An overview of the Chandra mission and goals, Chandra's namesake, top 10 facts.
Classroom activities, printable materials, interactive games & more.
Overview of X-ray Astronomy and X-ray sources: black holes to galaxy clusters.
All Chandra images released to the public listed by date & by category
Current Chandra press releases, status reports, interviews & biographies.
A collection of multimedia, illustrations & animations, a glossary, FAQ & more.
A collection of illustrations, animations and video.
Chandra discoveries in an audio/video format.
Why Astronomers Need Type 2 Quasars!
Episode 3 of Beyond The Veil:
April 3, 2000
The Search for Hidden Black Holes, a.k.a. Type 2 Quasars
If it looks like ducks and walks like ducks...
Many astronomers believe it is possible that type 2 quasars exist. Others maintain that they MUST exist in order to explain the mysterious X-ray background radiation.
In 1962, Riccardo Giacconi and his colleagues discovered that the X-ray sky was not dark. Rather it is bathed in a uniform, diffuse glow, called the X-ray background. A few years later, when quasars
were discovered, some scientists suspected that the X-ray background might be due to extremely distant quasars.
This was a plausible idea: the numbers of quasars and their power seemed to work out about right. But there was a problem. The spectrum of quasars does not match the spectrum of the background X
rays. The X-ray background has relatively more high-energy X rays than quasars.
You might say, the sources needed to explain the background looked like ducks and walked like ducks, but they did not quack like ducks. X-ray astronomers called this problem the "spectral
One possible solution to the spectral paradox was that something other than quasars are the source of the X-ray background.
Another possible solution was that some quasars might be a special class of quasars, called Type 2 quasars, that show an excess of high-energy X-rays, not because the quasar produces such an excess,
but because a cloud around the quasar filters out the low energy X-rays. If many such sources were found with different-sized clouds and at different distances, the X-ray background would be
In other words, if the ducks have sacks over their heads, they might not sound like ducks, but they really are ducks!
They might not sound like ducks, but they really are ducks...
To be continued...
Next week, Chandra implicates Type 2 quasars in the mystery of the X-ray background.
| Episode 2
March 27, 2000 :: According to the unified model for AGNs, it all depends on your point of view. The central black hole is assumed to be surrounded by a thick donut-shaped cloud of gas and dust. Episode 2