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Public Affairs
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

For more information, contact: 

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468 
daguilar{at}cfa.harvard.edu 

Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
clafon{at}cfa.harvard.edu

For Release: March 10, 2003

Release No.: 03-08

Twin Bursts Provide Several Firsts 
==================================

Gamma-ray bursts (GRBs) are the most violent explosions in the 
Universe, but little is known about them. In recent years, several 
theories have been put forward to explain these elusive explosions, 
but the mystery still remains.  Now, two recent bursts observed by 
astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) 
provide unique data that can not only help test previous models but 
also help theorists come up with a better picture of what GRBs really 
are. 

A highly polarized blast 
------------------------
Harvard astronomer David Bersier and his colleagues observed the first 
burst, GRB 020405, to measure the amount of polarization of light from 
its afterglow.  Polarization is a measure of the direction of 
vibration of light waves. Polarized light tends to vibrate in a
particular direction in the sky, while unpolarized light vibrates in 
all directions equally. 

GRB 020405 was observed with the MMT 6.5-meter telescope at Mount 
Hopkins, Arizona, on April 6, 2002, a day after the burst was first 
detected. Data from this burst indicated a polarization level of 
almost 10 percent, the highest level ever measured. A day later, a 
second group measured a polarization level of about two percent.
Interestingly, astronomers also observed a two percent polarization 
only hours before the 10 percent measurement, implying a rapid change 
in polarization on either side of the peak.

Utilizing the MMT was crucial to gathering enough light for the 
measurements. The telescope was outfitted with a very sensitive 
digital camera and a set of filters used to measure polarization. 
These filters are made of the same material used to make polarized 
sunglasses. 

Said Smithsonian astronomer Brian McLeod, who developed the camera 
equipment, "The key to making this measurement was having the camera 
installed on the MMT telescope for many different projects. GRBs are 
discovered only about once a month, so we can't just wait around with 
the telescope idle. When the GRB went off, we called the astronomers 
who happened to be using the telescope that night and asked them to 
point the telescope at the GRB." 

GRBs are believed to come from either the merger of two black holes or 
neutron stars, or from the explosion of a very massive star. Models 
show that these explosions appear very energetic because much of their 
energy is blasted outward in two narrow jets in opposite directions.

In a broad sense, these recent observations support such models, which 
predict some amount of varying polarization. But the group's 
observations also demonstrate that many details still need to be 
worked out. For instance, polarization from a GRB afterglow is 
expected to be highest when viewed from the edge of the jet. In some 
cases the polarization can be as high as 20 percent, implying that GRB 
020405 was indeed seen from near the edge of its jet. At this extreme 
viewing angle, calculations predict a gradual decrease in
polarization. Instead, the astronomers saw a significant decrease in 
the span of just one day. 

One by one, the group has ruled out errors resulting from observing 
instruments, dust (either in the host galaxy or in the Milky Way), and 
microlensing (the temporary brightening in light from a distant object 
when a dim star comes between it and the Earth). Bersier hopes that
comparing his results with those of other groups that observed this 
burst will help produce a more robust model of GRBs.

A rapidly varying blast 
-----------------------
If the first burst was rare -- as far as viewing angle is concerned -- 
the second burst was not far behind.  Discovered by the orbiting High 
Energy Transient Explorer (HETE) satellite on October 4, 2002, 
observations of GRB 021004 began less than 10 minutes after the blast.

Bersier and his colleagues wanted to see if the GRB light curve would 
show the same short-term variations seen in a burst the previous year. 
Sure enough, their observations demonstrated that the light from the 
burst fluctuated on a timescale of 15 - 30 minutes. Over the course of
several hours, the brightness of the afterglow repeatedly decreased 
and increased. Since several nearby stars did not exhibit this highly 
unusual behavior, the team concluded the variations to be genuine and 
intrinsic to the burst.

The rapid variations in the light curve, or "wiggles," are believed to 
be due to density variations in the interstellar matter. Since they 
appeared within hours of the GRB, astronomers theorize that the matter 
must be close to the GRB itself. This is a clue that the likely source 
of the GRB was a hypernova, or exploding star.

According to Bersier, "This second burst has provided us with the 
best-sampled light curve to date." The more than 100 data points 
revealed a gradually fading burst with a significant bump in the light 
curve. This sudden increase in energy while the afterglow was fading 
has puzzled astronomers. Though several models can help explain the 
surge of energy at the start of the blast and minor surges in the 
middle, no single model has been found to explain this extra energy 
during fading.  Bersier says more detailed work is needed before a 
completely accurate model emerges and suggests accounting for energy 
distributions in future models. 

The rapid brightness fluctuations were not the only thing that caught 
astronomers' interest. Watching this burst, Bersier and his colleagues 
were surprised to see the afterglow change its intrinsic color as it 
faded.  While one other burst has shown a similar color change, that 
burst is believed to have been affected by microlensing. No model can 
explain the color change seen in GRB 021004 yet.

On a fundamental level, findings from these two bursts will help 
answer some basic questions about the Universe.  Light from these 
bursts began its journey billions of years ago, when the Universe 
itself was a teenager. It was the time when clouds of dense gas 
combined violently to form new stars and new galaxies. Scientists hope 
that by observing the oldest visible phenomenon in the Universe, they 
will some day be able to answer how life itself began.

This research was reported within papers in the February 1, 2003, and 
February 20, 2003, issues of The Astrophysical Journal Letters.

Headquartered in Cambridge, Massachusetts, the Harvard- Smithsonian 
Center for Astrophysics (CfA) is a joint collaboration between the 
Smithsonian Astrophysical Observatory and the Harvard College 
Observatory. CfA scientists organized into six research divisions 
study the origin, evolution, and ultimate fate of the universe.

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