Over the past forty years, black hole candidates have revealed their secrets to those who can see their x-rays. Compact binary stars that contain black hole candidates are fundamentally x-ray sources. Active galactic nuclei (AGNs), which are likely powered by massive black holes, emit much of their energy as x-rays and gamma-rays. Our insight into the conditions around these objects comes from the variability and the energy spectrum of x-rays. The x-ray variability gives us a measure of a black hole's size through the dependence of variability on the light-travel time across a source. The x-ray spectrum tells us something about the temperature, density, and composition of the gas flowing into the black hole. Can Sgr A* give us a similar insight into itself through its x-ray emission? X-rays are seen from its vicinity, but they may be generated by stars orbiting the black hole rather than by the black hole itself.
The x-ray source associated with the central Galactic black hole is called CXOGC J174540.0−290027, a name based on the initial position in right ascension and declination given to the x-ray source (I sympathize with your revulsion of that name; x-ray maps of the Galactic center are nearly unreadable because every source has a nearly-identical name of this form). The x-ray source is located 0.27±0.18 arc seconds from Sgr A*, which is close enough for the x-rays to be from Sgr A*. Could this be a chance coincidence? This is the inevitable problem in high-energy astronomy, because the angular resolution of an x-ray telescope, which is not as fine as the resolution of an optical or a radio telescope, is often not sufficient to separate x-ray sources or to associate an x-ray source with a particular optical or radio source in regions dense in stars and binary systems.
The difficulty of correctly associating an x-ray source with a suspected massive black hole bedevils the study of other galaxies. The Andromeda galaxy (M31) provides a good example of the problems astronomers encounter. An x-ray source with a luminosity that varies from 2×1037 to 1038 ergs s−1 was found at the center of this nearby galaxy in 1979. As in our own Galaxy, the Andromeda galaxy appears to have a massive black hole at its center; observations at visible wavelengths of stellar velocities at the center of M31 suggest that these stars are orbiting a black hole of 3×107 solar masses. Inevitably astronomers suggested an association between the x-ray emission and the presumed black hole. The resolution of the best x-ray instrument used in those studies, however, was no better than 7 arc seconds; with M31 only 784,000 parsecs away from us, this resolution makes any x-ray source within 27 parsecs of the black hole appear to be x-ray emission from the black hole itself. When the Chandra x-ray telescope, the latest and best imaging x-ray telescope, was pointed towards the center of the Andromeda galaxy at the end of the 1990s, it returned an image that showed the single x-ray source of previous observations as five separate x-ray sources. One of these sources, emitting about 4×1036 ergs s−1, or 4% of the peak power seen previously for the five combined sources, is about 1 arc second from the galactic center. It could be the x-ray emission of the massive black hole at the center of the Andromeda galaxy, but the telescope's resolution can only place the x-ray source to within 4 parsecs of the black hole. Inside this distance, there is as much mass in individual stars as is in the black hole. With tens of millions of stars in this region, one must wonder whether the x-ray emission is from one or more stars or binary systems close to the black hole rather than from the black hole itself.
This same concern arises with the central Galactic black hole. Could the x-ray emission from Sgr A* be from a source that is in our line of sight with the black hole? The likelihood of a chance coincidence of CXOGC J174540.0−290027 with Sgr A* is less than 1 in 20, so we are most likely seeing x-rays emitted from the vicinity of the central Galactic black hole rather than a source a dozen parsecs in front of or behind the black hole. But this does not eliminate the possibility that the x-rays are from one or more stars or binary systems orbiting near the black hole.
The power radiated as x-rays from the region of the central Galactic black hole is modest, especially considering the size of the black hole. The absorption-corrected x-ray luminosity in the 2 to 10 keV band—dust absorbs the x-rays below 2 keV—is only 2×1033 ergs s-1, or half of the total power generated by the Sun. This small rate of power generation is a tremendous problem for those who would like to associate the x-rays with processes around the central black hole, because so many other x-rays sources within the Galaxy, including many stars, produce similar power output in x-rays. If several of these x-ray sources are orbiting close to the central Galactic black hole, they could easily produce all of the x-rays we see. The authors who first wrote of the x-rays observed by the Chandra x-ray telescope from the vicinity of Sgr A* spent much of their effort eliminating the most plausible of these x-ray sources by comparing the characteristics of the x-rays to the characteristics of prospective x-ray sources. The discussion that follows is based on their arguments.
The x-rays are variable. The brightness of CXOGC J174540.0−290027 can increase by a factor of 3 in only one hour. Because the light travel time across a region generating x-rays sets the minimum timescale for variability, this one hour timescale implies the x-rays are generated in a region that is less than 10 AU across. This size is much larger than the 0.1 AU for the radius of the black hole's last stable orbit, so the flaring is consistent with the generation of x-ray by gas flowing into the massive black hole.
The x-rays are characterized by a temperature of 2 keV (23 million degrees Kelvin). Many large stars, such as Wolf-Rayet stars (hydrogen-burning stars that have driven much of their original mass away as a wind) can produce x-rays, but the characteristic temperature of the x-rays is below 1 keV.
The x-rays are extended on the sky. Only half of the x-rays originating from a circle of radius 0.6 arc seconds, or from within a cylindrical volume of 4,600 AU (0.023 parsecs) radius. At twice this radius, the x-ray flux has fallen to a steady but high background level. While many objects orbit the black hole at 4,600 AU, the only objects that are visible are the B main-sequence stars, which are not strong x-ray sources. The numerous neutron stars and stellar-mass black holes believed to orbit Sgr A* cannot produce much of an x-ray flux when they are alone, because objects of several solar masses are poor at capturing gas compared to a million-solar-mass black hole. The rate at which an object captures an ambient gas is proportional to the square of the mass. This means that a 1 million solar-mass black hole captures 1 million times the gas of 1 million black holes and neutron stars of 1 solar mass. If black holes and neutron stars near Sgr A* can capture enough gas to be visible to us, then Sgr A* itself should be a brilliant source that transcends those sources.
X-ray binary systems are plausible x-ray sources around Sgr A*. With a high concentration of stars, close encounters between stars can lead to the creation of binary systems. This process is common within globular clusters, but it should be rare around Sgr A* because of the high stellar velocities. Theorists expect such binaries to be short lived. On the other hand, a single binary system could account for a large fraction of the x-rays from the Sgr A* region, particularly the x-rays seen in a flare.
The most difficult x-ray source to eliminate is the magnetically-active main-sequence star. These young, low-mass stars can produce a continuous x-ray output of 1031 ergs s−1, and some can produce x-ray flares that release 1033 ergs s−1. Several hundred of these stars can account for the continuous x-ray emission from Sgr A*, and a flare from a single star can account for the observed flares. The argument against these stars relies on an estimate from star creation theory of the number of small stars relative to the number of large stars; it is argued that the number of observed massive stars implies a density of magnetically-active stars that is too low to account for the x-rays. One certainly does not expect the smaller stars to drift into close orbit of Sgr A*. Mass segregation suggests that the numerous stars orbiting the black hole should be neutron stars and star-sized black holes, and the stars that are observed orbiting the black hole are large main-sequence stars. But without good observational evidence, the possibility that the observed x-rays are from small, magnetically-active stars cannot be eliminated.
Despite this inability to eliminate all possible stellar sources from consideration, most astrophysicist believe the x-rays are created by gas flowing into the black hole. The combination of an x-ray source extending thousands of AU from the black hole and x-ray flares from a region somewhat larger than the black hole's last stable orbit are certainly suggestive of emission by gas falling into the black hole.
So uncertainty persists about the source of the x-rays seen around Sgr A*. The probability of a chance coincidence is small, but not so small as to create certainty that we are seeing the central black hole in x-rays. Many sources of x-rays in the Galaxy are implausible sources to be orbiting Sgr A*, but not all sources are implausible. Finally, while not discussed on this page, the theories that explain the x-rays as the emission by gas falling onto the central black hole are themselves uncertain enough to admit skepticism of their validity. Most likely, the x-rays are produced by gas falling onto the central black hole, but one shouldn't be surprised if the x-rays are eventually attributed to stars orbiting the black hole.
Garcia, Michael R., Murry, Stephen S., Primini, Francis A., Foreman, William R., McClintock, Jeffrey E., and Jones, Christine. “A First Look at the Nuclear Region of M31 with Chandra.” The Astrophysical Journal 537 (1 July 2000): L23–L26.