A Chandra/HETGS Census of X-ray Variability From Sgr A* During 2012

TL;DR

This study systematically analyzes 2012 Chandra X-ray observations of Sgr A*, detecting 39 flares, characterizing their properties, and suggesting flares are the primary X-ray emission source from the black hole's inner accretion flow.

Contribution

First detailed high-resolution X-ray variability analysis of Sgr A* during 2012, providing flare statistics and insights into their contribution to X-ray emission.

Findings

39 X-ray flares detected with durations up to 8 ks.

Flare luminosity distribution follows a power law with index -1.9.

Flares account for one-third of total X-ray output.

Abstract

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 (XVP). With 38 High Energy Transmission Grating Spectrometer (HETGS) 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 ~1e34 erg/s to 2e35 erg/s. 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 NIR 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.