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Register a new userUHE neutrino and cosmic ray emission from GRBs: revising the models and clarifying the cosmic ray-neutrino connection
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Added by
Mauricio Bustamante
Publication date
2014
fields
Physics
and research's language is
English
Authors
Mauricio Bustamante
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Gamma-ray bursts (GRBs) have long been held as one of the most promising sources of ultra-high energy (UHE) neutrinos. The internal shock model of GRB emission posits the joint production of UHE cosmic ray (UHECRs, above 10^8 GeV), photons, and neutrinos, through photohadronic interactions between source photons and magnetically-confined energetic protons, that occur when relativistically-expanding matter shells loaded with baryons collide with one another. While neutrino observations by IceCube have now ruled out the simplest version of the internal shock model, we show that a revised calculation of the emission, together with the consideration of the full photohadronic cross section and other particle physics effects, results in a prediction of the prompt GRB neutrino flux that still lies one order of magnitude below the current upper bounds, as recently exemplified by the results from ANTARES. In addition, we show that by allowing protons to directly escape their magnetic confinement without interacting at the source, we are able to partially decouple the cosmic ray and prompt neutrino emission, which grants the freedom to fit the UHECR observations while respecting the neutrino upper bounds. Finally, we briefly present advances towards pinning down the precise relation between UHECRs and UHE neutrinos, including the baryonic loading required to fit UHECR observations, and we will assess the role that very large volume neutrino telescopes play in this.
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Gamma-ray bursts are short-lived, luminous explosions at cosmological distances, thought to originate from relativistic jets launched at the deaths of massive stars. They are among the prime candidates to produce the observed cosmic rays at the highest energies. Recent neutrino data have, however, started to constrain this possibility in the simplest models with only one emission zone. In the classical theory of gamma-ray bursts, it is expected that particles are accelerated at mildly relativistic shocks generated by the collisions of material ejected from a central engine. We consider neutrino and cosmic-ray emission from multiple emission regions since these internal collisions must occur at very different radii, from below the photosphere all the way out to the circumburst medium, as a consequence of the efficient dissipation of kinetic energy. We demonstrate that the different messengers originate from different collision radii, which means that multi-messenger observations open windows for revealing the evolving GRB outflows.
We perform GRMHD and RMHD simulations of weakly and highly magnetized gamma-ray burst (GRB) jets propagating in binary neutron star (BNS) merger ejecta. Using the simulations, we first find that mixing between the jet and cocoon, which is present in all types of jets, inhibits the formation of subphotospheric collisionless shocks. However, we show that a mild magnetization may lead to the formation of collisionless subshocks which allow efficient proton acceleration. We consider shear acceleration and diffusive shock acceleration at collimation shocks, internal shocks, shock breakout, and external shocks, to provide the first self-consistent estimate for neutrino and cosmic-ray (CR) signals from GRBs in BNS mergers. We find that short GRBs do not produce detectable neutrino signals with current-day facilities. Shock breakout yields $ sim 10 $ PeV neutrinos at viewing angles $sim20^circ $, independent of the jet magnetization. However, a neutrino signal from shock breakout is well below detection limits of current detectors. Such a signal would allow a coincident neutrino-$gamma-$ray detection, providing a testable prediction for shock breakout as a neutrino production site. Using the numerical modeling that fits GW170817 afterglow emission, we find that blast waves in BNS mergers can account for 5%-10% of the Galactic CR luminosity in the PeV-EeV energy range. Based on these estimates, the observed level of CR anisotropy places a constraint on the distance of the latest Galactic binary neutron star merger to $lesssim3$ kiloparsecs.
The Tibet ASgamma experiment just reported their measurement of sub-PeV diffuse gamma ray emission from the Galactic disk, with the highest energy up to 957 TeV. These gamma-rays are most likely the hadronic origin by cosmic ray interaction with interstellar gas in the Galaxy. This measurement provides direct evidence to the hypothesis that the Galactic cosmic rays can be accelerated beyond PeV energies. In this work, we try to explain the sub-PeV diffuse gamma-ray spectrum within cosmic rays diffusive propagation model. We find there is a tension between the sub-PeV diffuse gamma rays and the local cosmic ray spectrum. To describe the sub-PeV diffuse gamma-ray flux, it generally requires larger local cosmic-ray flux than measurement in the knee region. We further calculate the PeV neutrino flux from the cosmic ray propagation model. Even all of these sub-PeV diffuse gamma rays originate from the propagation, the Galactic neutrinos only account for less than ~15% of observed flux, most of which are still from extragalactic sources.
The first detection of high-energy astrophysical neutrinos by IceCube provides new opportunities for tests of neutrino properties. The long baseline through the Cosmic Neutrino Background (C$ u$B) is particularly useful for directly testing secret neutrino interactions ($ u$SI) that would cause neutrino-neutrino elastic scattering at a larger rate than the usual weak interactions. We show that IceCube can provide competitive sensitivity to $ u$SI compared to other astrophysical and cosmological probes, which are complementary to laboratory tests. We study the spectral distortions caused by $ u$SI with a large s-channel contribution, which can lead to a dip, bump, or cutoff on an initially smooth spectrum. Consequently, $ u$SI may be an exotic solution for features seen in the IceCube energy spectrum. More conservatively, IceCube neutrino data could be used to set model-independent limits on $ u$SI. Our phenomenological estimates provide guidance for more detailed calculations, comparisons to data, and model building.
We obtain the maximum diffuse neutrino intensity predicted by hadronic photoproduction models of the type which have been applied to the jets of active galactic nuclei or gamma ray bursts. For this, we compare the proton and gamma ray fluxes associated with hadronic photoproduction in extragalactic neutrino sources with the present experimental upper limit on cosmic ray protons and the extragalactic gamma ray background, employing a transport calculation of energetic protons traversing cosmic photon backgrounds. We take into account the effects of the photon spectral shape in the sources on the photoproduction process, cosmological source evolution, the optical depth for cosmic ray ejection, and discuss the possible effects of magnetic fields in the vicinity of the sources. For photohadronic neutrino sources which are optically thin to the emission of neutrons we find that the cosmic ray flux imposes a stronger bound than the extragalactic gamma ray background in the energy range between 10^5 GeV and 10^11 GeV, as previously noted by Waxman & Bahcall (1999). We also determine the maximum contribution from the jets of active galactic nuclei, using constraints set to their neutron opacity by gamma-ray observations. This present upper limit is consistent with the jets of active galactic nuclei producing the extragalactic gamma ray background hadronically, but we point out future observations in the GeV-to-TeV regime could lower this limit. We also briefly discuss the contribution of gamma ray bursts to ultra-high energy cosmic rays as it can be inferred from possible observations or limits on their correlated neutrino fluxes.
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