**1. Overview of the Mission**

SVOM (Space-based multiband astronomical Variable Objects Monitor) is a sinofrench mission that is dedicated to Gamma-Ray Burst (GRB) science and, more generally, to the discovery and multi-wavelength follow-up of transient sources. It is expected to be launched in mid 2023 for a nominal duration of three years, plus an extended duration of two additional years. The mission includes four space-based and three ground-based instruments (Figure 1) that, working together, will provide multi-wavelength follow-up of the targets in order to obtain a complete coverage of the GRB emission over seven decades in energy, from the trigger up to the very late phases of the afterglow (see Figure 2). In particular, SVOM will be able to trigger and locate GRBs, and distribute alerts and accurate localization in real time. In addition to that, SVOM will adopt an optimal pointing strategy for ground-based follow-up, fostering synergies with other space and groundbased facilities and allowing for a large fraction of GRBs detected and observed with redshift measurements.

SVOM is the result of a collaboration between the Chinese and French national space agencies, CNSA (China National Space Administration) and CNES (Centre national d'études spatiales), with the main contributions from the Institute of Research into the Fundamental Laws of the Universe (Irfu) and the Research Institute of Astrophysics and Planetology (IRAP) for France and the National Astronomical Observatory (NAOC) and the Beijing High Energy Institute (IHEP) for China.

In the following, we describe the instruments and the observational strategy (Section 2) and give an overview of the main science cases that can be addressed with SVOM (Section 3). Finally, we outline the crucial role that SVOM will play thanks to its characteristics and its synergies with the large ground-based facilities that will be operational in the next decade (Section 4).

**Citation:** Bernardini, M.G.; Cordier, B.; Wei, J.; on behalf of the SVOM Collaboration. The SVOM Mission. *Galaxies* **2021**, *9*, 113. https:// doi.org/10.3390/galaxies9040113

Academic Editors: Elena Moretti and Francesco Longo

Received: 16 November 2021 Accepted: 1 December 2021 Published: 4 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** The SVOM (Space-based multiband astronomical Variable Objects Monitor) mission: The satellite with the four instruments on board (ECLAIRs, GRM, MXT, and VT), and the three instruments on ground (GWAC and the two GFTs).

**Figure 2.** The ranges of energy and the typical timescales covered by the different instruments of the SVOM mission in space (**upper panel**) and on ground (**lower panel**).

#### **2. Instruments and Observational Strategy**

The SVOM satellite will be launched by the Chinese Long March 2C rocket from the Xichang launch base, and will be put into an orbit with a 30-degree inclination, an altitude of 625 km and an orbital period of 96 min. It will be equipped with four instruments, two French and two Chinese:

• The telecope ECLAIRs (France), a coded-mask instrument with a field of view of 2 sr, capable of triggering and locating GRBs with a precision of less than 12' in the 4–120 keV energy band;


These instruments will be supported by three ground-based facilities:


When a GRB is triggered within the field of view of ECLAIRs, the satellite automatically slews within a few minutes to repoint the two narrow-field instruments on board, MXT and VT, to provide fast detection and accurate localization of the possible X-ray and optical counterparts. The alert messages and the preliminary information collected are transmitted to the ground in about 10 s at Very High Frequency (VHF) in a frequency band between 137 and 138 MHz by an on-board antenna. The messages are then downlinked through a network of radio stations that, to ensure permanent contact with the satellite, will consist of ∼45 VHF antennae homogeneously deployed in the inter-tropical zone around the Earth, between latitudes −30◦ and +30◦ . They will relay every alert message from the satellite to the French Science Center (FSC) located at Saclay in France. After a first analysis at the FSC, the messages will be distributed to the whole scientific community through the GCN alert network or other networks available, in particular to the GFTs robotic telescopes that will refine the GRB position and give an initial indication of distance.

The rapid follow-up of SVOM-detected GRBs with ground-based optical telescopes in order to have a redshift measurement for a large fraction of them (∼2/3) is one of the scientific objectives of the mission. In order to avoid targets that are too close to the Sun and are thus not accessible by ground telescopes during their nighttime, an optimal pointing strategy will be adopted such that the optical axis of the instruments on board the satellite point in the direction opposite to the Sun. This choice implies that the Earth will hide the field of view of the instruments once per orbit, up to 50% of the period of revolution.
