Sources of Long-Period X-ray Pulsations before the Onset of Solar Flares

Abstract: According to Tan et al. (2016), long-period (with a period P = 1.9‒47.3 min and a duration of 1‒2 h) quasi-periodic pulsations (QPPs) are observed in soft Х-rays before a significant fraction (26‒46%) of “isolated” solar flares. The results were obtained from the GOES/XRS data without spatial resolution. In this paper, we analyzed the sources of such QPPs before 35 “isolated” X-class flares based on ”quick-look” RHESSI images in the 6–12 keV range and found that the events can be divided into two types. In type I events, the sources of all QPPs and the main flare are located in the same active region (AR) on the Sun, while in type II events the sources of at least part of the QPPs are located in a different AR than the flare AR. A more detailed analysis of two type I events and three type II events using RHESSI X-ray and SDO/AIA ultraviolet images shows that the sources of X-ray pulsations in one AR are located in different places (within ~20 Mm from each other and from the main flare) and their appearance corresponds to the appearance of new loop-shaped ultraviolet sources. We present observational arguments, which are not in favor of the mechanisms proposed by Tan et al. (2016) based on oscillations of coronal loops as LRC electric circuits or MHD oscillations of loops. The mechanism of oscillating reconnection seems to be more promising for explaining the QPPs under consideration. In type I events, it occurs in one AR, while in type II events it can occur in parallel in several separated ARs; to explain this it is necessary to assume the coherence of the subphotospheric emergence of magnetic fluxes in different parts of the Sun. This assumption requires further verification.