Particle Physics Division

The main objectives of the French-Chinese satellite SVOM (for Space-based Variable Objects Monitor) are studies of gamma-ray bursts, which are fascinating astrophysical events because of their unpredictable occurrence on the sky and the tremendous amount of energy they release in the form of gamma-rays, produced by the explosion of some types of very massive stars or by the coalescence of neutron stars. These events are revealed in gamma rays by the detection of a point source lasting a fraction of a second to a few tens of seconds, rarely a few minutes. The observation of the source at longer wavelengths (in X-rays, in the visible and even in radio waves) makes it possible to study the afterglow of the event during several hours and to estimate its characteristics, in particular its distance. In most cases, the event occurred in the very distant and therefore young universe.

To detect gamma-ray bursts on board SVOM, the French ECLAIRs telescope will continuously observe one-sixth of the sky (2 steradians) in the soft gamma-ray range (from 4 to 150 keV) with its telescope equipped with a coded mask mounted above a detector plane composed of 6400 cadmium telluride pixels. The detected photons will be processed in the UGTS (Unité de Gestion et de Traitements Scientifiques), the instrument's on-board computer, by the flight software developed at CEA.

This software will analyse the data in real time to detect a gamma-ray burst and localize it to better than 12 arcmin on the sky, i.e. on an area of the sky smaller than the Moon's apparent disc. It will then automatically re-point the satellite to this position, such that the afterglow of the burst can be quickly observed with the small field-of-view instruments on board: the French MXT telescope (in X-rays, whose focal plane was produced at the CEA) and the Chinese VT telescope (in the visible). The software will also produce a sequence of alerts sent by the satellite to the French VHF receiver network deployed around the world. These alerts will be received and processed at the French Science Centre (FSC) under the responsibility of the CEA, to alert the worldwide community of transient sky observatories about the occurrence of an interesting event.

Version 7.0.3 of the flight software, delivered in February 2023, is the culmination of several successive deliveries to CNES over the last two years, each one equipped with increased functionality. These versions were tested using a test bench designed at the CEA around a representative model of the UGTS computer, then validated at the CEA by an automatic test management system. They were used successively at CNES during test campaigns and for the calibration of the instrument in a vacuum chamber, using radioactive sources and an X-ray generator. This software contains all the essential functions for the management of the instrument, from the command and control of the detector to the acquisition of the photon by photon data for their delayed transmission to the ground, to the real-time detection of gamma-ray bursts and alert message generation by the scientific software.

The software is coupled to a programmable digital circuit of type FPGA, integrated in the UGTS, whose firmware was also produced at the CEA and delivered in 2021. This circuit alleviates the computational load of the processor, carrying out the real-time acquisition of the photons from the camera, the data encapsulation into packets and their sending to the satellite's mass-memory, as well as the data pre-processing for the scientific software.