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1. SN 2021hpr: A Normal Type Ia Supernova Showing Excess Emission in the Early Rising Phase

   https://doi.org/10.3847/1538-4357/adb3a4  

   Iskandar, Abdusamatjan; Wang, Xiaofeng; Esamdin, Ali; Zeng, Xiangyun; Pellegrino, Craig; Yan, Shengyu; Liu, Jialian; Filippenko, Alexei V; Howell, D Andrew; McCully, Curtis; et al (May 2025, The Astrophysical Journal)

   Abstract We present extensive optical observations of a nearby Type Ia supernova (SN Ia), SN 2021hpr, located in the spiral galaxy NGC 3147 at a distance of ∼45 Mpc. Our observations cover a phase within ∼1–2 days to ∼290 days after the explosion. SN 2021hpr is found to be a spectroscopically normal SN Ia, with an absoluteB-band peak magnitude of $M_{\max }\left(B\right)\approx -19.16\pm 0.14$mag and a postpeak decline rate of Δm₁₅(B) =  1.0 ± 0.01 mag. Early time light curves showed a ∼7.0% excess emission compared to a homogeneously expanding fireball model, likely due to SN ejecta interacting with a companion or immediate circumstellar matter (CSM). The optical spectra of SN 2021hpr are overall similar to those of normal SNe Ia, but characterized by prominent detached high-velocity features (HVFs) of Siiiand Caiiin the early phase. After examining a small sample of well-observed normal SNe Ia, we find that the HVFs are likely common for the subgroup with early excess emission. The association of an early bump feature with the HVFs could be attributed to density or abundance enhancement at the outer layer of the exploding star, likely as a result of interactions with companion/CSM or experiencing more complete burning. Nevertheless, the redshifted Feiiand Niiilines in the nebular-phase spectra of SN 2021hpr, contrary to the blueshift trend seen in other SNe Ia showing early bump features, indicate its peculiarity in the explosion that remains to be understood. 

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2. Flight of the Bumblebee: the Early Excess Flux of Type Ia Supernova 2023bee Revealed by TESS, Swift, and Young Supernova Experiment Observations

   https://doi.org/10.3847/1538-4357/ad0edb  

   Wang, Qinan; Rest, Armin; Dimitriadis, Georgios; Ridden-Harper, Ryan; Siebert, Matthew R.; Magee, Mark; Angus, Charlotte R.; Auchettl, Katie; Davis, Kyle W.; Foley, Ryan J.; et al (February 2024, The Astrophysical Journal)

   Abstract We present high-cadence ultraviolet through near-infrared observations of the Type Ia supernova (SN Ia) 2023bee atD= 32 ± 3 Mpc, finding excess flux in the first days after explosion, particularly in our 10 minutes cadence TESS light curve and Swift UV data. Compared to a few other normal SNe Ia with early excess flux, the excess flux in SN 2023bee is redder in the UV and less luminous. We present optical spectra of SN 2023bee, including two spectra during the period where the flux excess is dominant. At this time, the spectra are similar to those of other SNe Ia but with weaker Siii, Cii,and Caiiabsorption lines, perhaps because the excess flux creates a stronger continuum. We compare the data to several theoretical models on the origin of early excess flux in SNe Ia. Interaction with either the companion star or close-in circumstellar material is expected to produce a faster evolution than observed. Radioactive material in the outer layers of the ejecta, either from double detonation explosion or from a⁵⁶Ni clump near the surface, cannot fully reproduce the evolution either, likely due to the sensitivity of early UV observable to the treatment of the outer part of ejecta in simulation. We conclude that no current model can adequately explain the full set of observations. We find that a relatively large fraction of nearby, bright SNe Ia with high-cadence observations have some amount of excess flux within a few days of explosion. Considering potential asymmetric emission, the physical cause of this excess flux may be ubiquitous in normal SNe Ia. 

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3. The Young and Nearby Normal Type Ia Supernova 2018gv: UV-Optical Observations and the Earliest Spectropolarimetry

   Yang, Yi; Hoeflich, Peter A.; Baade, Dietrich; Maund, Justyn R.; Wang, Lifan; Brown, Peter. J.; Stevance, Heloise F.; Arcavi, Iair; Burke, Jamie; Cikota, Aleksandar; et al (March 2019, arXiv e-prints)

   The non-detection of companion stars in Type Ia supernova (SN) progenitor systems lends support to the notion of double-degenerate (DD) systems and explosions triggered by the merging of two white dwarfs. This very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN\,2018gv with the ESO Very Large Telescope at − 13.6 days relative to the B−band maximum light, or ∼ 5 days after the estimated explosion --- the earliest spectropolarimetric observations to date of any Type Ia SN. These early observations still show a low continuum polarization ( ≲ 0.2\%) and moderate line polarization (0.30 ± 0.04\% for the prominent \ion{Si}{2} λ6355 feature and 0.85 ± 0.04\% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼ 100 days after the maximum light indicate that SN\,2018gv is a normal Type Ia SN with similar spectrophotometric behavior to SN\,2011fe. 

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4. Ground-based and JWST Observations of SN 2022pul. I. Unusual Signatures of Carbon, Oxygen, and Circumstellar Interaction in a Peculiar Type Ia Supernova

   https://doi.org/10.3847/1538-4357/ad0975  

   Siebert, Matthew R; Kwok, Lindsey A; Johansson, Joel; Jha, Saurabh W; Blondin, Stéphane; Dessart, Luc; Foley, Ryan J; Hillier, D John; Larison, Conor; Pakmor, Rüdiger; et al (December 2023, The Astrophysical Journal)

   Abstract Nebular-phase observations of peculiar Type Ia supernovae (SNe Ia) provide important constraints on progenitor scenarios and explosion dynamics for both these rare SNe and the more common, cosmologically useful SNe Ia. We present observations from an extensive ground- and space-based follow-up campaign to characterize SN 2022pul, a super-Chandrasekhar mass SN Ia (alternatively “03fg-like” SN), from before peak brightness to well into the nebular phase across optical to mid-infrared (MIR) wavelengths. The early rise of the light curve is atypical, exhibiting two distinct components, consistent with SN Ia ejecta interacting with dense carbon–oxygen (C/O)-rich circumstellar material (CSM). In the optical, SN 2022pul is most similar to SN 2012dn, having a low estimated peak luminosity (M_B= −18.9 mag) and high photospheric velocity relative to other 03fg-like SNe. In the nebular phase, SN 2022pul adds to the increasing diversity of the 03fg-like subclass. From 168 to 336 days after peakB-band brightness, SN 2022pul exhibits asymmetric and narrow emission from [Oi]λλ6300, 6364 (FWHM ≈ 2000 km s⁻¹), strong, broad emission from [Caii]λλ7291, 7323 (FWHM ≈ 7300 km s⁻¹), and a rapid Feiiito Feiiionization change. Finally, we present the first ever optical-to-MIR nebular spectrum of an 03fg-like SN Ia using data from JWST. In the MIR, strong lines of neon and argon, weak emission from stable nickel, and strong thermal dust emission (withT≈ 500 K), combined with prominent [Oi] in the optical, suggest that SN 2022pul was produced by a white dwarf merger within C/O-rich CSM. 

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5. High-Cadence TESS and Ground-based Data of SN 2019esa, the Less Energetic Sibling of SN 2006gy ^∗

   https://doi.org/10.3847/1538-4357/ac8ea7  

   Andrews, Jennifer E.; Pearson, Jeniveve; Lundquist, M. J.; Sand, David J.; Jencson, Jacob E.; Bostroem, K. Azalee; Hosseinzadeh, Griffin; Valenti, S.; Smith, Nathan; Amaro, R. C.; et al (October 2022, The Astrophysical Journal)

   Abstract We present photometric and spectroscopic observations of the nearby ( D ≈ 28 Mpc) interacting supernova (SN) 2019esa, discovered within hours of explosion and serendipitously observed by the Transiting Exoplanet Survey Satellite (TESS). Early, high-cadence light curves from both TESS and the DLT40 survey tightly constrain the time of explosion, and show a 30 day rise to maximum light followed by a near-constant linear decline in luminosity. Optical spectroscopy over the first 40 days revealed a reddened object with narrow Balmer emission lines seen in Type IIn SNe. The slow rise to maximum in the optical light curve combined with the lack of broad H α emission suggest the presence of very optically thick and close circumstellar material (CSM) that quickly decelerated the SN ejecta. This CSM was likely created from a massive star progenitor with an M ̇ ∼ 0.2 M ☉ yr −1 lost in a previous eruptive episode 3–4 yr before eruption, similar to giant eruptions of luminous blue variable stars. At late times, strong intermediate-width Ca ii , Fe i , and Fe ii lines are seen in the optical spectra, identical to those seen in the superluminous interacting SN 2006gy. The strong CSM interaction masks the underlying explosion mechanism in SN 2019esa, but the combination of the luminosity, strength of the H α lines, and mass-loss rate of the progenitor seem to be inconsistent with a Type Ia CSM model and instead point to a core-collapse origin. 

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