Aims. We calculate the theoretical event rate of gamma-ray bursts (GRBs) from the collapse of massive first-generation (Population III; Pop III) stars. The Pop III GRBs could be super-energetic with ...the isotropic energy up to Eiso ≳ 1055−57 erg, providing a unique probe of the high-redshift Universe. Methods. We consider both the so-called Pop III.1 stars (primordial) and Pop III.2 stars (primordial but affected by radiation from other stars). We employ a semi-analytical approach that considers inhomogeneous hydrogen reionization and chemical evolution of the intergalactic medium. Results. We show that Pop III.2 GRBs occur more than 100 times more frequently than Pop III.1 GRBs, and thus should be suitable targets for future GRB missions. Interestingly, our optimistic model predicts an event rate that is already constrained by the current radio transient searches. We expect ~10−104 radio afterglows above ~0.3 mJy on the sky with ~1 year variability and mostly without GRBs (orphans), which are detectable by ALMA, EVLA, LOFAR, and SKA, while we expect to observe maximum of N < 20 GRBs per year integrated over at z > 6 for Pop III.2 and N < 0.08 per year integrated over at z > 10 for Pop III.1 with EXIST, and N < 0.2 for Pop III.2 GRBs per year integrated over at z > 6 with Swift.
We estimate a gamma-ray burst (GRB) formation rate based on the new relation between the spectral peak energy (E sub(p)) and the peak luminosity. The new relation is derived by combining the data of ...E sub(p) and the peak luminosities by BeppoSAX and BATSE, and it looks considerably tighter and more reliable than the relations suggested by the previous works. Using the new E sub(p)-luminosity relation, we estimate redshifts of the 689 GRBs without known distances in the BATSE catalog and derive a GRB formation rate as a function of the redshift. For the redshift range of 0 < z < 2, the GRB formation rate increases and is well correlated with the star formation rate, while it keeps constant toward z approx 12. We also discuss the luminosity function and the redshift dependence of the intrinsic luminosity (luminosity evolution).
We present one possible mechanism for the giant flares of the soft gamma-ray repeaters (SGRs) within the framework of the magnetar (superstrongly magnetized neutron star) model, motivated by the ...positive period increase associated with the August 27 event from SGR 1900+14. From second-order perturbation analysis of the equilibrium of the magnetic polytrope, we find that there exist different equilibrium states separated by the energy of the giant flares and the shift in the moment of inertia to cause the period increase. This suggests that, if we assume that global reconfiguration of the internal magnetic field of suddenly occurs, the positive period increase as well as the energy ≳1044 erg of the giant flares may be explained. The moment of inertia can increase with a release of energy, because the star shape deformed by the magnetic field can be prolate rather than oblate. In this mechanism, since oscillation of the neutron star will be excited, a ∼ ms-period pulsation of the burst profile and an emission of gravitational waves are expected. The gravitational waves could be detected by planned interferometers such as LIGO, VIRGO and LCGT.
Context. Most X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase \propto t super(-1/2) in the first few hours. Aims. This is not predicted by the ...standard afterglow model and needs an explanation. Methods. We discuss that the shallow decay requires an unreasonably high gamma-ray efficiency, \ga75{- }90\%, within current models, which is difficult to produce by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs similar to 10 super(3)-10 super(6) s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, \epsilon_{\rm e} \propto t greater than or equal to The former explanation predicts a very long precursor, while both prefer dim optical flashes from the reverse shock, as was recently reported. We also calculate the multi-wavelength afterglows and compare them with observations. Results. No optical break at the end of the shallow X-ray decay indicates a preference for the time-dependent microphysics model with additionally decaying magnetic fields, \epsilon_B \propto t greater than or equal to
We analyze Swift gamma-ray bursts (GRBs) and X-ray afterglows for three GRBs with spectroscopic redshift determinations: GRB 050401, XRF 050416a, and GRB 050525a. We find that the relation between ...spectral peak energy and isotropic energy of prompt emissions (the Amati relation) is consistent with that for the bursts observed in the pie-Swift era. However, we find that the X-ray afterglow light curves, which extend up to 10--70 days, show no sign of the jet break that is expected in the standard framework of collimated outflows. We do so by showing that none of the X-ray afterglow light curves in our sample satisfy the relation between the spectral and temporal indices that is predicted for the phase after jet break. The jet break time can be predicted by inverting the tight empirical relation between the peak energy of the spectrum and the collimation-corrected energy of the prompt emission (the Ghirlanda relation). We find that there are no temporal breaks within the predicted time intervals in X-ray band. This requires either that the Ghirlanda relation has a larger scatter than previously thought, that the temporal break in X-rays is masked by some additional source of X-ray emission, or that it does not happen for some unknown reason.
The recently discovered gamma-ray burst GRB 060218/SN 2006aj is classified as an X-ray flash with very long duration driven possibly by a neutron star. Since GRB 060218 is very near, 6140 Mpc, and ...very dim, the 1 yr observation by Swift suggests that the rate of GRB 060218-like events might be very high so that such low-luminosity (LL) GRBs might form a different population from the cosmological high-luminosity (HL) GRBs. We found that the high-energy neutrino background from LL GRBs could be comparable with that from HL GRBs. If each neutrino event is detected by IceCube, later optical-infrared follow-up observations such as those by Subaru and HST can possibly identify a Type Ibc supernova associated with LL GRBs, even if gamma rays and X-rays are not observed by Swift, This is in a sense a new window from neutrino astronomy, which might enable us to confirm the existence of LL GRBs and to obtain information about their rate and origin. We also suggest that LL GRBs are high-energy gamma-ray and cosmic-ray sources.
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