17 research outputs found
Magnetic Field and Flavor Effects on the Gamma-Ray Burst Neutrino Flux
We reanalyze the prompt muon neutrino flux from gamma-ray bursts (GRBs), at
the example of the often used reference Waxman-Bahcall GRB flux, in terms of
the particle physics involved. We first reproduce this reference flux treating
synchrotron energy losses of the secondary pions explicitly. Then we include
additional neutrino production modes, the neutrinos from muon decays, the
magnetic field effects on all secondary species, and flavor mixing with the
current parameter uncertainties. We demonstrate that the combination of these
effects modifies the shape of the original Waxman-Bahcall GRB flux
significantly, and changes the normalization by a factor of three to four. As a
consequence, the gamma-ray burst search strategy of neutrino telescopes may be
based on the wrong flux shape, and the constraints derived for the GRB neutrino
flux, such as the baryonic loading, may in fact be already much stronger than
anticipated.Comment: 4 pages, 3 figures. Minor clarifications. Final version to appear in
Phys. Rev.
UHE neutrino and cosmic ray emission from GRBs: revising the models and clarifying the cosmic ray-neutrino connection
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.Comment: 4 pages, 2 figures. To be published in Proceedings of the 6th Very
Large Volume Neutrino Telescope Workshop (VLVnT13), Stockholm, Sweden, 5-7
August, 201
Are gamma-ray bursts the sources of ultra-high energy cosmic rays?
We reconsider the possibility that gamma-ray bursts (GRBs) are the sources of
the ultra-high energy cosmic rays (UHECRs) within the internal shock model,
assuming a pure proton composition of the UHECRs. For the first time, we
combine the information from gamma-rays, cosmic rays, prompt neutrinos, and
cosmogenic neutrinos quantitatively in a joint cosmic ray production and
propagation model, and we show that the information on the cosmic energy budget
can be obtained as a consequence. In addition to the neutron model, we consider
alternative scenarios for the cosmic ray escape from the GRBs, i.e., that
cosmic rays can leak from the sources. We find that the dip model, which
describes the ankle in UHECR observations by the pair production dip, is
strongly disfavored in combination with the internal shock model because a)
unrealistically high baryonic loadings (energy in protons versus energy in
electrons/gamma-rays) are needed for the individual GRBs and b) the prompt
neutrino flux easily overshoots the corresponding neutrino bound. On the other
hand, GRBs may account for the UHECRs in the ankle transition model if cosmic
rays leak out from the source at the highest energies. In that case, we
demonstrate that future neutrino observations can efficiently test most of the
parameter space -- unless the baryonic loading is much larger than previously
anticipated.Comment: 55 pages, 23 figures, 1 table. Version accepted for publication in
Astroparticle Physics. Main analysis performed with TA data; for plots with
HiRes data, see v
Neutrino and cosmic-ray emission from multiple internal shocks in gamma-ray bursts
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.Comment: 12 pages, 7 figures. Matches published versio
Neutrino Decays over Cosmological Distances and the Implications for Neutrino Telescopes
We discuss decays of ultra-relativistic neutrinos over cosmological distances
by solving the decay equation in terms of its redshift dependence. We
demonstrate that there are significant conceptual differences compared to more
simplified treatments of neutrino decay. For instance, the maximum distance the
neutrinos have traveled is limited by the Hubble length, which means that the
common belief that longer neutrino lifetimes can be probed by longer distances
does not apply. As a consequence, the neutrino lifetime limit from supernova
1987A cannot be exceeded by high-energy astrophysical neutrinos. We discuss the
implications for neutrino spectra and flavor ratios from gamma-ray bursts as
one example of extragalactic sources, using up-to-date neutrino flux
predictions. If the observation of SN 1987A implies that \nu_1 is stable and
the other mass eigenstates decay with rates much smaller than their current
bounds, the muon track rate can be substantially suppressed compared to the
cascade rate in the region IceCube is most sensitive to. In this scenario, no
gamma-ray burst neutrinos may be found using muon tracks even with the full
scale experiment, whereas reliable information on high-energy astrophysical
sources can only be obtained from cascade measurements. As another consequence,
the recently observed two cascade event candidates at PeV energies will not be
accompanied by corresponding muon tracks.Comment: 20 pages, 6 figures, 1 table. Matches published versio
Systematics in the Interpretation of Aggregated Neutrino Flux Limits and Flavor Ratios from Gamma-Ray Bursts
Gamma-ray burst analyses at neutrino telescopes are typically based on
diffuse or stacked (i.e., aggregated) neutrino fluxes, because the number of
events expected from a single burst is small. The interpretation of aggregated
flux limits implies new systematics not present for a single burst, such as by
the integration over parameter distributions (diffuse fluxes), or by the low
statistics in small burst samples (stacked fluxes). We simulate parameter
distributions with a Monte Carlo method computing the spectra burst by burst,
as compared to a conventional Monte Carlo integration. With this approach, we
can predict the behavior of the flux in the diffuse limit as well as in low
statistics stacking samples, such as used in recent IceCube data analyses. We
also include the flavor composition at the detector (ratio between muon tracks
and cascades) into our considerations. We demonstrate that the spectral
features, such as a characteristic multi-peak structure coming from
photohadronic interactions, flavor mixing, and magnetic field effects, are
typically present even in diffuse neutrino fluxes if only the redshift
distribution of the sources is considered, with z \simeq 1 dominating the
neutrino flux. On the other hand, we show that variations of the Lorentz boost
can only be interpreted in a model-dependent way, and can be used as a model
discriminator. For example, we illustrate that the observation of spectral
features in aggregated fluxes will disfavor the commonly used assumption that
bursts with small Lorentz factors dominate the neutrino flux, whereas it will
be consistent with the hypothesis that the bursts have similar properties in
the comoving frame.Comment: 46 pages, 21 figures, 2 tables. Minor corrections in Sec. 3.3 (Gamma
dependence of model FB-D). Fixed some typo