Monster gamma-ray burst seen in cosmic neighbourhood
Sen—Astronomers have observed an unusually powerful gamma-ray burst, one of the biggest types of explosion in the Universe, in a relatively nearby region of space.
Usually such huge blasts are detected in the farthest reaches of the Universe and are very short-lived. After having long been a mystery, scientists have decided those are due to the creation of black holes or the merging of two super-dense neutron stars.
The new burst, reported as a “monster” event in the journal Science this week, was different. It was longer lasting and occurred “only” 3.75 billion light-years away which is considerably closer than the remote blasts. One of the brightest ever seen, and labelled GRB 130427A, it was detected by three space telescopes.
First the Gamma-ray Burst Monitor (GBM) aboard NASA’s Fermi Gamma-ray Space Telescope captured the initial wave of gamma rays on April 27. In its first three seconds alone, the “monster burst” was brighter than almost any burst previously observed.
The Swift X-ray space telescope, which picks up about 100 gamma-ray bursts a year, spotted it at virtually the same moment and automatically alerted ground-based observatories.
Then NASA’s newest X-ray observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), made the first detection of a high-energy burst afterglow after more than a day. The observations are said to challenge long-standing predictions.
The new burst had a different cause to the more remote, short-lived ones. A team led by astrophysicist Daniele Malesani, of the Dark Cosmology Center at the Niels Bohr Institute at the University of Copenhagen, deduced that it was caused by a supernova, the explosion of a massive star.
Dr Malesani said: “We suddenly saw a gamma-ray burst that was extremely bright - a monster gamma-ray burst. This one of the most powerful gamma-ray bursts we have ever observed with the Swift satellite.
A close-up and a wide-angle view of the gamma-ray burst, taken by the instruments on the Swift satellite. Credit: NASA/Niels Bohr Institute
“We follow the so-called afterglow, which usually lasts a few days or for several weeks, from both Swift and from the ground-based telescopes. In this case, the burst was so powerful that we could observe the afterglow for several months.”
By analysing the light from the afterglow with a spectroscope, the team discovered that the stellar culprit had a mass 20-30 times the mass of the Sun, but was only three or four times the size. It was rapidly rotating and was of a type called Wolf-Rayet stars.
Its location was pinned down to a particular galaxy and the astronomers found that it exploded when the Universe was 9.9 billion years old.
Sen asked Dr Malesani about the discovery. He told us: “There are two main categories of gamma-ray bursts (GRBs). They are distinguished by their duration, so they are called, with little imagination, short and long GRBs; short bursts last 0.01 to 2 seconds, while long GRBs last from about two to several hundreds of seconds. We have now learnt that these two classes are physically distinct objects.
“The GRB that we are dealing with our paper is a long GRB, so related to the explosion of a massive star - this was in fact confirmed by the observation of a so-called “supernova” coincident with the GRB, as seen a couple of weeks after the GRB, when the gamma-ray radiation had faded away leaving room for the supernova light.
The location of the gamma-ray burst in a scan of the sky by Fermi. Image Credit: NASA/DOE/Fermi LAT Collaboration
“In general, it is extremely rare for a supernova also to produce a GRB. It is estimated that only one in 10,000 does so. It is an active area of research to understand why. The problem is not settled yet, but we believe that special conditions are required to generate a GRB, for example, the nucleus of the progenitor star must have an exceptionally fast rotation.”
Dr Malesani told us: “The particularity of this event is that it is especially close to the Earth in comparison to other, very distant GRBs. It is still billions of light years away! Usually, GRBs that are discovered close by are exceptionally faint, compared to other GRBs. So, this one is the first “normally-bright” GRB which happens to be nearby. The fact that it is both nearby and normally bright makes it exceptionally bright in terms of the received flux on Earth, which allows us to study it in unprecedented detail.
“It is expected that only a few such events will be detected per century - so this is almost a once-in-a-lifetime opportunity for an astronomer’s career!”
Paul Hertz, director of NASA’s Astrophysics Division in Washington, said: “We expect to see an event like this only once or twice a century, so we’re fortunate it happened when we had the appropriate collection of sensitive space telescopes with complementary capabilities available to see it.”Overview animation of a gamma-ray burst. Credit: NASA