A CHANDRA/HETGS CENSUS OF X-RAY VARIABILITY FROM Sgr A* DURING 2012

We present the first systematic analysis of the X-ray variability of Sgr A∗ during the Chandra X-ray Observatory’s 2012 Sgr A∗ X-ray Visionary Project. With 38 High Energy Transmission Grating Spectrometer observations spaced an average of 7 days apart, this unprecedented campaign enables detailed study of the X-ray emission from this supermassive black hole at high spatial, spectral and timing resolution. In 3 Ms of observations, we detect 39 X-ray flares from Sgr A∗, lasting from a few hundred seconds to approximately 8 ks, and ranging in 2–10 keV luminosity from ∼1034 erg s−1 to 2 × 1035 erg s−1. Despite tentative evidence for a gap in the distribution of flare peak count rates, there is no evidence for X-ray color differences between faint and bright flares. Our preliminary X-ray flare luminosity distribution dN/dL is consistent with a power law with index −1.9+0.3 −0.4; this is similar to some estimates of Sgr A∗’s near-IR flux distribution. The observed flares contribute one-third of the total X-ray output of Sgr A∗ during the campaign, and as much as 10% of the quiescent X-ray emission could be comprised of weak, undetected flares, which may also contribute high-frequency variability. We argue that flares may be the only source of X-ray emission from the inner accretion flow.