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A new Hubble image of a powerful gamma-ray burst reveals a curious host galaxy

Feb 10, 1999: Two weeks ago one of the most powerful cosmic explosions ever recorded bathed the earth in gamma rays. The gamma-ray burst -- called GRB990123 -- was so intense that its visible light could have been seen through common binoculars. Since then the optical counterpart to GRB990123 has faded until it is 4 million times dimmer that it was during its peak.

Yesterday astronomers released a new Hubble photograph of the gamma-ray burst's fading optical counterpart. The relic fireball appears to be embedded in a faint, irregular galaxy.

Right, above: A Hubble telescope image of GRB 990123 taken between Feb 8 1999 23:06 UT and Feb 9 1999 03:21 UT. The image is of a square region about the gamma-ray burst, 3.2 arcseconds on each side. The optical counterpart, a point source at the center of the image, is seen to be superposed on an irregular galaxy, which could perhaps be an interacting system. More information.

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A little more than a day after the gamma-ray burst astronomers used the 10-meter Keck II telescope on Mauna Kea to analyze ultraviolet and visible light from the fading afterglow. They measured a redshift z=1.6 for the visible light, which implies that the explosion took place about 10 billion light years from Earth.

The discovery of a fuzzy blob near the location of GRB990123 marks the fourth uncontroversial association of a gamma-ray burst with what is apparently a distant galaxy. "These galaxies are so distant and so faint that it's difficult to say much about them," explains Dr. Tom Koshut, a gamma-ray astronomer at the NASA Marshall Space Flight Center. "We know that they are very far away because absorption lines in their optical spectra have a high redshift. But are they spiral galaxies, giant ellipticals, or something bizarre and unfamiliar? It can be very hard to tell."

Although the nature of the optical afterglows of gamma-ray bursts and their host galaxies remain a puzzle, they have already answered one important question about gamma-ray bursters: "Where are they?"

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Left: This image from the Hubble Space Telescope shows the optical afterglow from a gamma-ray burst detected in February 1997. The bright spot is thought to be an expanding fireball, and the weak diffuse emission (below and to the right) may be the distant host galaxy.

Until recently astronomers weren't sure whether gamma-ray bursts came from our own Solar System, the Milky Way galaxy, or much further away. With the discovery of optical counterparts it's now thought that most gamma-ray bursts come from the distant reaches of the cosmos. Last year Shri Kulkarni of Caltech and his colleagues found that a gamma-ray burst recorded in December 1997 came from a faint galaxy with a redshift of 3.4. That means that the burst originated over 12 billion light years away. Kulkarni noted that "The energy released by this burst in its first few seconds staggers the imagination."

The next question is "What causes gamma-ray bursts?" Scientists are unsure. In the absence of much hard data theorists have proposed a multitude of possible scenarios, from super-supernovae to mutually annihilating neutron stars. It is widely thought that the studies of gamma-ray burst afterglows and their host galaxies might provide some clues.

Right: This figure shows the how the brightness in gamma-rays, as observed with BATSE, varied during GRB 990123. The three intervals marked by vertical lines indicate the times during which the ROTSE optical telescope obtained its first three visible images. The ability to photograph a gamma-ray burst during the event is a new advance that astronomers hope will lead to greater understanding of these mysterious explosions. Credit: Dr. Michael Briggs, NASA/Marshall.

In the past, observations of visible light from gamma-ray bursts have all been made hours after the event had ended. The light by that time was so dim it could only be detected by the world's largest telescopes. Thanks to the development of the Gamma-ray burst Coordinates Network (GCN), optical telescopes can now be trained on bursts while the optical emission is still bright. This will result in better redshift measurements of the optical fireballs, and it will make astronomers better able to match up gamma-ray bursts with their host galaxies.

For now the secret of gamma-ray bursts remains veiled in distant, mysterious fuzzy blobs.