Two types of non-photonic signals that may be expected from GRBs are gravitational waves (GWs) and high energy neutrinos (HENUs). The most likely GW emitters are short GRBs [97], if these indeed arise from merging compact objects [12]. The Swift and Fermi localization of a short GRB would help to narrow the search window for gravitational waves from that object [98]. The detection of gravitational waves from a well-localized GRB would lead to a great scientific payoff for understanding the merger physics, the progenitor types, and the neutrons star equations of state. The rates of compact merger GW events in the advanced LIGO and VIRGO detectors may be at least several per year [99]. However, even if these events all give rise to gamma ray bursts, only a small fraction would be beamed towards us. Long GRBs, more speculatively, might be detectable in GWs if they go through a magnetar phase [100], or if the core collapse breaks up into substantial blobs [101]; more detailed numerical calculations of collapsar (long) GRBs lead to GW prospects which range from pessimistic [102] to modest [103].
High energy neutrinos may also be expected from baryon-loaded GRBs, if
sufficient protons are co-accelerated in the shocks. The most widely
considered paradigm involves proton acceleration and
p
interactions in internal shocks, resulting in prompt ~ 100 TeV HENUs
[104,
105].
Other interaction
regions considered are external shocks, with
p
interactions on reverse shock UV photons leading to EeV HENUs
[106];
and pre-emerging or choked jets in collapsars resulting in HENU precursors
[107].
An EeV neutrino flux is also expected from external shocks in very
massive Pop. III magnetically dominated GRBs
[108].
Current IceCube observations
[109]
are putting significant constraints on the simplest internal
shock neutrino emission model. More careful modeling of internal shocks
[110]
reveal that several years of observations will be needed for reliably
testing such models, while other types of models, such as photospheric
models
[111]
or modified internal shock models
[112]
are yet to be tested. However, the excitement in this field is palpable,
especially since the announcement of the detection by IceCube of
PeV neutrinos
[113]
whose origin is almost certainly astrophysical.
Acknowledgments: We are grateful to NASA NNX 13AH50G and the Royal Society (U.K.) for partial support.