Caption: Sagittarius A* (Sgr A*) and its environment as imaged in the X-ray band (fiducial range 0.1--100 Å) (and thus the image is false-color) by the Chandra X-ray Observatory (1999--).

Features:

1. Sgr A* is an intense compact radio source surrounding, more or less, the Milky Way or Galactic center black hole, a supermassive black hole (SMBH) of mass ∼ 4.3*10**6 M_☉. Sgr A* is also an X-ray source evidently.

   Note that the term Sgr A* is also commonly used as a synonym for the Milky Way or Galactic center black hole. We so use it here.

2. The electromagnetic radiation (EMR) from Sgr A* is provided by the interstellar medium (ISM) swirling into Sgr A*. The ISM gravitational potential energy gets converted to kinetic energy which viscosity and shocks convert to heat energy which is radiated as thermal radiation.

   There may be some non-thermal EMR too---but yours truly must look this up.

3. The ellipses mark light echoes from a strong burst of EMR from Sgr A* possibly caused by the accretion of a Mercury-sized object by Sgr A* (see CXO: Light Echo at Galactic Center).

4. Sgr A* is very nearly at the center of mass and the bottom of the gravitational well of the Milky Way. I'm NOT sure if we know how close it is to these things.

5. The formula and fiducial-value formulae for the Schwarzschild radius are

         R_sch = 2GM/c**2 = 2.9532 (M/M_☉) km = 1.9741*10**(-8) (M/M_☉) AU ≅ 2*10**(-8) (M/M_☉) AU

   from which it follows that Sgr A* has a Schwarzschild radius ≅ 0.1 AU which is approximately the radius of the event horizon.

6. The Event Horizon Telescope (EHT) is a project to image the event horizon regions of Sgr A* and other SMBHs. EHT uses a global array of radio telescopes to image the near environment SMBHs with the angular resolution of order 0.1 AU.

   EHT has already imaged the M87* event horizon region. M87* is the SMBH at the center of M87, a cD or giant elliptical galaxy (see m87_virgo.html). This image verifies to high probability the existence of event horizons. See the discussion of the M87* image in file m87_virgo.html.

7. Note that the near environment of Sgr A* is complex due to accreting ISM, and so EMR emission from near the event horizon requires radiative transfer modeling. Thus, the evidence for the event horizons from Sgr A* will NOT be as simple as one ideally would hope, but hopefully it will be convincing.