Nine billion light-years away

"Early in the morning of January 23, 1999, a robotic telescope in New Mexico picked up a faint flash of light in the constellation Corona Borealis. Though just barely visible through binoculars, it turned out to be the most brilliant explosion ever witnessed by humanity. We could see it nine billion light-years away, more than halfway across the observable universe.

If the event had taken place a few thousand light-years away, it would have been as bright as the midday sun, and it would have dosed Earth with enough radiation to kill off nearly every living thing. ..."The flash was another of the famous gamma-ray bursts, which in recent decades have been one of astronomy's most intriguing mysteries. ...

Before 1997, most of what we knew about gamma-ray bursts was based on observations from the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory. BATSE revealed that two or three gamma-ray bursts occur somewhere in the observable universe on a typical day."

Neil Gehrels, et.al., Scientific American, Majestic Universe, 2004,
"The Brightest Explosions in the Universe", pp. 65-6.