One of the important activities at the Galactic Center is that represented by the apparent supernova remnant, Sagittarius A East, which is relatively close to the central black hole. This non-thermal shell source has been well studied at radio wavelengths for some time (Ekers et al 1983; Yusef-Zadeh and Morris 1987b; Mezger et al 1989; Pedlar et al 1989). The radio to submillimeter studies reveal that Sgr A East lies largely behind Sgr A West, although perhaps not entirely; some portion of it must lie in front. In addition, the shell source surrounds Sgr A^* and Sgr A W in projection, though its center is displaced from Sgr A^* by about 2.5 pc. The likelihood that Sgr A East has affected the dynamics and the geometry of the CND has been discussed by Morris and Serabyn (1996) and by Yusef-Zadeh et al (1999).

The idea that Sgr A East is a supernova remnant has been debated in the literature because, while it has the appropriate morphology, its energy requirements are unusually large (e.g. Mezger et al 1989). This led some to consider that Sgr A East might have resulted from the release of energy from the central black hole, in spite of the fact that it is offset from Sgr A^* by several parsecs. One hypothesis that accounts for that offset posits that Sgr A East resulted from the tidal disruption of a star by the central black hole (Khokhlov and Melia 1996). Such an event should occur every 10⁴ to 10⁵ years, consistent with the 10⁴ year expansion age of the Sgr A East shell. When this happens, about half of the stellar mass is ejected as a spray into a large solid angle with an energy well in excess of that of a supernova. The subsequent expansion of the relativistic ejecta could thereby produce a shell source resembling Sgr A East. More recently, Sgr A East was studied in detail by Maeda et al (2002) with the Chandra X-Ray Observatory (CXO). The X-ray image (figure 1.1), when combined with the radio data, suggests that Sgr A East is a mixed-morphology supernova remnant, which means in this case that it has a spherical radio shell surrounding a centrally concentrated X-ray continuum source. In addition, the 6.7 keV iron emission line is concentrated toward the center of the shell, consistent with Sgr A East being a supernova remnant. The implied high metallicity and the placement of the iron line emission is not accounted for by hypotheses invoking stellar disruption or an energy release from the stellar black hole.

One of the reasons that Sgr A East is said to be unusually energetic for a supernova remnant is that it has apparently compressed the dust and gas in the interstellar medium around it, particularly on the Eastern side of its periphery, where a ridge of dense molecular gas abuts the edge of the shell. This ridge is apparently the compressed portion of the 50 km s⁻¹ molecular cloud (Serabyn et al 1992; Uchida et al 1998). Within this ridge, and aligned with the edge of the Sgr A East shell, is a string of compact H II regions, G-0.02-0.07, indicating that massive stars have recently formed within the compressed gas ridge. While one is tempted to conclude that the young stars responsible for these H II regions were formed as a result of the cloud compression by Sgr A East, there is a timescale problem: it takes substantially longer to form stars (∼10⁵ years) than the expansion time of Sgr A East.

The question of whether the shell of Sgr A East has overrun the central black hole is an important one for understanding the environment of the black hole. The shell of Sgr A East coincides roughly with the inner edge of the CND on the side opposite to the center of Sgr A East (although that shell is not terribly well defined there), so it is entirely possible that the inner parsec of the Galaxy lies within Sgr A East, where the gas in most of the volume is extremely hot (∼10⁷–10⁸ K). The unknown quantity is the line-of-sight displacement between them. If the shell has passed over the black hole within the past several hundred years, as considered

Figure 1.1. Smoothed X-ray image of Sgr A East (1.5–7.0 keV) with superimposed 20 cm radio contours, from Maeda et al (2002). See also color section.

by Maeda et al (2002), then because of the strongly enhanced density of the shell, this event may have led to an energetic accretion event. The dynamics of such an event, or of the gas presently surrounding the black hole if the shell has already swept through the center, are complicated by the strong ram pressure of the winds emanating from the hot stars in the central parsec. Indeed, most investigators assume that the black hole resides within the stellar wind bubble. However, the current Chandra X-ray picture, which shows continuum X-ray emission extended throughout this region, is consistent with the idea that much of the volume of the central parsec is filled with a hot gas.