The FOXSI-4 Sounding Rocket: High Resolution Focused X-ray Observations of the Sun

It has been firmly demonstrated that direct-focusing instruments can transform the way high-energy X-rays from astronomical objects, including the Sun, are measured. The NuSTAR spacecraft has increased the sensitivity to faint astrophysical sources by 100 times as compared with previous, indirect, imagers. The first three flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket established the usefulness and feasibility of a similar method optimized for the Sun, and showed that in addition to greater sensitivity, a vastly improved dynamic range can be obtained in this way. This technology stands ready to revolutionize understanding of solar flares by elucidating particle acceleration sites in the corona, studying how electrons propagate and deposit their energies, and how accelerated particles escape into interplanetary space. While the fundamental building blocks of solar hard X-ray (HXR) focusing are in place and ready for a spacecraft mission, concurrent development is required to prepare for the next generation of high-energy solar explorers, which will require higher rate capability and higher angular resolution to investigate finer-scale structure and to better complement instruments at other wavelengths.

FOXSI-4 features technological advances that enable high angular resolution as well as measurement of bright sources. In the first category, we will develop high-precision mirror production methods and finely pixelated Si CMOS sensors, and will demonstrate substrip/subpixel resolution in fine-pitch CdTe sensors. Secondly, we will demonstrate rate capability of these sensors sufficient for flare measurement, and will develop novel pixelated attenuators that optimize energy coverage even at high rates.

The experiment will demonstrate these technologies in NASA's first-ever solar flare campaign, flying in tandem with the Hi-C FLARE rocket. The campaign will position multiple rocket experiments awaiting an opportunistic signal and will launch the experiments for near-simultaneous observation of the flare.This campaign will allow for direct collaboration with the Parker Solar Probe (PSP) during one of its perihelia.