The Universe Lights Up on Beethoven's Birthday
RXTE Pinpoints Location of Gamma Ray Burst
Ludwig van Beethoven would have been impressed. On December 16, the
229th anniversary of the musician's birth, the Universe lit up in
gamma rays that, for a few seconds, outshone the entire sky.
The event was a gamma-ray burst and was soon dubbed the "Beethoven
Burst" by Dr. Brad Schaefer of Yale University. Satellites detect two
or three gamma-ray bursts a day, but a burst this bright only happens
once in maybe four years.
"This was by far the brightest burst we've detected in a long time,"
said Dr. Frank Marshall, a NASA astrophysicist at Goddard Space Flight
Center. "I knew we had to find its location quickly. Otherwise, all
the powerful optical and X-ray telescopes would not be able to study
this monster event.
Gamma-ray bursts are by far the most energetic events known in the
Universe, second only in power to the Big Bang. The gamma rays
themselves are invisible to the human eye; they are the most energetic
form of radiation, more powerful than optical light, ultraviolet
radiation and X-rays.
The cause of these bursts is not known. Most scientists believe they
come from the farthest reaches of the Universe, perhaps from the
merging of two black holes or from a massive star explosion. If a
burst ever originated nearby in our Galaxy, the bath of radiation
could cause mass extinction on Earth.
Catch a Bursting Star
The Beethoven Burst was one of those rare and spectacular explosions
that lasted long enough and shone bright enough for scientists to
study it in-depth. Although a little ingenuity didn't hurt.
Scientists have difficulty studying gamma-ray bursts because they
appear randomly and without warning, and they last only a few seconds.
To maximize their catch, scientists have devised a computer network
for satellites and ground-based telescopes that quickly spreads the
news of a burst so that at least one telescope can get a good look
before the burst fades.
The two main satellites that first detect gamma-ray bursts are NASA's
Compton Gamma-Ray Observatory (CGRO) and the Italian-Dutch BeppoSAX
satellite. When these satellites sense a burst, they transmit the
approximate burst location through the Gamma-Ray Burst Coordinates
Network (GCN). The GCN, operated by NASA Goddard Space Flight Center,
then notifies scores of telescopes around the globe.
On January 23, 1999, the GCN struck gold when it notified a robotic
telescope called ROTSE with a crude source location only ten seconds
into a burst. ROTSE was able to capture the first optical image of a
gamma-ray burst while it was bursting.
The Beethoven Burst, which goes by the more technical name GRB 991216,
was not seen by ROTSE because the event happened during the daytime
over New Mexico, where ROTSE is located. But in space, the Rossi
X-ray Timing Explorer (RXTE) got a long, hard look at Beethoven's
As Seconds Tick Away...
For gamma-ray bursts, there is the burst and there is the afterglow.
The afterglow can last for days and, as BeppoSAX discovered, glow in
the X-ray band. Most of what scientists know about gamma-ray bursts
come from studies of the afterglow.
RXTE doesn't catch many gamma-ray bursts. RXTE is an X-ray telescope
that usually spends its time observing neutron stars, X-ray pulsars
and possible black holes. Yet when RXTE got the signal from CGRO (via
the GCN) about the Beethoven Burst, the RXTE science team decided to
check it out.
The folks at RXTE knew they couldn't capture the gamma-ray burst while
it was actually going off. Bursts rarely last more than a minute, and
it would take several hours before RXTE could slew around to look for
the burst. RXTE's plan was to capture the afterglow of the burst and
notify the other telescopes through the GCN with a more precise burst
location. One such telescope is the powerful Keck telescope in
Hawaii. Keck is so powerful that it can detect optical light from
some bursts several days and even months afterwards. But
it needs the fine detail about the source location.
|Rossi X-ray Timing|
The first step for RXTE was to detect the gamma-ray burst. CGRO
churns out a crude burst location within a few seconds of a burst.
The brighter the burst, the better the estimate. Within about an
hour, though, CGRO had a better idea of where the Beethoven Burst came
from. RXTE control center picked up on this later location estimate,
still covering a large region of the sky and certainly not a pinpoint.
Fortunately, RXTE has a wide-enough field of view and some ingenious
software, so it can take CGRO's estimate of the source location and
make the best of it.
This RXTE software was developed after the RXTE launch when scientists
realized that gamma-ray bursts do indeed glow brightly for many hours
after the burst in X-rays. Essentially, it enables RXTE to do
something it was never meant to do.
"After receiving the improved BATSE position [an instrument aboard
CGRO], RXTE scientists used special planning software to plan a series
of spacecraft maneuvers to search for the X-ray afterglow of the
burst," said Dr. Marshall, one of the key scientists working with
RXTE. "The maneuvers are designed so that a new source anywhere in
the BATSE error circle will be detected twice. The location of the
new source can then be determined by correlating the X-ray count rate
with the orientation of the satellite."
Show me what RXTE saw !
A Prelude to Joy
RXTE saw the Beethoven afterglow within four hours and nailed down a
tighter location. Whereas the CGRO location was somewhere within a
4-degrees circle (about eight times the apparent width of the moon),
RXTE chiseled the source down to 0.04-by-0.3 degrees in size. Another
scan 10 hours later pinned it down to 0.04-by-0.08 degrees.
Without RXTE's efforts, the burst would have been lost. BeppoSAX was
not looking in the direction of the burst, and CGRO couldn't provide
anymore detail to the source location. RXTE's tighter constraints on
a source location allowed powerful ground telescopes to zoom in for a
closer look. These telescopes are built to observe faint sources in a
narrow field of view with great accuracy.
"It's a bit like the little engine that could," said Dr. Patricia
Boyd, a scientists who analyzes RXTE data at Goddard Space Flight
Center. "RXTE lost one of its antennae recently. Yet not only can we
continue to do our day-to-day science, we can also zoom around and
catch a gamma-ray burst."
The first ground-based observatory to catch the optical afterglow was
the MDM of Kitt Peak, Arizona, which recorded a fading magnitude 18.7
source. Several observatories have since trained on the burst and
recorded a steadily declining afterglow in optical and radio
Even the Chandra X-ray Observatory was able to catch the fading embers
just four days after the event, a remarkable bit of reprogramming for
so complex a facility. This was Chandra's debut in the gamma-ray
An Exuberant Beethoven Fan
An approximate value for the redshift of the Beethoven Burst puts it more
than 10 billion light years away, roughly 2 billion years after the
Big Bang. Yet what caused this burst? A neutron star smashing into a
black hole? A "hypernova," 100 times more powerful than the already
potent supernova? Or maybe a real gung-ho Beethoven fan?
Two new NASA satellites will radically improve scientists' ability to
study gamma-ray bursts and determine their origin. The High-Energy
Transient Explorer (HETE-2) will launch in late January, 2000,
followed by the Swift mission in 2003. These missions will help
scientists study the burst itself, not just the afterglow.
Current satellites have whetted scientists' appetite for gamma-ray
bursts. These bursts have been studied for 40 years, but only
recently -- through the information provided by the likes of CGRO and
BeppoSAX -- have scientists come to understand how powerful, how
distant and how pervasive these bursts really are.
HETE-2 and Swift will dedicate themselves to gamma-ray bursts. Their
primary goal is to relay incredibly precise burst locations within
seconds, not hours. Depending on how one measures accuracy (better
resolution or ability to find quick location), these satellites will
be 100 to 1,000 times better at studying gamma-ray bursts.
New Satellites Dedicated to Locating Bursts
HETE-2 will detect up to a thousand bursts a year and, for about 30 of
these bursts, provide very detailed information about their location
and spectra, or light characteristics, within minutes. This is
collaboration among MIT, France and Japan.
Swift, to be built by NASA Goddard, hopes to chop the relay time down
to 15 seconds with even more precise coordinates -- a 1 to 4
arc-minute position, tight enough for Keck and Chandra to use. Swift
also has telescopes on board that will determine an arc-second
position of a burst within a few minutes and also determine the
redshift, or distance, to the burst source.
With this type of rapid response and precision, scientists hope to
finally get a handle on the location and nature of gamma-ray bursts,
by far the most powerful events known.
No one knows when the next really big burst will come. Perhaps on
Mozart's birthday, perhaps on yours. When ever it comes, gamma-ray
astronomers plan to be ready.