LMC X-2
LMC X-2 is a low-mass X-ray binary located in the Large Magellanic Cloud, a satellite galaxy of Milky Way. It is one of the five brightest X-ray sources in the LMC and is the most luminous LMXB with a luminosity ranging from 0.3×1037 and 6×1038 ergs/s. This binary system consist of a neutron star accreating matter from an orbiting low-mass blue star. Its high luminosity is caused because of its X-ray emission is close to the Eddington limit or Eddington luminosity for a neutron star and because the Large Magellanic Cloud has lower metal abundances, allowing for higher Eddington luminosities and a higher accretion rate causing its high luminosity.
LMC X-2 is classified as a Z-source, a subtype of neutron star LMXB characterized by high accretion rates and a distinctive Z-shaped track in X-ray color-color and hardness-intensity diagrams. These tracks reflect transitions between three spectral states: the horizontal branch, normal branch, and flaring branch. It is the first Z-source identified outside the Milky Way, making it the eighth known Z-source overall.
Discovery
Main system
The main system of LMC X-2 was discovered around January 1971 by the Uhuru satellite during early satellite flights that identified the system as a point in the LMC along with 2 other points, which are now known as the supersoft X-ray binaries CAL 83 and CAL 87.Characteristics
System Characteristics
LMC X-2 consists of a neutron star and a companion evolved B-type star. The companion star orbits the neutron star in 8.15 hours from an inclination of around 70°, its semi-major axis along with the eccentricity is unknown from the lack of orbital information and observations.Physical Characteristics
Neutron star
This neutron star is the main star of the system LMC X-2, this has a diameter around 16 kilometers and a solar mass of 1.4 ± 0.6 by using the X-ray data of LMC X-2 and fit the data in a theoretical blackbody model which relates to the mass and radius along with its temperature.Companion star
The companion star of LMC X-2 is quite unknown because of it being outshined by the neutron star it is orbiting, this only has information of the mass and radius of around 1 solar masses and 6 to 10 solar radii.Observation
LMC X-2 is has been extensively studied by multiple X-ray observatories:- Four observations from Rossi X-ray Timing Explorer revealed the complete Z-diagram, confirming its Z-source classification through spectral state transitions and quasi-periodic oscillations.
- Archival data from XMM-Newton provided high-precision spectral analysis, with models incorporating blackbody emission and Comptonization. Luminosities indicate near-Eddington accretion.
- Approximately 140 ks of observations from Astrosat captured broad-band X-ray spectral evolution, showing changes in disk temperature, electron temperature, and optical depth along the Z-track.
- Other Observations from MAXI and Swift Space Telescope indicates persistent emission with occasional flares. No definitive orbital period has been identified despite photometric searches.