December 4

Look at the first two links in the December 4 figures area for
good gamma-ray-burst pages.

We're going to end the semester talking (mostly) about other sorts
of objects containing black holes. On December 4 we talked about 
black hole "supernovae" and about gamma-ray bursts. On December 9 we'll
talk about supermassive black holes at the centers of galaxies...

You should remind yourself about how we think a supernova actually happens.
What is it that causes the core to collapse? On what timescales
does the core collapse? What happens to the rest of the star while
the core collapses? Try
http://adansonia.as.arizona.edu/%7Eedo/astro203_fall2003/lectures_directory/oct23.txt
for a refresher.

On October 23 we pointed out that astronomers, not Nature, have trouble making
the "stars" in their computers actually blow up.

If the star's core is "too massive", it makes a black hole rather
than a neutron star. If the star's envelope wasn't stripped away
enough in normal stellar evolution (winds, mass loss), there's
so much material to push aside that the shock never reaches
the surface of the star and we never see a supernova. We'll get
back to that last point in a while...

OK, on to gamma ray bursts. As we said above, there are good websites
(and your text!) for reviewing the history of the discovery of gamma ray
bursts.

Suffice it to say that some rare celestial objects were shown, for the first
time in the 1960s, to emit prodigious numbers of gamma rays. these
gamma rays are emitted in very short periods of time, in a burst.
What sort of object makes a gamma ray burst? 

It really took until the 1990s to sort this out (1996 or 1997, really,
though there were hints before that). 

The problem is that gamma-ray telescopes have terrible vision. They
can tell you sortof in what direction a gamma ray photon has come from,
but not a whole lot more. So, if you want to use all of the tools
at your disposal, say, optical spectroscopy, to figure out the physical nature
of gamma ray bursts, you need to point your telescope PRECISELY, not
APPROXIMATELY. Another way of stating the problem is that there are
literally thousands of stars inside the "gamma ray positional error box."
You can't examine them all!

It wasn't until the 1990s that a satellite was launched that could
look simultaneously in the gamma ray part of the spectrum and the xray
part of the spectrum. The xray telescope has a much more precise
positional accuracy, allowing astronomers to see a non-gamma-ray
"afterglow" in the same spot as the gamma ray burst. We never can see
what's going on at the exact moment of the gamma ray burst because the
stupid burst only lasts a few seconds!

We'll get back to thre sotry of figuring out where the afterglows, and
thus the bursts, come from, in a bit.

Even without knowing a model for gamma ray bursts, one can deduce
something of their nature simply by plotting their position on the
sky.  Imagine that gamma ray bursts are all coming from massive stars
in the Milky Way. Well, massive stars in the Milky Way are highly confined
to the disk of the Milky Way, so the distribution of positions
of gamma ray bursts should look like the distribution of positions
of the Milky Way stars themselves. AND THEY DON'T! In fact, the
distribution of positions of gamma ray bursts is random, all points
on the sky are equally likely to have had a burst.

How do we make such a distribution? Well, one way is if the bursts
are coming from very far away, say from really distant galaxies. 
So they'd be rare events, perhaps a special kind of supernova,
or a merging pair of neutron stars, in a distant galaxy. That
would make a random distribution on the sky. The problem is, then
gamma ray bursts would have to be brighter than any event we've ever
seen, since they're far away and obey the inverse-square law of light.
If we can see them here, and they're far away, they have to be really
bright "there." So the problem is that we'll have trouble
making a model.

If gamma ray bursts are really really close, say, closer than the next
nearest star, they can be randomly distributed. Imagine that you have
a forest just outside of Tucson, and another just outside of
Phoenix. If you were halfwat between, say in Casa Grande, there would
be two directions to go to see a tree. But if you were inside the
Tucson forest, you'd see trees in all directions. The good news here
would be we'd be free of the need to invent a model to explain the
"strongest explosions in the Universe." The bad news is that we'd still
have to make a model to make the gamma ray bursts.

So in the late 1990s, astronomers on earth, in the optical part
of the spectrum, showed that a gamma ray burst had come from
a distant galaxy. In the intervening several years, all of the ones
we could follow up from the ground are coming from distant galaxies.
So we have to deal with the reality that gamma ray bursts are
really energetic. The good news is, then, that these bursts
can be quite rare/unlikely.

Well, it turns out that a couple of gamma ray burst afterglows are associated
with supernovae! So that gives us a good hint about possible mechanisms
for making the enrgy. In fact, they're associated with extremely energetic
super-supernovae, more correctly called "hypernovae."

So, this makes us star thiking of a rare form of supernova, one coming
from a really really massive star. The core of the star becomes a black
hole, and SOMEHOW the rest of the star falling onto the black hole
creates a gamma ray burst AND perhaps a supernova.

We're in the midst of figuring out the right model for a gamma ray
burst. The odds are that we're on the right track but that we're wrong
in some details. Science is messy when you're in the middle of the
mystery.

In fact, you might be interested in
http://www.as.arizona.edu:8080/Astro/1050695049/index_html

So, let's play "what if" and see where that leads us.
What if, we have a really massive star making a black hole in its
core? The outside of the star eventually notices, and falls in, making
an accretion disk around the black hole. It's so hot in the inner
part of the accretion disk that the inner part of the accretion disk puffs up.
Because it's hot, the matter is pretty much opaque to all forms of light.
Only neutrinos can escape.

This accretion disk is so puffed up that there's essentially a chimney of 
empty space ONLY at the north and south poles of the stars. Neutrinos
interact (annihilate each other) in this empty space, SOMEHOW making
gamma rays and a blast wave.

As this beam of gamma rays eats its way out through the star, it also
deposits huge amounts of energy into the star, thus eventually ripping
it apart. If the star is just right, out pops a gamma ray burst,
and then a while later (weeks?), out pops a supernova. Or, out pops
a gamma ray burst, and there isn't enough ripping-apart energy
to ever tear the star completely apart. This MIGHT explain why only SOME
gamma ray bursts are associated with supernovae.

What else is nice about this model? Well, since the enrgy is coming
out in 2 beams, we don't need to make as much energy as if the energy
were uniformly coming out in all directions. Roughly speaking, we just
have to acount for a supernova's-worth of energy, not "more energy than
anything else known, by a factor of 100."

So, we have a good hand-wavy model. It accounts for many things, but is far
from a mature model. We'll watch the front page of the New York Times
over the next decade or two to see how the story turns out.