Localizing flares to understand stellar magnetic fields and space weather in exo-systems

Ilin, Ekaterina; Poppenhäger, Katja; Alvarado-Gómez, Julián D.

doi:10.48550/arxiv.2112.09676

Cited by 2 publications

(2 citation statements)

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“…If we instead assume a steeper FFD (α = 2.0, based on observations of stellar flares in the optical; see e.g. the summary by Ilin et al 2021), the conditions are even more relaxed, yielding flare numbers that are lower by approximately a factor of 10.…”

Section: Nature Of the Flares On Kepler-63mentioning

confidence: 99%

X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

Coffaro

Stelzer

Orlando

2022

A&A

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The X-ray satellite XMM-Newton has so far revealed coronal cycles in seven solar-like stars. Of these, the youngest stars ϵ Eridani (~400 Myr) and ι Horologii (~600 Myr) display the shortest X-ray cycles and the smallest amplitudes, defined as the variation of the X-ray luminosity between the maximum and minimum of the cycle. The X-ray cycle of ϵ Eridani was characterised by applying a novel technique that allowed us to model the corona of a solar-like star in terms of magnetic structures, such as those observed on the Sun (active regions, cores of active regions, and flares), at various filling factors. The high surface coverage of the magnetic structures on ϵ Eridani (65%–95%) that emerged from that study was suggested to be responsible for the low cycle amplitude in the X-ray band. It was also hypothesised that the basal surface coverage with magnetic structures may be higher on the corona of solar-like stars while they are young. To investigate this hypothesis, we started the X-ray monitoring campaign of Kepler-63 in 2019. The star had been observed once before, in 2014, with XMM-Newton. With an age of 210 ± 45 Myr and a photospheric cycle of 1.27 yr, Kepler-63 is the youngest star so far to be observed in X-rays in order to reveal its coronal cycle. Our campaign comprised four X-ray observations of Kepler-63 spanning 10 months (i.e. three-fifths of its photospheric cycle). The long-term X-ray light curve did not reveal a periodic variation of the X-ray luminosity, but a factor two change would be compatible with the considerable uncertainties in the low signal-to-noise data for this relatively distant star. In the case of ϵ Eridani, we describe the coronal emission measure distribution (EMD) of Kepler-63 with magnetic structures such as those observed on the Sun. The best match with the observations is found for an EMD composed of cores and flares of GOES Class C and M following the canonical flare frequency distribution. More energetic flares are occasionally present but they do not contribute significantly to the quasi-stationary high-energy component of the emission measure probed with our modelling. This model yields a coronal filling factor of 100%. This complete coverage of the corona with X-ray-emitting magnetic structures is consistent with the absence of an X-ray cycle, confirming the analogous results derived earlier for ϵ Eridani. Finally, combining our results with the literature on stellar X-ray cycles, we establish an empirical relation between the cycle amplitude LX,max/LX,min and the X-ray surface flux, FX,surf. From the absence of a coronal cycle in Kepler-63, we infer that stars with higher X-ray flux than Kepler-63 must host an EMD that comprises a significant fraction of higher energy flares than those necessary to model the corona of Kepler-63, that is, flares of Class X or higher. Our study opens a new path for studies of the solar-stellar analogy and the joint exploration of resolved and unresolved variability in stellar X-ray light curves.

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Section: Nature Of the Flares On Kepler-63mentioning

confidence: 99%

X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

Coffaro

Stelzer

Orlando

2022

A&A

View full text Add to dashboard Cite

show abstract

“…If, instead, we assume a steeper FFD (α = 2.0, based on observations of stellar flares in the optical, see e.g. summary by Ilin et al 2021), the conditions are even more relaxed, yielding flare numbers that are lower by approximately a factor 10.…”

Section: Nature Of the Flares On Kepler-63mentioning

confidence: 99%

The X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

Coffaro,

Stelzer,

Orlando

2022

Preprint

View full text Add to dashboard Cite

The X-ray satellite XMM-Newton has so far revealed coronal cycles in seven solar-like stars. In this sample, the youngest stars Eridani (∼ 400 Myr) and ι Horologii (∼ 600 Myr) display the shortest X-ray cycles and the smallest amplitudes, defined as the variation of the X-ray luminosity between the maximum and minimum of the cycle. The X-ray cycle of Eridani was characterized by applying a novel technique that allowed to model the corona of a solar-like star in terms of magnetic structures observed on the Sun (active regions, cores of active regions and flares) at varying filling factors. The high surface coverage of Eridani with magnetic structures (65 − 95%) that emerged from that study was held responsible for the low cycle amplitude in the X-ray band. It was also hypothesized that the basal surface coverage with magnetic structures may be higher on the corona of solar-like stars while they are young. To investigate this hypothesis, we started in 2019 the X-ray monitoring campaign of Kepler-63. The star had been observed once before, in 2014, with XMM-Newton. With an age of 210 ± 45 Myr and a photospheric cycle of 1.27 yr, Kepler-63 is so far the youngest star observed in X-rays with the aim of revealing a coronal cycle. Our campaign comprised four X-ray observations of Kepler-63, spanning 10 months (i.e. 3/5 of its photospheric cycle). In the long-term X-ray lightcurve we did not reveal a periodic variation of the X-ray luminosity, but a factor two change would be compatible with the considerable uncertainties in the low signal-to-noise data of this relatively distant star. As for the case of Eridani, we described the coronal emission measure distribution (EMD) of Kepler-63 with magnetic structures observed on the Sun. The best match with the observations is found for an EMD composed of cores and flares of GOES Class C and M following the canonical flare frequency distribution. More energetic flares can be occasionally present but they do not contribute significantly to the quasi-stationary high-energy component of the emission measure probed with our modelling. This model yields a coronal filling factor of 100%. This complete coverage of the corona with X-ray emitting magnetic structures is consistent with the absence of an X-ray cycle, confirming the analogous results derived earlier for Eridani. Finally, combining our results with the literature on stellar X-ray cycles we establish an empirical relation between the cycle amplitude L X,max /L X,min and the X-ray surface flux, F X,surf . From the absence of a coronal cycle in Kepler-63 we infer that stars with higher X-ray flux than Kepler-63 must host an EMD that comprises a significant fraction of higher-energetic flares than those necessary to model the corona of Kepler-63, i.e. flares of Class X or higher. Our study opens new ground for studies of the solar-stellar analogy and the joint exploration of resolved and unresolved variability in stellar X-ray lightcurves.

show abstract

Localizing flares to understand stellar magnetic fields and space weather in exo-systems

Cited by 2 publications

References 59 publications

X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

The X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

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