FOXSI Success

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Revision as of 10:09, 19 December 2012

Introduction

RHESSI has been an enormous success story, as testified by the extent of these Nuggets, soon to enter their ninth year of publication. See the entry How does RHESSI work? for background information about RHESSI itself; RHESSI is an imager, rather than a true telescope, since it does not actually focus the X- and γ-rays it detects. This approach to imaging heretofore has been the only show in town between a few keV and a few MeV, but the lower-energy boundary has been rising recently because of technology developments. In general, to study solar activity and its fundamental non-thermal properties successfully, we need imaging spectroscopy for these hard radiations.

FOXSI is one of several offspring of the RHESSI spacecraft. While RHESSI continues to work well at the time of writing, it is an aging spacecraft that must be replaced by new-generation instrumentation. For hard X-rays the new developments in mirror and detector technology make it possible to do true focusing at tens of keV; indeed, the NuSTAR spacecraft was launched June 13, 2012, with just this capability. Whereas NuSTAR will observe the Sun, it is mainly a non-solar mission and has relatively poor angular resolution.

FOXSI

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.

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.

The data

Figure 1: Images of the flare from RHESSI (left) and FOXSI (right). Note the structured sidelobes in the RHESSI image, as well as the generally higher background level, as compared with the focusing optics of FOXSI.

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.

Conclusion

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.

References

[1] Hard X-ray emission from the solar corona