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First gamma-ray binary outside our Galaxy

The ATNF's Compact Array has helped to confirm the discovery of the first gamma-ray binary in another galaxy, which is also the most luminous one ever seen.

The dual-star system, dubbed LMC P3, contains a massive star and a crushed stellar core (a neutron star or black hole) that interact to produce a cyclic flood of gamma rays, the highest-energy form of light.

LMC P3 lies within the expanding debris of a supernova explosion located in the Large Magellanic Cloud (LMC), a small nearby galaxy about 163,000 light-years away.

In 2012 scientists using NASA's Chandra X-ray Observatory found a strong X-ray source within the supernova remnant and showed that it was orbiting a hot, young star many times the Sun's mass. The researchers concluded the compact object was either a neutron star or a black hole and classified the system as a high-mass X-ray binary (HMXB).

In 2015 a team led by Robin Corbet (NASA Goddard Space Flight Center) began looking for new gamma-ray binaries in data from NASA's Fermi space telescope.

The scientists discovered a 10.3-day cyclic change centered near one of several gamma-ray point sources recently identified in the LMC. One of them, called P3, was not linked to objects seen at any other wavelengths but was located near the HMXB. Were they the same object?

To find out, Corbet's team observed the binary in X-rays using NASA's Swift satellite, at radio wavelengths with the Australia Telescope Compact Array, and in visible light using the 4.1-meter Southern Astrophysical Research Telescope on Cerro Pachon in Chile and the 1.9-meter telescope at the South African Astronomical Observatory near Cape Town.

The Compact Array and Swift observations clearly reveal the same 10.3-day emission cycle seen in gamma rays by Fermi. They also indicate that the X-ray and radio emission occurs exactly out of phase with the gamma rays, confirming that LMC P3 is indeed the same system as that found by Chandra.

The star at the heart of LMC P3 has a surface temperature greater than 33,000 degrees Celsius, or more than six times hotter than the Sun's. The star is so luminous that pressure from the light it emits drives material from its surface, creating particle outflows with speeds of several million miles an hour.

In gamma-ray binaries, the compact companion is thought to produce a 'wind' of its own, one consisting of electrons accelerated to near the speed of light. The interacting outflows produce X-rays and radio waves throughout the orbit, but these emissions are detected most strongly when the compact companion travels along the part of its orbit closest to Earth.

Through a different mechanism, the electron wind also emits gamma rays. When light from the star collides with high-energy electrons, it receives a boost to gamma-ray levels. Called inverse Compton scattering, this process produces more gamma rays when the compact companion passes near the star on the far side of its orbit (as seen from our perspective).

The Fermi telescope has detected only five gamma-ray binaries in our Galaxy. But prior to Fermi's launch, gamma-ray binaries were expected to be quite numerous. Hundreds of HMXBs are cataloged, and these systems are thought to have started life as gamma-ray binaries following the supernova that formed the compact object.

It may be that we're detecting as gamma-ray binaries only those systems with the fastest rotating neutron stars at birth, whereas most neutron stars are born as slower rotators. It's also possible that gamma-ray binaries may have been missed because they are not 'on' much of the time, or because they emit only faintly in the region of the spectrum being searched on.

Source: NASA

"A Luminous Gamma-ray Binary in the Large Magellanic Cloud," R. H. D. Corbet et al., 2016 Oct. 1, Astrophysical Journal
[, preprint:].

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Added by Helen Sim on 2016-10-04

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