HUBS is being conceptualized to be a high throughput, high resolution spectroscopic X-ray mission, to be launched at around the same time as ESA's Athena mission, with complementary capabilities and scientific objectives. The scientific driver of HUBS is to quantify the properties and distribution of circumgalactic and intergalactic media, as well as the associated key processes such as stellar and AGN feedback, through high-resolution emission and absorption line spectroscopy. The results are expected to provide insights into the mechanisms of heating and metal transport, which are not understood, and thus aid in our understanding of galaxy formation and evolution. Because the emission is diffuse and the lines are expected to be very faint, basic technical requirementis of the HUBS design are low instrumental background and very large grasp (which is the product of effective areas and field of view).

A conceptual design of HUBS is as follows:

• High throughput, fast X-ray optics. It is to provide an overall effective area of ~1000 square cm and a field of view of and ~1 square degree, with a moderate spatial resolution of ~1 arcmin.
• High-resolution, high-quantum efficiency Imaging spectrometer. A 60x60 TES array are optimized for soft X-ray energies (0.1 to ~1 keV, where the most important lines of interest lie)). The energy resolution of each pixel is 2 eV @ 0.6 keV (where O VII and O VIII lines are located). A central 9x9 subarray of smaller pixels may be used to achieve sub-eV energy resolution for absorption line spectroscopy.
• Low-Earth, low-inclination orbit for low particle-induced background and high observing efficiency.

It is envisioned that HUBS will be operated in deep pointing (for absorption line spectroscopy), medium exposure (of selected fields), and all-sky survey modes, with a mission lifetime of at least five years.

More information
HUBS home page: http://hubs.phys.tsinghua.edu.cn

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