We present a population of 20 radio-luminous supernovae (SNe) with emission reaching $L_{ u}{sim}10^{26}-10^{29}rm{erg s^{-1} Hz^{-1}}$ 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/GRB171205A, and 19 core-collapse SNe detected at $approx (1-60)$ years after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the sub-sample of core-collapse events with spectroscopic classification at the time of explosion (73%). We interpret these findings into 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., $rho_{rm{CSM}}propto r^{-2}$) that is expected around massive stars and/or a departure 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 $dot M sim (10^{-4}-10^{-1})$ M$_{odot}$ yr$^{-1}$ (for an assumed wind velocity of $1000,rm{km,s^{-1}}$); 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 years after explosion with $L_{ u}{sim}10^{28}$ erg s$^{-1}$ Hz$^{-1}$ might constitute the first detections of an off-axis jet+cocoon system in a massive star. Future multi-wavelength 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.
We present multiwavelength observations of the Type II SN 2020pni. Classified at $sim 1.3$ days after explosion, the object showed narrow (FWHM $<250,textrm{km},textrm{s}^{-1}$) recombination lines of ionized helium, nitrogen, and carbon, as typicall
y 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 $dot{M}=(3.5-5.3)times10^{-3}$ M$_{odot}$ yr$^{-1}$ (assuming a wind velocity of $v_w=200,textrm{km},textrm{s}^{-1}$), estimated at a radius of $R_{rm in}=2.5times10^{14},rm{cm}$. 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 $8.2times10^{-3}$, respectively). Radio upper limits are also consistent with a dense CSM, and a mass-loss rate of $dot M>5 times 10^{-4},rm{M_{odot},yr^{-1}}$. During the first 4 days after first light, we also observe an increase in velocity of the hydrogen lines (from $sim 250,textrm{km},textrm{s}^{-1}$ to $sim 1000,textrm{km},textrm{s}^{-1}$), which suggests a complex CSM. The presence of dense and confined CSM, as well as its inhomogeneous structure, suggest 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.
We present X-ray and radio observations of the Fast Blue Optical Transient (FBOT) CRTS-CSS161010 J045834-081803 (CSS161010 hereafter) at t=69-531 days. CSS161010 shows luminous X-ray ($L_xsim5times 10^{39},rm{erg,s^{-1}}$) and radio ($L_{ u}sim10^{29
},rm{erg,s^{-1}Hz^{-1}}$) emission. The radio emission peaked at ~100 days post transient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blastwave. CSS161010 launched a mildly relativistic outflow with velocity $Gammabeta cge0.55c$ at ~100 days. This is faster than the non-relativistic AT2018cow ($Gammabeta csim0.1c$) and closer to ZTF18abvkwla ($Gammabeta cge0.3c$ at 63 days). The inferred initial kinetic energy of CSS161010 ($E_kgtrsim10^{51}$ erg) is comparable to that of long Gamma Ray Bursts (GRBs), but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ($sim0.01-0.1,rm{M_{odot}}$). This is consistent with the lack of observed gamma-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at ~150 Mpc in a dwarf galaxy with stellar mass $M_{*}sim10^{7},rm{M_{odot}}$ and specific star formation rate $sSFRsim 0.3,rm{Gyr^{-1}}$. This mass is among the lowest inferred for host-galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally-located intermediate mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare (rate $<0.4%$ of the local core-collapse supernova rate) H-rich transients that can launch mildly relativistic outflows.
We present comprehensive observations and analysis of the energetic H-stripped SN 2016coi (a.k.a. ASASSN-16fp), spanning the $gamma$-ray through optical and radio wavelengths, acquired within the first hours to $sim$420 days post explosion. Our campa
ign confirms the identification of He in the SN ejecta, which we interpret to be caused by a larger mixing of Ni into the outer ejecta layers. From the modeling of the broad bolometric light curve we derive a large ejecta mass to kinetic energy ratio ($M_{rm{ej}}sim 4-7,rm{M_{odot}}$, $E_{rm{k}}sim 7-8times 10^{51},rm{erg}$). The small [ion{Ca}{ii}] lamlam7291,7324 to [ion{O}{i}] lamlam6300,6364 ratio ($sim$0.2) observed in our late-time optical spectra is suggestive of a large progenitor core mass at the time of collapse. We find that SN 2016coi is a luminous source of X-rays ($L_{X}>10^{39},rm{erg,s^{-1}}$ in the first $sim100$ days post explosion) and radio emission ($L_{8.5,GHz}sim7times 10^{27},rm{erg,s^{-1}Hz^{-1}}$ at peak). These values are in line with those of relativistic SNe (2009bb, 2012ap). However, for SN 2016coi we infer substantial pre-explosion progenitor mass-loss with rate $dot M sim (1-2)times 10^{-4},rm{M_{odot}yr^{-1}}$ and a sub-relativistic shock velocity $v_{sh}sim0.15c$, in stark contrast with relativistic SNe and similar to normal SNe. Finally, we find no evidence for a SN-associated shock breakout $gamma$-ray pulse with energy $E_{gamma}>2times 10^{46},rm{erg}$. While we cannot exclude the presence of a companion in a binary system, taken together, our findings are consistent with a massive single star progenitor that experienced large mass loss in the years leading up to core-collapse, but was unable to achieve complete stripping of its outer layers before explosion.
We present our study of OGLE-2014-SN-073, one of the brightest Type II SN ever discovered, with an unusually broad lightcurve combined with high ejecta velocities. From our hydrodynamical modelling we infer a remarkable ejecta mass of $60^{+42}_{-16}
$~M$_odot$, and a relatively high explosion energy of $12.4^{+13.0}_{-5.9} times10^{51}$~erg. We show that this object belongs, with a very small number of other hydrogen-rich SNe, to an energy regime that is not explained by standard core-collapse (CC) neutrino-driven explosions. We compare the quantities inferred by the hydrodynamical modelling with the expectations of various exploding scenarios, trying to explain the high energy and luminosity released. We find some qualitative similarities with pair-instabilities SNe, although a prompt injection of energy by a magnetar seems also a viable alternative to explain such extreme event.
