NASA鈥檚 Swift space telescope has been used to directly measure the distance to fleeting cosmic explosions, called gamma-ray bursts, for the first time. The measurements are crucial for determining how much energy the bursts unleash.
Gamma-ray bursts (GRBs) are powerful blasts of high-energy gamma-ray photons thought to herald the birth of black holes. They typically last less than a minute but can produce afterglows at a range of wavelengths 鈥� from X-rays to radio waves 鈥� that linger for hours or weeks. The afterglows can reveal the distance and nature of the bursts, and Swift was designed to swivel towards them in about a minute.
Swift has detected two dozen GRBs since its launch in November 2004, and it has found X-ray afterglows for about 15 of them. But until recently, it saw no optical afterglows 鈥� light emitted in the visible spectrum 鈥� even when ground-based observers looking at the same bursts did. 鈥淲e were getting a little worried,鈥� admits Dale Frail, a Swift team member at the National Radio Astronomy Observatory in Socorro, New Mexico, US.
Advertisement
But Swift鈥檚 Ultraviolet/Optical Telescope (UVOT) has made up for lost time, catching the optical afterglow of a bright GRB on 18 March 2005 and a second one the very next day. 鈥淣ow we know there鈥檚 no problem 鈥� the first few events were just too faint for UVOT to detect,鈥� Frail told New 杏吧原创.
鈥淏locked鈥� wavelengths
Optical 鈥� as opposed to X-ray 鈥� afterglows are necessary for Swift to gauge the distance to a GRB. That is because hydrogen gas in space absorbs some wavelengths of the afterglow鈥檚 optical light, but not its X鈥搑ay emissions.
The expansion of the universe stretches light from faraway objects toward the red end of the spectrum, so both the X-ray afterglow and the partially 鈥渂locked鈥� optical wavelengths from distant GRBs appear 鈥渞edshifted鈥� in telltale ways.
鈥淏y looking at which colours we see and which we don鈥檛, we can get an estimate of the distance,鈥� says Neil Gehrels, the mission鈥檚 principal investigator at NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US.
鈥淩eally spectacular鈥�
Most GRBs observed originated about 9.3 billion light years away, while the most distant ever recorded lay 12.3 billion light years away. Swift has not yet broken that record, but one of its two recently measured bursts came close. The 18 March burst hailed from 9.2 billion light years out, while the 19 March event exploded at a distance of 11.6 billion light years.
That 19 March event was 鈥渞eally spectacular when you think about it,鈥� says Frail, who helped make the first-ever distance measurement to a GRB, in 1997. 鈥淚t鈥檚 so far away, yet it鈥檚 so bright we can still see it.鈥�
He says distance is the key to determining a GRB鈥檚 total energy output, as distant bursts that appear faint can be much more violent than those nearby that look brighter. He adds that Swift may be able to detect GRBs from the very edge of the observable universe, when the first stars exploded, becoming black holes. 鈥淵ou can look at GRBs as lighthouses for the early universe,鈥� he says.
