Richard Kelley

More than 40 years ago, X-ray astrophysicists sought a way to improve the energy resolution of non-dispersive X-ray spectrometers for the Advanced X-

Ray Astrophysics Facility (now Chandra). What emerged was something entirely new – the thermal detection of individual X-ray photons and the ability to measure their energies with extreme precision. Over the subsequent decades,

the technology has been developed and refined, and eventually incorporated into space-flight instruments. The X-ray microcalorimeter was used starting in the 1990’s to measure the spectrum of the diffuse X-ray background on a suborbital payload and has been in continuous use for laboratory atomic physics measurements. The Resolve X-Ray Spectrometer has now been deployed on the recently launched JAXA/NASA X-Ray Imaging and Spectroscopy Mission (XRISM) to measure the spectra of a wide

range of celestial objects. The microcalorimeter array on XRISM has an energy resolution of 4.5 eV and is nearly independent of energy over a range of 0.3-12 keV. The high spectral resolution provides extremely high sensitivity for line detections and the ability to measure energies to an accuracy of ~ 0.2 eV, corresponding to velocities as small as 10’s of km/sec in the X-ray band. Further, the fabrication of arrays of these devices allows for X-ray imaging spectroscopy of complex, extended sources, such as supernova remnants, galaxies, and clusters of galaxies. The XRISM mission is a pioneering mission that it will demonstrate the potential of this approach to X-ray spectroscopy. Much more ambitious missions are planned or foreseen that will have arrays with thousands of pixels with 1 eV resolution. In this talk we will tell the story of how we got here, show some of the impressive results that have been obtained with this technology, including from the XRISM observatory, and discuss what the future may hold for this way of doing spectroscopy.