Volume 533, September 2011
|Number of page(s)||7|
|Published online||26 August 2011|
Quasi-periodic pulsations in solar flares: new clues from the Fermi Gamma-Ray Burst Monitor
Max Planck Institute for Extraterrestrial Physics,
Giessenbachstrasse, Postfach 1312,
2 Green Cross Capital Pty Ltd, 495 Harris St, Ultimo, NSW 2007, Australia
3 Institute of Astro and Particle Physics, University Innsbruck, Technikerstrasse 25, 6176 Innsbruck, Austria
4 University of Alabama in Huntsville, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
5 Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545, USA
6 Universities Space Research Association, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
7 Space Science Office, VP62, NASA/Marshall Space Flight Center Huntsville, AL 35812, USA
Received: 14 April 2011
Accepted: 21 July 2011
Aims. In the past four decades, it has been observed that solar flares display quasi-periodic pulsations (QPPs) from the lowest, i.e. radio, to the highest, i.e. gamma-ray, frequencies in the electromagnetic spectrum. It remains unclear which mechanism creates these QPPs. In this paper, we analyze four bright solar flares that display compelling signatures of quasi-periodic behavior and were observed with the Gamma-Ray Burst Monitor (GBM ) onboard the Fermi satellite. Because GBM covers over three decades in energy (8 keV to 40 MeV), it is regarded as a key instrument in our attempt to understand the physical processes that drive solar flares.
Methods. We tested for periodicity in the time series of the solar flares observed by GBM by applying a classical periodogram analysis. However, in contrast to previous authors, we did not detrend the raw light curve before creating the power spectral density (PSD) spectrum. To assess the significance of the frequencies, we used a method that is commonly applied to X-ray binaries and Seyfert galaxies. This technique takes into account the underlying continuum of the PSD, which for all of these sources has a P(f) ~ f−α dependence and is typically labeled red-noise.
Results. We checked the reliability of this technique by applying it to observations of a solar flare that had been observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI ). These data contain, besides any potential periodicity from the Sun, a 4 s rotational period caused by the rotation of the spacecraft about its axis. We were unable to identify any intrinsic solar quasi-periodic pulsation but we did manage to reproduce the instrumental periodicity. Moreover, with the method adopted here, we do not detect significant QPPs in the four bright solar flares observed by GBM. We stress that for this kind of analyses it is of utmost importance to account appropriately for the red-noise component in the PSD of these astrophysical sources.
Key words: Sun: flares / methods: statistical / methods: data analysis / methods: observational
© ESO, 2011
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