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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The first direct detections of gravitational waves (GWs) from black hole (BH) mergers, GW150914, GW151226 and LVT151012, give a robust lower limit ∼ 70000 on the number of merged, highly-spinning BHs in our Galaxy. The total spin energy is comparable to all the kinetic energy of supernovae that ever happened in our Galaxy. The BHs release the spin energy to relativistic jets by accreting matter and magnetic fields from the interstellar medium (ISM). By considering the distributions of the ISM density, BH mass and velocity, we calculate the luminosity function of the BH jets, and find that they can potentially accelerate TeV-PeV cosmic-ray particles in our Galaxy with total power ∼ 10 37±3 erg s −1 as PeVatrons, positron factories and/or unidentified TeV gamma-ray sources. Additional ∼ 300 BH jet nebulae could be detectable by CTA (Cherenkov Telescope Array). We also argue that the accretion from the ISM can evaporate and blow away cold material around the BH, which has profound implications for some scenarios to predict electromagnetic counterparts to BH mergers.

Short gamma-ray bursts (sGRBs) are thought to be produced by binary NS mergers. While a sGRB requires a relativistic jet to break out of ejecta, the jet may be choked and fails to produce a successful sGRB. We propose a "delayed breakout" scenario where a late-time jet launched by a long-term engine activity can penetrate ejecta even if a prompt jet is choked. Observationally, such a late-time jet is supported by the long-lasting high-energy emissions in sGRBs. Solving the jet propagation in ejecta, we show that a typical late-time activity easily achieves the delayed breakout. This event shows not prompt γ-rays but long-time X-ray emissions for ∼ 10 2−3 s or even ∼ 10 4−5 s. Some delayed events may be already detected as soft-long GRBs without supernova signatures. In an optimistic case, a few events coincident with gravitational-waves (GWs) are detected by the second-generation GW detectors every year. X-ray followups of merger events without γ-rays will be a probe of long-lasting engine activities in binary mergers.

A γ-ray source must have a limited optical depth to pair production. This simple condition, called compactness, implies that gamma-ray bursts (GRBs) must involve a highly relativistic motion (Γ 100) giving the first and most important clue on their nature. So far, this condition has been discussed under the assumption that the γ-ray sources are viewed on-axis, that is, by an observer within the beaming cone of the relativistic source. Recently, following the detection of the weak short GRB 170817A, an extensive interest arose in the possibility that some γ-ray sources are viewed off-axis. We generalize here the compactness formalism for an arbitrary viewing angle taking several possible opacity processes and γ-ray spectra into account. We find that for a given observables (peak luminosity, temporal variability, and spectra) the minimal Lorentz factor, Γ min , is obtained, as expected, for an on-axis observer. More remarkably we find that compactness dictates also a maximal viewing angle, θ max 1/2Γ min . Our limit implies for regular GRBs a very small allowed viewing angle ( 10 −2 rad), making it extremely unlikely that they are viewed off-axis. For GRB 170817A we confirm earlier results that rule out the possibility that the observed γ-rays were seen by an on-axis observer as a regular short GRB. The short GRB 150101B was also suggested to be an off-axis event. We show that its maximal viewing angle 0.05 rad, which is inconsistent with the off-axis model. Finally we show that for low luminosity GRBs, compactness does not exclude by itself an off-axis model, but when combined with other consideration this option is strongly disfavored.

Studies on the self-leveling behavior in debris beds are crucial in the assessment of core-disruptive accidents (CDAs) that could occur in sodium-cooled fast reactors (SFRs). To clarify this behavior, a series of experiments have been performed in which nitrogen gas has been percolated uniformly through a particle bed. In these experiments, solid particles and water contained in a rectangular tank simulate respectively fuel debris and coolant. Based on the data obtained, an empirical model was developed to describe the transient variation in the bed inclination angle during the self-leveling process. Good agreement has been obtained between calculated and experimental values. Verification of the model has been confirmed through detailed analysis of the effects of experimental parameters such as particle size, particle density, and gas flow rate. Its applicability to extended conditions was further discussed by performing modeling simulations and comparing results against experimental data obtained from a larger-scale experimental system that employed a conventional boiling method. With further improvements, the model will be tested under more realistic reactor conditions and is expected to benefit future analyses and simulations of CDAs in SFRs.