ABSTRACT We present 3D full-sphere supernova simulations of non-rotating low-mass (∼9 M⊙) progenitors, covering the entire evolution from core collapse through bounce and shock revival, through shock breakout from the stellar surface, until fallback is completed several days later. We obtain low-energy explosions (∼0.5–1.0 × 1050 erg) of iron-core progenitors at the low-mass end of the core-collapse supernova (LMCCSN) domain and compare to a super-AGB (sAGB) progenitor with an oxygen–neon–magnesium core that collapses and explodes as electron-capture supernova (ECSN). The onset of the explosion in the LMCCSN models is modelled self-consistently using the vertex-prometheus code, whereas the ECSN explosion is modelled using parametric neutrino transport in the prometheus-HOTB code, choosing different explosion energies in the range of previous self-consistent models. The sAGB and LMCCSN progenitors that share structural similarities have almost spherical explosions with little metal mixing into the hydrogen envelope. A LMCCSN with less second dredge-up results in a highly asymmetric explosion. It shows efficient mixing and dramatic shock deceleration in the extended hydrogen envelope. Both properties allow fast nickel plumes to catch up with the shock, leading to extreme shock deformation and aspherical shock breakout. Fallback masses of $\mathord {\lesssim }\, 5\, \mathord {\times }\, 10^{-3}$ M⊙ have no significant effects on the neutron star (NS) masses and kicks. The anisotropic fallback carries considerable angular momentum, however, and determines the spin of the newly born NS. The LMCCSN model with less second dredge-up results in a hydrodynamic and neutrino-induced NS kick of >40 km s−1 and a NS spin period of ∼30 ms, both not largely different from those of the Crab pulsar at birth.

We report on the detection of a remarkable new fast high-energy transient found in the Chandra Deep Field-South, robustly associated with a faint (m R = 27.5 mag, z ph ∼2.2) host in the CANDELS survey. The X-ray event is comprised of 115 +12 −11 net 0.3-7.0 keV counts, with a light curve characterised by a ≈100 s rise time, a peak 0.3-10 keV flux of ≈5×10 −12 erg s −1 cm −2 , and a power-law decay time slope of −1.53 ± 0.27. The average spectral slope is Γ = 1.43 +0.23 −0.13 , with no clear spectral variations. The X-ray and multi-wavelength properties effectively rule out the vast majority of previously observed high-energy transients. A few theoretical possibilities remain: an "orphan" X-ray afterglow from an off-axis short-duration Gamma-ray Burst (GRB) with weak optical emission; a low-luminosity GRB at high redshift with no prompt emission below ∼20 keV rest-frame; or a highly beamed Tidal Disruption Event (TDE) involving an intermediate-mass black hole and a white dwarf with little variability. However, none of the above scenarios can completely explain all observed properties. Although large uncertainties exist, the implied rate of such events is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs, implying the discovery of an untapped regime for a known transient class, or a new type of variable phenomena whose nature remains to be determined.

A number of Type I (hydrogenless) superluminous supernova (SLSN) events have been discovered recently. However, their nature remains debatable. One of the most promising ideas is the shockinteraction mechanism, but only simplified semi-analytical models have been applied so far. We simulate light curves for several Type I SLSN (SLSN-I) models enshrouded by dense, non-hydrogen circumstellar (CS) envelopes, using a multi-group radiation hydrodynamics code that predicts not only bolometric, but also multicolor light curves. We demonstrate that the bulk of SLSNe-I including those with relatively narrow light curves like SN 2010gx or broad ones like PTF09cnd can be explained by the interaction of the SN ejecta with the CS envelope, though the range of parameters for these models is rather wide. Moderate explosion energy (∼ (2−4)·10 51 ergs) is sufficient to explain both narrow and broad SLSN I light curves, but ejected mass and envelope mass differ for those two cases. Only 5 to 10 M ⊙ of non-hydrogen material is needed to reproduce the light curve of SN 2010gx, while the best model for PTF09cnd is very massive: it contains almost 50 M ⊙ in the CS envelope and only 5 M ⊙ in the ejecta. The CS envelope for each case extends from 10R ⊙ to ∼ 10 5 R ⊙ (7 · 10 15 cm), which is about an order of magnitude larger than typical photospheric radii of standard SNe near the maximum light. We briefly discuss possible ways to form such unusual envelopes.