The first observed stellar X-ray flare oscillation: Constraints on the flare loop length and the magnetic field

Abstract

Abstract. We present the first X-ray observation of an oscillation during a stellar flare. The flare occurred on the active M-type dwarf AT Mic and was observed with XMM-Newton. The soft X-ray light curve (0.2-12 keV) is investigated with wavelet analysis. The flare's extended, flat peak shows clear evidence for a damped oscillation with a period of around 750 s, an exponential damping time of around 2000 s, and an initial, relative peak-to-peak amplitude of around 15%. We suggest that the oscillation is a standing magneto-acoustic wave tied to the flare loop, and find that the most likely interpretation is a longitudinal, slow-mode wave, with a resulting loop length of (2.5 ± 0.2) × 10 10 cm. The local magnetic field strength is found to be 105 ± 50 G. These values are consistent with (oscillation-independent) flare cooling time models and pressure balance scaling laws. Such a flare oscillation provides an excellent opportunity to obtain coronal properties like the size of a flare loop or the local magnetic field strength for the otherwise spatially-unresolved star.Key words. stars: coronae -stars: flare -stars: magnetic fields -stars: oscillations -X-rays: stars -stars: individual: AT Mic IntroductionOscillations in the solar corona have by now been observed for many years. Wavelength regimes ranging from radio to hard X-rays have been investigated in the search for evidence of waves. Table 1 in Aschwanden et al. (1999) provides a summary and description of the different periods found (0.02 to 1000 s). Most of these waves have been explained by magneto-hydrodynamic (MHD) oscillations in coronal loops, see Roberts (2000) for a detailed review of waves and oscillations in the corona.One of the most exciting aspects of "coronal seismology" is that it potentially provides us with the capability for determining the magnetic field strength in the corona (Roberts et al. 1984), as well as, in the stellar case, with information on otherwise unresolved spatial scales, e.g., flare loop lengths. It is notoriously difficult to measure the magnetic field strength in the corona because of the very high speeds of coronal electrons, which broaden spectral lines far beyond the width of Zeeman splitting. Techniques using both near-infrared emission lines and radio observations are successful, but have poor spatial resolution. Indirect methods are commonly used, such as the extrapolations of the coronal magnetic field from the photospheric magnetic field, which in turn can be measured using the Zeeman effect.Many of the observations of waves have been determined from variations in intensity. However, a huge step forward was achieved in solar coronal physics due to the high spatial resolution available with the Transition Region and Coronal Explorer (TRACE, Handy et al. 1999). Aschwanden et al. (1999) observed the first spatial displacement oscillations of coronal loops. It was suggested that these oscillations were triggered by a fast-mode shock from a flare site, and they were interpreted as standing fast kink-mode waves. Nakari...

Quasi-periodic modulation of solar and stellar flaring emission by magnetohydrodynamic oscillations in a nearby loop

“…QPP with similar parameters have been detected in stellar flares (see, e.g. Mathioudakis et al 2003;Mitra-Kraev et al 2005, and references therein). Typical periods of QPP are in the range of a few ms to several min.…”

Comparison of Damped Oscillations in Solar and Stellar X-Ray Flares

“…For example, Mitra-Kraev et al (2005) interpreted the soft X-ray QPP in a flare the dwarf AT Mic, as a standing slow magnetoacoustic oscillation. This interpretation allowed the authors to estimate the temperature of the flaring loop as 13 MK.…”

“…Particularly, stellar X-ray flares can provide proper parameters such as the emission measure, temperature, abundance, and density of the flaring loops (e.g., Raassen et al 2007;Pandey & Karmakar 2015), which are essential for the indirect determination of the coronal magnetic field strength. First attempts to use QPP for stellar coronal seismology have been made by Mitra-Kraev et al (2005); Pandey & Srivastava (2009);Anfinogentov et al (2013); Srivastava et al (2013).…”

Astrophysics in 2005

“…After the longitudinal MHD slowmode oscillations were discovered in soft X-rays with SOHO SUMER a few years ago, they could also be rediscovered in Yohkoh data (Mariska 2005(Mariska , 2006, and were also claimed to be discovered on the M-type dwarf AT Mic with the XMMNewton telescope (Mitra-Kraev et al 2005).…”