514 research outputs found

    Diffuse PeV neutrinos from gamma-ray bursts

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    The IceCube collaboration recently reported the potential detection of two cascade neutrino events in the energy range 1-10 PeV. We study the possibility that these PeV neutrinos are produced by gamma-ray bursts (GRBs), paying special attention to the contribution by untriggered GRBs that elude detection due to their low photon flux. Based on the luminosity function, rate distribution with redshift and spectral properties of GRBs, we generate, using Monte-Carlo simulation, a GRB sample that reproduce the observed fluence distribution of Fermi/GBM GRBs and an accompanying sample of untriggered GRBs simultaneously. The neutrino flux of every individual GRBs is calculated in the standard internal shock scenario, so that the accumulative flux of the whole samples can be obtained. We find that the neutrino flux in PeV energies produced by untriggered GRBs is about 2 times higher than that produced by the triggered ones. Considering the existing IceCube limit on the neutrino flux of triggered GRBs, we find that the total flux of triggered and untriggered GRBs can reach at most a level of ~10^-9 GeV cm^-2 s^-1 sr^-1, which is insufficient to account for the reported two PeV neutrinos. Possible contributions to diffuse neutrinos by low-luminosity GRBs and the earliest population of GRBs are also discussed.Comment: Accepted by ApJ, one more figure added to show the contribution to the diffuse neutrino flux by untriggered GRBs with different luminosity, results and conclusions unchange

    On the origin of >10 GeV photons in gamma-ray burst afterglows

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    Fermi/LAT has detected long-lasting high-energy photons (>100 MeV) from gamma-ray bursts (GRBs), with the highest energy photons reaching about 100 GeV. One proposed scenario is that they are produced by high-energy electrons accelerated in GRB forward shocks via synchrotron radiation. We study the maximum synchrotron photon energy in this scenario, considering the properties of the microturbluence magnetic fields behind the shock, as revealed by recent Particle-in-Cell simulations and theoretical analyses of relativistic collisionless shocks. Due to the small-scale nature of the micro-turbulent magnetic field, the Bohm acceleration approximation breaks down at such high energies. This effect leads to a typical maximum synchrotron photon of a few GeV at 100 s after the burst and this maximum synchrotron photon energy decreases quickly with time. We show that the fast decrease of the maximum synchrotron photon energy leads to a fast decay of the synchrotron flux. The 10-100 GeV photons detected after the prompt phase can not be produced by the synchrotron mechanism. They could originate from the synchrotron self-Compton emission of the early afterglow if the circum-burst density is sufficiently large, or from the external inverse-Compton process in the presence of central X-ray emission, such as X-ray flares and prompt high-latitude X-ray emission.Comment: 13 pages, 3 figures, accepted by ApJ Letter

    Understanding the multiwavelength observation of Geminga's TeV halo: the role of anisotropic diffusion of particles

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    In this letter we propose that the X-ray and the TeV observations in the vicinity of Geminga can be understood in the framework of anisotropic diffusion of injected electrons/positrons. This interpretation only requires the turbulence in the vicinity of Geminga to be sub-Alfv\'enic with the local mean magnetic field direction approximately aligned with our line of sight towards Geminga, without invoking extreme conditions for the environment, such as an extremely small diffusion coefficient and a weak magnetic field of strength <1μ<1\muG as suggested in previous literature.Comment: 7 pages, 4 figures, including Supplemental Material, PRL accepte

    How far are the sources of IceCube neutrinos? Constraints from the diffuse TeV gamma-ray background

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    The nearly isotropic distribution of the TeV-PeV neutrinos recently detected by IceCube suggests that they come from sources at distance beyond our Galaxy, but how far they are is largely unknown due to lack of any associations with known sources. In this paper, we propose that the cumulative TeV gamma-ray emission accompanying the production of neutrinos can be used to constrain the distance of these neutrino sources, since the opacity of TeV gamma rays due to absorption by the extragalactic background light (EBL) depends on the distance that these TeV gamma rays have travelled. As the diffuse extragalactic TeV background measured by \emph{Fermi} is much weaker than the expected cumulative flux associated with IceCube neutrinos, the majority of IceCube neutrinos, if their sources are transparent to TeV gamma rays, must come from distances larger than the horizon of TeV gamma rays. We find that above 80\% of the IceCube neutrinos should come from sources at redshift z>0.5z>0.5. Thus, the chance for finding nearby sources correlated with IceCube neutrinos would be small. We also find that, to explain the flux of neutrinos under the TeV gamma-ray emission constraint, the redshift evolution of neutrino source density must be at least as fast as the the cosmic star-formation rate.Comment: Accepted by ApJ, some minor changes made, 8 pages, 5 figure

    Interpretation of the unprecedentedly long-lived high-energy emission of GRB 130427A

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    High energy photons (>100 MeV) are detected by the Fermi/LAT from GRB 130427A up to almost one day after the burst, with an extra hard spectral component being discovered in the high-energy afterglow. We show that this hard spectral component arises from afterglow synchrotron-self Compton emission. This scenario can explain the origin of >10 GeV photons detected up to ~30000s after the burst, which would be difficult to be explained by synchrotron radiation due to the limited maximum synchrotron photon energy. The lower energy multi-wavelength afterglow data can be fitted simultaneously by the afterglow synchrotron emission. The implication of detecting the SSC emission for the circumburst environment is discussed.Comment: 4 pages, 2 figures, ApJL in pres

    Evidence of an Internal Dissipation Origin for the High-energy Prompt Emission of GRB 170214A

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    The origin of the prompt high-energy (>100>100MeV) emission of Gamma-ray Bursts (GRBs), detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, is still under debate, for which both the external shock origin and internal dissipation origin have been suggested. In the internal dissipation scenario, the high energy emission is expected to exhibit significant temporal variability, tracking the keV/MeV fast variable behavior. Here, we report a detailed analysis on the Fermi data of GRB~170214A, which is sufficiently bright in the high energy to enable a quantitative analysis of the correlation between high-energy emission and keV/MeV emission with high statistics. Our result shows a clear temporal correlation between high-energy and keV/MeV emission in the whole prompt emission phase as well as in two decomposed short time intervals. Such correlation behavior is also found in some other bright LAT GRBs, i.e., GRB 080916C, 090902B and 090926A. For these GRBs as well as GRB 090510, we also find the rapid temporal variability in the high-energy emission. We thus conclude that the prompt high-energy emission in these bright LAT GRBs should be due to internal origin.Comment: 12 pages, 4 figures, Accepted for publication in Ap

    Particle Acceleration in Mildly Relativistic Shearing Flows: the Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets

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    The origin of the extended X-ray emission in the large-scale jets of active galactic nuclei (AGNs) poses challenges to conventional models of acceleration and emission. Although the electron synchrotron radiation is considered the most feasible radiation mechanism, the formation of the continuous large-scale X-ray structure remains an open issue. As astrophysical jets are expected to exhibit some turbulence and shearing motion, we here investigate the potential of shearing flows to facilitate an extended acceleration of particles and evaluate its impact on the resultant particle distribution. Our treatment incorporates systematic shear and stochastic second-order Fermi effects. We show that for typical parameters applicable to large-scale AGN jets, stochastic second-order Fermi acceleration, which always accompanies shear particle acceleration, can play an important role in facilitating the whole process of particle energization. We study the time-dependent evolution of the resultant particle distribution in the presence of second-order Fermi acceleration, shear acceleration, and synchrotron losses using a simple Fokker--Planck approach and provide illustrations for the possible emergence of a complex (multicomponent) particle energy distribution with different spectral branches. We present examples for typical parameters applicable to large-scale AGN jets, indicating the relevance of the underlying processes for understanding the extended X-ray emission and the origin of ultrahigh-energy cosmic rays.Comment: 26 pages, 8 figures; to appear in Ap
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