Unpredictable Events

Abstract

Over the course of about a minute on Mar. 19, 2008, keen-eyed observers looking toward the constellation Boötes might have noticed the brief, mysterious appearance of a dim star that then quickly faded from view. At 2:13 a.m. EDT, instruments on two spacecraft detected a burst of high-energy gamma rays, a signature associated with the death of a star and the birth of a black hole. The star had reached a point in its energy-producing career where it essentially ran out of fuel. It began to collapse under its own weight and in its innermost regions formed a black hole. As gas rained toward the black hole, some of it was paradoxically diverted into a pair of oppositely directed, outwardly moving particle jets. Racing at speeds only a whisker slower than that of light itself, the jets sliced completely through the collapsing star – and one of them just happened to be pointed almost directly toward Earth. Even though most of this jet’s emission was radiation too energetic to see, a small amount was in the form of visible light, and anyone looking at the spot would have seen a dim star brightening to magnitude 5.3 before fading back to invisibility about 50 s later. We know this because robotic telescopes in Chile were looking, and they immediately slewed to the star’s position once NASA’s gamma-ray-sensing Swift satellite had determined it. What’s amazing about this event is that the explosion occurred not within our galaxy or even in any of its nearest neighbors. Instead, the dying star was 7.5 billion light-years away — so distant that the blast occurred when the universe was less than half its present age, long before our own Sun was born. Astronomers say that the event was so bright because the particle jet happened to be oriented almost directly into our line of sight. They expect similar alignments about once a decade, on average.