ChandraX‐Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy

Abstract

We report the results of the first-epoch observation with the ACIS-I instrument on the Chandra X-Ray Observatory of Sagittarius A* (Sgr A*), the compact radio source associated with the supermassive black hole (SMBH) at the dynamical center of the Milky Way. This observation produced the first X-ray (0.5-7 keV) spectroscopic image with arcsecond resolution of the central 17′ × 17′ (40 pc × 40 pc) of the Galaxy. We report the discovery of an X-ray source, CXOGC J174540.0-290027, coincident with Sgr A* within 0.″27 ± 0.″18. The probability of a false match is estimated to be ≲0.5%. The spectrum is well fitted either by an absorbed power law with photon index Γ ≈ 2.7 or by an absorbed optically thin thermal plasma with kT ≈ 1.9 keV and column density NH ≈ 1 × 1023 cm-2. The observed flux in the 2-10 keV band is ≈1.3 × 10-13 ergs cm-2 s-1, and the absorption-corrected luminosity is ≈2.4 × 1033 ergs s-1. The X-ray emission at the position of Sgr A* is extended, with an intrinsic size of ≈1.″4 (FWHM), consistent with the Bondi accretion radius for a 2.6 × 106 M☉ black hole. A compact component within the source flared by up to a factor of 3 over a period of ≈1 hr at the start of the observation. The search for Kα line emission from iron was inconclusive, yielding an upper limit on the equivalent width of 2.2 keV. Several potential stellar origins for the X-ray emission at Sgr A* are considered, but we conclude that the various properties of the source favor accretion onto the SMBH as the origin for the bulk of the emission. These data are inconsistent with "standard" advection-dominated accretion flow (ADAF) models or Bondi models, unless the accretion rate from stellar winds is much lower than anticipated. The central parsec of the Galaxy contains an ≈1.3 keV plasma with electron density ne ≈ 26ηcm-3, where ηf is the filling factor. This plasma should supply ≈10-6 M☉ yr-1 of material to the accretion flow at the Bondi radius, whereas measurements of linear polarization at 150 GHz and above limit the accretion rate near the event horizon to ≲10-8 M☉ yr-1, assuming an equipartition magnetic field. Taken together, the X-ray and radio results imply that outflows or convection are playing a role in ADAF models and subequipartition magnetic fields in Bondi models, or else the X-ray emission must be generated predominantly via the synchrotron self-Compton (SSC) process. The measured extent of the source and the detection of short timescale variability are evidence that the emission from Sgr A* contains both thermal and nonthermal emission components at comparable levels. We also discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane. Morphological evidence is presented that Sgr A* and the H II region Sgr A West lie within the hot plasma in the central cavity of Sgr A East, which we interpret as a supernova remnant that may have passed through the position of the SMBH, leading to a period of increased activity that ended within the past ≈300 yr. Similarly, we have discovered bright clumps of X-ray emission located on opposite sides of the Galactic plane, along a line passing through the central parsec of the Galaxy. The arrangement of these lobes suggests that Sgr A* may have experienced an earlier period of increased activity lasting several thousand years during which it expelled hot gas in a bipolar outflow oriented roughly perpendicular to the Galactic plane. Additionally, we present an analysis of stellar emission within the central parsec of the Galaxy.