The main objective of the SVOM mission is to detect and study gamma-ray bursts (GRBs). These powerful phenomena are attributed to massive star explosions (more than 20 times the mass of the Sun) and the merger of compact objects such as neutron stars or black holes. These extreme events lead to the birth of a black hole, which grows quickly by swallowing in a few seconds the matter in its immediate environment. This accretion is accompanied by the ejection of relativistic jets which, when directed towards the Earth, allow us to observe a GRB. The violent shocks that occur in the jets produce X- and gamma-ray streams, while the shock of the jets on the surrounding medium trigger an afterglow emission in X-ray, visible and radio waves. For a few seconds, the luminosity of a GRB in the visible domain can be a million times brighter than the brightest supernovae.
Guaranteeing multi-wavelength observation of about 80 GRBs of all kinds per year, the Chinese-French SVOM mission will thus make a unique contribution to the two most promising fields of research to emerge from recent advances: the role of GRBs in cosmology and understanding of the phenomenon itself.
Observation in the X-ray and gamma-ray domain is naturally essential because it is the only way to detect GRBs. But it only enables to observe part of this phenomenon, the prompt emission, which is the only emission source in this spectral domain.
To fully understand GRBs, it is necessary to extend spectral cover to the visible and near-infrared domains. Indeed, GRBs emit continuously in this spectral domain yielding essential information that is otherwise inaccessible, such as their distance and environment.
- To directly track the formation of stars in faraway galaxies to the extent that GRBs mark the final phase in the evolution of massive stars.
- To study the first generation of stars (population III stars) supposed to have formed particularly massive stars, potentially able to generate GRBs at the end of their short evolution.
- To enable the study of the entire medium in the foreground, including those of the host galaxy, to track the history of the re-ionization of the Universe and its enrichment in metals.
Fundamental physics objectives:
- To test some aspects of fundamental physics by bringing new constraints for a possible violation of the Lorentz invariance.
High-energy astrophysics objectives:
- To explore different populations of GRBs (such as "dark" bursts or X-ray flashes).
- To uncover the nature of short bursts.
- To study the nature of prompt emission.
- To study the relationship between prompt emission and the afterglow.
- To study the relationship between GRBs and supernova explosions.
- To explore the engine of GRBs, particularly a possible precursor.
- To study the physics of the relativistic ejections also at work in numerous astrophysics sites such as active galaxic nuclei (AGNs) or microquasars.
- To determine the nature of stars from which GRBs originate, to enable scientists to use GRBs to further their cosmology research.
To achieve the SVOM mission's science objectives, the envisioned measurement strategy combines space-based observation and associated ground facilities:
Onboard systems will:
- detect and locate GRBs and quickly transmit this information to the ground, which implies a wide-field instrument operating in the hard X-ray and low-energy gamma-ray band
- observe burst prompt emission in the high-energy band, which implies a wide-field instrument covering the wider spectral band from soft X-rays (~1 keV) to low-energy gamma-rays (a few MeV)
- and observe the early afterglow in the visible and near-infrared, which implies a narrow-field instrument
Ground facilities will:
- accurately measure burst celestial coordinates
- measure the photometric evolution of the afterglow in several spectral bands (from the visible to the near-infrared)
- and supply a photometric evaluation of its redshift
- Mission lifetime: minimum 3 years, possible extension by 2 years
- Orbit altitude: near 600 km, with an inclination of 30°