Cosmic X-ray Physics: Including a Sounding Rocket Investigation of Galactic X-ray Emission and Detector Development

Over half the normal matter in the Universe is thought to be diffusely distributed gas at temperatures around a million degrees, but current instrumentation for studying such material is relatively poor. We propose a suborbital investigation to improve our understanding of the Galactic diffuse X-ray background that includes the development of improved instrumentation for diffuse hot gas. The ultimate purpose of this is to determine the role of hot phases of the interstellar medium in mediating stellar feedback in star formation, in transport of metals, and in determining the structure and evolution of the Galaxy. This work will involve a flight of our existing X-ray Quantum Calorimeter sounding rocket payload from Australia to observe the Galactic center soft X-ray bulge with 5 eV spectral resolution in the 60-1500 eV range to attempt to determine its nature and emission mechanisms. This flight should also either confirm or put upper limits on the sterile neutrino model for the 3.5 keV signal observed near the Galactic center by XMM Newton. Our investigation also includes the development of detectors for a future sounding rocket experiment to obtain a scientifically useful spectrum of the emission in the 100-500 eV range coming from one million degree gas in the hot local bubble in the interstellar medium surrounding the Sun, with additional contributions from clumps of hot gas in the Galactic halo and interplanetary charge exchange on Solar wind ions. This will require an energy resolution of 1-2 eV FWHM and a total detector area of >2 square centimeters, a niche that larger microcalorimeter groups are not working on. Additional needs are infrared blocking filters with good transmission at these low X-ray energies and a calibration source capable of verifying the resolution and tracking the gain with sufficient accuracy. We are also assisting in a laboratory investigation of X-ray line emission from charge exchange to aid in interpreting the mixed astrophysical source of these X rays. All of this instrumentation development is relevant to a future probe-class mission to investigate intergalactic and circumgalactic hot gas. These are the supposed repository of the "missing baryons", and key to solving the "missing metals" problem. Characterization of circumgalactic and circumcluster regions at overdensities 200 is an essential part of understanding how matter and metals are transported into and out of galaxies, and will require a dedicated mission with a very large field of view, substantial effective area, high spectral resolution, and moderate spatial resolution. These investigations will provide the primary training for our graduate students and will involve a substantial number of undergraduates.