The existence of black holes (BHs) of mass ∼ 10 9 M ⊙ at z 6 is a big puzzle in astrophysics because even optimistic estimates of the accretion time are insufficient for stellar mass BHs of ∼ 10 M ⊙ to grow into such supermassive BHs. A resolution of this puzzle might be the direct collapse of supermassive stars with mass M ∼ 10 5 M ⊙ into massive seed BHs. We find that if a jet is launched from the accretion disk around the central BH, the jet can break out the star because of the structure of the radiation pressure-dominated envelope. Such ultra-long gamma-ray bursts with duration of ∼ 10 4 -10 6 s and flux of 10 −11 -10 −8 erg s −1 cm −2 could be detectable by Swift. We estimate an event rate of 1 yr −1 . The total explosion energy is 10 55 -10 56 erg. The resulting negative feedback delays the growth of the remnant BH by about 70 Myr or evacuates the host galaxy completely.

The process of unstable mass transfer in a stellar binary can result in either a complete merger of the stars or successful removal of the donor envelope leaving a surviving more compact binary. "Luminous red nova" (LRN) are the class of optical transients believed to accompany such merger/common envelope events. Past works typically model LRNe using analytic formulae for supernova light curves which make assumptions (e.g., radiation dominated ejecta, neglect of hydrogen recombination energy) not justified in stellar mergers due to the lower velocities and specific thermal energy of the ejecta. We present a one-dimensional model of LRN light curves, which accounts for these effects. Consistent with observations, we find that LRNe typically possess two light curve peaks, an early phase powered by initial thermal energy of the hot, fastest ejecta layers and a later peak powered by hydrogen recombination from the bulk of the ejecta. We apply our model to a sample of LRNe to infer their ejecta properties (mass, velocity, and launching radius) and compare them to the progenitor donor star properties from pre-transient imaging. We define a maximum luminosity achievable for a given donor star in the limit that the entire envelope is ejected, finding that several LRNe violate this limit. Shock interaction between the ejecta and pre-dynamical mass-loss, may provide an additional luminosity source to alleviate this tension. Our model can also be applied to the merger of planets with stars or stars with compact objects.

The gravitational waves from the neutron star merger event GW170817 were accompanied by an unusually weak short GRB 170817A, by an optical/IR macronova/kilonova and by a long lasting radio to X-rays counterpart. While association of short GRBs with mergers was predicted a long time ago, the luminosity of this prompt γ-ray emission was weaker by a few orders of magnitude than all known previous sGRBs and it was softer than typical sGRBs. This raise the question whether the γ-rays that we have seen were a regular sGRB viewed off-axis. We revisit this question following recent refined analyses of the γ-ray signal and the VLBI observations that revealed the angular structure of the relativistic outflow: observing angle of ∼ 20 • , a narrow jet with core 5 • and E iso > 10 52 ergs. We show here that: (i) The region emitting the observed γ-rays must have been moving with a Lorentz factor Γ 5; (ii) The observed γ-rays were not "off-axis" emission (viewing angle > 1/Γ) emerging from the core of the jet, where a regular sGRB was most likely produced; (iii) The γ-ray emission region was either "on-axis" (at an angle < 1/Γ) or if it was "off-axis" then the observing angle must have been small (< 5 • ) and the on-axis emission from this region was too faint and too hard to resemble a regular sGRB.

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