Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy

By F. Baganoff, Y. Maeda, M. Morris, M. Bautz, W. N. Brandt, W. Cui, J. Doty, Eric D. Feigelson, G. Garmire, S. Pravdo and 6 more. 2001

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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.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

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In Astronomy and Astrophysics

1. P. Hopkins
2. E. Quataert
3. Ramesh Narayan
4. L. Hernquist
5. R. Genzel
6. F. Yuan
7. T. Alexander
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In Nuclear and High Energy Physics

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Citations in the corpus

• First simultaneous NIR/X-ray detection of a flare from Sgr A (2004, methodology): “The IQ-state X-ray count rate in a 1.5” radius aperture of 5.3 ± 0.5×10−3 counts s−1 during the monitoring period is consistent with rates measured during previousChandra observations (Baganoff et al. 2001, 2003).”
• Radio and millimeter continuum surveys and their astrophysical implications (2009, result): “Chandra X-ray observations of Sgr A at the Galactic Center are suggestive of a considerably lower accretion rate compared to Bondi’s prediction (Baganoff et al. 2003), so that the very low ADAF radiative efficiency may not be required.”
• Timing the starburst–AGN connection (2010, background): “As pointed out by Davies et al. (2007), the Galactic centre also fits into this picture, where the winds from nearby young OB stars are not being accreted as efficiently as expected (Baganoff et al. 2003; Paumard et al. 2006).”
• Electron Acceleration around the Supermassive Black Hole at the Galactic Center (2001, background): “Actually, Sgr A*’s quiescent-state X-ray spectrum is obtained by averaging over a long observation period (Baganoff et al. 2003), which smooths out contribution from such weak flares.5 This aspect of the model therefore may explain why the occurrence frequency of infrared flares is more than twice…”
• Massive Black Hole Binary Evolution (2004, unknown): “For example, observations with the Chandra X-ray Observatory have revealed tenuous (n ≈ 10 − 100 cm), hot (T ≈ 1 keV) plasma within a parsec of the ∼ 4 × 10M⊙ Milky Way SBH [7].”
• Diffuse X-Ray Emission in a Deep Chandra Image of the Galactic Center (2004, background): “It is likely that these represent an outflow from the central parsec (Baganoff et al. 2003; Morris et al. 2003).”
• RADIATIVE MODELS OF SGR A* FROM GRMHD SIMULATIONS (2009, methodology): “Sgr A* is frequently monitored at all available wavelengths: in radio since its discovery in 1974 (Balick & Brown 1974), and more recently in submillimeter, near-infrared (NIR), and X-rays.”
• Polarimetry of near-infrared flares from Sagittarius A* (2006, background): “…al. 2006) suggests that the emission with 1033−34 erg/s flares arises from a compact source within a few ten Schwarzschild radii (RS) of the MBH (Baganoff et al. 2001; Baganoff et al. 2003; Eckart et al. 2004; Genzel et al. 2003a; Ghez et al. 2004; Eisenhauer et al. 2005, and references therein).”
• Predictions for high-frequency radio surveys of extragalactic sources (2004, background): “Chandra X-ray observations of Sgr A, at the Galactic Center, are suggestive of a considerably lower, compared to Bondi’s, accretion rate (Baganoff et al. 2003), so that the very low ADAF radiative efficiency may not be required.”
• Refining the fundamental plane of accreting black holes (2006, methodology): “For Sgr A∗, we include the hard X-ray flare by Baganoff et al. (2001), as the are may be due to jet emission (see e.g., Markoff et al. 2001b).”
• Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A* (2003, background): “The IR limit plus an X-ray detection from Chandra (Baganoff et al. 2001, 2003) indicate that Sgr A* is quite dim overall, with a bolometric luminosity of only L ≈ 1036 ergs s−1 ≈ 3× 10−9LEdd.”
• THE SUBMILLIMETER BUMP IN Sgr A* FROM RELATIVISTIC MHD SIMULATIONS (2010, background): “We have not computed IR or X-ray emission, and therefore do not fit to the quiescent X-ray luminosity from Baganoff et al. (2003), or to the upper limits to the IR emission.”