2019ApJ...886...96H

We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600x and 245x greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology-it lasts 5.7 ks (∼2 hr), with a rapid rise time of 1500 s and a decay time of 2500 s. The second flare lasts 3.4 ks, with rise and decay times of 1700 and 1400 s. These luminous flares are significantly harder than quiescence: the first has a power-law spectral index Γ = 2.06 ± 0.14 and the second has Γ = 2.03 ± 0.27, compared to Γ = 3.0 ± 0.2 for the quiescent accretion flow. These spectral indices (as well as the flare hardness ratios) are consistent with previously detected Sgr A* flares, suggesting that bright and faint flares arise from similar physical processes. Leveraging the brightest flare's long duration and high signal-to-noise, we search for intraflare variability and detect excess X-ray power at a frequency of ν ≃ 3 mHz, but show that it is an instrumental artifact and not of astrophysical origin. We find no other evidence (at the 95% confidence level) for periodic or quasi-periodic variability in either flares' time series. We also search for nonperiodic excess power but do not find compelling evidence in the power spectrum. Bright flares like these remain our most promising avenue for identifying Sgr A*'s short timescale variability in the X-ray, which may probe the characteristic size scale for the X-ray emission region.