The Arthur C. Clarke Gamma-Ray Burst

On 19 March 2007 Swift discovered four gamma-ray bursts. This is a record for Swift. There have been several times when Swift has discovered three gamma-ray bursts in one day, but this is the first time that four were found. Even more impressive is that during that time Swift actually found five gamma-ray bursts over a 24-hour period, but the fifth was technically found on 20 March 2008.

The really amazing event, though, was the second gamma-ray burst, named GRB 080319B with the "B" standing for the second burst of the day. This burst was so bright that it was briefly visible to the naked eye. The optical afterglow reached a visual magnitude of about 5.3 for a few seconds. Anyone who was in a dark site and looking at Boötes at about 6:12 UT (about 2:12 EDT) could have seen the gamma-ray burst as a faint flash that faded over a period of about 30 seconds. This was the brightest optical afterglow that has ever been seen, and one of the most energetic gamma-ray bursts ever detected. GRB 080319B is even more impressive when one considers that it was located half-way across the Universe. The light from GRB 080319B was emitted 7.5 billion years ago, over three billion years before the Sun was born.

So, why is this column titled the "Arthur C. Clarke Gamma-Ray Burst"? Sir Arthur C. Clarke, the acclaimed science fiction author who popularized the idea of communications satelites, died on 19 March 2008 at the age of 91. To quote the Wikipedia entry for Sir Arthur C. Clarke:

Clarke served in the Royal Air Force as a radar instructor and technician from 1941-1946, proposed satellite communication systems in 1945 which won him the Franklin Institute Stuart Ballantine Gold Medal in 1963 and a nomination in 1994 for a Nobel Prize, and 1999 for literature, and became the chairman of the British Interplanetary Society from 1947-1950 and again in 1953.[7] Later, he helped fight for the preservation of lowland gorillas. He won the UNESCO-Kalinga Prize for the Popularization of Science in 1961. Clarke was knighted in 1998. He emigrated to Sri Lanka in 1956 largely to pursue his interest in scuba diving, and lived there until his death.

Because of this there has been a call to officially name GRB 080319B the "Arthur C. Clarke Burst". This is a nice idea, and I support it, but unfortunately there is no mechanism for naming gamma-ray bursts beyond giving them a catalogue number. Gamma-ray bursts are named for the date that they occurred. GRB 080319B is the second gamma-ray burst to be detected on 2008-03-19. Informally the burst is being referred to as the "Arthur C. Clarke Burst" by some astronomers, but this is not an official name. Sir Arthur inspired a lot of astronomers, and there is a desire to honour him this way. Some astronomers at NASA's Goddard Space Flight Centre prepared the following tribute.

Swift Sees a Star Explode

NASA's Swift mission has caught an exploding in the act of exploding. On 9 Jan 2008 Swift was taking routine observations of the supernova SN2007uy in the galaxy NGC 2770 when something unexpected happened. A second supernova erupted as Swift was watching. The supernova SN2008D (the "D" indicates that this is the fourth supernova to be discovered in 2008) observed in real time. The graph below shows the Swift observations of the X rays from SN2008D. Notice that they start at near zero at time zero. This is when the star exploded. The X-ray intensity rises rapidly until about 60 seconds after the explosion, and then slowly fades away.

Swift has seen stars explode before. Most gamma-ray bursts are believed to be the initial stages of supernovae, and Swift detects about 100 of them a year. SN2008D, however, was unique. At 90 million light years away it is located very close to us, as cosmic distances go, so we have been able to get detailed observations that are not possible with the more distant gamma-ray bursts. Another critical difference is that only a small fraction of supernovae product gamma-ray bursts. Estimates are that only about one in one thousand supernovae produce gamma-ray bursts, so those supernovae are very unusual. SN2008D, however, was a normal Type Ibc supernova. This is the first time that we have witnessed the birth of a normal supernova.

NASA's press release on SN2008D describes the event in detail.

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.

A New Type of Supernova?

The gamma-ray burst GRB 080503 occurred at about 0830 EDT on
Saturday, 3 May 2008, and woke me up. It turned out to be a
short-hard burst with no optical afterglow. Unfortunately the burst
was not too far from the Sun, so it is not easy to get follow-up
observations. Initially the burst behaved as a normal short-hard
burst does. It had an X-ray afterglow which faded quite
quickly, but no optical afterglow. However, on 4 May things got
interesting.

About one day after the burst observers
at Gemini-North detected a new source at the location of
the X-ray afterglow that was not there immediately after the
burst went off. This new source got brighter and then started fading
after about one day after the burst. This was unusual. In general
the optical afterglows fade after the first few minutes, and do not
get brighter with time. There have been a handful of exceptions, but
usually an optical afterglow that gets brighter a day or more after
the burst is an indication that there is a supernova associated with
the gamma-ray burst. It is generally believed that the so-called
long-soft gamma-ray bursts are caused by Type Ib/c sueprnovae, but no
one has seen a supernova associated with a short-hard gamma-ray burst.

Some people have interpreted GRB 080503 as being a
mini-supernova. A mini-supernova, also known as
a macronova, has been predicted by at least two research
groups over the past decade. The idea is that when two neutron stars
merge there is an explosion and neutron-rich material is ejected. The
initial explosion produces the gamma-ray burst, and then neutron decay
in the ejecta produces an optical transient that looks like a
supernova. The optical light gets brighter over a few days, and then
fades away. The details of the decay of the ejecta are not well
known, and there seem to be at least three different decay channels
that can produce the optical light. As far as I can tell no-one knows
much about this. The idea is that GRB 080503 is one of these
mini-supernovae.

This is an extreme suggestion. The late brightening may be due
to something mundane, like a the gamma-ray burst expanding into clumpy
dust and gas, or material from an accretion disc spiralling into
whatever is left after the two neutron stars merged. We do not know
enough about the physics of merging neutron starts be be able to make
confident predictions here. However, it is possible that astronomers
at the Gemini-North telescope discovered a new type of
supernova last Sunday.

The source marked "s2" in the cyan circle is the
suspected optical afterglow of GRB 080503. The blue circle marks the
location of the X-ray afterglow that was detected
by Swift. This circle shows the
accuracy that the X-ray source can be located to. The source
marked "s1" may be the host galaxy of GRB 080503, but this
is uncertain. The distance to this gamma-ray burst is not known.
This image was taken with the Gemini-North telescope.

Welcome to Cosmic Gamma-Ray Bursts

Welcome to the Cosmic Gamma-Ray Burst blog. The idea of this blog is to be place for up-to-date information about gamma-ray bursts. I hope to add articles about once a week, or as events unfold. Here is a good general audience introduction to gamma-ray bursts. Sonoma handles the publicity for NASA's Swift mission.