LMC P3
LMC P3 is a gamma-ray binary star system located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way Galaxy approximately 163,000 light-years from Earth. It is the first gamma-ray binary discovered outside the Milky Way and the most luminous such system known, with gamma-ray emissions exceeding 10³⁶ ergs per second in the high-energy range above 100 MeV. The system consists of a massive O-type star orbiting a compact object, likely a neutron star, in an eccentric 10.3-day orbit, producing periodic high-energy emissions through interactions between the stellar wind and relativistic particles from the compact object. LMC P3 resides within the supernova remnant DEM L241 and serves as a key object for studying particle acceleration and binary evolution in low-metallicity environments.
Discovery
LMC P3 was initially identified as a high-mass X-ray binary in 2012 through observations with NASA's Chandra X-ray Observatory, located within the supernova remnant DEM L241 in the LMC. In 2015, analysis of data from NASA's Fermi Gamma-ray Space Telescope revealed a 10.301-day periodicity in gamma-ray emissions, confirming its classification as a gamma-ray binary—the first detected beyond the Milky Way. This discovery stemmed from a broader study of gamma-ray sources in the LMC, highlighting LMC P3 as an exceptionally powerful emitter. Follow-up observations across multiple wavelengths, including X-rays, radio, and optical, further characterized the system.Very high-energy gamma-ray emissions above 100 GeV were detected in 2017 using the High Energy Stereoscopic System telescope array in Namibia, marking LMC P3 as the first extragalactic VHE gamma-ray binary. These observations showed variability tied to the orbital phase, with VHE emissions occurring during approximately 20% of the orbit.
Characteristics
The binary system comprises an O5 III blue supergiant star, with an estimated mass of 33.5 solar masses and a surface temperature over 33,000 °C, orbiting a compact object that is likely a neutron star with a mass around 1.4 solar masses. The massive supergiant star expels a dense stellar wind at speeds of millions of miles per hour, while the compact object accelerates particles to relativistic speeds, possibly via a pulsar wind or jets.The supernova remnant DEM L241 is believed to be the birthplace of the compact object, formed from the explosion of a massive progenitor star.
Emissions
Emissions from LMC P3 span radio, optical, X-ray, and gamma-ray wavelengths, modulated by the orbital cycle:- Gamma-ray Emissions: Detected by Fermi at energies above 100 MeV and VHE emissions above 100 GeV, observed by H.E.S.S., occur near inferior conjunction and are anti-correlated with HE emissions.
- X-ray Emissions: Variable X-ray flux observed by Chandra, Swift, and XMM-Newton, peaking at periastron when the compact object interacts with the stellar disk.