Electroformed X-ray optics development for the FOXSI-4 sounding rocket experiment

We have been developing X-ray optics for a fourth FOXSI (Focusing Optics X-ray Solar Imager) sounding rocket experiment, FOXSI-4, which will be launched in 2024 and part of a flare campaign to constrain flare acceleration mechanisms. Our idea is to apply our original ultra-precise electroforming technique which has been developed for X-ray focusing systems in synchrotron radiation facilities to X-ray optics for astrophysics. Technologically, to fabricate larger mirrors in diameter and height (typically 10 /100 mm in diameter / height for ground X-ray focusing systems) is the most difficult challenge. As a first step, we fabricated a Wolter-I full-shell Ni test sample with height, diameter, and, thickness of 60, 60, and, 1 mm whose design parameters are optimized for FOXSI-4. We conducted X-ray irradiation tests at 15 keV and obtained focused images from the entire area successfully for the first time. Even though the resultant angular resolution in HPD is >30 arcsec due to remaining low- / mid-frequency figure errors in an axial direction, a sharp inner core with an FWHM value of ~5 arcsec is observed because of a very low figure error in a circumferential direction as expected from the inner surface profiles. It is also found that spot scan measurements with a beam size of 1 mm x 5 mm reveal a spatial variation in their performances and some of the areas achieved ~20 arcsec in HPD. Design, fabrication, and evaluation of the mirror module assembly are currently in progress and the impact of the mounting process will also be evaluated using X-ray. We will show recent status of our development and future plans.