An image of LMC P3, the most luminous gamma-ray binary in another galaxy taken by Nasa's Fermi Gamma-ray Space Telescope.
LMC P3 (circled) is located in a supernova remnant called DEM L241 in the Large Magellanic Cloud, a small galaxy about 163,000 light-years away. The system is the first gamma-ray binary discovered in another galaxy and is the most luminous known in gamma rays, X-rays, radio waves and visible light.NASA

Nasa researchers found the first gamma-ray binary star system in another galaxy, and the most luminous one, for the first time using the Fermi Gamma-ray Space Telescope.

A cyclic flood of gamma rays was observed by the scientists, which is the outcome of an interaction between a crushed stellar core and a massive star that is a part of the dual-star system referred to as LMC P3. LMC stands for Large Magellanic Cloud, a small galaxy in which the binary star system has been found.

"Fermi has detected only five of these systems in our own galaxy, so finding one so luminous and distant is quite exciting," lead researcher Robin Corbet at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said.

"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," Corbet added.

These rare systems comprise a black hole or neutron, which emits energy in the form of gamma rays. The LMC P3 is so far the second dual star system that has been discovered by the Fermi Gamma-ray Space Telescope.

The LMC P3 is located amid the increasing fragments of a supernova explosion in the galaxy, said to be 163,000 light years away. Researchers using Nasa's sophisticated telescope, the Chandra X-ray Observatory, in 2012 found a source of strong X-ray within the supernova remnant and figured out that it was a young and hot orbiting star.

The mass of this star was found to be many times greater than that of the sun. It was concluded that this object was either a black hole or a neutron star. This system was classified as a high-mass X-ray binary (HMXB).

The team led by Corbet started searching for new gamma-ray binaries in Fermi data by hunting for the periodic changes characteristic of these systems in 2015.

The scientists made the following discoveries: The LMC underwent a 10.3 day cyclic change which centred near one out of many gamma-ray point sources. It was also found that one of the gamma ray sources — P3 — was located near the HMXB and was not related to any of the wavelengths.

Further investigations were made by Corbet's team using NASA's Swift satellite.

"They observed the binary in X-rays , at radio wavelengths with the Australia Telescope Compact Array near Narrabri and in visible light using the 4.1-meter Southern Astrophysical Research Telescope on Cerro Pachón in Chile and the 1.9-meter telescope at the South African Astronomical Observatory near Cape Town," according to a statement released by Nasa on September 29.

The Swift satellite had also found the same phenomenon of the 10.3-day emission cycle. The brightest X-ray emission was found occurring opposite the gamma-ray peak.

"The optical observations show changes due to binary orbital motion, but because we don't know how the orbit is tilted into our line of sight, we can only estimate the individual masses," said Jay Strader, a team member and an astrophysicist at Michigan State University in East Lansing.

"The star is between 25 and 40 times the sun's mass, and if we're viewing the system at an angle midway between face-on and edge-on, which seems most likely, its companion is a neutron star about twice the sun's mass. If, however, we view the binary nearly face-on, then the companion must be significantly more massive and a black hole," Strader added.

The surface at the heart of LMC P3 was found to have a temperature more than 60,000 degrees Fahrenheit (33,000 degrees Celsius), or more than six times hotter than the sun. The star was extremely luminous and the light's pressure drove materials from the surface, and created very speedy particle outflows. The speed of these outflows is estimated to be several million miles per hour.

"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," Dubus concluded.

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