We examine the bright radio synchrotron counterparts of low-luminosity gamma-ray bursts (llGRBs) and relativistic supernovae (SNe) and find that they can be powered by spherical hypernova (HN) explosions. Our results imply that radio-bright HNe are d
riven by relativistic jets that are choked deep inside the progenitor stars or quasi-spherical magnetized winds from fast-rotating magnetars. We also consider the optical synchrotron counterparts of radio-bright HNe and show that they can be observed as precursors several days before the SN peak with an r-band absolute magnitude of M_r ~ -14 mag. While previous studies suggested that additional trans-relativistic components are required to power the bright radio emission, we find that they overestimated the energy budget of the trans-relativistic component by overlooking some factors related to the minimum energy of non-thermal electrons. If an additional trans-relativistic component exists, then a much brighter optical precursor with M_r ~ -20 mag can be expected. Thus, the scenarios of radio-bright HNe can be distinguished by using optical precursors, which can be detectable from < 100 Mpc by current SN surveys like the Kiso SN Survey, Palomar Transient Factory, and Panoramic Survey Telescope & Rapid Response System.
HD dominates the cooling of primordial clouds with enhanced ionization, e.g. shock-heated clouds in structure formation or supernova remnants, relic HII regions of Pop III stars, and clouds with cosmic-ray (CR) irradiation. There, the temperature dec
reases to several 10 K and the characteristic stellar mass decreases to $sim 10 {rm M}_{odot}$, in contrast with first stars formed from undisturbed pristine clouds ($sim 100 {rm M}_{odot}$). However, without CR irradiation, even weak far ultra-violet (FUV) irradiation suppresses HD formation/cooling. Here, we examine conditions for HD cooling in primordial clouds including both FUV and CR feedback. At the beginning of collapse, the shock-compressed gas cools with its density increasing, while the relic HII region gas cools at a constant density. Moreover, shocks tend to occur in denser environments than HII regions. Owing to the higher column density and the more effective shielding, the critical FUV intensity for HD cooling in a shock-compressed gas becomes $sim 10$ times higher than in relic HII regions. Consequently, in the shock-compressed gas, the critical FUV intensity exceeds the background level for most of the redshift we consider ($6 lesssim z lesssim 15$), while in relic HII regions, HD cooling becomes effective after the CR intensity increases enough at $z lesssim 10$. Our result suggests that less massive ($sim 10 {rm M}_{odot}$) Pop III stars may be more common than previously considered and could be the dominant population of Pop III stars.
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.
Long GRBs (LGRBs) have typical duration of ~ 30 s and some of them are associated with hypernovae, like Type Ic SN 1998bw. Wolf-Rayet stars are the most plausible LGRB progenitors, since the free-fall time of the envelope is consistent with the durat
ion, and the natural outcome of the progenitor is a Type Ic SN. While a new population of ultra-long GRBs (ULGRBs), GRB 111209A, GRB 101225A, and GRB 121027A, has a duration of ~ 10^4 s, two of them are accompanied by superluminous-supernova (SLSN) like bumps, which are <~ 10 times brighter than typical hypernovae. Wolf-Rayet progenitors cannot explain ULGRBs because of too long duration and too bright SN-like bump. A blue supergiant (BSG) progenitor model, however, can explain the duration of ULGRBs. Moreover, SLSN-like bump can be attributed to the so-called cocoon-fireball photospheric emissions (CFPEs). Since a large cocoon is inevitably produced during the relativistic jet piercing though the BSG envelope, this component can be a smoking-gun evidence of BSG model for ULGRBs. In this paper, we examine u, g, r, i, and J-band light curves of three ULGRBs and demonstrate that they can be fitted quite well by our BSG model with the appropriate choices of the jet opening angle and the number density of the ambient gas. In addition, we predict that for 121027A, SLSN-like bump could have been observed for ~ 20 - 80 days after the burst. We also propose that some SLSNe might be CFPEs of off-axis ULGRBs without visible prompt emission.
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.
Recent numerical simulations suggest that Population III (Pop III) stars were born with masses not larger than $sim 100 M_{odot}$ but typically $sim 40M_{odot}$. By self-consistently considering the jet generation and propagation in the envelope of t
hese low mass Pop III stars, we find that a Pop III blue super giant star has the possibility to raise a gamma-ray burst (GRB) even though it keeps a massive hydrogen envelope. We evaluate observational characters of Pop III GRBs and predict that Pop III GRBs have the duration of $sim 10^5$ sec in the observer frame and the peak luminosity of $sim 5 times 10^{50} {rm erg} {rm sec}^{-1}$. Assuming that the $E_p-L_p$ (or $E_p-E_{gamma, rm iso}$) correlation holds for Pop III GRBs, we find that the spectrum peak energy falls $sim$ a few keV (or $sim 100$ keV) in the observer frame. We discuss the detectability of Pop III GRBs by future satellite missions such as EXIST and Lobster. If the $E_p-E_{gamma, rm iso}$ correlation holds, we have the possibility to detect Pop III GRBs at $z sim 9$ as long duration X-ray rich GRBs by EXIST. On the other hand, if the $E_p-L_p$ correlation holds, we have the possibility to detect Pop III GRBs up to $z sim 19$ as long duration X-ray flashes by Lobster.
