Detector development for high spectral resolution observations of diffuse hot gas

We propose to develop microcalorimeter detectors based on superconducting transition edge thermometers specifically aimed at constructing a sounding rocket instrument to investigate hot gas in the Galactic interstellar medium. Current observations show that gas at temperatures of 800,000 to ~2 million kelvin is widely distributed in and perhaps around the Galaxy, but little is actually known about its location, structures, and geometry, nor about its role in Galactic evolution. Material near 1 million degrees seems particularly abundant and apparently fills much of the Galactic disk in the Solar neighborhood. Further study of this gas requires disentangling multiple emissions sources along a line of sight, including substantial foreground emission from charge exchange on the Solar wind. This spectral region around 150 to 350 eV is so line-rich that resolving individual emission lines to properly understand the different components requires better than 2 eV spectral resolution. Collecting enough photons to obtain a scientifically useful spectrum in a sounding rocket flight will require a total detector area of 2–3 square centimeters; to get this with an affordable number of readout channels for a sounding rocket budget calls for individual pixels around 1mm x 1mm. The development of these detectors is quite orthogonal to the extensive and very good current work on arrays for XRISM, Athena, and Lynx. The requirements that the pixels must be large and very high resolution, but need be optimized only for energies up to 600 eV puts them in a quite different regime with a different figure of merit for the transition edge sensors, probably leading to different construction and geometry. We do work closely with the major developers of more conventional arrays and will take advantage of their knowledge and insights, but this is really a different development problem. Current work suggests that a sensor geometry without the usual ‘zebra stripes’ and careful attention to superconducting contact geometry and external B fields can produce the required high thermometer sensitivity with acceptable levels of current sensitivity and excess noise. But no one has really pushed in this direction before. Beyond our goal of building a rocket payload with these detectors, the same pixels in a larger array would be optimal for a future NASA mission to study diffuse hot gas in circumgalactic and intergalactic space. Knowledge of the movement and metallicity of hot gas in circumgalactic environments is crucial to understanding material transport into and out of galaxies. Athena will technically be able to do such observations, but this is not what it is optimized for, and it will not do a significant number of the extremely long exposures required. With the more specialized detectors developed for our rocket experiment, a straightforward probe-class mission could be constructed with 20 times the mapping speed of Athena for emission lines, about half the speed for absorption lines, and a small fraction of the cost.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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