The sounding rocket experiment FOXSI-4, successfully launched in April 2024, conducted the world's first focusing imaging spectroscopic observation of a solar flare in x-rays. It aims to investigate the magnetic energy release and the magnetic to other forms of energy conversion mechanism caused by magnetic reconnection in solar flares. For soft x-ray observations, a non-dispersive type imaging-spectroscopy, that is, photon counting, was performed using back-illuminated CMOS sensors. The sensor has a sensitive layer of fully depleted silicon which is 25μm-thick and can achieve a high-speed continuous exposure at a rate of ~250fps. To evaluate the sensor's performance, we irradiated the sensor with monochromatic x-rays from 0.8 to 10keV at synchrotron facilities. Based on this data set, we evaluated the relationship between incident x-ray photon energy and the output signals from the sensor, known as a response matrix for use in analyzing flight data. We confirmed that the sensor outputs signals mainly proportional to the incident photon energy with little charge loss. We also analyzed charge sharing, which refers to the spread of electrons produced by a single photon across multiple pixels. The energy resolution is better than 400eV FWHM for the energy range of up to 10keV, which is sufficient to diagnose the spectrum of a solar flare.
X-ray imagery of the solar corona with arcsecond-level angular resolution is a key to understand various activities of hot plasmas in the corona. We report our development activities of precision Wolter mirrors for future soft X-ray imaging observations of the Sun. The Wolter mirrors under development are fabricated by direct polishing of glass-ceramic substrates. We fabricated an engineering Wolter mirror of 3-m focal length whose mirror area was 40 mm x 50 mm (along optical-axis and cylindrical directions, respectively) for each of the parabola and hyperbola sections using EEM (Elastic Emission Machining) precision polishing technology. X-ray imaging performance of the mirror was then evaluated with 8-keV parallel X-rays at SPring-8/BL29XUL. For the mirror area whose tilt up to >6 degrees along the cylindrical direction, the mirror showed imaging performance of ~0.25 arcsec FWHM (~0.55 arcsec HPD) with the scattering level ~1 × 10-4 of the PSF peak at 30-arcsec off-axis position.
FOXSI-4, the fourth flight of a US-Japan joint-sounding rocket experiment FOXSI, launched successfully in April 2024 and conducted the world’s first X-ray focusing-imaging spectroscopy of a large solar flare. It aims to reveal the mechanism of releasing and conversing magnetic energy caused by magnetic reconnection. FOXSI used high-precision Wolter type-I optics to directly focus X-rays, thus ensuring a high dynamic range. However, light entering from outside the field of view (13’ away from the on-axis position in FOXSI-4) can reach the focal plane, called stray light. For removing the stray light, FOXSI was equipped with a pre-collimator. FOXSI-4’s one has a porous honeycomb structure with a hole diameter of 0.7 mm and hole depth of 190 mm, whose aspect ratio is 1:270, thereby removing stray light. The wall thickness of the honeycomb structure is set at 0.09 mm, ensuring an aperture ratio of more than 60 % as a goal. This performance is very important, especially for analyzing dark regions (e.g., magnetic diffusion regions). Since it is hard to manufacture the required high aspect ratio of 1:270 by a machining process, we used a metal 3D printer. The performance of the pre-collimator mounted on the FOXSI-4 optics was evaluated with the parallel X-ray light at the Straylight Test Facility in NASA/MSFC. The evaluated aperture ratio and aspect ratio were 42 % and 1:370, respectively. Though the aperture ratio was smaller than the designed one, it was almost the same as that of FOXSI-3 which collected enough X-ray photons (more than 10M photons). While the aspect ratio was higher than the designed one, the corresponding field of view such as 550 arcsec could adequately cover the entire flaring region. Thus, this collimator had enough capability for a flare observation and was adopted as a flight product for FOXSI-4.
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