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 present a catalog of 9017 X-ray sources identified in Chandra observations of a 2×0.8 • field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Msec of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 × 10 32 erg s −1 (0.5-8.0 keV; 90% confidence) over an area of one square degree, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Msec) around Sgr A * . The positions of 60% of our sources are accurate to <1 ′′ (95% confidence), and 20% have positions accurate to <0. ′′ 5. We search for variable sources, and find that 3% exhibit flux variations within an observation, 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7-285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically-accreting cataclysmic variable. We compare the spatial distribution of Xray sources to a model for the stellar distribution, and find 2.8σ evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l≈0.1 • .

On the basis of 3 years of deep observations of the Galactic center with the Chandra X-Ray Observatory, we report the discovery of changes in the intensities and morphologies of two hard X-ray nebulosities. The nebulosities are dominated by fluorescent iron emission and are coincident with molecular clouds. The morphological changes are manifest on parsec scales, which requires that these iron features are scattered X-rays from a 2 or 3 year long outburst of a point source (either Sgr A or an X-ray binary) with a luminosity of at least ergs s . * 37 Ϫ110 The variability precludes the hypotheses that these nebulae either are produced by keV electrons bombarding molecular clouds or are iron-rich ejecta from supernovae. Moreover, the morphologies of the reflection nebulae implies that the dense regions of the clouds are filamentary, with widths of ≈0.3 pc and lengths of ≈2 pc.

We report on the discovery of fast-moving X-ray-emitting ejecta knots in the Galactic Oxygen-rich supernova remnant Puppis A from XMM-Newton observations. We find an X-ray knotty feature positionally coincident with an O-rich fast-moving optical filament with blue-shifted line emission located in the northeast of Puppis A. We extract spectra from northern and southern regions of the feature. Applying a one-component non-equilibrium ionization model for the two spectra, we find high metal abundances relative to the solar values in both spectra. This fact clearly shows that the feature originates from metal-rich ejecta. In addition, we find that line emission in the two regions is blue-shifted. The Doppler velocities derived in the two regions are different with each other, suggesting that the knotty feature consists of two knots that are close to each other along the line of sight. Since fast-moving O-rich optical knots/filaments are believed to be recoiled metal-rich ejecta, expelled to the opposite direction against the high-velocity central compact object, we propose that the ejecta knots disclosed here are also part of the recoiled material.

We report observations of the remnant of Supernova 1987A with the High Resolution Camera (HRC) onboard the Chandra X-ray Observatory. A direct image from the HRC resolves the annular structure of the X-ray remnant, confirming the morphology previously inferred by deconvolution of lower resolution data from the Advanced CCD Imaging Spectrometer. Detailed spatial modeling shows that the a thin ring plus a thin shell gives statistically the best description of the overall remnant structure, and suggests an outer radius 0.96 ′′ ±0.05 ′′ ±0.03 ′′ for the X-ray-emitting region, with the two uncertainties corresponding to the statistical and systematic errors, respectively. This is very similar to the radius determined by a similar modeling technique for the radio shell at a comparable epoch, in contrast to previous claims that the remnant is 10-15% smaller at X-rays than in the radio band. The HRC observations put a flux limit of 0.010 cts s −1 (99% confidence level, 0.08-10 keV range) on any compact source at the remnant center. Assuming the same foreground neutral hydrogen column density as towards the remnant, this allows us to rule out an unobscured neutron star with surface temperature T ∞ > 2.5 MK observed at infinity, a bright pulsar wind nebula or a magnetar.