How to Find a Gamma-Ray Burst

How does one find a gamma-ray burst? The first thing that
happens is that a gamma-ray burst explodes and beams a jet of gamma
rays (which are essentially the same as the gamma-rays from a nuclear
explosion) across the Universe. If that beam is pointed towards Earth
then a gamma-ray detector on a spacecraft somewhere in the Solar
System may see it. Swift is a satellite orbiting
about 600 km above the Earth, and its mission is to discover and study
gamma-ray bursts. It has a gamma-ray detector called the Burst Alert
Telescope, or BAT for short, that provides our first notice that a
gamma-ray burst has gone off. There are several other spacecraft that
can detect gamma-ray bursts, but the vast majority of bursts are
detected by Swift. Gamma-rays can not be focused like
normal light can, so determining where on the sky they are coming from
requires doing something clever. BAT consists of an array of
gamma-ray detectors with a mask in front of them. The mask contains
thousands of lead tiles in a random pattern that block gamma-rays from
reaching the detector. This is done so that the detectors see the
shadow that is cast by this mask. Think about a shadow. When the Sun
is high overhead the shadow is short. When the Sun is low in the sky
the shadow is long. The shadow is always pointing away from the Sun.
In fact, the shape and direction of the shadow tells us (after
applying some clever maths) exactly where the Sun is in the sky. The
same is true for the shadow cast by the mask on the gamma-ray
detector. The details of the shape of the shadow tell us where the
gamma-ray burst is.

However, there is a catch. The green circle shows the region
that the BAT thinks that the gamma-ray burst is located in. The BAT
can only localize a gamma-ray burst to between about one and three
arcminutes on the sky. That is about 1% of the size of the Full Moon.
That is pretty good, but we can do a lot better. The next step is
that the Swift satellite moves so that it is
pointing an X-ray telescope and an ultraviolet/optical
telescope in the direction of the burst. The X-ray telescope
detects X rays, the same as those that your dentist uses to
find cavities in your teeth, coming from the gamma-ray burst. Unlike
gamma-rays X rays can be focused, which means that they can be
localized on the sky very accurately. In
fact Swift's X-Ray Telescope (or XRT) can
determine the position of a gamma-ray burst to better than five
arcseconds, which is not a lot larger than a star appears to be. The
blue circle shows the location of the gamma-ray burst as determined by
the XRT.

The final step is to see if there is any optical light coming
from the gamma-ray burst. This is a job
for Swift's Ultraviolet/Optical Telescope, which is
affectionately known as UVOT. UVOT looks to see if there is anything
new at the location of the gamma-ray burst that was not there before.
To do this one looks at a picture of the sky taken with the UVOT (the
picture on the left) and a picture of the same part of the sky that
was taken at some point in the past (the picture on the right). The
BAT and XRT positions (green and blue circles respectively) are
overlaid on both pictures, and an astronomer (or automated software)
looks to see if there is anything at that location in the UVOT picture
that was not there before. If there was then we have discovered the
optical afterglow of a gamma-ray burst. In the example shown here
(which is GRB 080506, a gamma-ray discovered on 6 May 2008) there is
an optical afterglow (marked with a red circle). Only about 40% of
gamma-ray bursts have optical afterglows. The reason for this is not
well understood.

By combining the power of three telescopes that work in three
different energy ranges (gamma rays, X rays, and optical light)
astronomers are able to zoom in on a gamma-ray burst. Once a precise
position is know it is sent to observatories around the world, which
swing into action and observe the burst.

GRB 080506 turned out to be a fairly normal gamma-ray burst, but who knows
what the next one will be like.