A New Sounding Rocket Instrument for High Resolution Spectroscopy of the Diffuse X-ray Background at 70-1000 eV

There is accumulating evidence that the Sun’s location in a very low-density cavity mostly filled with 1.0 x 10⁶ K gas may not be unusual. Particularly as it may represent a common phase of the Galactic disk, we would like to understand the current state and history of this hot gas, but this has turned out to be a difficult observational problem. Emission from gas at these temperatures is mostly below 300 eV, and is distributed in a dense forest of lines that will require 1-2 eV spectral resolution to isolate. The emission is dominated by L lines of multi-electron ions, where the accuracy of calculated oscillator strengths is suspect and measurements are few. This Local Hot Bubble emission is heavily contaminated by foreground emission from charge exchange of Solar wind ions on neutral H and He in interplanetary space. In some directions there is also a contribution from large clumps of gas at nearly the same temperature, but located somewhere in the Galactic halo. High spectral resolution and good statistics are necessary to isolate better-understood lines and sort out these contributions in order to determine the relative abundances, ionization state, and temperature distribution that could tell us how this region was formed. There is currently no capability for making the necessary observations. We are working on a sounding rocket instrument with a wide field of view and an array of large pixels with TES sensors that will have 1-2 eV FWHM energy resolution and a few cm² str throughput. We will show some of the design challenges and current progress on this instrument.