STATISTICS OF X-RAY FLARES OF SAGITTARIUS A∗: EVIDENCE FOR SOLAR-LIKE SELF-ORGANIZED CRITICALITY PHENOMENA

Abstract

X-ray flares have routinely been observed from the supermassive black hole at our Galactic center, Sagittarius A∗ (Sgr A∗). The nature of these flares remains largely unclear, despite many theoretical models. In this paper, we study the statistical properties of the Sgr A∗ X-ray flares by fitting the count rate (CR) distribution and the structure function of the light curve with a Markov Chain Monte Carlo method. With the 3-million-second Chandra observations accumulated in the Sgr A∗ X-ray Visionary Project, we construct the theoretical light curves through Monte Carlo simulations. We find that the 2-8 keV X-ray light curve can be decomposed into a quiescent component with a constant CR of 6 × 10-3 count s-1 and a flare component with a power-law fluence distribution dN dE ∝ E-αE with αE = 1.65 ± 0.17. The duration-fluence correlation can also be modeled as a power law T ∝ EαET with αET < 0.55 (95% confidence). These statistical properties are consistent with the theoretical prediction of the self-organized criticality system with the spatial dimension S = 3. We suggest that the X-ray flares represent plasmoid ejections driven by magnetic reconnection (similar to solar flares) in the accretion flow onto the black hole.