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Context. Type Ibn supernovae (SNe) are characterized by narrow helium (He I) lines from photons produced by the unshocked circumstellar material (CSM). About 80 SNe Ibn have been discovered to date and only a handful of them have extensive observational records. Thus, many open questions remain regarding the progenitor system and the origin of CSM. Aims. Here, we investigate potential correlations between the spectral features of the prominent He Iλ5876 Å line and the optical and X-ray light curve properties of Type Ibn SNe (SNe Ibn). Methods. We compiled the largest sample of 61 SNe Ibn to date, of which 24 SNe have photometric and spectroscopic data available from the Young Supernova Experiment and 37 SNe benefit from archival datasets. We fit 24 SNe Ibn with sufficient photometric coverage (Btozbands) using semi-analytical models fromMOSFiT. Results. We demonstrate that the light curves of SNe Ibn are more diverse than previous analyses suggest, with absoluter-band peak magnitudes (r_max) of −19.4 ± 0.6 mag, along with rise (from −10 days to peak,γ₋₁₀) and decay rates (from peak to +10 days;γ₊₁₀) of −0.08 ± 0.06 and 0.08 ± 0.03 mag/day, respectively. We find that the majority of SNe Ibn in the subsample are consistent with a low-energy explosion (< 10⁵¹erg) of a star with a compact envelope surrounded by ~0.1 M_⊙of helium-rich CSM. The inferred ejecta masses are small (M_ej~ 1 M_⊙) and expand with a velocity of ~5000 km/s. Our spectroscopic analysis shows that the mean velocity of the narrow component of the He Ilines, associated with the CSM, peaks at ~1100 km/s. Conclusions. The mean CSM and ejecta masses inferred for a subsample of SNe Ibn indicate that their progenitors are not massive (~10 M_⊙) single stars at the moment of explosion; rather, they are likely to be binary systems. This finding is in agreement with detections of potential companion stars of SNe Ibn progenitors and inferred CSM properties from stellar evolution models. 

Abstract We present an ultraviolet to infrared search for the electromagnetic (EM) counterpart to GW190425, the second ever binary neutron star merger discovered by the LIGO-Virgo-KAGRA Collaboration. GW190425 was more distant and had a larger localization area than GW170817, so we use a new tool,Teglon, to redistribute the GW190425 localization probability in the context of galaxy catalogs within the final localization volume. We derive a 90th percentile area of 6688 deg², a ∼1.5× improvement relative to the LIGO/Virgo map, and show howTeglonprovides an order-of-magnitude boost to the search efficiency of small (≤1 deg²) field-of-view instruments. We combine our data with a large, publicly reported imaging data set, covering 9078.59 deg²of unique area and 48.13% of the LIGO/Virgo-assigned localization probability, to calculate the most comprehensive kilonova (KN), short gamma-ray burst (sGRB) afterglow, and model-independent constraints on the EM emission from a hypothetical counterpart to GW190425 to date under the assumption that no counterpart was found in these data. If the counterpart were similar to AT 2017gfo, there would be a 28.4% chance of it being detected in the combined data set. We are relatively insensitive to an on-axis sGRB, and rule out a generic transient with a similar peak luminosity and decline rate as AT 2017gfo to 30% confidence. Finally, across our new imaging and publicly reported data, we find 28 candidate optical counterparts that we cannot rule out as being associated with GW190425, finding that four such counterparts discovered within the localization volume and within 5 days of merger exhibit luminosities consistent with a KN. 

Abstract We present a comprehensive analysis of the photometric and spectroscopic evolution of SN 2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow (≈300–600 km s⁻¹) absorption lines of hydrogen, helium, calcium, and iron together with broad helium emission lines with a full width at half-maximum (FWHM) of ∼6000 km s⁻¹. For a steady, wind mass-loss regime, light-curve modeling results in an ejecta mass of ∼8M_⊙and circumstellar material (CSM) mass below 1M_⊙, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of ≈2M_⊙yr⁻¹. This mass-loss rate is 3 orders of magnitude larger than derived for normal Type II supernovae. We estimate that the bulk of the CSM of SN 2021foa must have been expelled within half a year, about 12 yr ago. Our analysis suggests that SN 2021foa had a helium-rich ejecta that swept up a dense shell of hydrogen-rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the redshifted emission of the hydrogen and helium lines, which results in an observed blueshift (∼−3000 km s⁻¹). Strong mass-loss activity prior to explosion, such as those seen in SN 2009ip-like objects and SN 2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star. 

Abstract The modern study of astrophysical transients has been transformed by an exponentially growing volume of data. Within the last decade, the transient discovery rate has increased by a factor of ∼20, with associated survey data, archival data, and metadata also increasing with the number of discoveries. To manage the data at this increased rate, we require new tools. Here we presentYSE-PZ, a transient survey management platform that ingests multiple live streams of transient discovery alerts, identifies the host galaxies of those transients, downloads coincident archival data, and retrieves photometry and spectra from ongoing surveys.YSE-PZalso presents a user with a range of tools to make and support timely and informed transient follow-up decisions. Those subsequent observations enhance transient science and can reveal physics only accessible with rapid follow-up observations. Rather than automating out human interaction,YSE-PZfocuses on accelerating and enhancing human decision making, a role we describe as empowering the human-in-the-loop. Finally,YSE-PZis built to be flexibly used and deployed;YSE-PZcan support multiple, simultaneous, and independent transient collaborations through group-level data permissions, allowing a user to view the data associated with the union of all groups in which they are a member.YSE-PZcan be used as a local instance installed via Docker or deployed as a service hosted in the cloud. We provideYSE-PZas an open-source tool for the community. 

ABSTRACT After correcting for their light-curve shape and colour, Type Ia supernovae (SNe Ia) are precise cosmological distance indicators. However, there remains a non-zero intrinsic scatter in the differences between measured distance and that inferred from a cosmological model (i.e. Hubble residuals or HRs), indicating that SN Ia distances can potentially be further improved. We use the open-source relational data base kaepora to generate composite spectra with desired average properties of phase, light-curve shape, and HR. At many phases, the composite spectra from two subsamples with positive and negative average HRs are significantly different. In particular, in all spectra from 9 d before to 15 d after peak brightness, we find that SNe with negative HRs have, on average, higher ejecta velocities (as seen in nearly every optical spectral feature) than SNe with positive HRs. At +4 d relative to B-band maximum, using a sample of 62 SNe Ia, we measure a 0.091 ± 0.035 mag (2.7σ) HR step between SNe with Si ii λ6355 line velocities ($$v_{Si\, rm{\small II}}$$) higher/lower than −11 000 km s−1 (the median velocity). After light-curve shape and colour correction, SNe with higher velocities tend to have underestimated distance moduli relative to a cosmological model. The intrinsic scatter in our sample reduces from 0.094 to 0.082 mag after making this correction. Using the Si ii λ6355 velocity evolution of 115 SNe Ia, we estimate that a velocity difference >500 km s−1 exists at each epoch between the positive-HR and negative-HR samples with 99.4 per cent confidence. Finally at epochs later than +37 d, we observe that negative-HR composite spectra tend to have weaker spectral features in comparison to positive-HR composite spectra. 

Abstract We present the Keck Infrared Transient Survey, a NASA Key Strategic Mission Support program to obtain near-infrared (NIR) spectra of astrophysical transients of all types, and its first data release, consisting of 105 NIR spectra of 50 transients. Such a data set is essential as we enter a new era of IR astronomy with the James Webb Space Telescope (JWST) and the upcoming Nancy Grace Roman Space Telescope (Roman). NIR spectral templates will be essential to search JWST images for stellar explosions of the first stars and to plan an effective Roman SN Ia cosmology survey, both key science objectives for mission success. Between 2022 February and 2023 July, we systematically obtained 274 NIR spectra of 146 astronomical transients, representing a significant increase in the number of available NIR spectra in the literature. Here, we describe the first release of data from the 2022A semester. We systematically observed three samples: a flux-limited sample that includes all transients <17 mag in a red optical band (usually ZTFror ATLASobands); a volume-limited sample including all transients within redshiftz< 0.01 (D≈ 50 Mpc); and an SN Ia sample targeting objects at phases and light-curve parameters that had scant existing NIR data in the literature. The flux-limited sample is 39% complete (60% excluding SNe Ia), while the volume-limited sample is 54% complete and is 79% complete toz= 0.005. Transient classes observed include common Type Ia and core-collapse supernovae, tidal disruption events, luminous red novae, and the newly categorized hydrogen-free/helium-poor interacting Type Icn supernovae. We describe our observing procedures and data reduction usingPypeIt, which requires minimal human interaction to ensure reproducibility. 

Abstract We present multiwavelength data of SN 2020acct, a double-peaked stripped-envelope supernova (SN) in NGC 2981 at ∼150 Mpc. The two peaks are temporally distinct, with maxima separated by 58 rest-frame days and a factor of 20 reduction in flux between. The first is luminous (M_r = −18.00 ± 0.02 mag) and blue (g − r = 0.27 ± 0.03 mag) and displays spectroscopic signatures of interaction with hydrogen-free circumstellar material. The second peak is fainter (M_r = −17.29 ± 0.03 mag) and has some spectroscopic similarities to an evolved stripped-envelope SN, with strong forbidden [Ca ii] and [O ii] features. No other known double-peaked SN exhibits a light curve similar to that of SN 2020acct. We find the likelihood of two individual SNe occurring in the same star-forming region within that time to be highly improbable, while an implausibly fine-tuned configuration would be required to produce two SNe from a single binary system. We find that the peculiar properties of SN 2020acct match models of pulsational pair instability (PPI), in which the initial peak is produced by collisions of shells of ejected material, shortly followed by core collapse. Pulsations from a star with a 72M_⊙helium core provide an excellent match to the double-peaked light curve. The local galactic environment has a metallicity of 0.4Z_⊙, a level where massive single stars are not expected to retain enough mass to encounter the PPI. However, late binary mergers or a low-metallicity pocket may allow the required core mass. We measure the rate of SN 2020acct–like events to be <3.3 × 10⁻⁸Mpc⁻³yr⁻¹atz= 0.07, or <0.1% of the total core-collapse SN rate. 

Abstract We present multiwavelength observations of the Type II SN 2020pni. Classified at ∼1.3 days after explosion, the object showed narrow (FWHM intensity <250 km s −1 ) recombination lines of ionized helium, nitrogen, and carbon, as typically seen in flash-spectroscopy events. Using the non-LTE radiative transfer code CMFGEN to model our first high-resolution spectrum, we infer a progenitor mass-loss rate of M ̇ = ( 3.5 – 5.3 ) × 10 − 3 M ⊙ yr −1 (assuming a wind velocity of v w = 200 km s −1 ), estimated at a radius of R in = 2.5 × 10 14 cm. In addition, we find that the progenitor of SN 2020pni was enriched in helium and nitrogen (relative abundances in mass fractions of 0.30–0.40 and 8.2 × 10 −3 , respectively). Radio upper limits are also consistent with dense circumstellar material (CSM) and a mass-loss rate of M ̇ > 5 × 10 − 4 M ☉ yr − 1 . During the initial 4 days after first light, we also observe an increase in velocity of the hydrogen lines (from ∼250 to ∼1000 km s −1 ), suggesting complex CSM. The presence of dense and confined CSM, as well as its inhomogeneous structure, indicates a phase of enhanced mass loss of the progenitor of SN 2020pni during the last year before explosion. Finally, we compare SN 2020pni to a sample of other shock-photoionization events. We find no evidence of correlations among the physical parameters of the explosions and the characteristics of the CSM surrounding the progenitors of these events. This favors the idea that the mass loss experienced by massive stars during their final years could be governed by stochastic phenomena and that, at the same time, the physical mechanisms responsible for this mass loss must be common to a variety of different progenitors.