1. Introduction
¶ Solar flares and CMEs often have severe impacts on the planetary
magnetospheres and ionospheres.
¶ Recent studies have identified possible extreme solar energetic events
with energies ~1034 erg in the past millennia. Strong XUV radiation
and CMEs associated with flares may intimately contribute to the
formation and disruption of habitable worlds.
¶ Observations of the young solar-type stars in various phases of evolution
will provide insights into the time history of our Sun and its heliosphere.
¶ Recent observations of superflares on young solar-type stars indicate
stellar CMEs. To study them, time-resolved optical spectroscopic observations
are currently the only approach.
Previous study:
Namekata et al (2022) Nat. Astron. 6, 241
- EK Doraconis observations with H-alpha and WL in 2020 Apr.
- First detection of H-alpha spectra of superflare.
- a blueshifted absorption as evidence of a gigantic filament eruption.
This work: Multi-wavelength observation to probe :
- white light : photosphere --- TESS
- H-alpha : chromosphere --- Seimei Telescope
- X-ray : corona --- NICER
2. Observation and Data Reduction
- G1.5V : a young solar-type start at an age of 50-125 Myr.
- Effective temp. of 5560-5700 K
- Radius : 0.94 R⊙
- Mass : 0.95 M ⊙
- Inclination angle: 60 deg.
- one of the best proxies of an infant Sun at the Hadean preiod on Earth
(~4Gyr ago).
2.2 TESS
2.3 Seimei Telescope
- Seimei Telescope (Kyoto univ.)
- 3.8 m optical telescope in operation since 2019.
- H-alpha spectra using Kyoto Okayama Optical Low-dispersion Spectrograph
with optical-fiber Integral Field Unit (KOOLS-IFU)
- examples of H-alpha spectra of EK Dra --> Fig. 1
2.4 NICER
-
NICER: Neutron Star Interior Composition ExploRer (NASA)
- An International Space Station (ISS) payload (no data at day time).
- launched on 3 June 2017, with a SpaceX Falcon 9 rocket.
- X-ray (0.2-12 keV) "concentrator" optics and silicon-drift detectors
(not an imager).
- energy resolved light curves (resolution: 85eV at 1keV, 137ev at 6keV)
Coordinated observation among 3 instruments was performed :
2022 April 10 - 21 (12 days).
Data coverage is shown in Fig.2.
3. Analysis and Results
3.1 Light Curves
Data coverage plot in Fig.2
3.2 TESS WLF Analysis
Identify significant WL enhancement in the TESS light curve,
occurred simultaneously with H-alpha and X-ray flares.
- E1 : BJD-2459680.0329 (2022/4/10 12:47:23 UT)
- E2 : BJD-2459685.9980 (2022/4/16 11:57:07 UT)
- E3 : BJD-2459687.0119 (2022/4/17 12:17:08 UT)
Obtained light curves : Fig.3(E1), Fig.4(E2) & Fig.5(E3).
Time lag of Optical and X-ray light curves : consistent with Neupert effect
Calculation of bolometric WL flare energy : assuming 10,000K black body
radiation spectrum. ---> Table 2
3.3 H-alpha Flare Analysis
Time evolution of pre-flare subtracted H-alpha spectra :
- E1 (Fig.6) : significant blueshifted emission profiles during the flare.
- E2 (Fig.7) : manifests emission in the H-alpha line center + blueshifted
emission 10-15min after the start of the flare.
- E3 (Fig.8) : does not display any significant H-alpha line asymmetry.
Spectral fitting of the blueshifted components ---> Velocity estimate of
the blueshifted component.
Light curve of EW (pre-flare state subtracted) ---> H-alpha radiated energy.
(EK Dra continuum flux multiplied by the enhanced H-alpha EW and
integrated in time.)
Derived parameters are listed in Table 3 and 4.
3.4 X-ray Flare Analysis
Spectral analysis of the increased intensity during the flares (for each
block of data).
---> Temperatures and Emission Measures (Assuming emission spectrum from
collisionally ionized diffuse gas, etc.)
---> flare radiation fluxes and energies in various energy range.
"bolometric X-ray" is defined as the quantities integrated over 0.1-100 keV.
Derived parameters are listed in Table 5, 6 & 7.
4. Time Evolution, Energetics, and Length Scale
4.1 Time Evolution of Optical and X-Ray Flares
4.2 Flare Radiative Energy Partition
Comparison of bolometric flare energy among WL H-alpha and X-ray
4.3. Length Scale of Flare Loop
5. Prominence Eruptions on a Young Solar-type Star
5.1. Discovery of Blueshifted Emission Spectra as Evidence
of Stellar Prominence Eruptions
"we concluded that we discovered prominence eruptions on a solar-type
star for the first time (undoubtedly for flare E1)."
5.2. Properties of Prominence Eruptions
5.3. Candidate of Possible X-Ray Dimming
Example of dimming after a flare.
Analysis of E1 X-ray data after the flare (block 5 and 6).
"This supports that the observed postflare decrease in soft X-ray
count rate can be related to the escape of hot coronal plasma
associated with a CME."
6. Discussion and Conclusion
nearly 4 pages of description ....
6.4 Future Work
- origin of prominence (maps of starspots)
- modeling of prominence eruption
- more event detection and analysis.
Appendix
Solar CME spectral analysis.
Otsu et al. (2022)
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