I7: Gamma-Ray Bursts: a Unique Laboratory for Modern Astrophysics
Gamma-Ray Bursts (GRBs) are transient gamma-ray flashes lasting from a fraction of a second to several tens of seconds. They appear unpredictably at random directions on the sky, and their elusive nature made them hardly detectable at other wavelengths for tens of years after their discovery in the late ‘60s of the last century. It was only in the late ‘90s that longer wavelength counterparts to GRBs could be detected for the first time: in X-rays, thanks to the BeppoSAX satellite, and subsequently in the optical and radio wavelengths. Thanks to the observations of these long lasting counterparts, the so-called afterglows, GRBs have been associated to cosmological sources (0.1<z<~9), and in few cases GRBs of the long category (i.e. lasting for more than 2 s) could be securely linked to Supernovae of type Ib/c (see e.g. Zhang B.-B. et al., 2011, ApJ, 730, 141, for a recent review on GRBs).
Despite the recent progresses in GRB science, obtained in particular thanks to the Swift and Fermi satellites, there are still many open questions. One concerns the mechanisms that power these extreme explosions (in a handful of seconds the isotropic equivalent energy emitted by GRBs spans from 1050 to 1054 erg, making them the most luminous events in the Universe), which is still unclear after more than four decades since their discovery. In particular the content of the relativistic flow that produces the GRBs, remains to be investigated: especially in terms of its bulk Lorentz factor, its magnetization, its baryon loading and their consequences on the possibility of GRBs being the sources of Ultra High Energy Cosmic Rays (UHECRs).
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 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.
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal (Abbott et al., Phys. Rev. Lett. 116, 061102). The signal matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a significance greater than 5.1σ. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger. This event is designated as GW150914. No electromagnetic counterpart was expected in the case of the merging of stellar masses black hole. Nevertheless, the increased sensitivity of the new generation of gravitational wave detector, let us hope for the coincident detection of a gravitation wave and the signal of a short GRB, this would provide “smoking gun” evidence for the binary nature of these GRB progenitor.
Space based Variable astronomical Object Monitor. It is a sino-french satellite to study Gamma-Ray Bursts, to launched in 2021. It’s been developed in France by CNES in close collaboration with CEA Irfu/SAp (AIM), APC, IRAP, LAM. French will provide a wide field coded mask camera (ECLAIRs, 4-150 keV), and a narrow field focussing X-ray telescope (MXT, 0.2-10 keV).
Cherenkov Telescope Array. It’s the next generation ground based next generation ground-based very high energy gamma-ray instrument (E> 10 GeV). It will serve as an open observatory to a wide astrophysics community and will provide a deep insight into the non-thermal high-energy universe.
The ANTARES Collaboration ha built a large area water Cherenkov detector in the deep Mediterranean Sea, optimised for the detection of muons from high-energy astrophysical neutrinos.
TThe VIRGO and LIGO are gravitational waves detectors. In their advanced version, expected to be operational in 2017, those detectors are expected to be able to localize neutrons star mergers up to redshift of 0.1 that are the best candidates for short GRB progenitors. A picture of VIRGO is shown below.
The Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling at the speed of light and each with energies beyond 1018 eV. The Layout of the Pierre Auger Observatory is shown below. Blue radial lines: view sectors of fluorescence detector (FD, 4×6=24) Black dot: Cherenkov ground station (Surface detectors, 1600) Red points: sites with specialized equipment (lasers, etc…).
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