The Astrophysics Spectator

Issue 2.28, July 27, 2005

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The basic layout of the site is as survey paths, which can be found under the Surveys link at the top of this and most other pages on this site. Each survey begins with a basic overview of the subject. Part of this overview include simulators of astrophysical phenomena that allow the reader to experiment with the phenomena. The later pages in a survey present the subject in greater and more mathematical depth. A path ends with research pages that describe current research projects and results in astrophysics.

The links at the top of each page are Home, which is the current home page of this site, Commentary, which is an index of short essays on topics loosely related to astrophysics, Surveys, which is the index of survey paths, Research, which is the index of research pages and the page leading to recent news items, Background, which is the index page for all background information on astrophysics, including survey pages, simulator pages, tables, bibliographic references, and lists of web resources, Previously, which is an index of previous home pages, and Site Info, which describes the site and its author, and gives contact information.

On the home page is found an addition link. This is the Store link, which leads to reviews of worthwhile books on astronomy and other relates subjects. Links on these pages enable the reader to buy these books from Amazon.com, which helps to financially sustain this web site.

Each Wednesday, a new issue of The Astrophysics Spectator is published that comprises a new home page, a new commentary, whatever news the author notices, and background, research, and simulator pages added to the survey paths. The home page acts as an index to the newly added pages. This site also has an RSS channel, whose link is given at the bottom of the right-hand column of this page.

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

Commentary

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)

Background

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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