Two mysterious X-ray flares have been discovered coming from a point beyond the Milky Way where there is no known source capable of producing them. When not flaring, the sources appear to be normal accreting neutron-star or black-hole X-ray binaries, but these cannot explain what the scientists observed.
In their study published in Nature, a team led by Jimmy Irwin, of the University of Alabama, identified two X-ray flares (bursts of X-rays) on the outskirts of two nearby galaxies – one near the Virgo galaxy NGC 4636, and the other near Centaurus A, NGC 5128.
Previously, scientists had found two very brief X-ray flares with high luminosity near the galaxy NGC 4697. Irwin and colleagues sought to find similar flares by re-examining archived Chandra X-ray observations for 70 galaxies near the Milky Way.
They found two more flare sources. One flared just once, while the other produced five flares. All flares lasted less than a minute and decayed over the course of around an hour. The flares were extremely bright – indeed one was more luminous than what can be attained by any normal neutron star. Furthermore their apparent location makes no sense – the flares appear to be located in old stellar populations.
Normally the source of flares can be identified by looking at how long they last for and whether they repeat. Flares that do not repeat and that last about a minute normally indicate the death of huge stars. But this requires a population of young stars, which the proposed source location lacks. Flares that do repeat are also placed under certain constraints in terms of what can produce them – none of which can match what scientists observed.
So what caused them? The team said the sources do not appear to self-destruct in the process of flaring, as is associated with a gamma ray burst or supernovae. Other than that, they said the nature "remains uncertain". They suggest the source could be a black hole undergoing a process currently unidentified.
In a related News & Views article, Sergio Campana, from the National Institute for Astrophysics (INAF) in Italy, said there are two "viable explanations" for the black hole interpretation. "One possibility, which is suggested by the authors, is that there exists an intermediate-mass black hole (100–1,000 times the mass of the Sun) at the centre of each source that, for some unknown reason, emits flares that last for about an hour," he wrote.
"Alternatively, one could envisage a lower-mass black hole whose X-ray emissions are beamed directly towards Earth. A binary system that has a highly eccentric orbit might explain the repeated flares detected from the source near NGC 5128. The flares from such a binary system would be strictly periodic, because sporadic surges of accretion would occur at the point of closest approach."
He added more observations would help solve the flare mystery, specifically the frequency at which they occur. "Now that we know these strange objects are out there, they will remain on the watch list and more examples will be searched for," he concluded.