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Tracking cosmic explosions with a “lobster eye”

​Coupled to a novel multi-pore optic inspired by the vision of some crustaceans, the X-ray camera to be used in the MXT telescope of the Franco-Chinese satellite SVOM was designed and assembled by the Irfu and recently delivered to the Cnes. SVOM is scheduled for launch in late 2022.

Published on 3 June 2021

The Franco-Chinese SVOM (Space-based multiband astronomical Variable Objects Monitor) mission will study the most powerful explosions in the Universe as well as the fusions of dense stars via their gamma-ray emissions (gamma-ray bursts). It combines gamma-ray, X-ray and optical telescopes onboard a satellite with a ground segment, including a network of robotic optical and infra-red telescopes.

Gamma-ray bursts appear as a very brief emission of gamma rays. This short duration (lasting only a few seconds) requires astrophysicists to devise a complex strategy in order to observe them. The gamma radiation will first be detected and localized by ECLAIRs (X-rays and gamma rays) and GRM (Gamma Ray Burst Monitor) instruments, providing the first alert. The Microchannel X-ray Telescope (MXT) will then be used to supply more precise localizations required by the large ground-based telescopes.

The MXT is equipped with a revolutionary optic : this optical system is composed of multiple microscopic square channels (40 µm on a side) assembled in a mosaic of plates. Each plate has about 600,000 facets. This unique geometry, reminiscent of a lobster's eye, offers the advantage of a large field of view (one square degree), or 4 times the apparent surface of the Sun.

The pnCCD detector, composed of a monolithic block of silicon and divided into 256 x 256 pixels, is integrated on a ceramic plate and surrounded by a thick shield to reduce the impact of cosmic radiation. It is equipped with a system of three thermoelectric coolers that maintain its temperature between -75°C and -60°C. The entire unit is inserted in a housing that contains a filter wheel used for calibration and protection purposes. The different parts of the camera were designed by Irfu, manufactured by Irfu or its subcontractors, and assembled in clean rooms at Irfu to avoid any contamination.

After two test models, the team was able to deliver the flight model to the Cnes (French Space Agency), in spite of the current sanitary restrictions. The next steps will include mounting the camera in the telescope by the Cnes in Toulouse, and subsequently calibrating it in Munich in November 2021, in the presence of Irfu scientists. The MXT will then be sent to China for its final integration on the SVOM satellite.

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