The origin of the ultrahigh-energy (UHE) cosmic rays (CRs) from the second knee (∼ 6 × 10 17 eV) above in the CR spectrum is still unknown. Recently, there has been growing evidence that a peculiar type of supernovae, called hypernovae, are associated with sub-energetic gamma-ray bursts (GRBs), such as SN1998bw/GRB980425 and SN2003lw/GRB031203. Such hypernovae appear to have high (up to mildly relativistic) velocity ejecta, which may be linked to the sub-energetic GRBs. Assuming a continuous distribution of the kinetic energy of the hypernova ejecta as a function of its velocity E k ∝ (Γβ) −α with α ∼ 2, we find that 1) the external shock wave produced by the high velocity ejecta of a hypernova can accelerate protons up to energies as high as 10 19 eV; 2) the cosmological hypernova rate is sufficient to account for the energy flux above the second knee; and 3) the steeper spectrum of CRs at these energies can arise in these sources. In addition, hypernovae would also give rise to a faint diffuse UHE neutrino flux, due to pγ interactions of the UHE CRs with hypernova optical-UV photons. PACS numbers: 98.70.Sa, 97.60.Bw 98.70.Rz,
The recent detection of delayed X-ray flares during the afterglow phase of gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the deceleration radius characterizing the beginning of the forward shock afterglow emission. Given the observed temporal overlapping between the flares and afterglows, there must be inverse Compton (IC) emission arising from such flare photons scattered by forward shock afterglow electrons. We find that this IC emission produces GeV-TeV flares, which may be detected by GLAST and ground-based TeV telescopes. We speculate that this kind of emission may already have been detected by EGRET from a very strong burst-GRB940217. The enhanced cooling of the forward shock electrons by the X-ray flare photons may suppress the synchrotron emission of the afterglows during the flare period. The detection of GeV-TeV flares combined with low energy observations may help to constrain the poorly known magnetic field in afterglow shocks. We also consider the self-IC emission in the context of internal-shock and external-shock models for X-ray flares. The emission above GeV from internal shocks is low, while the external shock model can also produce GeV-TeV flares, but with a different temporal behavior from that caused by IC scattering of flare photons by afterglow electrons. This suggests a useful approach for distinguishing whether X-ray flares originate from late central engine activity or from external shocks. 5 Similar processes have been studied, such as the IC scattering between the reverse shock photons (electrons) and forward shock electrons (photons) (Wang et al. 2001), the IC scattering of prompt MeV photons of GRBs by reverse shock electrons (Beloborodov 2005) and by afterglow electrons(Fan et al. 2005).
The increasingly deep limit on the neutrino emission from gamma-ray bursts (GRBs) with IceCube observations has reached the level that could put useful constraints on the fireball properties. We first present a revised analytic calculation of the neutrino flux, which predicts a flux an order of magnitude lower than that obtained by the IceCube collaboration. For benchmark model parameters (e.g. the bulk Lorentz factor is Γ = 10 2.5 , the observed variability time for long GRBs is t ob v = 0.01s and the ratio between the energy in accelerated protons and in radiation is η p = 10 for every burst) in the standard internal shock scenario, the predicted neutrino flux from 215 bursts during the period of the 40-string and 59-string configurations is found to be a factor of ∼ 3 below the IceCube sensitivity. However, if we accept the recently found inherent relation between the bulk Lorentz factor and burst energy, the expected neutrino flux increases significantly and the spectral peak shifts to lower energy. In this case, the non-detection then implies that the baryon loading ratio should be η p 10 if the variability time of long GRBs is fixed to t ob v = 0.01s. Instead, if we relax the standard internal shock scenario but keep to assume η p = 10, the non-detection constrains the dissipation radius to be R 4 × 10 12 cm assuming the same dissipation radius for every burst and benchmark parameters for fireballs. We also calculate the diffuse neutrino flux from GRBs for different luminosity functions existing in the literature. The expected flux exceeds the current IceCube limit for some luminosity functions, and thus the non-detection constrains η p 10 in such cases when the variability time of long GRBs is fixed to t ob v = 0.01s.
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