Gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 10(6) to 10(7) solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar.

We present the results of radiation‐magnetohydrodynamic simulations of the formation and expansion of H ii regions and their surrounding photodissociation regions (PDRs) in turbulent, magnetized, molecular clouds on scales of up to 4 pc. We include the effects of ionizing and non‐ionizing ultraviolet radiation and X‐rays from population synthesis models of young star clusters. For all our simulations we find that the H ii region expansion reduces the disordered component of the magnetic field, imposing a large‐scale order on the field around its border, with the field in the neutral gas tending to lie along the ionization front, while the field in the ionized gas tends to be perpendicular to the front. The highest pressure‐compressed neutral and molecular gas is driven towards approximate equipartition between thermal, magnetic and turbulent energy densities, whereas lower pressure neutral/molecular gas bifurcates into, on the one hand, quiescent, magnetically dominated regions and, on the other hand, turbulent, demagnetized regions. The ionized gas shows approximate equipartition between thermal and turbulent energy densities, but with magnetic energy densities that are 1–3 orders of magnitude lower. A high velocity dispersion (∼8 km s−1) is maintained in the ionized gas throughout our simulations, despite the mean expansion velocity being significantly lower. The magnetic field does not significantly brake the large‐scale H ii region expansion on the length and time‐scales accessible to our simulations, but it does tend to suppress the smallest scale fragmentation and radiation‐driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H ii region. However, the relative luminosity of ionizing and non‐ionizing radiation has a much larger influence than the presence or absence of the magnetic field. When the star cluster radiation field is relatively soft (as in the case of a lower mass cluster, containing an earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms.

The central engine of short gamma-ray bursts (sGRBs) is hidden from direct view, operating at a scale much smaller than that probed by the emitted radiation. Thus we must infer its origin not only with respect to the formation of the trigger -the actual astrophysical configuration that is capable of powering a sGRB -but also from the consequences that follow from the various evolutionary pathways that may be involved in producing it. Considering binary neutron star mergers we critically evaluate, analytically and through numerical simulations, whether the neutrino-driven wind produced by the newly formed hyper-massive neutron star can allow the collimated relativistic outflow that follows its collapse to actually produce a sGRB or not. Upon comparison with the observed sGRB duration distribution, we find that collapse cannot be significantly delayed (≤ 100 ms) before the outflow is choked, thus limiting the possibility that long-lived hyper-massive remnants can account for these events. In the case of successful breakthrough of the jet through the neutrino-driven wind, the energy stored in the cocoon could contribute to the precursor and extended emission observed in sGRBs.

We present the first three‐dimensional radiation‐magnetohydrodynamic simulations of the photoionization of a dense, magnetized molecular globule by an external source of ultraviolet radiation. We find that, for the case of a strong ionizing field, significant deviations from the non‐magnetic evolution are seen when the initial magnetic field threading the globule has an associated magnetic pressure that is greater than 100 times the gas pressure. In such a strong‐field case, the photoevaporating globule will adopt a flattened or ‘curled up’ shape, depending on the initial field orientation, and magnetic confinement of the ionized photoevaporation flow can lead to recombination and subsequent fragmentation during advanced stages of the globule evolution. We find suggestive evidence that such magnetic effects may be important in the formation of bright, bar‐like emission features in H ii regions. We include simple but realistic fits to heating and cooling rates in the neutral and molecular gas in the vicinity of a high‐mass star cluster, and show that the frequently used isothermal approximation can lead to an overestimate of the importance of gravitational instability in the radiatively imploded globule. For globules within 2 pc of a high‐mass star cluster, we find that heating by stellar X‐rays prevents the molecular gas from cooling below 50 K.