An international team of astronomers has spotted a very rare object in a nearby galaxy: a gamma ray binary, a pair of stars that are emitting gamma rays. This is the first one ever discovered outside the Milky Way, and the most powerful ever detected so far.
The system, named LMC P3, is located in the Large Magellanic Cloud galaxy, 163,000 light-years from Earth. It consists of a large heavy star that is orbited every 10.3 days by a very dense neutron star. With each orbit, as the stars get close, LMC P3 emits a huge amount of gamma-rays, which were detected by NASA’s Fermi observatory.
"Fermi has detected only five of these systems in our own galaxy, so finding one so luminous and distant is quite exciting," said lead researcher Robin Corbet at NASA's Goddard Space Flight Center in Greenbelt, in a statement.
"Gamma-ray binaries are prized because the gamma-ray output changes significantly during each orbit and sometimes over longer time scales. This variation lets us study many of the emission processes common to other gamma-ray sources in unique detail."
The system is a supernova remnant, sitting at the center of a large expanding cloud of debris. The larger star is estimated to be between 25 and 40 solar masses, while the neutron star could be about two times the mass of the Sun. There are some uncertainties, though.
In the paper, published in the Astrophysical Journal, the team discusses how the brightness of the system in gamma rays is quite puzzling. The researchers suggest that it might be a combination of internal properties of the neutron stars and maybe peculiar dynamics of the orbit, but there’s no conclusive evidence.
"It is certainly a surprise to detect a gamma-ray binary in another galaxy before we find more of them in our own," said Guillaume Dubus, a team member at the Institute of Planetology and Astrophysics of Grenoble in France.
"One possibility is that the gamma-ray binaries Fermi has found are rare cases where a supernova formed a neutron star with exceptionally rapid spin, which would enhance how it produces accelerated particles and gamma rays."
More observations with other instruments will hopefully clarify how this peculiar system gets so bright.
