UPDATE: A message posted “on behalf of the Swift-XRT team” on NASA’s Gamma-ray Coordinates Network (GCN) system at 8:57 a.m. BST on 28 May says that the astronomers now “do not believe this source to be in outburst”. Swift team member Kim Page, a nova and γ-ray-burst astronomer at the University of Leicester, UK, told Nature that the source had been initially mistaken for a new outburst, and that its intensity had been overestimated owing to measurement error. Instead, she says, it was a relatively common, persistent X-ray source — possibly a globular cluster — that had previously been catalogued. (See this post from Page’s Leicester colleague Phil Evans.)
NASA’s Swift satellite has detected a burst of high-energy γ-rays coming from the Andromeda galaxy, the closest large galaxy to the Milky Way. The rare cosmic explosion is likely to deliver a flood of data to astronomers, who are swivelling their telescopes to capture its aftermath.
Swift watches for γ-ray bursts and, if it detects one, the satellite automatically redirects to try to capture the source. The trigger went off at 9:21 p.m. Universal Time on 27 May; three minutes later, the X-ray telescope aboard Swift was already observing a bright X-ray glow where none had existed before.
News of the event rippled across the astronomical community. Within minutes the Swift data servers had crashed, leaving the official news mirrored in unofficial locations.
The closeness of the blast — just 766,500 parsecs (2.5 million light-years) away, a neighbour in cosmic terms — had astronomers speculating whether neutrino observatories, such as the IceCube detector in Antarctica, might pick up the event.
The burst may have originated when two ultra-dense neutron stars collided. If so, it probably would have generated gravitational waves — ripples in the fabric of space time, predicted by Einstein — zooming across the cosmos. Unfortunately, the machines best suited to detect such gravitational waves are currently offline. The US Laser Interferometer Gravitational-wave Observatory (LIGO) is in the midst of a multi-year, US$200-million upgrade to a more sensitive system. Two weeks ago, astrophysicist Gabriela Gonzalez of Louisiana State University in Baton Rouge speculated on the possibility that a nearby supernova — an exploding star, sometimes connected with γ-ray bursts — could go off during the LIGO upgrade. “My nightmare is that it happens before we turn on,” she said.
Another possibility, says astrophysicist Robert Rutledge of McGill University in Montreal, Canada, is that the blast is an ‘ultra-luminous’ X-ray source, a class of objects less bright than a galaxy heart but more bright than ordinary stars. If so, then the X-rays are likely to be visible for days to come, rather than half a day as one might expect from a γ-ray burst.
Whatever caused it, the Andromeda blast occurred some 2.5 million years ago. Its energy has been travelling towards us ever since.