Nature of Energy Release and Transfer for Solar and Stellar Flares Using High-Sensitivity Hard X-Ray Instrumentation

Abstract

Sensitive measurements of solar and stellar flares in the hard X-ray regime are necessary for investigating energy release and transfer during flaring events, as hard X-rays provide insight into the acceleration of electrons and emission of high-temperature plasmas. The research presented here seeks to develop and harness the powerful capabilities of hard X-ray focusing optics to probe faint events that have previously been elusive, ranging from small-scale solar flares to bright X-ray flares on distant stars. In exploring these uncharted regimes, this work probes some of the most intriguing mysteries of the stars, from coronal heating to the formation of planetary systems. Due to previous technological challenges with focusing hard X-rays, the recent state-of-the-art solar-dedicated instrument in the hard X-ray regime, RHESSI, utilized an indirect imaging technique, which is fundamentally limited in sensitivity and dynamic range. By instead using a direct imaging technique, the structure and evolution of small-scale solar events can be investigated in greater depth. The Focusing Optics X-ray Solar Imager (FOXSI), a hard X-ray photon counting instrument flown on three sounding rocket campaigns, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. In this thesis, the FOXSI technological approach and the development of the instrument through these campaigns is outlined, with an emphasis on the most recent campaign, FOXSI-3. Along with novel hard X-ray focusing technology, the FOXSI instrument utilizes fine-pitch silicon (Si) and cadmium telluride (CdTe) semiconductor strip detectors to measure the energy and position of each incident photon. CdTe detectors offer improved capabilities for detecting faint high-energy emission compared to Si due to a higher quantum efficiency above 10 keV. The characterization of the FOXSI-3 CdTe detector response, including gain, efficiency, and energy resolution, is presented here. During the FOXSI-2 rocket flight, two sub-A class solar microflares were observed. With the direct imaging technique of FOXSI, detailed imaging and spectral analyses could be performed on microflares over an order of magnitude fainter than the faintest microflares observed by RHESSI. Through this work, the energy transfer for these sub-A class microflares was found to be consistent with that of the standard model for solar flares. Additionally, observed spatial and temporal complexity indicate that flares of this small size more closely resemble the structure and dynamics of large flares than the single energy release of a nanoflare. In addition to faint solar microflares, observations of extreme flares on distant young stellar objects, observed by the Nuclear Spectroscopic Telescope Array (NuSTAR), were analyzed. NuSTAR is the first astrophysical satellite to use focusing optics for the hard X-ray regime and offers unprecedented sensitivity >10 keV, making these observations the first of their kind. Through spectral analysis and a study of flare energetics, the energy transfer for these bright flares was also found to be consistent with the standard model for solar and stellar flares. Additionally, an emission feature at 6.4 keV offers evidence of interaction between flare radiation and circumstellar material, which could have implications for planet formation. With advances in hard X-ray instrumentation, we move one step closer to answering some of the biggest questions in solar and stellar physics.