STIX, the Hard X-Ray Telescope on board Solar Orbiter

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Nugget
Number: 410
1st Author: Andrea Francesco BATTAGLIA
2nd Author: Säm KRUCKER
Published: 28 June 2021
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Contents

Introduction

The Solar Orbiter is ESA's new flagship space mission, launched in February 2020 with the objective to explore and to understand how the Sun creates and drives the inner heliosphere. To do that, Orbiter combines measurements from 10 different instruments: 4 in situ detectors and 6 remote-sensing instruments. Among the remote-sensing instruments, the Spectrometer/Telescope for Imaging X-rays (STIX; Ref. [1]) is the latest Hard X-Ray (HXR) telescope to study solar flares, and will supply most of the high-energy perspective of solar flares that will occur in solar cycle 25. Its unique opportunity to approach the Sun closely and to perform joint observations with the other Solar Orbiter instruments will provide new inputs into understanding the magnetic energy release and particle acceleration in solar flares.

The STIX Instrument: Brief Comparison With RHESSI

In general, the diagnostics capabilities of STIX resemble those of its predecessor, RHESSI. Indeed, microflares observed during the Solar Orbiter commissioning phase (June 2020) suggest that STIX has a similar detection threshold [Ref. 2] (Battaglia et al., 2021), since microflares with temperatures down to ~8 MK can be detected. However, one advantage of STIX is the constant background in time compared to RHESSIs often strongly varying background, which implies a more simplified detection of small events.

STIX is designed to observe solar flares from 4 to 150 keV at a nominal energy resolution (FWHM) of 1 keV at 6 keV, at a time resolution between 0.1 to 1 s. Data handling makes STIX somewhat different from RHESSI or other HXR instruments. RHESSI had enough telemetry capability for time-tagging every photon detected. Solar Orbiter's deep-space trajectory means that STIX gets much less telemetry. To allocate it in the best way, STIX first sends low-resolution quicklook (QL) data to the ground. From these data, the STIX team selects the most scientifically promising intervals, and "data requests" are sent back up to STIX in order specific downloads of data at chosen temporal and spectral resolutions.


First STIX Results: Spectroscopy

In order to compare the observations with previous works, Figure 1 shows STIX isothermal flare temperature estimates as a function of the emission measure of flares observed up to 6-Mar-2021. In the same figure, the emission measures and temperatures derived with X-ray observations of different instruments is also shown. This figure shows how the lower end of STIX roughly corresponds to that of RHESSI, and also it highlights, via NuSTAR and FOXSI samples, the advantages of microflare observations with HXR focusing optics. Moreover, the scatter plot in Figure 1 shows also that STIX observations are consistent with a strong result in flare X-ray diagnostics: there is a strong overall correlation between emission measure and temperature that extends across the entire magnitude range.


Figure 1: Comparison of the emission measures and temperatures of all STIX flares observed up to 6-Mar-2021 in at least two science energy bands, with the same parameters derived from other X-ray observations: RHESSI, FOXSI and NuSTAR. All flares are from active regions, except for the faintest ones, the quiet-Sun events reported in [3] and [4].

The STIX team is in the middle of the calibration effort (June 2021) and a stable version of both the spectral fitting software and imaging pipline will be available for the start of the nominal mission, in November 2021. However, in order to give a first impression to the reader, in Figure 2 we show preliminary spectroscopic results for a GOES class B6 microflare (SOL2020-06-07T21:43) as observed by STIX. We first we note how the STIX light curves (left) peak during the rise phase of GOES (highlighted by the derivative, in black). The earlier peak of the higher energies suggests the existence of a nonthermal component produced by accelerated electrons. The earlier peak of the STIX lower energies relative to GOES also is constent with greater STIX sensitivity to higher-temperature plasmas; from known flare behavior this also explains the faster STIX decay time as the result of cooling.

Figure 2: STIX data for the B6-class microflare SOL2020-06-07T21:43: left, the time series, with GOES in the upper panel and different STIX energy bands in the lower, all normalized. Note the match between GOES time derivative and STIX harder X-rays. Right, spectral fits as described in the text.

The right panel of Figure 2 shows fits to count spectra at two different times. The spectrum on the left, obtained at the nonthermal peak time, fits well to a combination of thermal and non-thermal components. The addition of the nonthermal component is justified by EOVSA microwave observations revealing a gyrosynchrotron bursts (D. Gary, private communication). The spectrum on the right is obtained at the thermal peak and is fitted with a single isothermal function. Note the slight excess at higher photon energies, suggesting the presence of weak nonthermal emission even in this later phase.

First STIX Results: Imaging

Finally, we describe preliminary imaging results for the same event (Figure 3), comparing with AIA images taken from a very different perspective. A sketch at upper left show these perspectives, and sketches on both panels show the different apparent sizes of the solar disk. As was the case for RHESSI, STIX has an indirect imaging system that requires extensive calibration before a robust imaging pipeline can be established. At present we are only able to describe source shape and size but not locations. The map on the left side of Figure 3 shows an AIA image (131Å) in red and in black (faintly seen!) the chromospheric flare ribbons as seen by the 1600Å filter. The green line is a 3D reference flare loop, a semi-circle perpendicular to the solar surface and connects the two flare ribbons. The map to the right displays the same AIA 1600Å map, but rotated to take into account the different vantage point of Solar Orbiter. Note how the loop changes appearance. The blue region is the STIX 6-10 keV source, an elongated forward-fit ellipse.

Figure 3: STIX image in blue (right panel) compared with AIA 131Å and AIA 1600Å (left). The green line shows a representative flare loop from the two different perspectives.

Conclusions

In conclusion, we have demonstrated that STIX works as designed. As a rule of thumb, STIX detects flares as small as the GOES A class. For flares above the GOES B class, detailed spectral and imaging analyses can be performed, and we expect great improvements in the imaging for more energetic events with improved software.

References

[1] "The Spectrometer/Telescope for Imaging X-rays (STIX)"

[2] "STIX X-ray microflare observations during the Solar Orbiter commissioning phase"

[3] "NuSTAR Detection of X-Ray Heating Events in the Quiet Sun"

[4] E. Lastufka et al., in preparation 2021

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