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1. Luminous Late-time Radio Emission from Supernovae Detected by the Karl G. Jansky Very Large Array Sky Survey (VLASS)

   https://doi.org/10.3847/2041-8213/ac375e  

   Stroh, Michael C.; Terreran, Giacomo; Coppejans, Deanne L.; Bright, Joe S.; Margutti, Raffaella; Bietenholz, Michael F.; De Colle, Fabio; DeMarchi, Lindsay; Duran, Rodolfo Barniol; Milisavljevic, Danny; et al (December 2021, The Astrophysical Journal Letters)

   Abstract We present a population of 19 radio-luminous supernovae (SNe) with emission reaching L ν ∼ 10 26 –10 29 erg s −1 Hz −1 in the first epoch of the Very Large Array Sky Survey (VLASS) at 2–4 GHz. Our sample includes one long gamma-ray burst, SN 2017iuk/GRB 171205A, and 18 core-collapse SNe detected at ≈1–60 yr after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the subsample of core-collapse events with spectroscopic classification at the time of explosion (79%). We interpret these findings in the context of the expected radio emission from the forward shock interaction with the circumstellar medium (CSM). We conclude that these observations require a departure from the single wind–like density profile (i.e., ρ CSM ∝ r −2 ) that is expected around massive stars and/or from a spherical Newtonian shock. Viable alternatives include the shock interaction with a detached, dense shell of CSM formed by a large effective progenitor mass-loss rate, M ̇ ∼ 10 − 4 – 10 − 1 M ⊙ yr −1 (for an assumed wind velocity of 1000 km s −1 ); emission from an off-axis relativistic jet entering our line of sight; or the emergence of emission from a newly born pulsar-wind nebula. The relativistic SN 2012ap that is detected 5.7 and 8.5 yr after explosion with L ν ∼ 10 28 erg s −1 Hz −1 might constitute the first detections of an off-axis jet+cocoon system in a massive star. However, none of the VLASS SNe with archival data points are consistent with our model off-axis jet light curves. Future multiwavelength observations will distinguish among these scenarios. Our VLASS source catalogs, which were used to perform the VLASS cross-matching, are publicly available at https://doi.org/10.5281/zenodo.4895112 . 

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2. Final Moments. I. Precursor Emission, Envelope Inflation, and Enhanced Mass Loss Preceding the Luminous Type II Supernova 2020tlf

   https://doi.org/10.3847/1538-4357/ac3f3a  

   Jacobson-Galán, W. V.; Dessart, L.; Jones, D. O.; Margutti, R.; Coppejans, D. L.; Dimitriadis, G.; Foley, R. J.; Kilpatrick, C. D.; Matthews, D. J.; Rest, S.; et al (January 2022, The Astrophysical Journal)

   Abstract We present panchromatic observations and modeling of supernova (SN) 2020tlf, the first normal Type II-P/L SN with confirmed precursor emission, as detected by the Young Supernova Experiment transient survey. Pre-SN activity was detected in riz -bands at −130 days and persisted at relatively constant flux until first light. Soon after discovery, “flash” spectroscopy of SN 2020tlf revealed narrow, symmetric emission lines that resulted from the photoionization of circumstellar material (CSM) shed in progenitor mass-loss episodes before explosion. Surprisingly, this novel display of pre-SN emission and associated mass loss occurred in a red supergiant (RSG) progenitor with zero-age main-sequence mass of only 10–12 M ⊙ , as inferred from nebular spectra. Modeling of the light curve and multi-epoch spectra with the non-LTE radiative-transfer code CMFGEN and radiation-hydrodynamical code HERACLES suggests a dense CSM limited to r ≈ 10 15 cm, and mass-loss rate of 10 −2 M ⊙ yr −1 . The luminous light-curve plateau and persistent blue excess indicates an extended progenitor, compatible with an RSG model with R ⋆ = 1100 R ⊙ . Limits on the shock-powered X-ray and radio luminosity are consistent with model conclusions and suggest a CSM density of ρ < 2 × 10 −16 g cm −3 for distances from the progenitor star of r ≈ 5 × 10 15 cm, as well as a mass-loss rate of M ̇ < 1.3 × 10 − 5 M ☉ yr − 1 at larger distances. A promising power source for the observed precursor emission is the ejection of stellar material following energy disposition into the stellar envelope as a result of gravity waves emitted during either neon/oxygen burning or a nuclear flash from silicon combustion. 

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3. The Early Phases of Supernova 2020pni: Shock Ionization of the Nitrogen-enriched Circumstellar Material

   https://doi.org/10.3847/1538-4357/ac3820  

   Terreran, G.; Jacobson-Galán, W. V.; Groh, J. H.; Margutti, R.; Coppejans, D. L.; Dimitriadis, G.; Kilpatrick, C. D.; Matthews, D. J.; Siebert, M. R.; Angus, C. R.; et al (February 2022, The Astrophysical Journal)

   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. 

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4. SN 2014C: VLBI image shows a shell structure and decelerated expansion

   https://doi.org/10.1093/mnras/staa4003  

   Bietenholz, Michael F; Bartel, Norbert; Kamble, Atish; Margutti, Raffaella; Matthews, David Jacob; Milisavljevic, Danny (February 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We report on new Very Long Baseline Interferometry radio measurements of supernova (SN) 2014C in the spiral galaxy NGC 7331, made with the European VLBI Network ∼5 yr after the explosion, as well as on flux density measurements made with the Jansky Very Large Array (VLA). SN 2014C was an unusual SN, initially of Type Ib, but over the course of ∼1 yr, it developed strong H α lines, implying the onset of strong interaction with some H-rich circumstellar medium (CSM). The expanding shock-front interacted with a dense shell of circumstellar material during the first year, but has now emerged from the dense shell and is expanding into the lower density CSM beyond. Our new VLBI observations show a relatively clear shell structure and continued expansion with some deceleration, with a suggestion that the deceleration is increasing at the latest times. Our multifrequency VLA observations show a relatively flat power-law spectrum with Sν ∝ ν−0.56 ± 0.03, and show no decline in the radio luminosity since t ∼ 1 yr. 

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5. Luminous Radio Emission from the Superluminous Supernova 2017ens at 3.3 yr after Explosion

   https://doi.org/10.3847/2041-8213/acf1fd  

   Margutti, Raffaella; Bright, J S; Matthews, D J; Coppejans, D L; Alexander, K D; Berger, E; Bietenholz, M; Chornock, R; DeMarchi, L; Drout, M R; et al (September 2023, The Astrophysical Journal Letters)

   Abstract We present the results from a multiyear radio campaign of the superluminous supernova (SLSN) SN 2017ens, which yielded the earliest radio detection of an SLSN to date at the age of ∼3.3 yr after explosion. SN 2017ens was not detected at radio frequencies in the first ∼300 days but reachedL_ν≈ 10²⁸erg s⁻¹cm⁻²Hz⁻¹atν∼ 6 GHz, ∼1250 days post explosion. Interpreting the radio observations in the context of synchrotron radiation from the supernova shock interaction with the circumstellar medium (CSM), we infer an effective mass-loss rate $\dot{M}\approx {10}^{-4}{M}_{☉}{\mathrm{yr}}^{-1}$atr∼ 10¹⁷cm from the explosion’s site, for a wind speed ofv_w= 50–60 km s⁻¹as measured from optical spectra. These findings are consistent with the spectroscopic metamorphosis of SN 2017ens from hydrogen poor to hydrogen rich ∼190 days after explosion reported by Chen et al. SN 2017ens is thus an addition to the sample of hydrogen-poor massive progenitors that explode shortly after having lost their hydrogen envelope. The inferred circumstellar densities, implying a CSM mass up to ∼0.5M_☉, and low velocity of the ejection suggest that binary interactions (in the form of common-envelope evolution and subsequent envelope ejection) play a role in shaping the evolution of the stellar progenitors of SLSNe in the ≲500 yr preceding core collapse. 

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