Newborn black holes in collapsing massive stars can be accompanied by a fallback disk. The accretion rate is typically super-Eddington and strong disk outflows are expected. Such outflows could be directly observed in some failed explosions of compac
t (blue supergiants or Wolf-Rayet stars) progenitors, and may be more common than long-duration gamma-ray bursts. Using an analytical model, we show that the fallback disk outflows produce blue UV-optical transients with a peak bolometric luminosity of ~10^(42-43) erg s^-1 (peak R-band absolute AB magnitudes of -16 to -18) and an emission duration of ~ a few to ~ 10 days. The spectra are likely dominated intermediate mass elements, but will lack much radioactive nuclei and iron-group elements. The above properties are broadly consistent with some of the rapid blue transients detected by Pan-STARRS and PTF. This scenario can be distinguished from alternative models using radio observations within a few years after the optical peak.
We investigate the shock acceleration of particles in massive galaxy mergers or collisions, and show that cosmic rays (CRs) can be accelerated up to the second knee energy ~0.1-1 EeV and possibly beyond, with a hard spectral index Gamma ~ 2. Such CRs
lose their energy via hadronuclear interactions within a dynamical timescale of the merger shock, producing gamma rays and neutrinos as a by-product. If ~ 10 % of the shock dissipated energy goes into CR acceleration, some local merging galaxies will produce gamma-ray counterparts detectable by CTA. Also, based on the concordance cosmology, where a good fraction of the massive galaxies experience a major merger in a cosmological timescale, the neutrino counterparts can constitute ~ 20-60 % of the isotropic background detected by IceCube.
We investigate the possible origin of extended emissions (EEs) of short gamma-ray bursts with an isotropic energy of ~ 10^(50-51) erg and a duration of a few 10 s to ~ 100 s, based on a compact binary (neutron star (NS)-NS or NS-black hole (BH)) merg
er scenario. We analyze the evolution of magnetized neutrino-dominated accretion disks of mass ~ 0.1 M_sun around BHs formed after the mergers, and estimate the power of relativistic outflows via the Blandford-Znajek (BZ) process. We show that a rotation energy of the BH up to > 10^52 erg can be extracted with an observed time scale of > 30 (1+z) s with a relatively small disk viscosity parameter of alpha < 0.01. Such a BZ power dissipates by clashing with non-relativistic pre-ejected matter of mass M ~ 10^-(2-4) M_sun, and forms a mildly relativistic fireball. We show that the dissipative photospheric emissions from such fireballs are likely in the soft X-ray band (1-10 keV) for M ~ 10^-2 M_sun possibly in NS-NS mergers, and in the BAT band (15-150 keV) for M ~ 10^-4 M_sun possibly in NS-BH mergers. In the former case, such soft EEs can provide a good chance of ~ 6 yr^-1 for simultaneous detections of the gravitational waves with a ~ 0.1 deg angular resolution by soft X-ray survey facilities like Wide-Field MAXI.
Recently, Thornton et al. reported the detection of four fast radio bursts (FRBs). The dispersion measures indicate that the sources of these FRBs are at cosmological distance. Given the large full sky event rate ~ 10^4 sky^-1 day^-1, the FRBs are a
promising target for multi-messenger astronomy. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which are produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, emission duration and event rate, can be consistently explained in this scenario. As a result, we predict that some FRBs can accompany type Ia supernovae (SNe Ia) or X-ray debris disks. Simultaneous detection could test our scenario and probe the progenitors of SNe Ia, and moreover would provide a novel constraint on the cosmological parameters. We strongly encourage future SN and X-ray surveys that follow up FRBs.
Metal-poor massive stars may typically end up their lives as blue supergiants (BSGs). Gamma-ray bursts (GRBs) from such progenitors could have ultra-long duration of relativistic jets. For example Population III (Pop III) GRBs at z ~ 10-20 might be o
bservable as X-ray rich events with a typical duration of T_90 ~ 10^4(1+z) sec. Recent GRB111209A at z = 0.677 has an ultra long duration of T_90 ~ 2.5*10^4 sec so that it have been suggested that the progenitor might be a metal-poor BSGs in the local universe. Here, we suggest luminous UV/optical/infrared emissions associated with such a new class of GRB from metal poor BSGs. Before the jet head breaks out the progenitor envelope, the energy injected by the jet is stored in a hot-plasma cocoon, which finally emerges and expands as a baryon-loaded fireball. We show that the photospheric emissions from the cocoon fireball could be intrinsically very bright (L_peak ~ 10^(42-44) erg/sec) in UV/optical bands (E_peak ~ 10 eV) with a typical duration of ~ 100 days in the rest frame. Such cocoon emissions from Pop III GRB might be detectable in infrared bands at ~ years after Pop III GRBs at up to z ~ 15 by up-coming facilities like JWST. We also suggest that GRB111209A might have been rebrightening in UV/optical bands up to an AB magnitude of < 26. The cocoon emissions from local metal-poor BSGs might have been already observed as luminous supernovae without GRB since they can be seen from the off-axis direction of the jet.
We analyze the effects of the back reaction due to a conformal field theory (CFT) on a black hole spacetime with negative cosmological constant. We study the geometry numerically obtained by taking into account the energy momentum tensor of CFT appro
ximated by a radiation fluid. We find a sequence of configurations without a horizon in thermal equilibrium (CFT stars), followed by a sequence of configurations with a horizon. We discuss the thermodynamic properties of the system and how back reaction effects alter the space-time structure. We also provide an interpretation of the above sequence of solutions in terms of the AdS/CFT correspondence. The dual five-dimensional description is given by the Karch-Randall model, in which a sequence of five-dimensional floating black holes followed by a sequence of brane localized black holes correspond to the above solutions.
