Quasi-Periodic Pulsations in Solar and Stellar Flares: Re-Evaluating Their Nature in the Context of Power-Law Flare Fourier Spectra

Abstract

The nature of quasi-periodic pulsations in solar and stellar flares remains debated. Recent work has shown that power-law-like Fourier power spectra, also referred to as 'red' noise processes, are an intrinsic property of solar and stellar flare signals, a property that many previous studies of this phenomenon have not accounted for. Hence a re-evaluation of the existing interpretations and assumptions regarding QPP is needed. Here we adopt a Bayesian method for investigating this phenomenon, fully considering the Fourier power law properties of flare signals. Using data from the PROBA2/LYRA, Fermi/GBM, Nobeyama Radioheliograph and Yohkoh/HXT instruments, we study a selection of flares from the literature identified as QPP events. Additionally we examine optical data from a recent stellar flare that appears to exhibit oscillatory properties. We find that, for all but one event tested, an explicit oscillation is not required in order to explain the observations. Instead, the flare signals are adequately described as a manifestation of a power law in the Fourier power spectrum, rather than a direct signature of oscillating components or structures. However, for the flare of 1998 May 8, strong evidence for the existence of an explicit oscillation with P ≈ 14-16 s is found in the 17 GHz radio data and the 13-23 keV Yohkoh HXT data. We conclude that, most likely, many previously analysed events in the literature may be similarly described in terms of power laws in the flare Fourier power spectrum, without the need to invoke a narrowband, oscillatory component. As a result the prevalence of oscillatory signatures in solar and stellar flares may be less than previously believed. The physical mechanism behind the appearance of the observed power laws is discussed.

Above the Noise: The Search for Periodicities in the Inner Heliosphere

“…Several articles, particularly in the context of quasi-periodic pulsations (QPPs) associated with solar flares, now incorporate the fundamental power-law nature of power spectra in the analysis of coronal time series, but they use several different approaches (e.g. Ireland, McAteer, and Inglis, 2015;Inglis, Ireland, and Dominique, 2015;Auchère et al, 2016;Pugh, Broomhall, and Nakariakov, 2017). We based our analysis on Auchère et al (2016), who proposed a new method for deriving global confidence levels, and we tested it (together with an appropriate background-noise model) using long, high-cadence light curves derived from specific SDO/AIA observations.…”

“…A growing recent consensus (e.g. Inglis, Ireland, and Dominique, 2015;Ireland, McAteer, and Inglis, 2015;Auchère et al, 2016) strongly cautions against the application of white-noise confidence levels for similar time series that have also been detrended, in the context of solar oscillations.…”

“…Inglis, Ireland, and Dominique, 2015;Ireland, McAteer, and Inglis, 2015;Auchère et al, 2016), noting the likelihood of false detections of periodic features close to the cutoff frequency of the detrending filter. In the Appendix, we note that some of the bins observed above confidence lie close to but below the 15-hour period of the detrending filter.…”

“…A growing body of examples across a range of astrophysical phenomena, including X-ray binaries (van der Klis, 1995), Seyfert galaxies (Lawrence et al, 1987;Markowitz et al, 2003), gamma-ray bursts (Ukwatta et al, 2009;Cenko et al, 2010), and quasiperiodic pulsations (QPPs) during solar flares (e.g. Inglis, Ireland, and Dominique, 2015) yield power-law-like spectra. Contributions by erratic, aperiodic changes in brightness lead to spectral power P of the form P ∝ f −α for frequencies f .…”

“…Several recent articles (e.g. Gruber et al, 2011;Inglis, Ireland, and Dominique, 2015;Ireland, McAteer, and Inglis, 2015;Auchère et al, 2016) have invalidated several reported QPP observations (e.g. from Inglis and Nakariakov, 2009;Nakariakov et al, 2010) that originally incorporated detrending and did not account for the power-law nature of the recovered spectra.…”