Science Nugget: First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX - Solar Orbiter

First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX

(Solar Orbiter Nugget #48 by Natália Bajnoková (1), Iain G. Hannah (1), Kristopher Cooper (2), Säm Krucker (3,4), Brian W. Grefenstette (5), David M. Smith (6), Natasha L. S. Jeffrey (7), Jessie Duncan (8))

1. Introduction

Hard X-rays (HXR) serve as an important diagnostic for plasma heating and particle acceleration [1]. Therefore, they are crucial for studying flare energy release processes, which are not fully understood. Microflares (GOES B class and smaller flares [2]) are good candidates for studying these processes as they occur more frequently and usually with simpler configurations than large flares. Two HXR instruments that have been observing microflares in recent years are NuSTAR [3] and Solar Orbiter/ STIX [4]. There are several advantages for conducting joint HXR microflare observations with NuSTAR and STIX:

Combined observations of the same event from the two telescopes give us better constraint on the underlying physics of the energy release processes. This is especially important for events observed at limits of these instrument’s capabilities – in our case STIX has lower sensitivity than NuSTAR, but NuSTAR has limited throughput limiting observations of brighter events (due to effects like pile-up, where two or more photons are detected as a single photon).
Observing the same flare from different angles allow us to probe the 3D structure of the flare, and if behind the limb from one viewpoint we can also simultaneously observe the faint coronal source and bright lower atmosphere footpoints [5].

In this work we present the first results of a simultaneous microflare observations with NuSTAR and STIX.

2. Overview of observations

On 2020 June 6 and 7, NuSTAR and STIX jointly observed an active region AR12765 located close to the southeast solar limb. The instruments jointly observed 3 microflares (GOES B1, A7 and B6) that we divided into 5-time ranges over which we performed joint spectral fitting and imaging. During the observations the instruments were at approximately 45° separation (shown in the right panel in Figure 1) and both observed the microflares on-disk, thus observing the same X-ray sources. An example of STIX and NuSTAR lightcurves from a June 6 joint microflare are shown in the left panel in Figure 1.

Figure 1. (left) NuSTAR and STIX lightcurves from June 6 2020 GOES B1 class microflare. Region in grey highlights the even integration time. (right) The NuSTAR (Earth)–STIX–microflare AR positioning during the joint observations.

3. Joint spectral fitting and imaging

We have jointly fitted spectra from 5 different times from the 3 microflares using the new python X-ray fitting package sunkit-spex (https://github.com/sunpy/sunkit-spex). As NuSTAR contains two separate telescopes (FPMA and FPMB) this simultaneous fitting involves the two NuSTAR spectra and the STIX spectra, with added model scaling parameters to deal with small systematics between instruments. In addition, some of the microflares were bright enough to produce extremely low < 1% detector livetime in NuSTAR, meaning that that pile-up correction method had to be developed in sunkit-spex to model and fit this at the same time as the flare models.

An example of this joint spectral fitting is shown in Figure 2. In this case NuSTAR and STIX spectra were jointly fitted with a thermal model, with an additional non-thermal thick-target component to the NuSTAR spectra as it has more counts above background at higher energies. Despite the microflare being very faint for STIX and very bright for NuSTAR, consistent model parameters were found, which are in the expect range for HXR microflares [6]. In addition, the systematic differences between STIX and NuSTAR FPMA were only small, about 6%.

Figure 2. Example of joint spectral fitting for the June 6 microflare. NuSTAR contains two spectra from focal plane modules A and B that were pile-up corrected (the pile-up models are shown in green and red).

We were also able to perform joint imaging for this event. Images of NuSTAR and STIX HXR emission are shown overlain on 131 Å AIA images in Figure 3. With NuSTAR, we observed two non-thermal sources (orange) at either ends of an elongated thermal source (blue). This indicates the standard flare configuration consisting of non-thermal footpoints connected by hot flare loops. With STIX we were able to reconstruct a single lower energy (thermal) source originating from the hot flare loops (as suggested from the spectral fitting) that aligns well with the NuSTAR emission, reprojected to the STIX viewpoint.

Figure 3. Joint imaging for the June 6 microflare. The left panel shows NuSTAR image with 60, 70, 80 and 95% contour levels overlaid on a flare-time 131 Å SDO/AIA image. The right panel shows STIX image with matching contour levels to the NuSTAR ones as well as the reprojected NuSTAR contours.

A similar analysis was performed on the other microflare, again finding consistent spectral fits and imaged structures STIX and NuSTAR. The thermal parameters found from the spectral fitting are shown in Figure 4, in the context of previous HXR microflare studies. These events clearly bridge the gap between previous NuSTAR and STIX studies.

Figure 4. Summary of temperature and emission measure parameters from various NuSTAR [7, 8, 9, 10], STIX [11], RHESSI [6, 12], and FOXSI [13] microflare and flare studies. The microflares from this study are marked as red crosses in bold.

4. Conclusions

We were able to successfully perform the first joint spectral and imaging analysis of microflares observed with NuSTAR and STIX. In the events studied we were able to get a reliable fit to the spectra, which might not have been possible individually as both instruments were operating at the limit of the capabilities. Work is ongoing, aiming for joint observations in more optimal conditions. The ideal NuSTAR–STIX configuration would be: a) Fainter GOES A class microflare observed during STIX perihelion (better sensitivity to faint emission) or b) GOES B class microflare observed as occulted for NuSTAR and on-disk for STIX which would allow us to detect the faint coronal energy release site with NuSTAR, and the resulting bright lower atmosphere emission with STIX.

This study has been published in Bajnoková, N., Hannah, I. G., Cooper, K., et al. 2024, MNRAS, 533, 3742, doi: 10.1093/mnras/stae2029162

Affiliations

(1) School of Physics & Astronomy, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK
(2) School of Physics & Astronomy, University of Minnesota Twin Cities, Minneapolis, MN 55455, USA
(3) School of Engineering, University of Applied Sciences and Arts Northwestern Switzerland, CH-5210 Windisch, Switzerland
(4) Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
(5) Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
(6) Santa Cruz Institute of Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
(7) Department of Mathematics, Physics & Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK
(8) NASA Marshall Space Flight Center, ST13, Huntsville, AL 35812, USA

References

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[12] Warmuth A., Mann G., 2016, A&A, 588, A115
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