We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient, AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L _pk ∼ 4 × 10⁴⁴ erg s⁻¹, exceeding that of superluminous supernovae (SNe), before declining as L ∝ t ⁻². Initial spectra at δt ≲ 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1c and temperatures reaching T ∼ 3 × 10⁴ K. Later spectra revealed a persistent optically thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s⁻¹ with no evidence for ejecta cooling. Our broadband monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blast wave with v _sh ∼ 0.1c with a dense environment ( for v _w = 1000 km s⁻¹). While these properties exclude ⁵⁶Ni-powered transients, our multiwavelength analysis instead indicates that AT 2018cow harbored a “central engine,” either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼10⁵⁰–10^51.5 erg over ∼10³–10⁵ s and resides within low-mass fast-moving material with equatorial–polar density asymmetry (M _ej,fast ≲ 0.3 M _☉). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black holes are disfavored by the large environmental density probed by the radio observations.

We present multi-wavelength observations of SN 2014C during the first 500 days. These observations represent the first solid detection of a young extragalactic stripped-envelope SN out to high-energy X-rays ∼40 keV. SN 2014C shows ordinary explosion parameters (E _k ∼ 1.8 × 10⁵¹ erg and M _ej ∼ 1.7 M _⊙). However, over an ∼1 year timescale, SN 2014C evolved from an ordinary hydrogen-poor supernova into a strongly interacting, hydrogen-rich supernova, violating the traditional classification scheme of type-I versus type-II SNe. Signatures of the SN shock interaction with a dense medium are observed across the spectrum, from radio to hard X-rays, and revealed the presence of a massive shell of ∼1 M _⊙ of hydrogen-rich material at ∼6 × 10¹⁶ cm. The shell was ejected by the progenitor star in the decades to centuries before collapse. This result challenges current theories of massive star evolution, as it requires a physical mechanism responsible for the ejection of the deepest hydrogen layer of H-poor SN progenitors synchronized with the onset of stellar collapse. Theoretical investigations point at binary interactions and/or instabilities during the last nuclear burning stages as potential triggers of the highly time-dependent mass loss. We constrain these scenarios utilizing the sample of 183 SNe Ib/c with public radio observations. Our analysis identifies SN 2014C-like signatures in ∼10% of SNe. This fraction is reasonably consistent with the expectation from the theory of recent envelope ejection due to binary evolution if the ejected material can survive in the close environment for 10³–10⁴ years. Alternatively, nuclear burning instabilities extending to core C-burning might play a critical role.

We present X-ray and radio observations of the Fast Blue Optical Transient CRTS-CSS161010 J045834−081803 (CSS161010 hereafter) at t = 69–531 days. CSS161010 shows luminous X-ray (L _x ∼ 5 × 10³⁹ erg s⁻¹) and radio (L _ν ∼ 10²⁹ 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 blast wave. CSS161010 launched a mildly relativistic outflow with velocity Γβc ≥ 0.55c at ∼100 days. This is faster than the non-relativistic AT 2018cow (Γβc ∼ 0.1c) and closer to ZTF18abvkwla (Γβc ≥ 0.3c at 63 days). The inferred initial kinetic energy of CSS161010 (E _k ≳ 10⁵¹ erg) is comparable to that of long gamma-ray bursts, but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ( ). This is consistent with the lack of observed γ-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 _* ∼ 10⁷ M _⊙ and specific star formation rate sSFR ∼ 0.3 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 radio observations of 23 optically discovered tidal disruption events (TDEs) on timescales of ∼500–3200 days postdiscovery. We detect nine new TDEs that did not have detectable radio emission at earlier times, indicating a late-time brightening after several hundred (and up to 2300) days; an additional seven TDEs exhibit radio emission whose origin is ambiguous or may be attributed to the host galaxy or an active galactic nucleus. We also report a new rising component in one TDE previously detected in the radio at ∼10³ days. While the radio emission in some of the detected TDEs peaked on a timescale ≈2–4 yr, over half of the sample still show rising emission. The range of luminosities for the sample is ∼10³⁷–10³⁹ erg s⁻¹, about 2 orders of magnitude below the radio luminosity of the relativistic TDE Sw J1644+57. Our data set indicates ∼40% of all optical TDEs are detected in radio hundreds to thousands of days after discovery, and that this is probably more common than early radio emission peaking at ∼10² days. Using an equipartition analysis, we find evidence for a delayed launch of the radio-emitting outflows, with delay timescales of ∼500–2000 days, inferred velocities of ≈0.02–0.15c, and kinetic energies of ∼10⁴⁷–10⁴⁹ erg. We rule out off-axis relativistic jets as a viable explanation for this population, and conclude delayed outflows are a more likely explanation, possibly from delayed disk formation. We conclude late radio emission marks a fairly ubiquitous but heretofore overlooked phase of TDE evolution.

We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of tidal disruption event (TDE) AT2018hyz, spanning 970–1300 d after optical discovery. In conjunction with earlier deeper limits, including those at ≈700 days, our observations reveal rapidly rising emission at 0.8–240 GHz, steeper than F _ν ∝ t ⁵ relative to the time of optical discovery. Such a steep rise cannot be explained in any reasonable scenario of an outflow launched at the time of disruption (e.g., off-axis jet, sudden increase in the ambient density), and instead points to a delayed launch. Our multifrequency data allow us to directly determine the radius and energy of the radio-emitting outflow, and we find from our modeling that the outflow was launched ≈750 days after optical discovery. The outflow velocity is mildly relativistic, with β ≈ 0.25 and ≈0.6 for a spherical geometry and a 10° jet geometry, respectively, and the minimum kinetic energy is E _K ≈ 5.8 × 10⁴⁹ and ≈6.3 × 10⁴⁹ erg, respectively. This is the first definitive evidence for the production of a delayed mildly relativistic outflow in a TDE; a comparison to the recently published radio light curve of ASASSN-15oi suggests that the final rebrightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past nonrelativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw J1644+57. We suggest that such delayed outflows may be common in TDEs.