Dissecting X-ray–Emitting Gas Around the Center of Our Galaxy

The Galaxy Center in X-rays At the center of our Galaxy there is a black hole 4-million-fold more massive than the Sun. Wang et al. (p. 981; see the Perspective by Schnittman) report x-ray data on the accretion flow around this supermassive black hole, revealing how it interacts with its surroundings. The data rule out the possibility that the quiescent (that is, flare-free) x-rays observed are produced by coronal emission from a population of stars at the center of the Galaxy and also rule out the possibility that there is a pure radiatively inefficient accretion flow with no outflows. X-ray observations of the center of our Galaxy reveal the interplay between the massive black hole there and its surroundings. [Also see Perspective by Schnittman] Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy’s SMBH provides an instructive exception, and we present a close-up view of its quiescent x-ray emission based on 3 megaseconds of Chandra observations. Although the x-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission on the basis of the lack of predicted iron (Fe) Kα emission. The extremely weak hydrogen (H)–like Fe Kα line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.