During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates which can exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history, or, at longer wavelengths, by its own emission. We model the evolution of TDE disk size and accretion rate, by accounting for the time-dependent fallback rate, for the influence of wind losses in the early, advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body due to Lense-Thirring effect, and precession may manifest itself as quasi-periodic modulation of light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t= 500 d corresponding to a disk transition from the advective state to the gas-pressure dominated state. 3

Gamma-ray burst (GRB) X-ray flares are believed to mark the late-time activity of the central engine.We compute the temporal evolution of the average flare luminosity in the common rest-frame energy band of 44 GRBs taken from the large Swift 5-yr data base. Our work highlights the importance of a proper consideration of the threshold of detection of flares against the contemporaneous continuous X-ray emission. In the time-interval 30 < t < 1000 s, we find ∝ t^(−2.7±0.1); this implies that the flare isotropic energy scaling is Eiso,flare ∝ t^(−1.7). The decay of the continuum underlying the flare emission closely tracks the average flare luminosity evolution, with a typical flare-to-steep-decay luminosity ratio Lflare/Lsteep = 4.7. This suggests that flares and continuum emission are deeply related to one another. We infer on the progenitor properties considering different models. According to the hyperaccreting black hole scenario, the average flare luminosity scaling can be obtained in the case of rapid accretion (t_acc<

GRB 090417B was an unusually long burst with a T 90 duration of at least 2130 s and a multi-peaked light curve at energies of 15-150 keV. It was optically dark and has been associated with a bright star-forming galaxy at a redshift of 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst and thus an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B -2cannot be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. The Swift/XRT X-ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80 pc from the burst along the line of sight. Our results suggest that this dust sheet imparts an extinction of A V 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a gamma-ray burst that is dark due to the localized dust structure in its host galaxy.

The gamma‐ray burst (GRB) prompt emission is believed to be from highly relativistic electrons accelerated in relativistic shocks. From the GRB high‐energy power‐law spectral indices β observed by the Burst and Transient Source Experiment (BATSE) Large Area Detectors (LAD), we determine the spectral index, p, of the electrons' energy distribution. Both the theoretical calculations and numerical simulations of the particle acceleration in relativistic shocks show that p has a universal value ≈2.2–2.3. We show that the observed distribution of p during GRBs is not consistent with a δ‐function distribution or a universal p value, with the width of the distribution ≥0.54. The distributions of p during X‐ray afterglows are also investigated and found to be inconsistent with a δ‐function distribution. The p distributions in blazars and pulsar wind nebulae are also broad, inconsistent with a δ‐function distribution.

The nature of ultra-luminous X-ray sources (ULXs) has long been plagued by an ambiguity about whether the central compact objects are intermediate-mass (IMBH, ∼ > 10 3 M ⊙ ) or stellar-mass (a few tens M ⊙ ) black holes (BHs). The high luminosity (≃ 10 39 erg s −1 ) and super-soft spectrum (T ≃ 0.1 keV) during the high state of the ULX source X-1 in the galaxy M101 suggest a large emission radius ( ∼ > 10 9 cm), consistent with being an IMBH accreting at a sub-Eddington rate. However, recent kinematic measurement of the binary orbit of this source and identification of the secondary as a Wolf-Rayet star suggest a stellar-mass BH primary with a super-Eddington accretion. If that is the case, a hot, optically thick outflow from the BH can account for the large emission radius and the soft spectrum. By considering the interplay of photons' absorption and scattering opacities, we determine the radius and mass density of the emission region of the outflow and constrain the outflow mass loss rate. The analysis presented here can be potentially applied to other ULXs with thermally dominated spectra, and to other super-Eddington accreting sources.