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Milky Way Galaxy

The Appearance of Sgr A*

The central Galactic black hole Sagittarius A* stands out from the surrounding radio sources at the Galactic center as a bright point source with a nonthermal radio spectrum. This signature is seen in a larger number of the distant active galactic nuclei (AGN), which are universally believed to be powered by massive black holes. When Sgr A* was first found during radio observations of the Galactic center, its nonthermal spectrum immediately compelled researchers to associate the source with the AGNs of distant galaxies and to suggest that Sgr A* is a massive black hole.[1] Since the 1974 publication of that discovery, researchers have shown that Sgr A* is a black hole candidate. The stated hope of those studying Sgr A* had been that the physical processes generating the tremendous power of an AGN are at work at a dramatically-reduced level within our own galaxy; studying the relatively nearby central Galactic black hole would then give us insights into the physics of the extremely distant AGNs.

So far the x-ray binaries have provided the best insight into the AGNs. Many x-ray binary systems contain a black hole candidate of 3 solar masses or more orbiting a fusion-powered star. Gas flows from the companion star into a gas disk around the black hole. The disk itself converts gravitational potential energy into radiation. At the outer edges of the disk, the heat is radiated as optical or infrared radiation. At the inner edge of the disk, the heat is radiated away as ultraviolet radiation, x-rays, and gamma-rays.

The basic theory for the active galactic nuclei is similar to the theory for x-ray binary systems that contain black holes: energy is generated as gas flows through an accretion disk orbiting a black hole. Where the two theories differ is in the mass of the black hole, in the amount of gas flowing onto the black hole, and in the source feeding gas onto the black hole. The black hole in the AGN theory has a mass ranging from 1 million to 1 billion solar masses. Gas flows onto the massive black hole in the AGN theory at a much greater rate that it does in an x-ray binary system. These two conditions enable the AGN to generate from 102 to 109 time the energy of the brightest x-ray binaries. Finally, the source of gas is not a single star, but the millions of stars orbiting the black hole. The accretion disk in the AGN theory is fed with gas lost by orbiting stars as well as by interstellar gas.

The hope had been that this theory is also at work in Sgr A*. But should we expect the same theory to apply to objects generating such different levels of power? The total energy generated by the central black hole is estimated at less than 1037 ergs s?1, or less than 2,500 times the power output of the Sun. This power output may at first glance seem tremendous, but when compared to the power output of the AGNs, it is a trifle. The power output of the AGNs ranges from 106 to 1013 times the power of the Sun. An AGN therefore generates from 1 thousand to 10 billion times the power of Sgr A*.

Even by the standards of the Milky Way Galaxy, the power generated by Sgr A* is quite modest. The largest main-sequence stars and the OB supergiant stars produce many times the energy of Sgr A*. The largest B main-sequence stars orbiting Sgr A* produce ten times the energy of the black hole. The star Rigel in Orion ( Orionis) produces an order of magnitude more energy than Sgr A*. The Crab nebula, which is powered by a spinning neutron star, generates about 1038 ergs s?1, or 10 times the energy of Sgr A*. Many of the x-ray pulsars and x-ray binaries produce more power than Sgr A*. So Sgr A* is outshone by numerous other Galactic objects despite being the largest object in the Milky Way Galaxy.

Besides the lower power, the central Galactic black hole differs fundamentally from AGNs in the type of radiation it emits. Sagittarius A* is fundamentally a radio source; at higher frequencies, it is muted. Its power output at radio frequencies is estimated to be several hundred times the power output of the Sun.[2] For decades this was the only radiation seen from Sgr A*, despite efforts to observe the object at infrared and x-ray frequencies. Only with the recent improvements in x-ray and infrared instrumentation have have these efforts produced fruit.

Highly variable infrared emission has been seen from Sgr A* in recent years.[3] The infrared power can fluctuate by a factor of 2 in less than an hour. The power released during the peak infrared emission is somewhat less than the power released as radio waves, and at other times it can be much less than the radio emission.

X-rays were first detected from the region surrounding Sgr A* in 1999 with the Chandra X-ray Observatory.[4] The source of these x-rays, named CXOGC J174540.0?290027, is thought to be hot gas surrounding Sgr A*. The association with Sgr A* is bases on likelihood calculations that the x-rays are associated with the central black hole rather than nearby stars or a previously unseen coincident object. There is always the chance that the x-ray source is being misidentified; in this case, that chance is placed at less than 5%.

The x-rays come from an extended area spanning about 10,000 AU. This x-ray emission is generally steady, although occasional flares of about 1 hour duration occur. These flares, which are necessarily from a much smaller region than the steady x-ray emission (a light travel time of 1 hour implies a 7 AU maximum size for the emission region), emit an x-ray flux that is of order 3 times the steady x-ray flux. The power emitted as x-rays in the steady emission is very low, only 60% of the total solar luminosity. It is this low power that presents a problem for interpreting Sgr A* as an ultra-low-luminosity AGN. In the AGNs, the x-rays account for about 10% of the radiated power. In the central black hole, the x-rays account for less than 0.1% of the radiated power. If an accretion disk is responsible for the x-ray in both types of object, its is acting in a much different manner around Sgr A* than around an AGN.

Superficially, Sgr A* resembles a radio-loud AGN, but the very different x-ray characteristics have raised doubts whether the processes in the two types of sources are entirely the same. The energy generated by Sgr A* is still probably from gas flowing onto the black hole, but the structure of this flow appears to be much different than the steady accretion disks envisioned for AGNs. The current theories for Sgr A* now assume that the gas does not flow through the type of accretion disk is found around AGNs and around black holes in x-ray binaries. The hope that we could learn about AGN physics by studying Sgr A* is vain.

[1]Balick, Bruce, and Brown, Robert L. ?Intense Sub-Arcsecond Structure in the Galactic Center.? The Astrophysical Journal 194 (1 December 1974): 265?270.

[2]Morris, M., ande Serabyn, E. ?The Galactic Center Environment.? Annual Review of Astronomy and Astrophysics 34 (1996): 645?701.

[3]Ghez, A. M., Wright, S. A., Mathews, K., Thompson, D., Le Mignant, D., Tanner, A., Hornstein, S. D., Morris, M., Becklin, E. E., and Soifer, B. T. ?Variable Infrared Emission from the Supermassive Black Hole at the Center of the Milky Way.? The Astrophysical Journal Letters 601 (1 February 2004): L159?L162.

[4]Baganoff, F. K., et al. ?Chandra X-ray Spectroscopic Imaging of Sagittarius A* and the Central parsec of the Galaxy.? The Astrophysical Journal 591 (10 July 2003): 891?915.

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