The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment aims to investigate fundamental questions about the high-energy Sun through direct imaging and spectroscopy of hard X-rays. The experiment utilizes Wolter-I type nested hard X-ray mirrors and fine-pitch semiconductor detectors, which are separated by a 2m focal length. Tol date, FOXSI has had two successful flights, on 2012 November 02 and 2014 December 11, demonstrating that the technology can measure small-scale energy releases (microflares and aggregated nanoflares) from the solar corona. The third flight for FOXSI is scheduled for August 2018. Significant improvements have been made on the FOXSI instrumentation, including upgraded optic modules with more nested mirror shells; specially designed collimators to mitigate the number of single bounce photons (ie., ghost rays) reaching the focal plane detector; and fine-pitch double-sided CdTe strip detectors to replace some of the Si-based hard X-ray detectors for better efficiency for hard X-rays. Furthermore, a CMOS based soft X-ray (SXR) instrument, “Phoenix”, will be added to FOXSI-3 by replacing one hard X-ray detector with a photon-counting SXR sensor. This will enable evaluation of the Sun via imaging spectroscopy simultaneously over a large X-ray energy range covering soft to hard X-rays. This paper will describe the overall instrument design of the FOXSI-3 experiment, which will be sensitive to solar soft and hard X-rays in the 1 – 20 keV range, as well as give a summary of insightful results and lessons from the first two flights. Possible observations for FOXSI-3 will also be discussed.
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment conducts direct imaging and spectral observation of the Sun in hard X-rays, in the energy range 4 to 20 keV. These high-sensitivity observations are used to study particle acceleration and coronal heating. FOXSI is designed with seven grazing incidence optics modules that focus X-rays onto seven focal plane detectors kept at a 2m distance. FOXSI-1 was flown with seven Double-sided Si Strip Detectors (DSSD), and two of them were replaced with CdTe detectors for FOXSI-2. The upcoming FOXSI-3 flight will carry DSSD and CdTe detectors with upgraded optics for enhanced sensitivity. The detectors are calibrated using various radioactive sources. The detector’s spectral response matrix was constructed with diagonal elements using a Gaussian approximation with a spread (sigma) that accounts for the energy resolution of the detector. Spectroscopic studies of past FOXSI flight data suggest that the inclusion of lower energy X-rays could better constrain the spectral modeling to yield a more precise temperature estimation of the hot plasma. This motivates us to carry out an improved calibration to better understand the finer-order effects on the spectral response, especially at lower energies. Here we report our improved calibration of FOXSI detectors using experiments and Monte-Carlo simulations.
In high energy solar astrophysics, imaging hard X-rays by direct focusing offers higher dynamic range and greater sensitivity compared to past techniques that used indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors together with seven high-sensitivity semiconductor detectors to observe the Sun in hard X-rays through direct focusing. The FOXSI rocket has successfully flown twice and is funded to fly a third time in summer 2018. The Wolter-I geometry consists of two consecutive mirrors, one paraboloid and one hyperboloid, that reflect photons at grazing angles. Correctly focused X-rays reflect once per mirror segment. For extended sources, like the Sun, off-axis photons at certain incident angles can reflect on only one mirror and still reach the focal plane, generating a background pattern of singly reflected rays (i.e., ghost rays) that can limit the sensitivity of the observation to faint, focused sources. Understanding and mitigating the impact of the singly reflected rays on the FOXSI optical modules will maximize the instruments’ sensitivity to background-limited sources. We present an analysis of the FOXSI singly reflected rays based on ray-tracing simulations and laboratory measurements, as well as the effectiveness of different physical strategies to reduce them.
The Focusing Optics X-ray Solar Imager (FOXSI) is, in its initial form, a sounding rocket experiment designed to apply the technique of focusing hard X-ray (HXR) optics to the study of fundamental questions about the high-energy Sun. Solar HXRs arise via bremsstrahlung from energetic electrons and hot plasma produced in solar flares and thus are one of the most direct diagnostics of are-accelerated electrons and the impulsive heating of the solar corona. Previous missions have always been limited in sensitivity and dynamic range by the use of indirect (Fourier) imaging due to the lack of availability of direct focusing optics, but technological advances now make direct focusing accessible in the HXR regime (as evidenced by the NuSTAR spacecraft and several suborbital missions). The FOXSI rocket experiment develops and optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. To date, FOXSI has completed two successful flights on 2012 November 02 and 2014 December 11 and is funded for a third flight. This paper gives a brief overview of the experiment, which is sensitive to solar HXRs in the 4-20 keV range, describes its first two flights, and gives a preview of plans for FOXSI-3.