Solar Hard X-rays with Insight

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Introduction

This Nugget reports solar observations from the Insight/HXMT hard X-ray/gamma-ray observatory, launched 15-Jun-2017 (ref. [1]). Its three major instruments cover low, medium, and high energies with large effective areas, and it arrived in orbit in time to observe the major flare SOL2017-09-06 (Ref. [2]). Its instruments use modulation optics, as did RHESSI, but at degree-scale angular resolutions and thus not helpful for details of solar sources. This Nugget reports another interesting flare at the limb, SOL2017-07-03 (M1.3), observed also by RHESSI and Fermi/GBM (Figure 1)

Figure 1: Mmulti-wavelength light curves of the M1.3 flare: (upper panel) GOES soft X-rays (middle panel) hard X-ray light curves from RHESSI, Fermi, and HXMT (the 50-1000 keV range).

Observations of SOL2017-07-03

The observations reveal a footpoint-occulted flare with bright impulsive hard X-ray emission (Figure 1), but interestingly not much flux at 100 keV. AIA 131 Å images show a fine stable plasma filament (Figure 2(A/B/C)(, and a mini-cusp structure appearing at the peak of the hard X-ray phase, plus a filament eruption. We did not see a loop-top hard X-ray source in this case, and only a single footpoint region (north). The non-detection of a south footpoint region suggests electron energy losses in the lower atmosphere. Hard X-rays above the 50-100 keV band were not detected by RHESSI. by the RHESSI detector, and the absence of coronal emission suggests no effective trapping. This scenario is supported by emission measure maps (Figure 3) derived from SDO/AIA data in the manner discussed in Ref. [3].

Figure 1: Composite images using data from SDO/AIA 131 Å observations in 9 integration time ranges from 16:12:00 to 16:15:36 UT, solar limb marked with gray line, integration time 24 seconds. The overlaid contours show hard X-ray images based on RHESSI data in three energy bands: 6-10 keV, 10-30 keV and 30-100 keV as marked with red, green and blue solid lines respectively. The contour levels for 6-10 keV, 10-30 keV and 30-100 keV are 30%, 60% and 90%. The gray dash-dotted lines shows the reconnection current sheet. The yellow arrow in panel (D) marks the first appearance of the erupting filament.

Figure 1: Emission-measure maps derived from SDO/AIA observations in six integration time ranges from 16:13:12 to 16:15:36 UT. The background is 0.3-10 MK integrated EM map, overlaid with the contours of both 0.3-10 MK and >10 MK integrated EM map with contour levels 30%, 60%, 90%, marked with sky blue and magenta respectively. The hard X-ray image contours based on RHESSI data in three energy bands: 6-10 keV, 10-30 keV and 30-100 keV marked with red, green and blue solid lines respectively. The contour levels for 6-10 keV, 10-30 keV and 30-100 keV are 30%, 60% and 90%

In a flare such as this one, we envision lower-coronal magnetic reconnection releasing vast amoutns of energy, heating the ambient cooler plasma and simultaneously accelerating electrons. The hard X-ray spike is very short, though; only about 30 s above 50 keV, as shown in Figure 1. This and the imaging imply an acceleration site very close to the bottom of the corona. In some manner the dissipation of this energy injected hot plasma into large-scale loops loops, without significant nonthermal signatures.

Conclusion and Summary

Different hard X-ray telescopes provide the insight to the particle accelerations and high energy emission properties in flares, and the joint X-ray observations of multiple instruments should be very helpful to understand the energy conversion and plasma response during the flares in wide energies and large spatial ranges. The Insight/HXMT solar observations can play a role in this owing their unique properties.

References

[1] "Overview to the Hard X-ray Modulation Telescope (Insight-HXMT) Satellite"

[2] "Non-thermal Electron Energization During the Impulsive Phase of an X9.3 Flare Revealed by Insight-HXMT"

[3] "X-ray fine structure of a limb solar flare revealed by Insight-HXMT, RHESSI and Fermi"