Why Astronomers Need Type 2 Quasars!

Episode 3 of Beyond The Veil:
The Search for Hidden Black Holes, a.k.a. Type 2 Quasars

Just Ducks
If it looks like ducks and walks like ducks...
April 3, 2000 :: 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.

Recent Headlines
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 paradox."

One possible solution to the spectral paradox was that something other than quasars are the source of the X-ray background.

Bagged Ducks
They might not sound like ducks, but they really are ducks...
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 explained.

AGN 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
In other words, if the ducks have sacks over their heads, 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.


Return to Chandra Chronicles

Highlights and Feature Stories from the Chandra Project
Archived reports of past Chandra Chronicles
Receive the Chandra Digest
Updated: April 24, 2007