Blast From Empty Space Poses Scientific Mystery

admin@mysteriesunsealed.com — Sat, 29 Dec 2007 18:14:47 GMT

Just when space scientists thought they had solved the mystery of the brightest explosions in the universe, along comes one that has the experts befuddled.

The confounding "gamma ray burst" was detected by space-based instruments and the Swift orbiting gamma ray observatory on Jan. 25, 2007. At first it stood out only because it was rather bright -- one of the brightest ever recorded. Since then, however, the burst -- dubbed GRB 070125 -- has proven far more interesting in other ways.

A telescope imaged the afterglow of the mysterious burst called GRB 070125 on January 26, 2007. At right, an image taken of the same field on February 16 with the Keck I telescope reveals no trace of an afterglow, or a host galaxy. The white cross in the zoom-in view marks the GRB's location. The two nearest galaxies, and their distances, are marked with arrows.

Unlike the other hundred or so similar gamma ray bursts discovered -- most of which shine out momentarily with incredible brightness from crowded stellar nurseries -- GRB 070125 appears to be all by itself in space without any signs of a galaxy around it. That goes against everything astronomers have learned about gamma ray bursts over the last ten years.

"It's out in the middle of nowhere," said gamma ray burst investigator Derek Fox of Pennsylvania State University. Fox coauthored a report on GRB 070125 that will be published in The Astrophysical Journal.

The current theory holds that gamma ray bursts like GRB 070125 are given off by super-jumbo-sized stars that run out of fuel and violently collapse to form black holes, explained Neil Gehrels, principal investigator of NASA's Swift telescope.

Such huge stars can only be created in very gas and dust-rich parts of galaxies where lots of other stars are also being born. So it makes no sense to find such a star living and dying in the empty space between galaxies. Nor is there time for such stars to travel out into intergalactic space, said Gehrels, since mega stars are also the shortest-lived.

The mystery of GRB 070125 only deepened as astronomers narrowed in on the burst's location, about 9.5 billion light-years away, with more powerful telescopes. They were still looking for the light from the galaxy which theory predicted must be there, but no galaxy appeared.

The researchers also collected the visible light from the burst's afterglow with the Gemini North 8-meter telescope in Hawaii, then split the light into its spectrum of colors to look for telltale "absorption" lines. These dark lines are sure signs of material in the burst's home galaxy filtering the light of the burst.

"We usually see very deep absorption features," said Caltech astronomer Brad Cenko, lead author of the report. "In this case we didn't see any. That in and of itself is pretty unique."

Nor did any other researchers have any luck.

"There are quite a few groups that have looked at this place," confirmed Gehrels. "You can really tell for sure that there's nothing going on there."

The next step, says Cenko, is to win some time using the Hubble Space Telescope to look for the missing galaxy. If it's not there, it's possible they could see some signs that two nearby galaxies, already observed with the Keck I telescope, have been colliding and leaving a string of lonely stars of all ages and sizes between them, he said.

"That's the leading hypothesis," Cenko said.

Then again, they could find something else entirely unexpected. As Cenko added, "There are still lots of surprises in this field."

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