The primary scientific objective is to conduct a census of baryons in the warm-hot circumgalactic and intergalactic media and thus to directly address the issue of “missing baryons” in the local universe. The results are expected to impact our understanding of galaxy formation. Secondary objectives are many, including hot interstellar medium, diffuse X-ray background, supernova remnants, as well as charge exchange processes in the solar system.
The most effective approach is to carry out high-resolution spectroscopy in the UV and soft X-ray bands, where the warm-hot baryons are expected to manifest themselves in emission and absorption lines. HUBS mission will focus on the hotter phase of the media. It is being designed to employ superconducting transition-edge sensors (TESs) in photon- counting mode, to achieve both high throughput and high resolution. The key technologies include: TES array, SQUID readout and multiplexing, magnetic shielding, cryogen-free cooling system (to reach an operating temperature of about 50 mK), fast X-ray optics (with a field-of-view of about 1 square degree), and so on. Three modes of operations are envisioned: deep pointing, medium surveys, and all-sky survey .
For HUBS mission, the most relevant waveband is between 0.1 and 2 keV (similar to ROSAT). The most prominent spectral lines of interest are those of O VII and O VIII at around 0.6 keV, due partly to the high abundance of oxygen. In general, the thermal emission of million-degree gas is expected to be dominated by lines in this energy range.