Daichi Hiramatsu

We present spectrophotometric observations of the Type Ibn supernova SN 2020able, along with a comprehensive analysis and modelling of its evolution. The transient occurred in SDSS J092602.93+243115.1, a faint host galaxy characterised by significantly subsolar metallicity (12 + log(O/H) ≃ 8.28 dex). The early photometric evolution and high peak luminosity ( L _peak ≃ 4.4 × 10 ⁴³ erg s ⁻¹ ) indicate an efficient conversion of ejecta kinetic energy into radiation via interaction with the circumstellar medium. Light-curve modelling suggests that this interaction involves ejecta with a mass of M _ej ≃ 1.3 M _☉ impacting a shell of M _CSM ≃ 0.6 M _☉ . However, interaction is found to be geometrically and temporally confined within a few days after explosion, with the later evolution of the transient being dominated by the free expansion of the ejecta. Spectroscopic analysis reveals a complex, stratified, circumstellar environment. In addition to the innermost shocked region, we identify two additional He-rich shells: a dense inner one ( n _e ≃ 1.4 × 10 ⁹ cm ⁻³ ) located at R ≳ 9.3 × 10 ¹⁴ cm, and a more extended outer component reaching R _out ≃ 3.3 × 10 ¹⁵ cm and expanding at approximately 1.2 × 10 ³ km s ⁻¹ . The physical parameters of the former suggest it was produced by a discrete, pre-supernova eruptive event similar to that of SN 2006jc, which expelled approximately 1.8 × 10 ⁻² M _☉ of material. Conversely, the expansion velocity of the outermost shell is consistent with the escape velocity of Wolf–Rayet stars, pointing towards a late WN- to WC-type progenitor as the most viable candidate for SN 2020able.

The emission mechanism and host galaxy preference of optical/UV tidal disruption events (TDEs) are still not entirely understood. We present observations of the TDE AT 2022csn, which is one of the most distant ( d _L ∼ 726 Mpc) and luminous ( L peak = 2.48 7 − 0.067 + 0.073 × 1 0 44 erg s − 1 ) optical/UV TDEs observed to date. Although it is a spectroscopically normal H + He TDE, it shows some photometric peculiarities, exhibiting a pronounced double-peaked light curve (with peaks separated by 18.30 ± 2.84 days in the g band) and lying in the low-temperature and large-radius end of the optical/UV TDE population. The host galaxy of AT 2022csn shows evidence for a significant starburst within the last gigayear, consistent with hosts of other optical/UV TDEs, as well as narrow emission lines, consistent with hosts of Type II AGN. Interaction between the TDE and a preexisting AGN accretion disk might explain the peculiar photometric properties. However, it is puzzling that a TDE would be visible in a Type II AGN, where according to the AGN unification picture the central region around the supermassive black hole is obscured. We suggest a few scenarios to reconcile this. AT 2022csn together with AT 2019ahk, which shows similar properties, may belong to a new subset of low-temperature, high-radius TDEs in galaxies with Type II AGN emission features.

Hydrogen-rich superluminous supernovae without narrow emission lines (SLSNe-II) are rare transients whose powering mechanisms remain debated, particularly the role of circumstellar medium (CSM) interaction. We present photometric and spectroscopic observations of SN 2019cqc to investigate its power source and progenitor mass loss. We model the lightcurve using MOSFiT with the csm and csmni models to constrain the explosion and CSM parameters. SN 2019cqc peaked at M _g = −20.21 ± 0.07 and emitted ∼1.7 × 10 ⁵⁰ erg of energy in the form of radiation. Photometric and spectroscopic modeling indicates that CSM interaction dominates in both scenarios, with best-fit CSM and ejecta masses of ∼4.5 M _⊙ and ∼32 M _⊙ , respectively. The inferred CSM properties imply an extreme eruptive mass loss at a rate of ∼0.2–0.3 M _⊙ , yr ⁻¹ in the years preceding the core collapse, consistent with a variable progenitor of luminous blue variable progenitor. We observe a persistent blueshifted asymmetry in the H α emission line. At early times (≲+110 days), this is attributed to electron scattering with the bulk velocity of ejecta, while the profile at late epochs (≳ +402 days onward) suggests the subsequent formation of dust in the ejecta. Additionally, we identify a distinct, short-lived feature at ∼4600 Å, likely a blend of ionized C iii /N iii lines powered by the interaction of the SN-shock with the extended atmosphere.

Mapping how the explosion properties of Type II supernovae (SNe II) relate to the properties of their progenitors can provide strong constraints for understanding the final evolutionary stages of massive stars. Type IIP SNe, linked to the explosions of single red supergiant (RSG) stars, have recently been found to require some form of interaction with circumstellar material (CSM) to reproduce the rapid rise to the plateau often seen in their light curves. In this work, we present observations and analysis of the Type IIP SN 2020bij, characterized by a slow rise to its plateau as well as high expansion velocities. We identify four other SNe IIP from the literature (ASASSN-14 kg, SN 2018fif, SN 2021yja, and SN 2023axu) with similarly slowly rising light curves and find that they also show high expansion velocities. Using both analytical and numerical models, all five events can be explained with weak to no CSM interaction. We therefore propose that these events constitute a new subclass of Type IIP SNe that could be associated with relatively confined CSM. Early and dense photometric coverage of future SNe IIP together with early spectroscopic observations will further map this subclass and its physical properties. Understanding such rare events could be key to constraining the diversity of late-stage mass loss in RSGs.

We present spectroscopic and photometric observations of SN 2021ukt, a peculiar short-plateau object that was originally identified as a Type IIn supernovae (SN) and later underwent an unprecedented transition to a Type Ib (possibly Type IIb) SN. The early time light curves of SN 2021ukt exhibit an ∼25 day plateau. Such a short phase of hydrogen recombination suggests a rather thin H-rich outer envelope of the progenitor star. The narrow Balmer emission lines in the spectra of SN 2021ukt during the first week indicate the interaction between the expanding ejecta and the immediate circumstellar material (CSM). This H α line is observed throughout its helium-rich ejecta-dominated phase and nebular phase, suggesting persistent interaction with a radially extended CSM profile. We explore synthetic light curves among grids of parameters generated by MESA + STELLA . We also compare the spectrophotometric evolution of SN 2021ukt with several SNe that exhibit a short plateau and persistent ejecta–CSM interaction. An estimate of the progenitor mass of SN 2021ukt is made based on the flux ratio between [Ca ii ] λλ 7291, 7324 and [O i ] λλ 6300, 6364 during its nebular phase. Our analysis suggests that the progenitor star of SN 2021ukt has a zero-age main-sequence mass of M _ZAMS ≈ 12 M _⊙ , a mass of radioactive ⁵⁶ Ni synthesized in the SN ejecta of M _Ni ≈ 0.05 M _⊙ , and a mass of the H-rich envelope of M _H,env ≲ 0.5 M _⊙ . This study adds to the growing sample of transitional SNe, reinforcing evidence for a continuum of underrepresented progenitors whose evolutionary pathways lie between those of standard SN models.

Although Type Ia supernovae (SNe Ia) are found in all galaxies types, recent local Hubble constant measurements have disfavored using SNe Ia in early-type/quiescent galaxies, aiming instead for better consistency with SNe Ia in star-forming/late-type host galaxies calibrated by Cepheid distances. Here, we investigate the feasibility of a parallel distance ladder using SNe Ia exclusively in quiescent, massive ( log M * / M ⊙ ≥ 10 ) host galaxies, calibrated by tip of the red giant branch (TRGB) distances. We present TRGB measurements to four galaxies: three measured from the Hubble Space Telescope with the Advanced Camera for Surveys F814W filter and one measured from the JWST NIRCam F090W filter. Combined with literature measurements, we define a TRGB-calibrator sample of five high-mass, early-type galaxies that hosted well-measured SNe Ia: NGC 1316 (SN 2006dd), NGC 1380 (SN 1992A), NGC 1404 (SN 2007on, SN 2011iv), NGC 4457 (SN 2020nvb), and NGC 4636 (SN 2020ue). We jointly standardize these calibrators with a fiducial sample of 124 Hubble-flow SNe Ia from the Zwicky Transient Facility that are matched in host-galaxy and light-curve properties. Results with this homogenized subsample show a Hubble residual scatter under 0.11 mag, lower than usually observed in cosmological samples of the full SN Ia distribution. We obtain a measurement of the Hubble constant, H ₀ = 75.3 ± 2.9 km s ⁻¹ Mpc ⁻¹ , including statistical and estimated systematic uncertainties, and discuss the potential to further improve the precision of this approach. As calibrator and SN samples grow, we advocate that future cosmological applications of SNe Ia use subsamples matched in host-galaxy and SN properties across redshifts.

Calcium-strong transients (CaSTs) are a subclass of faint and rapidly evolving supernovae (SNe) that exhibit strong calcium features and notably weak oxygen features. The small but growing population of CaSTs exhibits some aspects similar to thermonuclear SNe but others similar to massive star core-collapse events, leading to intriguing questions on their physical origins. SN 2025coe is one of the nearest CaSTs discovered to date, and our coordinated multiwavelength observations obtained days to weeks postexplosion reveal new insights into these enigmatic transients. With the most robust near-IR (NIR) spectroscopic time series of a CaST collected to date, SN 2025coe shows spectral signatures characteristic of Type Ib SNe (SNe Ib; i.e., He-rich stripped-envelope SNe (SESNe)). SN 2025coe is the third X-ray-detected CaST and our analysis of Neil Gehrels Swift Observatory X-ray data suggests interaction with 0.12 ± 0.11 M _⊙ of circumstellar material (CSM) extending to at least 2 × 10 ¹⁵ cm (∼30,000 R _⊙ ), while our analysis of the 1–240 GHz radio nondetections gives an outer radius of that CSM of at most ∼5 × 10 ¹⁵ cm. This inferred nearby high-density CSM extending out to (3.5 ± 1.5) × 10 ¹⁵ cm is similar to that seen in the other two X-ray-detected CaSTs, and its presence suggests that either intensive mass loss from a massive star or some exotic pre-SN mass ejection may be a common feature of this subclass. Our work also expands upon recent studies of the optical properties of SN 2025coe and explores our current understanding of different progenitor systems that could possibly produce CaSTs.

We present the largest uniform study to date of Type IIn supernovae (SNe IIn), focusing in this first paper on the multiband optical light curves of 490 SNe IIn. The sample, constructed from multiple surveys, extends to z ≈ 0.8, with the majority of events at z ≲ 0.3. We construct uniform multiband and bolometric light curves using Gaussian process regression, and determine key observed properties in the rest frame (e.g., peak luminosity, timescales, radiated energy). We find that SNe IIn span broad ranges in peak luminosity (∼10 ⁴² –10 ⁴⁴ erg s ⁻¹ ) and timescales (∼20–300 days above 50% of peak luminosity), but the sample divides into two clear groups in the luminosity-timescale phase space around the median peak luminosity (≈10 ⁴³ erg s ⁻¹ ): faint–fast and luminous–slow groups. This leads to a strong bimodality in the radiated energy distribution, with peaks at ∼10 ⁴⁹ and ∼2 × 10 ⁵⁰ erg, with the latter events having a characteristic timescale of ∼100 days, and the former appearing to bifurcate into two branches with timescales of ∼40 and ∼70 days. Therefore, SNe IIn exhibit at least two dominant groupings, and perhaps three, which are likely reflective of different progenitor and/or circumstellar medium (CSM) formation pathways. We do not find any obvious transition in SN IIn properties at the arbitrary cutoff of ≈−20 mag used for the designation “Type IIn superluminous supernovae” or “SLSN-IIn,” and we argue that this classification should be abandoned. The absence of SNe IIn with timescales of ≲15 days defines the region occupied by fast transients with evidence for interaction with a hydrogen-poor CSM.

We present a summary of gravitational-wave (GW) follow-up using the Las Cumbres Observatory global network of telescopes during the third (O3) and fourth (O4) observing runs of the GW detectors. As in O2, we implemented the Gehrels et al. (2016) galaxy-targeted strategy. Here, we test its efficacy in O3 and O4 and analyze the Las Cumbres Observatory response time and depth for nine GW alerts that showed a possibility of having an electromagnetic counterpart (GW190425, GW190426_152155, S190510g, GW190728_064510, GW190814, S190822c, GW191216_213338, S240422ed, and S250206dm). We find that Las Cumbres Observatory is able to begin observations in response to GW alerts within minutes of the alert, with the observations being deep enough to detect possible GW170817-like kilonovae out to a median distance of 250 Mpc. In this sense, a global rapid-response network of telescopes like Las Cumbres is an excellent GW follow-up facility. However, the galaxy-targeted follow-up strategy was much less efficient in O3 and O4 than originally predicted, given the larger-than-assumed GW localizations. We conclude that coordination between various facilities to include both wide-field and rapid-response capabilities is required to achieve efficient and comprehensive follow-up of GW events.

Core-collapse supernovae are explosions of massive stars. While most massive stars end as iron-core-collapse supernovae, less massive stars are expected to explode as electron-capture supernovae (ECSNe), defining the low-mass boundary of core-collapse supernovae. ECSNe were proposed ∼40 yr ago, and first-principles simulations predict their successful explosions with low energies of ∼10 ⁵⁰ erg. Nevertheless, only one convincing candidate, SN 2018zd, has been proposed other than SN 1054, the progenitor of the Crab Nebula. We search for ECSN candidates among Type II SNe from the literature and a public Zwicky Transient Facility sample, using a color-based diagnostic, selecting ten candidates with blue colors at the middle of the plateau. We classify three as gold , for which a spectrum around the middle of the plateau disfavors strong interaction with the circumstellar medium that would make the SN bluer, and seven as silver without such spectra. Comparing the observed multicolor light curves with radiation-hydrodynamical models, we infer the explosion energies, (0.4–1.7) × 10 ⁵⁰ erg for the gold candidates and (0.4–2.7) × 10 ⁵⁰ erg including the silver candidates, consistent with first-principles predictions and the mass-loss rates, 3 × 10 ⁻³ –3 × 10 ⁻² M _⊙ yr ⁻¹ for the gold candidates, which remain similar when the silver candidates are included, and are higher than those expected for the early super-asymptotic-giant-branch phase. The ECSN occurrence ratios among SNe II are inferred as 3 . 0 − 2.9 + 10.6 % and 15 . 7 − 12.7 + 17.3 % from the gold and silver candidates, respectively, which we interpret as lower and upper limits. To robustly identify ECSNe and refine this ratio, spectroscopic follow-ups of ECSN candidates around the middle of the plateau are essential.