Several dozen satellites around Earth, and one orbiting Mars, detected a flash of high-energy photons 鈥� known as gamma rays 鈥� on 27 December 2004. The 0.25-second flash was so bright it overwhelmed the detectors on many satellites 鈥� making an energy measurement impossible 鈥� and disrupted some radio communication on Earth.
鈥淚t was so bright, it came right through the body of the Swift satellite, even though Swift wasn鈥檛 pointed at the object,鈥� says John Nousek, mission director for NASA鈥檚 Swift spacecraft 鈥� launched especially to detect gamma-ray bursts (GRBs) 鈥� at Pennsylvania State University, US.
The brief flash was followed by a fainter afterglow of gamma rays lasting for about 500 seconds, which showed a recurring signal every 7.5 seconds. That signal led scientists using Europe鈥檚 INTEGRAL spacecraft to trace the source of the 鈥渟uperflare鈥� to a dead star 鈥� called a neutron star 鈥� known to spin at that rate. Measurements of the distance to the star 鈥� called SGR 1806-20, range from 30,000 to 50,000 light years from Earth.
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That relatively small distance, coupled with an accurate energy measurement by NASA鈥檚 RHESSI satellite, means the explosion was not as powerful 鈥� at source 鈥� as more distant bursts linked with black holes. Nevertheless, it 鈥渕ay have sterilised any planets within a few light years of it鈥�, says Rob Fender, an astronomer at Southampton University, UK, who is studying the lingering radio emission from the flare. 鈥淭his may be a once-in-a-lifetime event both for astronomers and for the neutron star itself.鈥�
Clean credit card
But Christopher Thompson, an astrophysicist at the Canadian Institute for Theoretical Physics, says that may not be so. The neutron star in question is rare magnetar, with a magnetic field so strong it could wipe a credit card clean from a distance of 160,000 kilometres. And this magnetar is even rarer yet, one of three 鈥渟oft gamma repeaters鈥� (SGRs) in the Milky Way.
SGRs tend to release low-energy flares of gamma rays sporadically, but more energetic bursts have been observed twice before 鈥� in 1998 and 1979. But the energy in the initial 0.25-second burst from the most recent flare was 100 times that of the two previous superflares.
But Thompson, who worked on the most accepted magnetar model, says: 鈥淚 wasn鈥檛 shocked at the energy it was putting out. The total energy implied by the models is enough to power a dozen or more of these events in the life of one magnetar.鈥�
Superflares may occur when tightly wrapped magnetic fields inside the magnetar start to 鈥渦ntwist鈥�. This briefly rips loose some magnetic fields outside the star, releasing a 鈥渇ireball鈥� of particles, and light that astronomers see as a bright flash of gamma rays.
Extreme distances
If this flare had been even farther away 鈥� up to 100 million light years or so 鈥� it would have looked 鈥渋ndistinguishable鈥� from a short GRB 鈥� a cosmic phenomenon that has baffled astronomers for years. Short GRBs are blasts of high-energy gamma rays that last less than two seconds. Astronomers are unsure of their cause but think they have a different origin than 鈥渓ong鈥� GRBs 鈥� lasting for several seconds or minutes 鈥� which are thought to be created during the birth of black holes.
This latest observation leads David Palmer, a Swift team member at Los Alamos National Laboratory in New Mexico, US, to say: 鈥淚鈥檓 fairly confident that soft gamma repeaters account for at least some short gamma-ray bursts.鈥�
Neil Gehrels, principal investigator for Swift at NASA, says Swift should be able to help settle the debate about short GRBs. Swift will study both SGRs and short GRBs, having the capability to quickly respond to short GRBs in order to locate them in space.
But he laments: 鈥淚t鈥檚 very unlikely we鈥檙e going to see another one of these supergiant flares.鈥�
