STIX X-ray microflare observations during commissioning phase

Andrea Francesco Battaglia

27th April 2021

Some wonderful pictures of the STIX instrument

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From the instrument paper: Krucker et al. 2020

This presentation will focus on the first STIX results paper

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Part of the A&ASolar OrbiterSpecial Issue

Quicklook at the lowest STIX energy channel

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Overview of the time period considered in the paper, with the lowest Quicklook (QL) energy channel (4-10 keV)

A total of 69 flares were detected with the 4-10 keV QL lightcurve, which are marked by vertical bars in the central panel, as follows:

- black: seen in a single STIX science energy channel (difficult to see in the QL lightcurve);- red: seen at multiple STIX science energy bins. The flare temperature could be derived (see slide 5);- blue: three flares discussed in detail in the paper.

All flares are from AR 12765,which only became visible fromSolar Orbiter after June 5: flaresfrom earlier times were occultedby the solar limb and not visibleby STIX

Flare temperature estimates

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Comparison of the Emission Measure (EM) and Temperature of STIX microflares detected during the commissioning phase with previously published microflare and flare observations in X-rays above 2.5 keV.The black dashed curves mark flares at equal GOES class as labeled.

Isothermal flare temperature estimates at the STIX thermal peak time using the ratio between the 6-7 keV and5-6 keV channels

Consistent with previous X-ray observations

STIX has a similar lower detection threshold to RHESSI.Advantage of STIX: constant background in time that allows detection of smaller events

Same figure… but up-to-date

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Correlation between EM and temperature: strong resultin X-ray observations

Here we consider all the flares observed by STIX for which the temperature could be estimated

STIX can observe microflares down to plasma temperaturesof about 8 MK

GOES B6 microflare on June 7, 2020 - Morphology

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GOES B6 microflare on June 7, 2020 – Time evolution

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GOES, AIA 94 A and Fe XVIII have slower decay phase than STIX:

- they have sensitivity to plasma at emperatures below the STIX sensitivity range

STIX peaks before GOES. This can be due to two reasons:

- higher sensitivity of STIX to hotter plasma;- presence of nonthermal components

In this case: STIX detected a nonthermal component(see slide 9 about spectroscopy)

GOES B6 microflare on June 7, 2020 – Preliminary spectroscopy

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Nonthermal peak time

Single thermal +Broken power-law =Total fit

Residuals

~ 2 minutes later

Single thermal

Traces ofnonthermalemission

GOES B6 microflare on June 7, 2020 – Preliminary imaging

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Microflare as seen from different vantage points: Earth (left) and Solar Orbiter (right)

Imaging system not yet ready! Calibration of the visibility phases still in progress (for this reason,the X-ray source has been manually positioned at the top of the semi-circle)

Size and orientation of the source areconsistent with theflare ribbons

Reference semi-circle perpendicular to the solar surface, drawn in order to guide the eye

Red: AIA 131 ABlack: AIA 1600 A

Blue: STIX elliptical GaussianBlack: rotated AIA 1600 A

Thank you!

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Questions?Contact me: andrea.battaglia@fhnw.ch