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A Seven-Day Multi-Wavelength Flare Campaign on AU Mic III: Quiescent and Flaring Properties of the X-ray Spectra and Chromospheric lines

Yuta Notsu, Isaiah I. Tristan, Rachel A. Osten, Alexander Brown, Adam F. Kowalski, Carol A. Grady

TL;DR

The paper presents a comprehensive 7-day, multiwavelength campaign on AU Mic, combining X-ray spectra from XMM-Newton with chromospheric optical spectra and UV/optical photometry to study quiescent and flaring phases. It shows that quiescent corona exhibits an EM distribution spanning log T ~ 6.3–7.6 with EM$_{ m tot} \approx (2.5$–$3.6)\times10^{52}$ cm$^{-3}$ and inverse-FIP abundances, while flares reveal a range of energetics ($E_{ m X}$ from 2×10^{31} to 4×10^{33} erg) and loop scales (B ~ 50 G–1.5 kG, L ~ 3×10^{8}–2×10^{10} cm). The standout Flare 23 shows Neupert-type behavior with symmetric chromospheric line broadenings up to ~600 km s$^{-1}$ in Hβ, consistent with intense electron-beam heating modeled by RADYN; other major flares (11, 15) exhibit longer durations and larger loop sizes. Across the chromospheric lines studied, no clear blue/red wing asymmetries are found, though symmetric broadening tied to continuum evolution is evident. The results support the standard flare model for active M dwarfs and highlight the importance of multiwavelength, time-resolved spectroscopy for constraining flare energetics and atmospheric responses in exoplanet-hosting stars.

Abstract

We present the X-ray quiescent and flaring properties from a unique, 7-day multiwavelength observing campaign on the M1 flare star AU Mic. Combining the XMM-Newton X-ray spectra with the chromospheric line and broadband NUV and optical continuum observations provides a dataset that is one of the most comprehensive to date. We analyze the sample of 38 X-ray flares and study in detail the X-ray flare temperature ($T$) and emission measure (EM) evolutions of three largest flares with the X-ray flare energies of $>10^{33}$ erg. The $T-\mathrm{EM}$ evolution tracks and multi-wavelength emission evolutions of the largest-amplitude Neupert-type flare reveal that the so-called ``Flare H-R diagram" is consistent with thermal coronal flare emission evolution. The two other more gradual and longer duration X-ray flares are interpreted as having larger size scales. None of the 17 H$α$ and H$β$ flares show clear blue/red wing asymmetries, including the ones associated with the potential X-ray dimming event previously reported. The above largest-amplitude Neupert flare shows clear symmetric H$α$ and H$β$ broadenings with roughly $\pm$400 and $\pm$600 km s$^{-1}$, respectively, which are synchronized with the optical/NUV continuum emission evolution. Radiative hydrodynamic modeling results suggest that electron beam heating parameters that have been used to reproduce M-dwarf flare NUV/optical continuum emissions can reproduce these large broadenings of H$α$ and H$β$ lines. These results suggest that these most energetic M-dwarf flares are associated with stronger magnetic field flux densities and larger size scales than solar flares but can be interpreted in terms of the standard flare model.

A Seven-Day Multi-Wavelength Flare Campaign on AU Mic III: Quiescent and Flaring Properties of the X-ray Spectra and Chromospheric lines

TL;DR

The paper presents a comprehensive 7-day, multiwavelength campaign on AU Mic, combining X-ray spectra from XMM-Newton with chromospheric optical spectra and UV/optical photometry to study quiescent and flaring phases. It shows that quiescent corona exhibits an EM distribution spanning log T ~ 6.3–7.6 with EM– cm and inverse-FIP abundances, while flares reveal a range of energetics ( from 2×10^{31} to 4×10^{33} erg) and loop scales (B ~ 50 G–1.5 kG, L ~ 3×10^{8}–2×10^{10} cm). The standout Flare 23 shows Neupert-type behavior with symmetric chromospheric line broadenings up to ~600 km s in Hβ, consistent with intense electron-beam heating modeled by RADYN; other major flares (11, 15) exhibit longer durations and larger loop sizes. Across the chromospheric lines studied, no clear blue/red wing asymmetries are found, though symmetric broadening tied to continuum evolution is evident. The results support the standard flare model for active M dwarfs and highlight the importance of multiwavelength, time-resolved spectroscopy for constraining flare energetics and atmospheric responses in exoplanet-hosting stars.

Abstract

We present the X-ray quiescent and flaring properties from a unique, 7-day multiwavelength observing campaign on the M1 flare star AU Mic. Combining the XMM-Newton X-ray spectra with the chromospheric line and broadband NUV and optical continuum observations provides a dataset that is one of the most comprehensive to date. We analyze the sample of 38 X-ray flares and study in detail the X-ray flare temperature () and emission measure (EM) evolutions of three largest flares with the X-ray flare energies of erg. The evolution tracks and multi-wavelength emission evolutions of the largest-amplitude Neupert-type flare reveal that the so-called ``Flare H-R diagram" is consistent with thermal coronal flare emission evolution. The two other more gradual and longer duration X-ray flares are interpreted as having larger size scales. None of the 17 H and H flares show clear blue/red wing asymmetries, including the ones associated with the potential X-ray dimming event previously reported. The above largest-amplitude Neupert flare shows clear symmetric H and H broadenings with roughly 400 and 600 km s, respectively, which are synchronized with the optical/NUV continuum emission evolution. Radiative hydrodynamic modeling results suggest that electron beam heating parameters that have been used to reproduce M-dwarf flare NUV/optical continuum emissions can reproduce these large broadenings of H and H lines. These results suggest that these most energetic M-dwarf flares are associated with stronger magnetic field flux densities and larger size scales than solar flares but can be interpreted in terms of the standard flare model.

Paper Structure

This paper contains 20 sections, 9 equations, 44 figures.

Table of Contents

  1. Introduction
  2. Observations, Data, and Flare lightcurve overview
  3. Observation Data
  4. Multi-wavelength flare lightcurve atlas
  5. X-ray spectral analysis
  6. Quiescent phase X-ray spectra
  7. Time-averaged X-ray flare spectra
  8. Multiwavelength time-evolution analysis of remarkable flares
  9. Flare 23: The large Neupert-type flare with the largest multi-wavelength coverage in this campaign
  10. Flares 11&15: The two long duration X-ray flares
  11. Flares 35&36: Two flares preceding with the potential X-ray dimming event
  12. Flare 65: Another energetic flare showing clear broadenings of chromospheric lines
  13. Discussions
  14. Rotational modulations of the quiescent emissions
  15. Coronal properties from quiescent phase X-ray data
  16. ...and 5 more sections

Figures (44)

  • Figure 1: Summary lightcurves of the AU Mic X-ray campaign that overlap with the SMARTS/CHIRON spectroscopic data (2018 October 10 -- 25). (a)&(b) H$\alpha$ and H$\beta$ equivalent width (E.W.) values from the SMARTS/CHIRON spectroscopic data with the time cadence of 65 seconds. Gray numbers (in (a)) and gray dashed lines (in all the panels) show H$\alpha$ flares and their timings, respectively, which are identified in T23 (cf. Table 6 of T23). (c)&(d) XMM EPIC-pn X-ray (0.2 -- 12 keV) and XMM OM UVW2 data are plotted in units of counts per second, and with 30 and 10 sec binnings, respectively. Flares 11 and 15, which are discussed in detail in Section \ref{['subsec:ana_flare_11_and_15']}, are marked with the gray numbers in (c). (e)&(f) LCOGT U-band and SMARTS V-band photometric data are plotted in relative flux units, and with the time cadence of 46 and 47 seconds, respectively. (g) VLA Ku band data in the unit of flux density (mJy) and with the time binning of 10 seconds (from T25).
  • Figure 2: (a) XMM EPIC-pn X-ray (0.2 -- 12 keV) lightcurve of AU Mic for the data period of Obs-ID 0822740301 (2018 October 10 13:13:59.760 -- 2018 October 12 01:42:19.177 UTC), plotted in units of counts per second, and with 30 sec binning. The red numbers show flares identified by T23, and the start, peak, and end times of these flares (from Table 6 of T23) are shown with green and orange colored regions. (b) Same as (a) but for the EPIC-MOS1 (0.2 -- 12 keV) lightcurve and the combined one of RGS1 and RGS2 data. (c)&(d) XMM OM UVW2, LCOGT U-band, and VLA Ku band lightcurves as in Figure \ref{['fig:allEW_AUMic_XMM_opt']}(d)--(g), but for the data period of the X-ray data in (a)&(b). The flare peak times in the XMM X-ray data (in (a)) are also plotted as gray dashed lines in (c)&(d).
  • Figure 3: (a) H$\alpha$ and H$\beta$ equivalent width (E.W.) lightcurves as in Figure \ref{['fig:allEW_AUMic_XMM_opt']}(a)&(b), but for the data of 2018 October 13 and 14, which overlap with the X-ray data in (b)&(c). (b)&(c) The X-ray lightcurve data as Figure \ref{['fig:X-ray_Ha_obsid_0822740301_lc']}(a)&(b) but for Obs-ID 0822740401 (2018 October 12 13:06:33.808 to 2018 October 14 01:36:22.232 UTC). (d),(e),&(f) XMM OM UVW2, LCOGT U-band, SMARTS V-band and VLA Ku band lightcurves as in Figure \ref{['fig:allEW_AUMic_XMM_opt']}(d)--(g) and Figure \ref{['fig:X-ray_Ha_obsid_0822740301_lc']}(c)&(d), but for the data period of the X-ray data in (b)&(c)
  • Figure 4: Same as Figure \ref{['fig:X-ray_Ha_obsid_0822740401_lc']}, but for the period of the XMM X-ray data Obs-ID 0822740501 (2018 October 14 12:21:18.787 to 2018 October 16 00:04:00.273 UTC).
  • Figure 5: Same as Figure \ref{['fig:X-ray_Ha_obsid_0822740401_lc']}, but for the period of the XMM X-ray data Obs-ID 0822740601 (2018 October 16 23:39:21.362 to 2018 October 17 18:17:33.004 UTC ). No overlapping VLA Ku band data exist here (cf. Figure \ref{['fig:allEW_AUMic_XMM_opt']}).
  • ...and 39 more figures