The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study “missing” baryons in the universe. Unlike dark matter, baryonic matter is made of elements in the periodic table, and can be directly observed through the electromagnetic signals that it produces. Stars contain only a tiny fraction of the baryonic matter known to be present in the universe. Additional baryons are found to be in diffuse (gaseous) form, in or between galaxies, but a significant fraction has not yet been seen. The latter (“missing” baryons) are thought to be hiding in low-density warm-hot ionized medium (WHIM), based on results from theoretical studies and recent observations, and be distributed in the vicinity of galaxies (i.e., circumgalactic medium) and between galaxies (i.e., intergalactic medium). Such gas would radiate mainly in the soft X-ray band and the emission would be very weak, due to its very low density. HUBS is optimized to detect the X-ray emission from the hot baryons in the circumgalactic medium, and thus fill a void in observational astronomy. The goal is not only to detect the “missing” baryons, but to characterize their physical and chemical properties, as well as to measure their spatial distribution. The results would establish the boundary conditions for understanding galaxy evolution. Though highly challenging, detecting “missing” baryons in the intergalactic medium could be attempted, perhaps in the outskirts of galaxy clusters, and could shed significant light on the large-scale structures of the universe. The current design of HUBS will be presented, along with the status of technology development.
In the area of space astronomy, some electronic detectors should be cooled to liquid helium temperature level to improve their sensitivity and reduce the background noise. A hybrid J-T cooler has been developed for future space application by our laboratory. The J-T loop is precooled by two-stage thermally coupled pulse tube cooler. There is no moving part at low temperature in this system which features low vibration electromagnetic interference. The hybrid J-T cooler has been experimental tested and cooling capacity of about 34.5mW@4.32K is achieved when supply pressure is 1.97MPa. Besides, when sufficient precooling power is provided for the J-T loop cooling capacity of 102.3mW at 5.02K is achieved. In this experiment, three oil free linear compressors are used to drive the J-T loop and a GM cooler is used to provide the precooling power for the J-T cooler. This J-T cooler will be the potential cryocooler for the future space detectors requiring cooling power of 100mW at < 6K.
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