A close-up of the nearest supermasive black hole --- Sgr A*

Supermassive black holes (SMBHs) are typically low in radiation luminosity. But the collective mechanical feedback from such radiatively inefficient SMBHs could rival or even exceed that from active galactic nuclei and could hence strongly affect galaxy evolution.  Our own Galaxy’s SMBH, Sgr A*, provides an excellent laboratory to study this radiatively inefficient accretion state. I will review some of the recent progresses in understanding Sgr A*. In particular,  a close-up view of its x-ray emission based on recent 3 megaseconds of Chandra observations is providing new insights into the interplay of the SMBH with its environment. The X-ray spectrum of Sgr A* shows multiple emission lines of highly ionized ions. This, together with the lack of the neutral iron K-alaph line, suggests that the accretion of the SMBH is truly in a hot mode. A spectral modeling further suggests that the accretion must be accompanied by an outflow, which likely removes more than 99% of the material initially captured by the SMBH. This conclusion is confirmed in a fitting of the spatially-resolved X-ray emission structure with a simulated accretion/outflow flow, which also enables us to determine its angular momentum. Both the fitted position and inclination angles of the flow are consistent with those of the surrounding disk of massive stars, confirming that their stellar winds are feeding the SMBH.  If time allows, I will discuss what we are learning from time variability analysis and ongoing multi-wavelength monitoring of Sgr A*,  as well as a comparison with other nearby low-luminosity SMBHs.