This event, dubbed GRB 090423, is the most distant cosmic explosion ever seen. At 3:55 a.m. EDT on April 23, Swift detected a ten-second-long gamma-ray burst of modest brightness. It quickly pivoted to bring its ultraviolet/optical and X-ray telescopes to observe the burst location. Swift saw a fading X-ray afterglow but none in visible light. “The burst most likely arose from the explosion of a massive star,” said Derek Fox at Pennsylvania State University. “We’re seeing the
demise of a star — and probably the birth of a black hole — in one of the universe’s earliest stellar generations.”
Gamma-ray bursts are the universe’s most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets — driven by processes not fully understood — punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths. Beyond a certain distance, the expansion of the universe shifts all optical emission into longer infrared wavelengths. While a star’s ultraviolet light could be similarly shifted into the visible region, ultraviolet-absorbing hydrogen gas grows thicker at earlier times.
Within three hours of the burst, Nial Tanvir at the University of Leicester, U.K., and his colleagues, reported detection of an infrared source at the Swift position using the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. At the same time, Fox led an effort to obtain infrared images of the afterglow using the Gemini North Telescope on Mauna Kea. The source appeared in longer-wavelength images but was absent in an image taken at the shortest wavelength of 1 micron. This “drop out” corresponded to a distance of about 13 billion light-years. As Fox spread the word about the record distance, telescopes around the world slewed toward GRB 090423 to observe the afterglow before it faded away. NASA’s Goddard Space Flight Center manages Swift.
