages, 4 figures; accepted to AJ

Flares are thought to be the rapid releases of magnetic energy through magnetic reconnection in the corona. Blue asymmetries (enhancement of the blue wing) in chromospheric lines have been widely observed during flares on M dwarfs. They are thought to be caused by the upward motions of cool plasma (e.g., chromospheric evaporations, filament/prominence eruptions). As observed on the Sun, stellar filament/prominence eruptions can evolve into stellar CMEs (coronal mass ejections). Here we report 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\(\alpha\) flares from the OISTER observations performed between 2019-01-16 and 2019-01-18. Among them, 3 H\(\alpha\) flares were associated with white-light flares. However, one of them did not show clear brightening in continuum; during this flare, the H\(\alpha\) 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. By assuming that the blue-asymmetry in H\(\alpha\) 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. Although, the estimated mass is comparable to expectations from the empirical relation between the flare X-ray energy and mass of upward-moving material for solar CMEs, 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., {"references":["Airapetian, V. S., et al. (2020), Int. J. Astrobiology, 19, 136. doi:10.1017/S1473550419000132","Argiroffi C., et al. (2019), Nature Astron., 3, 742. doi:10.1038/s41550-019-0781-4","Gopalswamy N., et al., (2003), ApJ, 586, 562. doi:10.1086/367614","Honda S., et al. (2018), PASJ, 70, 62. doi:10.1093/pasj/psy055","Moschou S.-P., et al. (2019), ApJ, 877, 105. doi:10.3847/1538-4357/ab1b37","Shibata, K., et al. (1995), ApJ Lett., 451, L83. doi:10.1086/309688","Vida, K., et al. (2016), A&A, 590, A11. doi:10.1051/0004-6361/201527925","Vida, K., et al. (2019), A&A, 623, A49. doi:10.1051/0004-6361/201834264","Yamashiki, Y. A., et al. (2019), ApJ, 881, 114. doi:10.3847/1538-4357/ab2a71","Yashiro S., Gopalswamy N. (2009), in IAU Symp. 257, Universal Heliophysical Processes, ed. Gopalswamy N., Webb D. F. (Cambridge: Cambridge University Press), 233. doi:10.1017/S1743921309029342"]}

We developed a high-speed image reduction pipeline using Graphics Processing Units (GPUs) as hardware accelerators. Astronomers desire detecting EM counterpart of gravitational-wave sources as soon as possible for sharing positional information to organize systematic follow-up observations. Therefore, high-speed image processing is important. We developed a new image reduction pipeline for our robotic telescope system, which uses a GPU via a Python package CuPy to achieve high-speed image processing. As a result, the processing speed was increased by more than a factor of forty to that of the current pipeline, while maintaining the same functions., 24 pages, 18 figures, 6 tables, accepted for publication in PASJ, code available at this http URL https://github.com/MNiwano/Eclaire

We report multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with Seimei Telescope (6150--7930 {\AA}), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700--7500 {\AA}), 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$\alpha$, 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$\alpha$ emission line full width at 1/8 maximum dramatically increases to 14 {\AA} from 8 {\AA} in the low-resolution spectra (R$\sim$ 2000) accompanied with the large white-light flares, (2) some weak H$\alpha$/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$\alpha$ 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$\alpha$ 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$\alpha$ 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., Comment: 18 pages, 17 figures, Accepted for Publication in PASJ

We present our ALMA Band 8 observations of a damped Ly${\alpha}$ absorption (DLA) system at $z$=3.150 observed in the spectrum of the quasar Q2233+131 at $z$=3.295. The optical counterpart of this DLA has been identified and it shows a double-peaked Ly${\alpha}$ emission line. Since one possible origin of DLAs at high redshift is an outflowing gas from star-forming galaxies, DLA2233+131 provides a good laboratory to investigate the nature of high-$z$ DLAs. Motivated by this, we have carried out ALMA band 8 observations to study the [C II] line in this system. However, we do not detect any significant emission line in the observed pass bands. Instead, we have serendipitously found three submm continuum sources in the observed sky area. One appears to be the quasar Q2233+131 itself while the other two sources are newly identified submm galaxies (SMGs), called SMG1 and SMG2 in this paper. They are located at a separation of 400.7 and 800.1 from Q2233+131, respectively. Their 646 ${\mu}$m fluxes are 6.35 mJy and 6.43 mJy, respectively, being higher than that of Q2233+131, 3.62 mJy. Since these two SMGs are not detected in the optical images obtained with the Hubble Space Telescope and the Subaru Telescope, they have a very red spectral energy distribution. It is, therefore,suggested that they are high-redshift galaxies or very dusty galaxies at intermediate redshift although we cannot rule out the possibility that they are optically very faint SMG analogs at low redshift. Follow up observations will be necessary to explore the nature of this interesting region., Comment: 11 pages, 4 figures, accepted to PASJ

We report on the multi-wavelength photometry of the 2018 superoutburst in EG Cnc. We have detected stage A superhumps and long-lasting late-stage superhumps via the optical photometry and have constrained the binary mass ratio and its possible range. The median value of the mass ratio is 0.048 and the upper limit is 0.057, which still implies that EG Cnc is one of the possible candidates for the period bouncer. This object also showed multiple rebrightenings in this superoutburst, which are the same as those in its previous superoutburst in 1996--1997 despite the difference in the main superoutburst. This would represent that the rebrightening type is inherent to each object and is independent of the initial disk mass at the beginning of superoutbursts. We also found that $B-I$ and $J-K_{\rm S}$ colors were unusually red just before the rebrightening phase and became bluer during the quiescence between rebrightenings, which would mean that the low-temperature mass reservoir at the outermost disk accreted with time after the main superoutburst. Also, the ultraviolet flux was sensitive to rebrightenings as well as the optical flux, and the $U-B$ color became redder during the rebrightening phase, which would indicate that the inner disk became cooler when this object repeated rebrightenings. Our results thus basically support the idea that the cool mass reservoir in the outermost disk is responsible for rebrightenings., Comment: Accepted for publication in PASJ. 13 pages and 9 figures

著者人数: 61名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 鈴木, 大介), Number of authors: 61 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Suzuki, Daisuke), Accepted: 2019-09-25, 資料番号: SA1190156000

Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R, Corrected Figure 8 / Table 4 to use final fits. Includes machine-readable photometry table (hopefully for real this time)

We present optical observations of the Type Ia supernova (SN) 2019ein, starting two days after the estimated explosion date. the spectra and light curves show that SN 2019ein belongs to a high-velocity (HV) and broad-line group with a relatively rapid decline in the light curves (Delta m(15)(B) = 1.36 0.02 mag) and a short rise time (15.37 0.55 days). the Si ii lambda 6355 velocity, associated with a photospheric component but not with a detached high-velocity feature, reached similar to 20,000 km s(-1) 12 days before the B-band maximum. the line velocity, however, decreased very rapidly and smoothly toward maximum light, to similar to 13,000 km s(-1), which is relatively low among HV SNe. This indicates that the speed of the spectral evolution of HV SNe Ia is correlated with not only the velocity at maximum light, but also the light-curve decline rate, as is 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 an 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 m(15)(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 Ni-56 (or Delta m(15)) and the extent of the O-Ne-C burning layer provides an important constraint on the explosion mechanism(s) of HV and NV SNe., Google Summer of Code initiative; ESA's Summer of Code in Space program; Optical and Near-infrared Astronomy Inter-University Cooperation Program; Ministry of Education, Culture, Sports, Science and Technology (MEXT) of JapanMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT) [17H06362]; Institute for Cosmic Ray Research (ICRR); JSPS KAKENHIMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [19K23461, 18H04585, 18H05223, 17H02864, 17K14253]; Turkish Scientific and Technical Research CouncilTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [TuBITAK-2211C, TuBITAK-2214A], This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. the spectral data of comparison SNe awere downloaded from the SUSPECT25 (Richardson et al. 2001) and WISeREP26 (Yaron & Gal-Yam 2012) databases. This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory's High Energy Astrophysics Division. This research made use of TARDIS, a community-developed software package for spectral synthesis in supernovae (Kerzendorf & Sim 2014; Kerzendorf et al. 2019). the development of TARDIS received support from the Google Summer of Code initiative and from ESA's Summer of Code in Space program. TARDIS makes extensive use of Astropy and PyNE. This work is supported by the Optical and Near-infrared Astronomy Inter-University Cooperation Program. Part of this work was financially supported by Grants-in-Aid for Scientific Research 17H06362 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was supported by the joint research program of the Institute for Cosmic Ray Research (ICRR). M.K. acknowledges support by JSPS KAKENHI Grant (19K23461). K.M. acknowledges support by JSPS KAKENHI Grant (18H04585, 18H05223, 17H02864). M.Y. is partly supported by JSPS KAKENHI Grant (17K14253). U. B. acknowledges the support provided by the Turkish Scientific and Technical Research Council (TuBITAK-2211C and TuBITAK-2214A).

The first detected gravitational wave from a neutron star merger was GW170817. In this study, we present J-GEM follow-up observations of SSS17a, an electromagnetic counterpart of GW170817. SSS17a shows a 2.5-mag decline in the $z$-band from 1.7 days to 7.7 days after the merger. Such a rapid decline is not comparable with supernovae light curves at any epoch. The color of SSS17a also evolves rapidly and becomes redder for later epochs; the $z-H$ color changed by approximately 2.5 mag in the period of 0.7 days to 7.7 days. The rapid evolution of both the optical brightness and the color are consistent with the expected properties of a kilonova that is powered by the radioactive decay of newly synthesized $r$-process nuclei. Kilonova models with Lanthanide elements can reproduce the aforementioned observed properties well, which suggests that $r$-process nucleosynthesis beyond the second peak takes place in SSS17a. However, the absolute magnitude of SSS17a is brighter than the expected brightness of the kilonova models with the ejecta mass of 0.01 $\Msun$, which suggests a more intense mass ejection ($\sim 0.03 \Msun$) or possibly an additional energy source., Comment: 7 pages, 5 figures. Published in PASJ (Publications of the Astronomical Society of Japan)

Context.Spectropolarimetry is a powerful technique for investigating the physical properties of gas and solid materials in cometary comae without mutual contamination, but only a few spectropolarimetric studies have been conducted to extract each component.Aims.We attempt to derive the continuum (i.e., scattered light from dust coma) polarization degree of comet 2P/Encke, free of the influence of molecular emissions. The target is unique in that its orbit is dynamically decoupled from Jupiter, like the main-belt asteroids, but it ejects gas and dust like ordinary comets.Methods.We observed the comet using the Hiroshima Optical and Near-Infrared Camera attached to the Cassegrain focus of the 150 cm Kanata telescope on UT 2017 February 21 when the comet was at the solar phase angle ofα= 75°.7.Results.We find that the continuum polarization degree with respect to the scattering plane isPcont, r= 33.8 ± 2.7% at the effective wavelength of 0.82μm, which is significantly higher than those of cometary dust in a high-Pmaxgroup at similar phase angles. Assuming that an ensemble polarimetric response of the dust of 2P/Encke as a function of phase angle is morphologically similar with those of other comets, its maximum polarization degree is estimated toPmax≳ 40% atαmax≈ 100°. In addition, we obtain the polarization degrees of the C2swan bands (0.51–0.56μm), the NH2αbands (0.62–0.69μm), and the CN-red system (0.78–0.94μm) in a range of 3–19%, which depend on the molecular species and rotational quantum numbers of each branch. The polarization vector is aligned nearly perpendicularly to the scattering plane with an average of 0°.4 over a wavelength range of 0.50–0.97μm.Conclusions.From the observational evidence, we conjecture that the high polarization degree of 2P/Encke might be attributable to a dominance of large dust particles around the nucleus, which have remained after frequent perihelion passages near the Sun.