We present results of our Chandra observation with ACIS-I centered on the position of Sagittarius A * (Sgr A * ), the compact nonthermal radio source associated with the massive black hole (MBH) at the dynamical center of the Milky Way Galaxy. We have obtained the first high spatial resolution (≈ 1 ′′ ), hard X-ray (0.5-7 keV) image of the central 40 pc (17 ′ ) of the Galaxy.We have discovered an X-ray source, CXOGC J174540.0−290027, coincident with the radio position of Sgr A * to within 0. ′′ 35, corresponding to a maximum projected distance of 16 light-days for an assumed distance to the center of the Galaxy of 8.0 kpc. We received 222 ± 17 (1σ) net counts from the source in 40.3 ks. The source is detected with high significance, S/N ≃ 37σ, despite the highly elevated diffuse X-ray background in the central parsec of the Galaxy. Due to the low number of counts, the spectrum is well fit either by an absorbed power-law model with photon index Γ = 2.7 +1.3 −0.9 (N (E) ∝ E −Γ photons cm −2 s −1 keV −1 ) and column density N H = (9.8 +4.4 −3.0 ) × 10 22 cm −2 (90% confidence interval) or by an absorbed optically thin thermal plasma model with kT = 1.9 +0.9 −0.5 keV and N H = (11.5 +4.4 −3.1 ) × 10 22 cm −2 . Using the power-law model, the measured (absorbed) flux in the 2-10 keV band is (1.3 +0.4 −0.2 ) × 10 −13 ergs cm −2 s −1 , and the absorption-corrected luminosity is (2.4 +3.0 −0.6 ) × 10 33 ergs s −1 . The X-ray source coincident with Sgr A * is resolved, with an apparent diameter of ≈ 1 ′′ . We report the possible detection, at the 2.7σ significance level, of rapid continuum variability on a timescale of several hours. We also report the possible detection of an Fe Kα line at the ≃ 2σ level. The long-term variability of Sgr A * is constrained via comparison with the ROSAT /PSPC observation in 1992. The origin of the X-ray emission (MBH vs. stellar) and the implications of our observation for the various proposed MBH emission mechanisms are discussed. The current observations, while of limited signalto-noise, are consistent with the presence of both thermal and nonthermal emission components in the Sgr A * spectrum.We also briefly discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane and present morphological evidence that Sgr A * and Sgr A West lie within the hot plasma in the central cavity of Sgr A East. Over 150 point sources are detected in the 17 ′ × 17 ′ field of view. Our survey of X-ray sources is complete down to a limiting 2-10 keV absorbed flux of F X ≈ 1.7 × 10 −14 ergs cm −2 s −1 . For sources at the distance of the Galactic Center, the corresponding absorption-corrected luminosity is L X ≈ 2.5 × 10 32 ergs s −1 . The complete flux-limited sample contains 85 sources. Finally, we present an analysis of the integrated emission from the detected point sources and the diffuse emission within the central 0.4 pc (10 ′′ ) of the Galaxy.

The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hours exposure in two observations. We detect 1075 X-ray sources, most with sub-arcsecond positional accuracy. Ninety-one percent of the sources are spatially associated with known stellar members of the cluster, and an additional 7% are newly identified deeply embedded cloud members. This provides the largest Xray study of a pre-main sequence stellar population and covers the initial mass function from brown dwarfs up to a 45 M ⊙ O star. Source luminosities span 5 orders of magnitude from log L x ≃ 28.0 to 33.3 erg s −1 in the 0.5 − 8 keV band, plasma energies range from 0.2 to >10 keV, and absorption ranges from log N H < 20.0 to ∼ 23.5 cm −2 . Comprehensive tables providing X-ray and stellar characteristics are provided electronically.We examine here the X-ray properties of Orion young stars as a function of mass; other studies of astrophysical interest will appear in companion papers. Results include: (a) the discovery of rapid variability in the O9.5 31 M ⊙ star θ 2 A Ori, and several early B stars, inconsistent with the standard model of X-ray production in small shocks distributed throughout the radiatively accelerated wind; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G-through M-type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the 'saturation' level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray -2emission; (e) evidence that T Tauri plasma temperatures are often very high with T ≥ 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main sequence very low mass objects showing flaring levels similar to those seen in more massive T Tauri stars and a decline in magnetic activity as they evolve into L-and T-type brown dwarfs.

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.

To address the role of energetic processes in the solar nebula, we provide a detailed characterization of magnetic flaring in stellar analogs of the pre-main-sequence Sun based on two 0.5 day observations of the Orion Nebula cluster obtained with the Chandra X-Ray Observatory. The sample consists of 43 stars with masses between 0.7 and 1.4 M and ages from less than 0.3 to '10 Myr. We find that the X-ray luminosities measured in the 0.5-8 keV band are strongly elevated over main-sequence levels with an average log L X h i¼ 30:3 ergs s À1 and log L X =L Ã h i¼ À3:9. The X-ray emission is strongly variable within our exposures in nearly all solar analogs; about 30 flares with 29:0 ergs s À1 < log L X ðpeakÞ < 31:5 ergs s À1 on timescales from 0.5 to more than 12 hr are seen during the Chandra observations. Analogs of the 1 Myr old premain-sequence Sun exhibited X-ray flares that are 10 1.5 times more powerful and 10 2.5 times more frequent than the most powerful flares seen on the contemporary Sun. Radio observations indicate that acceleration of particles to relativistic energies is efficient in young stellar flares. Extrapolating the solar relationship between X-ray luminosity and proton fluence, we infer that the young Sun exhibited a 10 5 -fold enhancement in energetic protons compared to contemporary levels. Unless the flare geometries are unfavorable, this inferred proton flux on the disk is sufficient to produce the observed meteoritic abundances of several important short-lived radioactive isotopes. Our study thus strengthens the astronomical foundation for local proton spallation models of isotopic anomalies in carbonaceous chondritic meteorites. The radiation, particles, and shocks produced by the magnetic reconnection flares seen with Chandra may also have flash-melted meteoritic chondrules and produce excess 21 Ne seen in meteoritic grains.

About 1000 X-ray emitting young pre-main sequence (PMS) stars distributed in mass from ∼0.05 M ⊙ brown dwarfs to a ∼ 50 M ⊙ O star are detected in an image of the Orion Nebula obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory. This is the richest field of sources ever obtained in X-ray astronomy. Individual X-ray luminosities in the Orion Nebula Cluster range from the sensitivity limit of < 2 × 10 28 erg s −1 to ∼ 10 32 erg s −1 . ACIS sources include 85 − 90% of V < 20 stars, plus a lower but substantial fraction of deeply embedded stars with extinctions as high as A V ≃ 60.The relationships between X-ray and other PMS stellar properties suggest that X-ray luminosity of lower-mass PMS stars depends more on mass, and possibly stellar rotation, than on bolometric luminosity as widely reported. In a subsample of 17 unabsorbed stars with mass ≃ 1 M ⊙ , X-ray luminosities are constant at a high level around L x ≃ 2 × 10 30 erg s −1 for the first ≃ 2 My while descending the convective Hayashi track, but diverge during the 2−10 My phase with X-ray emission plummeting in some stars but remaining high in others. This behavior is consistent with the distribution of X-ray luminosities on the zero-age main sequence and with current theories of their rotational history and magnetic dynamos.The sources in the Becklin-Neugebauer/Kleinman-Low (BN/KL) region of massive star formation are discussed in detail. They include both unabsorbed and embedded low-mass members of the Orion Nebula Cluster, the luminous infrared Source n, and a class of sources without optical or infrared counterparts that may be new magnetically active embedded PMS stars. Several X-ray sources are also variable radio emitters, an association often seen in magnetically active PMS stars. Faint X-ray emission is seen close to, but apparently not coincident with the Becklin-Neugebauer object. Its nature is not clear.