Hot Universe Baryon Surveyor (HUBS) is a proposed Chinese space mission to search for the so-called “missing baryons”. HUBS will focus on soft X-ray detection. The central part of the HUBS telescope is a soft X-ray spectrometer that uses a large transition-edge sensors array to detect the photon emission from a warm-hot intergalactic medium. The detector array comprises more than 3600 pixels. To read such a large number of pixels, a multiplexed readout technique is obligatory. We aim to develop a time-division multiplexed (TDM) readout system for HUBS. We choose TDM because it is the most mature and common one among various multiplexed readout techniques. We started by developing a proto-type TDM system that uses a single-stage SQUID readout. The basic multiplexing unit is composed of a SQUID series array (SSA) in parallel with a SQUID-based superconducting/normal conducting switch (SN switch). The SSA is composed of 16 individual SQUID cell that adopts a 1st order serial gradiometer design. The switch is also made of SQUID cells connected in series. The SQUID cell for a switch can comprise two Josephson junctions (JJs) like a usual DC-SQUID. It can also take the form of a Zappe interferometer that consists of four JJs. We will present the design and the simulation results of the sensor SQUID array and the SN switches.
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.
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