Twin gamma-ray bursts provide several firsts
Posted: March 11, 2003

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.

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.

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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