Oscillations in stellar superflares

Abstract

Two different mechanisms may act to induce quasi-periodic pulsations (QPP) in wholedisk observations of stellar flares. One mechanism may be magneto-hydromagnetic (MHD) forces and other processes acting on flare loops as seen in the Sun. The other mechanism may be forced local acoustic oscillations due to the high-energy particle impulse generated by the flare (known as 'sunquakes' in the Sun). We analyze shortcadence Kepler data of 257 flares in 75 stars to search for QPP in the flare decay branch or post-flare oscillations which may be attributed to either of these two mechanisms. About 18 percent of stellar flares show a distinct bump in the flare decay branch of unknown origin. The bump does not seem to be a highly-damped global oscillation because the periods of the bumps derived from wavelet analysis do not correlate with any stellar parameter. We detected damped oscillations covering several cycles (QPP), in seven flares on five stars. The periods of these oscillations also do not correlate with any stellar parameter, suggesting that these may be a due to flare loop oscillations. We searched for forced global oscillations which might result after a strong flare. To this end, we investigated the behaviour of the amplitudes of solar-like oscillations in eight stars before and after a flare. However, no clear amplitude change could be detected. We also analyzed the amplitudes of the self-excited pulsations in two δ Scuti stars and one γ Doradus star before and after a flare. Again, no clear amplitude changes were found. Our conclusions are that a new process needs to be found to explain the high incidence of bumps in stellar flare light curves, that flare loop oscillations may have been detected in a few stars and that no conclusive evidence exists as yet for flare induced global acoustic oscillations (starquakes).

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

“…A systematic study of QPPs in stellar white light flares was recently performed by Balona et al (2015); Pugh et al (2016). Typically, stellar QPP are seen as a periodic, decaying variation of the signal after the flare peak, which resembles SUMER oscillations in solar flares.…”

“…The similarity in solar and stellar flares, including the QPPs may give us hints to understand the nature of the flares and associated oscillations, as well as coronal plasma properties for various stars including the Sun (Balona et al 2015;Chang et al 2015). In particular, properties of oscillating loops could be estimated by applying an appropriate model to the frequency and amplitude modulation of the perturbation, which are independent on the sharpness and strength of the sources.…”

Quasi-Periodic Pulsations During the Impulsive and Decay Phases of an X-Class Flare

“…These pulsations, known as quasi-periodic pulsations (QPPs), are variations of flux as a function of time. Their nature has been examined in many previous studies of both solar (e.g., Parks & Winckler 1969;Kane et al 1983;Asai et al 2001;Fleishman et al 2008;Nakariakov & Melnikov 2009;Reznikova & Shibasaki 2011;Li et al 2015) and stellar (e.g., Balona et al 2015;Pugh et al 2016) observations. In a typical event, the emission from a solar flare is seen to pulsate with a characteristic timescale ranging from 1 s up to several minutes.…”

Seismological insights into solar and stellar magnetic activity cycles

“…This not only implies a link between stellar flares and stellar magnetic activity but that the same physical processes are involved in solar and stellar flares. Balona et al [52] demonstrated that QPPs can be observed in Kepler flare light curves. Pugh et al [51] searched the entire Kepler short cadence database for flares with evidence of QPPs and Figure 10 shows an example of one such flare.…”