The Astrophysics Spectator

Issue 2.28, July 27, 2005

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July 27, 2005

I continue the exploration of x-ray astronomy this week with a page on galaxies with bright nuclei, generally referred to as Active Galactic Nuclei (AGN). These galaxies include the quasars. The nuclei of these galaxies produce massive amounts of radiation that can vary on times of only an hour. High-speed jets shoot away for many millions of parsecs from some AGNs. One of the principle tools for studying these objects is the x-ray telescope.

The theory behind the AGN is similar to the theory behind x-ray binary systems that contain a black-hole candidate: a black hole at the center of a galaxy that is surrounded by an accretion disk. All of the radiation we see from the AGN is created by the accretion disk as mass flows through the disk to the black hole. The other feature of AGNs, the jets, are created from matter accelerated away from the accretion disk along the disk's axis of rotation. These characteristics of the AGN are shared with some x-ray binary stars, so the basic physics is similar for these two types of object. The big difference is in the mass of the black hole, for while the black hole candidate in an x-ray binary is several solar masses in size, the black hole powering an AGN is orders of magnitude larger, as large as several times 109 solar masses.

Under this theory for the AGN, x-rays are produced by the inner part of the accretion disk and by the material accelerated into the jet. When we observe AGNs with x-ray telescopes, we are looking at the interface between the accretion disk and the black hole. X-ray emission lines of highly-ionized atoms are the primary diagnostic of conditions in this region of the AGN. These lines contain information about the motion of gas near the black hole. But x-ray lines are also produced by clouds several parsecs from the black hole; these clouds produce absorption lines when they stand between the accretion disk and the observer, and they produce emission lines when they are to the side. The lines produced by these clouds are diagnostics of the environment around the black hole.

This week I include a commentary on fluke results. A surprising number of statistically significant detections turn out to be statistical flukes. This is because the scientific community performs a large number of experiments each year, and each experiment contains an element of chance. I was pondering this element of science after reading an editorial in The Wall Street Journal about research that quantifies this effect for highly-publicized medical studies.

Jim Brainerd


Flukes. Many new results found in astronomy turn out to be statistical flukes. This is not surprising when we consider that scientific discovery has its elements of chance. Perform enough experiments within the community, and inevitably low-probability statistical fluctuations will appear and be published in the scientific literature. This is one reason why we repeat experiments. (continue)


X-Rays from AGNs. Many galaxies have active nuclei, which simply means that something at the centers of these galaxies produces a tremendous amount of energy, much of it in the x-ray band. The theory is that this something is an accretion disk orbiting a black hole with a mass anywhere from several million times the mass of the Sun to over a billion times the mass of the Sun. X-ray astronomers are currently trying to place limits on this theory. (continue)


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