We examine the spectrum of diffuse emission detected in the 17 ′ by 17 ′ field around Sgr A * during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with originating in a two-temperature plasma, as well as a prominent lowionization Fe K-α line. The cooler, kT ≈ 0.8 keV plasma differs in surface brightness across the image between (0.2 − 1.8) × 10 −13 erg cm −2 s −1 arcmin −2 (observed, 2-8 keV). This soft plasma is probably heated by supernovae, along with a small contribution from the winds of massive Wolf-Rayet and O stars. The radiative cooling rate of the soft plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 3 × 10 5 y. The hotter, kT ≈ 8 keV component is more spatially uniform, with a surface brightness of (1.5 − 2.6) × 10 −13 erg cm −2 s −1 arcmin −2 (observed; 2-8) keV. The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because neither supernova remnants nor WR/O stars are observed to produce thermal plasma hotter than kT ≈ 3 keV. Moreover, a kT ≈ 8 keV plasma would be too hot to be bound to the Galactic center, and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy is ∼ 10 40 erg s −1 . This corresponds to the entire kinetic energy of one supernova every 3000 y, which is unreasonably high. However, alternative explanations for the kT ≈ 8 keV diffuse emission are equally unsatisfying. The hard X-rays are unlikely to result from undetected point sources, because no known population of stellar object is numerous enough to the observed surface brightness. There is also no evidence that non-thermal mechanisms for producing the hard emission are operating, as the expected shifts in the line energies and ratios from their collisional equilibrium values are not observed. We are left to conclude that either there is a significant shortcoming in our understanding of the mechanisms that heat the interstellar medium, or that a population of faint (< 10 31 erg s −1 ), hard X-ray sources that are a factor of 10 more numerous than CVs remains to be discovered.

We report on the X-ray emission from the shell-like, nonthermal radio source Sgr A East (SNR 000.0+00.0), located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-Ray Observatory. This is the first time Sgr A East has been clearly resolved from other complex structures in the region. The X-ray-emitting region is concentrated within the central '2 pc of the larger radio shell. The spectrum shows strong K lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature $2 keV, absorption column $1 Â 10 23 H cm À2 , luminosity $8 Â 10 34 ergs s À1 in the 2-10 keV band, and gas mass $2 1=2 M with a filling factor . The plasma appears to be rich in heavy elements, overabundant by roughly a factor of 4 with respect to solar abundances, and shows a spatial gradient of elemental abundance; the spatial distribution of iron is more compact than that of the lighter elements. The gas mass and elemental abundance of the X-ray emission support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR), perhaps produced by the Type II supernova explosion of a massive star with a main-sequence mass of 13-20 M . The combination of the radio and X-ray morphologies classifies Sgr A East as a new metal-rich '' mixed morphology '' (MM) SNR. The size of the Sgr A East radio shell is the smallest of the known MM SNRs, which strongly suggests that the ejecta have expanded into a very dense interstellar medium. The ejecta-dominated chemical compositions of the plasma indicate that the ambient materials should be highly homogeneous. We thus evaluate a simplified dynamical evolution model where an SNR was formed about 10,000 yr ago and expanded into an ambient medium with a homogeneous density of 10 3 cm À3 . The model roughly reproduces most of the observed properties in the X-ray and radio wavelengths. A comparison with the radio observations requires the dense ambient medium to be ionized, but a luminous X-ray irradiator with an expected X-ray luminosity of $10 40 ergs s À1 is not currently present. The presence of the ionized gas may be explained if the massive black hole (MBH) associated with the compact, nonthermal radio source Sgr A* was bright in X-rays about 300 yr ago but is presently dim. It is possible that the dust/molecular ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past and may have removed material from the black hole vicinity, leading to its present quiescent state. This may be a specific example of the intimate relationship between an SNR and MBH accretion activity in galactic nuclei.

Sgr AÃ , the massive black hole at the center of the Galaxy, varies in radio through X-ray emission on hourly timescales. The flare activity is thought to arise from the innermost region of an accretion flow onto Sgr A Ã . We present simultaneous light curves of Sgr A Ã in radio, submillimeter and X-rays that show a possible time delay of 110 AE 17 minutes between X-ray and 850 m suggesting that the submillimeter flare emission is optically thick. At radio wavelengths, we detect time lags of 20:4 AE 6:8, 30 AE 12, and 20 AE 6 minutes between the flare peaks observed at 13 and 7 mm (22 and 43 GHz) in three different epochs using the VLA. Linear polarization of 1% AE 0:2% and 0:7 AE 0:1% is detected at 7 and 13 mm, respectively, when averaged over the entire observation on 2006 July 17. A simple model of a bubble of synchrotron-emitting electrons cooling via adiabatic expansion can explain the time delay between various wavelengths, the asymmetric shape of the light curves, and the observed polarization of the flare emission at 43 and 22 GHz. The derived physical quantities that characterize the emission give an expansion speed of v exp $ 0:003Y0:1 c, magnetic field of B $ 10Y70 G, and particle spectral index p $ 1Y2. These parameters suggest that the associated plasma cannot escape from Sgr A Ã unless it has a large bulk motion.

We present a catalog of 34 diffuse features identified in X-ray images of the Galactic center taken with the Chandra X-ray Observatory. Several of the features have been discussed in the literature previously, including 7 that are associated with a complex of molecular clouds that exhibits fluorescent line emission, 4 that are superimposed on the supernova remnant Sgr A East, 2 that are coincident with radio features that are thought to be the shell of another supernova remnant, and one that is thought to be a pulsar wind nebula only a few arcseconds in projection from Sgr A * . However, this leaves 20 features that have not been reported previously. Based on the weakness of iron emission in their spectra, we propose that most of them are non-thermal. One long, narrow feature points toward Sgr A * , and so we propose that this feature is a jet of synchrotron-emitting particles ejected from the supermassive black hole. For the others, we show that their sizes (0.1-2 pc in length for D=8 kpc), X-ray luminosities (between 10 32 and 10 34 erg s −1 , 2-8 keV), and spectra (power laws with Γ = 1 − 3) are consistent with those of pulsar wind nebulae. Based on the star formation rate at the Galactic center, we expect that ∼20 pulsars have formed in the last 300 kyr, and could be producing pulsar wind nebulae. Only one of the 19 candidate pulsar wind nebulae is securely detected in an archival radio image of the Galactic center; the remainder have upper limits corresponding to L R 10 31 erg s −1 . These radio limits do not strongly constrain their natures, which underscores the need for further multi-wavelength studies of this unprecedented sample of Galactic X-ray emitting structures.