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

Abstract

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.

Sgr A*, the best-sampled of all AGN?

“…It shows the central storm field, blue-and red-shifted on both sides of the (central) BD, and co-aligned with the central twin-jet. The latter has been detected more recently by [Baganoff et al, 2003] at X-rays, and added in purple (by us), as a thin, straight, radial feature passing through Sgr A* (at the center), almost vertically in Galactic coordinates, of length one lyr, starting at a distance of one lyr from Sgr A*. It has the expected position, orientation, width (of order 1%), and length of a central jet launched from the rotation center of our Galaxy, together with a plausible brightness.…”

“…It is contained inside of Sgr A East, the large storage bubble which discharges to the North and South into the central chimney. The jet proper cannot be recognised on this scale, but has been mapped by [Baganoff et al, 2003] at X-rays, projecting onto Sgr A West, to the South of Sgr A*, as a 0.3 pc-long straight-line segment; cf. fig.5.…”

Stellar winds near massive black holes – the case of the S-stars

“…These winds provide gaseous material that can be accreted by the black hole which is thought to be the source of X-ray emission close to Sgr A * . Baganoff et al (2003) used Chandra X-ray spectroscopic imaging of the central parsec of the galaxy to measure the Bondi accretion rate of the central black hole. They found that the X-ray emission at the position of Sgr A * is extended and consistent with the accretion radius for a ∼ 10 6 M⊙ black hole (∼ 12000 au).…”

Testing for periodicities in near-IR light curves of Sgr A*

“…Multi-wavelength detections of the radio point source at sub-millimeter, X-ray, and infrared wavelengths have also been made, showing that the luminosity associated with the black hole is many orders of magnitudes below that of active galactic nuclei (AGN) with comparable masses [5]. These observations have also shown that the emission from Sgr A* is variable [e.g., [6][7][8][9][10]. Although it is now easily detected in its bright states when its flux increases by up to an order of magnitude over time scales of 1 to 3 hours, Sgr A* is difficult to detect in its faintest states at X-ray wavelengths because of the strong diffuse background, and in the near-infrared because of confusion with nearby stellar sources [6,9].…”

“…These observations have also shown that the emission from Sgr A* is variable [e.g., [6][7][8][9][10]. Although it is now easily detected in its bright states when its flux increases by up to an order of magnitude over time scales of 1 to 3 hours, Sgr A* is difficult to detect in its faintest states at X-ray wavelengths because of the strong diffuse background, and in the near-infrared because of confusion with nearby stellar sources [6,9]. Advances in adaptive optics (AO) technology have offered improved sensitivity to infrared emission from Sgr A* against the stellar background, such that observations in its faint states are now possible [11].…”