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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$^2$., 25 pages, 10 figures, 3 tables, published at https://academic.oup.com/ptep/article/2021/5/05A104/6122445

This paper introduces a second-generation balloon-borne hard X-ray polarimetry mission, XL-Calibur. X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as pulsars and binary black hole systems. The XL-Calibur contains a grazing incidence X-ray telescope with a focal plane detector unit that is sensitive to linear polarization. The telescope is very similar in design to the ASTRO-H HXT telescopes that has the world’s largest effective area above ~10 keV. The detector unit combines a low atomic number Compton scatterer with a CdZnTe detector assembly to measure the polarization making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. It also contains a CdZnTe imager at the bottom. The detector assembly is surrounded by the improved anti-coincidence shielding, giving a better sensitivity. The pointing system with arcsecond accuracy will be achieved.

We present optical and near-infrared observations of SN 2019ehk, which was initially reported as a Type Ib supernova (SN). We show that it evolved to a Ca-rich transient according to its spectral properties and evolution in late phases. However, it shows a few properties distinct from those of the canonical Ca-rich transients: a short-duration first peak in the light curve, high peak luminosity, and association with a star-forming environment. Indeed, some of these features are shared with iPTF14gqr and iPTF16hgs, which are candidates for a special class of core-collapse SNe: the so-called ultra-stripped envelope SNe, i.e., a relatively low-mass He (or C+O) star explosion in a binary as a precursor of short-period double neutron star (NS) binaries. The estimated ejecta mass (0.4M (circle dot)) and explosion energy (1.7 x 10(50) erg) are consistent with this scenario. The analysis of the first peak suggests the existence of dense circumstellar material in the vicinity of the progenitor, implying a CCSN origin. Based on this analysis, we suggest SN 2019ehk is another candidate for a low-mass He star explosion. It might create a double NS binary, but with a wide separation. These candidates for low-mass stripped envelope SNe, including ultra-stripped envelope SN candidates, seem to form a subpopulation among Ca-rich transients, associated with young population. We propose that the key to distinguishing this population is the early first peak in their light curves., Japan Society for the Promotion of Science (JSPS) through KAKENHI [JP17H02864, JP18H04585, JP18H05223, JP20H00174, JP20H04737, JP18H03720, JP16H02158]; Japan Society for the Promotion of Science [JP17K14253]; Turkish Scientific and Technical Research Council [TuBTAK-2211C, TuBTAK-2214A]; National Aeronautics and Space Administration, The authors acknowledge Raffaella Margutti, Dan Milisavljevic, and Wynn Jacobson-Galan for stimulating discussion, after their manuscript and this manuscript on SN 2019ehk were both posted on arXiv. The GMOS observation, S19A-051, was conducted via the time exchange program between Gemini and the Subaru Telescope. The FOCAS observation, S19B-055, was conducted at the Subaru Telescope, and the authors thank Kentaro Aoki and the Subaru telescope staff for the observation. 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. This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Keiichi Maeda acknowledges support provided by the Japan Society for the Promotion of Science (JSPS) through KAKENHI Grants JP17H02864, JP18H04585, JP18H05223, JP20H00174, and JP20H04737. Masayuki Yamanaka is supported by the Grants-in-Aid for Young Scientists of the Japan Society for the Promotion of Science (JP17K14253). Koji S. Kawabata acknowledges support provided by the Japan Society for the Promotion of Science (JSPS) through KAKENHI Grant JP18H03720. Umut Burgaz acknowledges support provided by the Turkish Scientific and Technical Research Council (TuBTAK-2211C and 2214A) Tomoki Morokuma acknowledges support provided by the Japan Society for the Promotion of Science (JSPS) through KAKENHI Grants JP16H02158 and JP18H05223.

XL-Calibur is a hard X-ray (15-80 keV) polarimetry mission operating from a stabilised balloon-borne platform in the stratosphere. It builds on heritage from the X-Calibur mission, which observed the accreting neutron star GX 301-2 from Antarctica, between December 29th 2018 and January 1st 2019. The XL-Calibur design incorporates an X-ray mirror, which focusses X-rays onto a polarimeter comprising a beryllium rod surrounded by Cadmium Zinc Telluride (CZT) detectors. The polarimeter is housed in an anticoincidence shield to mitigate background from particles present in the stratosphere. The mirror and polarimeter-shield assembly are mounted at opposite ends of a 12 m long lightweight truss, which is pointed with arcsecond precision by WASP - the Wallops Arc Second Pointer. The XL-Calibur mission will achieve a substantially improved sensitivity over X-Calibur by using a larger effective area X-ray mirror, reducing background through thinner CZT detectors, and improved anticoincidence shielding. When observing a 1 Crab source for $t_{\rm day}$ days, the Minimum Detectable Polarisation (at 99% confidence level) is $\sim$2$\%\cdot t_{\rm day}^{-1/2}$. The energy resolution at 40 keV is $\sim$5.9 keV. The aim of this paper is to describe the design and performance of the XL-Calibur mission, as well as the foreseen science programme., Comment: 19 pages, 17 figures. Accepted for publication in Astroparticle Physics

We analyze the ultraviolet (UV) and X-ray data of NGC 1275 obtained with {\it Swift}/UVOT, XRT, BAT and {\it Fermi} Large Area Telescope over about 10 years to investigate the origin of the nuclear emission from NGC 1275. We confirm that the UV and soft/hard X-ray fluxes gradually increased along with the GeV gamma rays. At times, short-term variations in the UV or soft X-ray spectral regions showed rapid variations correlated with the GeV gamma-rays. However there was no significant correlation between the UV and soft X-rays. The UV spectrum had a narrow spectral shape that could be represented by single-temperature blackbody radiation. These results could possibly indicate that the long-term variability of UV and X-ray emissions is caused by the jet, while the emissions from the accretion disk contribute to the UV and X-ray bands to some extent., 38 pages, 14 figures. Accepted in ApJ

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).

Publisher's version (útgefin grein), We present the Fermi Large Area Telescope (LAT) observations of the binary neutron star merger event GW170817 and the associated short gamma-ray burst (SGRB) GRB 170817A detected by the Fermi Gamma-ray Burst Monitor. The LAT was entering the South Atlantic Anomaly at the time of the LIGO/Virgo trigger (t GW) and therefore cannot place constraints on the existence of high-energy (E > 100 MeV) emission associated with the moment of binary coalescence. We focus instead on constraining high-energy emission on longer timescales. No candidate electromagnetic counterpart was detected by the LAT on timescales of minutes, hours, or days after the LIGO/Virgo detection. The resulting flux upper bound (at 95% C.L.) from the LAT is 4.5 × 10−10 erg cm−2 s−1 in the 0.1–1 GeV range covering a period from t GW + 1153 s to t GW + 2027 s. At the distance of GRB 170817A, this flux upper bound corresponds to a luminosity upper bound of 9.7 × 1043 erg s−1, which is five orders of magnitude less luminous than the only other LAT SGRB with known redshift, GRB 090510. We also discuss the prospects for LAT detection of electromagnetic counterparts to future gravitational-wave events from Advanced LIGO/Virgo in the context of GW170817/GRB 170817A., This work performed in part under DOE Contract DE-AC02-76SF00515.

The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include NASA and the Department of Energy in the United States, the Commissariat a l’Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucleaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’Etudes Spatiales in France. This work was performed in part under DOE Contract DE- AC02-76SF00515.