A demonstration of STIX hard X-ray imaging spectroscopy capabilities for an X-class flare (SOL2021-10-28)

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Nugget
Number:	426
1st Author:	Andrea BATTAGLIA, Hannah COLLIER,
2nd Author:	and Säm KRUCKER
Published:	7 February 2021
Next Nugget:	TBD
Previous Nugget:	A solar flare driven by thermal conduction observed in mid-infrared
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Contents 1 Introduction 2 Calibration 3 The flare 4 Conclusions 5 References

Introduction

The Solar Orbiter observatory, launched just two years ago, carries a hard X-ray imager (STIX)] that now extends RHESSI's hard X-ray record. Although not as powwerful as RHESSI in some regards, STIX has remarkable advantages - it approaches the Sun closely, and it can observe stereoscopically when combined with Earth-bound instruments.

This Nugget reports on the first X-class flare observed, SOL2021-10-28.

Calibration

STIX consists of two-grid shadow optics, as did RHESSI, but with the innovation of Moiré patterns. Its sophisticated simplicity required much effort on in-flight calibration of the amplitudes and phases of the visibility functions provided by the grids (Ref. [1]) SOL2021-10-28, as luck would have it, occurred at a solar radial distance outside the nominal operating window (within 0.75 AU), and so the alignment in the analysis below is ad hoc, but all future observations will have proper STIX aspect solutions.

The flare

At the time of the flare, Solar Orbiter was rather far away from the Sun (0.80 AU), but the viewing angle was only 4 degrees away from the Sun-Earth line. This small separation angle makes the comparison with observations taken from Earth relatively straightforward, but somewhat limited due to projection effects. However, for emissions originating from a single height layer, the images can be rotated from one vantage point to another precisely. In particular STIX can be compared accurately with UV observations of the flare ribbons provided by SDO/AIA. On the other hand, emission from flares loops, which have complicated 3-dimensional structures, cannot be coaligned accurately. Therefore, STIX and AIA images are best compared in a two-panel figure showing the images side by side, with the chromospheric emissions being shown in both panels, as in Figure 1

For this Nugget, we highlight imaging results obtained of the main hard X-ray burst integrated from 15:27:24.6 to 15:28:09.6 UT (see Figure 1, top; STIX times have been adjusted to account for the different light travel time to Earth). We used the CLEAN imaging algorithm to reconstruct STIX images with a CLEAN beam size of 16.4 arcsec FWHM. AIA observations of SOL2021-10-28 reveal a classic two-ribbon flare geometry (Figure 1, bottom right). In AIA 131 Å images, the flare ribbon structure is clearly seen, while the hot temperature response of 131A also shows the flare loops connecting the ribbons. As expected, AIA UV 1600 &Aring: outlines the flare ribbons. The main UV sources show corresponding non-thermal hard X-ray counterparts as outlined by the 16-70 keV contours (blue). The thermal X-ray contours in red (9-10 keV) reveal the hottest part of the flare arcade that connects the hard X-ray footpoint sources.

Spectral fitting of the flare-integrated count spectrum shows a good fit with the standard thermal and thick target model (see Figure 2; fit parameters are given in the Figure). The spectral fits confirm that the imaging range used for the non-thermal image in Figure 1 is appropriate, as the 16-70 keV range does not contain any significant thermal contribution. Furthermore, imaging spectroscopy results at narrow energy ranges shown in Figure 2 (right) confirms that the transition between the thermal and non-thermal component seen in the spectral fit also corresponds to the transition in the change in the image morphology from coronal loop sources to chromospheric footpoints.

Conclusions

The STIX team is very pleased with the STIX imaging performance, and we expect the calibration to further improve in the future, particularly as the calibration of the finest subcollimators becomes available as well as the coarser ones.. Considering that SOL2021-10-28 was observed at 0.8 AU, while the main Solar Orbiter science window near perihelion will be at 0.3 AU, we expect similar image quality as shown here for low M-class flares. Imaging down to GOES A class is also feasible. The first close perihelion (0.3 AU) will be at the end of March 2022.

STIX data are available from our Website. Imaging software is currently available in IDL and features several different imaging algorithms (Back projection, CLEAN, MEM_GE, EM, forward fit; see Ref. [2], although the software package is still far from streamlined. Therefore, it is currently best to collaborate directly with a STIX team member when working with STIX data.

References

[1] "Imaging from STIX visibility amplitudes"; "Multiple electron acceleration instances during a series of solar microflares observed simultaneously at X-rays and microwaves"

[2] P. Massa et al., in preparation (2022).