A gamma-ray binary discovered in the Large Magellanic Cloud, a dwarf galaxy approximately 163,000 light-years away, is the first object of its kind ever found in a galaxy other than the Milky Way and is the most luminous one ever seen.
LMC P3 (circled) is located in the supernova remnant DEM L241 in the Large Magellanic Cloud. Image credit: NASA’s Goddard Space Flight Center.
Gamma-ray binaries contain either a neutron star or a black hole and radiate most of their energy in the form of gamma rays.
Only a handful of such systems have been previously discovered, all within our Milky Way Galaxy.
Remarkably, the newly-discovered object, LMC P3, is the most luminous such system known in gamma rays, X-rays, radio waves and visible light, and it’s only the second one discovered with NASA’s Fermi Gamma-ray Space Telescope.
LMC P3 is located in a supernova remnant called DEM L241 in the Large Magellanic Cloud.
A paper describing the discovery was published online this week in the Astrophysical Journal (arXiv.org preprint).
Observations from the LAT (magenta line) show that gamma rays from LMC P3 rise and fall over the course of 10.3 days. The companion is thought to be a neutron star. Illustrations across the top show how the changing position of the neutron star relates to the gamma-ray cycle. Image credit: NASA’s Goddard Space Flight Center.
“We report the discovery with the Fermi Large Area Telescope (LAT) of a luminous gamma-ray binary in the Large Magellanic Cloud from a search for periodic modulation in all sources in the third Fermi LAT catalog. This is the first such system to be found outside the Milky Way,” the authors said.
“The system has an orbital period of 10.3 days and is associated with a massive O5III star located in the supernova remnant DEM L241, previously identified as the candidate high-mass X-ray binary (HMXB) CXOUJ053600.0-673507.”
“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,” added co-author Dr. Jay Strader, of Michigan State University.
“The star is between 25 and 40 solar masses, 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.”
Both objects form when a massive star runs out of fuel, collapses under its own weight and explodes as a supernova.
The star’s crushed core may become a neutron star, with the mass of half a million Earths squeezed into a ball no larger than Washington, D.C. Or it may be further compacted into a black hole, with a gravitational field so strong not even light can escape it.
The surface of the star in LMC P3 has a temperature exceeding 60,000 degrees Fahrenheit (33,000 degrees Celsius).
The star is so luminous that pressure from the light it emits actually drives material from the surface, creating particle outflows with speeds of several million miles an 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 co-author Dr. Guillaume Dubus, from 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.”
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R.H.D. Corbet et al. 2016. A Luminous Gamma-ray Binary in the Large Magellanic Cloud. ApJ 829, 105; doi: 10.3847/0004-637X/829/2/105
