E6: Gamma-Ray Bursts as Multi-messengers

In addition, while there is a consensus on progenitors of long GRBs, as being very massive stars, the situation is less clear for what concerns the short GRBs. The most popular models involve the possibility of a coalescence of two compact objects (black holes or neutron stars), but a direct proof of this model is still lacking.

The high-energy part of the GRB spectrum

The observation by the Fermi/LAT of several GRB photons with energies reaching 50-100 GeV in the source frame is an encouraging sign for GRB detections with the next generation of instruments operating in the GeV and TeV domain. The extrapolation of Fermi/LAT spectral measurements to the VHE domain is difficult due to the limited knowledge of GRB properties at these energies (see Gehrels & Razzaque Frontiers of Physics 2013). In particular, it remains unclear whether the hard component observed in the spectra of several bright Fermi GRBs is a common property at GeV energies. In addition, intrinsic spectral cut-offs similar to the case of GRB 090926A are expected at 1-100 GeV energies, and are strongly related to the value of the GRB-jet Lorentz factor. For these reasons, current estimates of the GRB detection rate by future TeV experiments, like CTA, suffer from important uncertainties and range from ~0.5 to ~2 GRBs per year (Inoue et al., Astropart. Phys. 43, 252, 2013). Nonetheless, during the lifetime of SVOM, a few but invaluable GRB detections are thus expected in the 0.05-0.5 TeV range from CTA, especially in the light of the recent bright GeV afterglow detected for GRB 130427A (Tam et al., ApJ, 77, L13, 2013).

Joint time-resolved spectral analyses based on SVOM, and CTA data will also help to pinpoint the acceleration and emission processes occurring in GRB jets. Studies of the GRB prompt-emission phase from sub-MeV to sub-TeV energies can help to distinguish between leptonic and hadronic models, and to investigate the possibility of GRBs being a source of ultra-high-energy cosmic rays, and answer the long-standing question of the origin of the cosmic rays observed on Earth with energies larger than 10¹⁰GeV.

GRBs and the new messengers

To date neutrino and gravitational wave signals detected by current experiments could not be correlated with any precise astrophysical source. Thanks to their short duration and their high flux, GRBs are the best electromagnetic counterpart candidates, and a simultaneous detection would represent the start of a new era for those two research fields, namely the astronomy era.

Neutrinos: Neutrinos are unique messengers to study the high-energy Universe as they are neutral and stable, interact weakly and therefore travel directly from their point of creation to the Earth without absorption. Neutrinos could play an important role in understanding the mechanisms of cosmic-ray acceleration, and their detection from a cosmic source would be a direct evidence of the presence of hadronic acceleration. Indeed, HE neutrinos are produced in a beam-dump scenario via meson (mainly pion π) decay, when the accelerated hadrons (protons or nuclei) interact with ambient matter or dense photon fields. The emitted neutrinos typically have an energy which is about 10% of the energy of the interacting protons. The production of neutrinos of 10¹⁴ eV then necessitates the acceleration of protons up to 10¹⁵ eV and is therefore expected independently of the question to know if GRBs are the source of UHECRs. Depending on the details of the model considered, these high-energy neutrinos are emitted in coincidence with, or as a precursor signal to gamma-ray emission.

Gravitational Waves: A new generation of gravitational wave detectors, Advanced LIGO, Advanced Virgo and Kagra, are currently under construction and will reach their design sensitivity around 2018-2020. Thanks to the ten-fold improvement in sensitivity with respect to the previous generation, these detectors are likely to make the first direct detection of gravitational waves (GW). GRBs are believed to be an important source of GW. The most favored models for the central engine of the GRB are coalescence of two compact objects (neutrons stars, NS, or black holes, BH) for the short GRBs and massive star death or core-collapse supernovae for the long GRBs. Post-Newtonian theory predicts a distinctive GW chirp signal from the inspiral stage of NS-NS or NS-BH coalescences, so that the detection of such a signal associated with a short GRB would provide “smoking gun” evidence for the binary nature of the GRB progenitor. The merger and post-merger GW signal may reveal the mass and spin of the newly-formed object, whether and when a black hole is formed, and may also place constraints on the nuclear equation of state of dense matter (T. Hinderer, et al., Phys. Rev. D 81, 123016, 2010).