FOXSI Success

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Nugget
Number: 189
1st Author: Lindsay Glesener
2nd Author: Säm Krucker
Published: 21 December 2012
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Introduction

The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload flew for the first time on November 2, 2012 from the White Sands Missile Range. FOXSI carries grazing-incidence hard X-ray (HXR) focusing optics paired with silicon strip detectors to observe solar X-rays from 4-15 keV. During the 6.5-minute observation interval several targets were observed, including active regions, the quiet Sun, and a B-class flare.

FOXSI is one of several offspring of the RHESSI spacecraft. RHESSI observations of partially occulted flares revealed that coronal looptop HXR sources are probably present in almost all flares (Krucker & Lin 2008). However, these looptop sources are only rarely observed in on-disk flares because of the high dynamic range (or ability to image bright and faint sources together) that is required. Coronal and footpoint sources differ in brightness by 1-2 orders of magnitude, which is often just beyond RHESSI’s capabilities. In addition, even better sensitivity is needed to paint clear pictures of energetic electrons in the corona, where flare particle acceleration is thought to occur.

Direct focusing optics can improve in these areas by focusing HXR down to a small detector volume, increasing sensitivity. The point spread function of such optics tends to be quite narrow, leading to a dynamic range of 100-1000 for sources more than 30 arcseconds apart. FOXSI is an effort to develop HXR focusing optics for solar observations on a rocket payload, with an eye toward a spaceborne observer featuring these optics somewhere down the road.

Figure 1:

While the primary science target for FOXSI’s first flight was HXR from the quiet Sun, which would be well below RHESSI’s sensitivity level, the fortunate occurrence of a microflare during the observation interval offers the opportunity to compare capabilities with RHESSI. In Figure 1 a RHESSI image (made using the CLEAN technique and subcollimators 3,5,6,7,8, and 9) is shown side by side with an unprocessed FOXSI image of the same time interval and energy range. The RHESSI image shows artifacts over the entire field of view, inhibiting the imaging of faint sources in the presence of a bright one; the FOXSI image is free of such artifacts.

With the first solar HXR imaging spectroscopy achieved using focusing optics, FOXSI’s first flight met its comprehensive flight requirements. A second flight in 2014 will examine the Sun with even greater sensitivity.

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