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1. A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients

   https://doi.org/10.3847/1538-4357/acc533  

   Ho, Anna Y. Q. ; Perley, Daniel A. ; Gal-Yam, Avishay ; Lunnan, Ragnhild ; Sollerman, Jesper ; Schulze, Steve ; Das, Kaustav K. ; Dobie, Dougal ; Yao, Yuhan ; Fremling, Christoffer ; et al ( June 2023 , The Astrophysical Journal)

   We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum light 1 day <t_1/2< 12 days, of which 28 have blue (g−r≲ −0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had predominantly featureless spectra and luminous radio emission: AT2018lug (The Koala) and AT2020xnd (The Camel). Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad Hαin emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (eight) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 24 of the transients. All X-ray and radio observations resulted in nondetections; we rule out AT2018cow-likemore »X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs and use ZTF’s SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate.

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2. ASASSN-18am/SN 2018gk: an overluminous Type IIb supernova from a massive progenitor

   https://doi.org/10.1093/mnras/stab629  

   Bose, Subhash ; Dong, Subo ; Kochanek, C S ; Stritzinger, M D ; Ashall, Chris ; Benetti, Stefano ; Falco, E ; Filippenko, Alexei V ; Pastorello, Andrea ; Prieto, Jose L ; et al ( March 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of MV ≈ −20 mag that is in between normal core-collapse SNe and superluminous SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting with circumstellar material (CSM), and their powering mechanism is debated. ASASSN-18am declines extremely rapidly for a Type II SN, with a photospheric-phase decline rate of ∼6.0 mag (100 d)−1. Owing to the weakening of H i and the appearance of He i in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution shows significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesized 56Ni mass $M_{\rm Ni} \sim 0.4\, \rm {M_{\odot }}$ and ejecta with high kinetic energy Ekin = (7–10) × 1051 erg. Introducing a magnetar central engine still requires $M_{\rm Ni} \sim 0.3\, \rm {M_{\odot }}$ and Ekin = 3 × 1051 erg. The high 56Ni mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high 56Ni yields. The earliest spectrum shows ‘flash ionization’ features, from which we estimatemore »a mass-loss rate of $\dot{M}\approx 2\times 10^{-4} \, \rm \rm {M_{\odot }}\,yr^{-1}$. This wind density is too low to power the luminous light curve by ejecta–CSM interaction. We measure expansion velocities as high as 17 000 $\rm {\, km\, s^{-1}}$ for Hα, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of 1.8–3.4 M⊙ using the [O i] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main-sequence mass of 19–26 M⊙.« less
3. The Lick Observatory Supernova Search follow-up program: photometry data release of 70 SESNe

   https://doi.org/10.1093/mnras/stac723  

   Zheng, WeiKang ; Stahl, Benjamin E. ; de Jaeger, Thomas ; Filippenko, Alexei V. ; Wang, Shan-Qin ; Gan, Wen-Pei ; Brink, Thomas G. ; Altunin, Ivan ; Baer-Way, Raphael ; Bigley, Andrew ; et al ( March 2022 , Monthly Notices of the Royal Astronomical Society)

   We present BVRI and unfiltered (Clear) light curves of 70 stripped-envelope supernovae (SESNe), observed between 2003 and 2020, from the Lick Observatory Supernova Search follow-up program. Our SESN sample consists of 19 spectroscopically normal SNe Ib, 2 peculiar SNe Ib, six SNe Ibn, 14 normal SNe Ic, 1 peculiar SN Ic, 10 SNe Ic-BL, 15 SNe IIb, 1 ambiguous SN IIb/Ib/c, and 2 superluminous SNe. Our follow-up photometry has (on a per-SN basis) a mean coverage of 81 photometric points (median of 58 points) and a mean cadence of 3.6 d (median of 1.2 d). From our full sample, a subset of 38 SNe have pre-maximum coverage in at least one passband, allowing for the peak brightness of each SN in this subset to be quantitatively determined. We describe our data collection and processing techniques, with emphasis toward our automated photometry pipeline, from which we derive publicly available data products to enable and encourage further study by the community. Using these data products, we derive host-galaxy extinction values through the empirical colour evolution relationship and, for the first time, produce accurate rise-time measurements for a large sample of SESNe in both optical and infrared passbands. By modelling multiband light curves, we find that SNe Ic tend to have lower ejectamore »masses and lower ejecta velocities than SNe Ib and IIb, but higher 56Ni masses.

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4. SN 2020acat: an energetic fast rising Type IIb supernova

   https://doi.org/10.1093/mnras/stac1192  

   Medler, K. ; Mazzali, P. A. ; Teffs, J. ; Ashall, C. ; Anderson, J. P. ; Arcavi, I. ; Benetti, S. ; Bostroem, K. A. ; Burke, J. ; Cai, Y-Z ; et al ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   The ultraviolet (UV) and near-infrared (NIR) photometric and optical spectroscopic observations of SN 2020acat covering ∼250 d after explosion are presented here. Using the fast rising photometric observations, spanning from the UV to NIR wavelengths, a pseudo-bolometric light curve was constructed and compared to several other well-observed Type IIb supernovae (SNe IIb). SN 2020acat displayed a very short rise time reaching a peak luminosity of $\mathrm{{\rm Log}_{10}}(L) = 42.49 \pm 0.17 \, \mathrm{erg \, s^{-1}}$ in only ∼14.6 ± 0.3 d. From modelling of the pseudo-bolometric light curve, we estimated a total mass of 56Ni synthesized by SN 2020acat of MNi = 0.13 ± 0.03 M⊙, with an ejecta mass of Mej = 2.3 ± 0.4 M⊙ and a kinetic energy of Ek = 1.2 ± 0.3 × 1051 erg. The optical spectra of SN 2020acat display hydrogen signatures well into the transitional period (≳ 100 d), between the photospheric and the nebular phases. The spectra also display a strong feature around 4900  Å that cannot be solely accounted for by the presence of the Fe ii 5018 line. We suggest that the Fe ii feature was augmented by He i 5016 and possibly by the presence of N ii 5005. From both photometric and spectroscopic analysis, we inferred that the progenitor of SN 2020acat was an intermediate-mass compact star with an MZAMS of 15–20 M⊙.
5. Ultraviolet Spectroscopy and TARDIS Models of the Broad-lined Type Ic Supernova 2014ad

   https://doi.org/10.3847/1538-4357/ac8989  

   Kwok, Lindsey A. ; Williamson, Marc ; Jha, Saurabh W. ; Modjaz, Maryam ; Camacho-Neves, Yssavo ; Foley, Ryan J. ; Garnavich, Peter ; Maeda, Keiichi ; Milisavljevic, Dan ; Pandya, Viraj ; et al ( September 2022 , The Astrophysical Journal)

   Few published ultraviolet (UV) spectra exist for stripped-envelope supernovae and none to date for broad-lined Type Ic supernovae (SNe Ic-bl). These objects have extremely high ejecta velocities and are the only supernova type directly linked to gamma-ray bursts (GRBs). Here we present two epochs of HST/STIS spectra of the SN Ic-bl 2014ad, the first UV spectra for this class. We supplement this with 26 new epochs of ground-based optical spectra, augmenting a rich spectral time series. The UV spectra do not show strong features and are consistent with broadened versions of other SN Ic spectra observed in the UV. We measure Feii5169 Å velocities and show that SN 2014ad has even higher ejecta velocities than most SNe Ic both with and without observed GRBs. We construct models of the SN 2014ad UV+optical spectra usingtardis, a 1D Monte Carlo radiative-transfer spectral synthesis code. The models fit the data well at multiple epochs in the optical but underestimate the flux in the UV, likely due to simplifying assumptions. We find that high densities at high velocities are needed to reproduce the spectra, with ∼3M_⊙of material atv> 22,000 km s⁻¹, assuming spherical symmetry. Our nebular line fits suggest a steep density profilemore »at low velocities. Together, these results imply a higher total ejecta mass than estimated from previous light-curve analysis and expected from theory. This may be reconciled by a flattening of the density profile at low velocity and extra emission near the center of the ejecta.

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