Issue 4.15

The Astrophysics Spectator

September 19, 2007

X-rays are emitted by the cores of many galaxies. Is this radiation created by stars and compact binary systems within the core of each galaxy, or by a massive black hole of over 1 million solar masses sitting at each galaxy's center? This question is difficult to resolve because of the angular resolution of x-ray telescopes. Early x-ray telescopes could observe distant galaxies with a resolution of only several arc seconds, which corresponds to a spatial resolution of a couple of dozen parsecs for nearby galaxies such as the Andromeda galaxy. With the Chandra x-ray telescope, the x-ray telescope with the current best resolution, neighboring galaxies can be observed with an angular resolution of better than 1 arc second, bringing the spatial resolution down to several parsecs for the nearby galaxies. With this improved resolution, one finds that the x-ray sources identified with earlier instruments are composes of several distinct x-ray sources; most of these sources are binary star systems, and perhaps one of these sources is a massive black hole. With tens of millions of stars orbiting a galaxy's central black hole within the resolution of the best x-ray telescopes, we cannot definitively say that we see a galaxy's central black hole as an x-ray source.

The difficulty of distinguishing x-ray sources in distant galaxies underlines the importance of Sgr A*, the central Galactic black hole, in developing a broad understanding of the centers of typical galaxies. At a distance that is 1% of the distance to the Andromeda galaxy, Sgr A* is observed in the x-ray with a spatial resolution of 0.02 parsecs. X-rays are emitted from the region around Sgr A*, but at a surprisingly low power, much lower than the power in x-rays radiated by the cores of the Andromeda galaxy and other galaxies. If the power generated by Sgr A* is typical, then other source are intruding in our study of massive black holes in other galaxies, and the black holes in these other galaxies are still hidden from us at x-ray energies.

With this issue of the web site, a page is added that discusses the x-ray emission from the vicinity of Sgr A*. The emphasis of the article is on whether stars orbiting the central black hole are producing the observed x-rays.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for October 3.

Milky Way Galaxy

Sgr A* as an X-ray Source. Strong x-ray emission is the hallmark of most black hole candidates. To the surprise of all astrophysicists, the massive black hole at the center of our Galaxy is a weak x-ray source. It is more like a large star than like the many black hole candidates we see scattered throughout our own Galaxy in compact binary systems and scattered throughout the universe within the cores of active galaxies. Because the power generated around Sgr A* in x-rays is so modest, the question arises whether the x-rays are from stars orbiting the black hole rather than from the black hole itself. (continue)