FIGARONET in short

The fast transient Universe has become a major and mature field of study. It includes the most powerful explosive sources that have been discovered, Gamma-Ray Bursts. This phenomenon encompasses a variety of sources that are, or at least are suspected to be, linked with violent astronomical phenomena such as core-collapse supernovae or binary neutron star mergers. While the later are among the best candidates for the first direct detection of gravitational waves, the former may well produce high energy neutrinos that can be detectable with the next generation of detectors. GRBs are also extreme panchromatic cosmic sources, in the sense that they have been detected from radio decimetric wavelength to several tens of GeV. To add to the complexity, it is not well understood whether during their formation GRBs host a transient magnetar, i.e. a highly magnetized, rapidly spinning neutron star.

The general purpose of FIGARONET is to be an international network of astronomers and physicists to collaborate on the best strategy for the follow-up of these sources at all wavelengths, to optimize the coordinated studies of the wealth of data that are acquired, and to determine the objectives and design the instrumentation that will be needed to complement upcoming GRB missions and GW experiments.

GRBs are associated with the sudden release of about 1050 - 1054 ergs (equivalent isotropic energy). Though they emit at all wavelengths, their energy spectrum peak lies between few tens of keV and few MeV. The prompt phase lasts from several milliseconds to minutes, and, in rare cases, hours; it is followed by an afterglow, that can be observed for days to months, and consists of several phases of monotonic decay (proportional to t-1 - t-2) with the presence of a plateau, break(s), flares, ending by a steeper decay (Nousek et al., 2008, Klotz et al., 2009).

In the current standard scenario GRBs are explained by a powerful explosion that results in the launch of powerful jets with bulk Lorentz factors of the order of few hundreds to thousand; these jets will interact with the interstellar medium or the wind left previously by the progenitor. This will result in highly relativistic shocks, producing the afterglow from synchrotron and inverse Compton emission from electrons stochastically accelerated. The prompt emission is the result of both the internal, mildly relativistic shocks that occur inside the jet when shells traveling at different speeds interact each other, or from the reverse shock propagating backward.

The progenitor source can be of several types:

  • In the case of short-hard GRBs (sGRBs), the coalescence of a binary system of neutron stars, or eventually neutron star - black hole, is favored. These sources are strong emitters of gravitational waves (GW), particularly during the last stages of the merger event, and they might be detected by the Advanced Virgo / Advanced LIGO (AdV/ALIGO) network of detectors. This hypothesis on the origin of sGRBs has been recently reinforced with the observation of a kilonova associated with GRB 130603B (Tanvir et al., 2013, Berger et al., 2013), a feature that was predicted.
  • Long GRBs (lGRBs) are believed to be the result of the death of a massive star, which produces the collapse of the stellar core and the launch of the jets. These sources are seen at larger distances than sGRBs, up to z = 9.4 and are linked to SN Ib,c IIn. Pop III stars in the early Universe might also be progenitors of lGRBs, giving rise to the reonization of the Universe.
Both types of GRBs are predicted to be strong emitters of high-energy neutrinos, and can be detected by the future KM3NET detector. They can be also the sites of production of ultra-high energy cosmic rays (up to 128 GeV photons for GRB 130427A as detected by Fermi-LAT), that can be observed by Fermi-LAT or Cerenkov telescopes.

Until now GRBs are detected through their high-energy prompt radiation. While the detection by space-based instruments remains crucial, many of the progress in the understanding of GRBs have been made thanks to accompanying observations either in space (the discovery of afterglows in X-ray for instance), on ground (association with SNs, reverse shock...) or both (kilonova observations). Therefore it is of paramount importance to have an efficient network for the follow up of GRBs, that covers the new rising astronomy of non-photonic messengers. This is the exact purpose of FIGARONET.