There's a simple reason why a curious neutron star in the M15 globular star cluster has shown two faces over the years, beaming an X-ray portrait as perplexing as Mona Lisa's smile. The reason: That's not one star system, but two.
Astronomers discovered one neutron star system in M15, called 4U2127, with the Einstein X-ray satellite in 1984. Characteristic of a low-mass X-ray binary, 4U2127 contains a city-sized neutron star orbiting a "living" hydrogen-burning star slightly smaller than our Sun, named AC211. Escaped gas from AC211 falls onto the neutron star, attracted by its strong center of gravity. The transfer of gas, called an accretion disk, glows hot in X rays.
The data revealed that the neutron star itself was not directly visible in X-ray light because it was hidden behind the accretion disk. This neat picture was put into doubt when the Japanese Ginga X-ray satellite saw luminous X-ray bursts from the region in 1990. The length of the burst and other light characteristics implied that the surface of the neutron star was directly visible, a complete contradiction to earlier observations.
Using Chandra's HETGS instrument, White and Angelini discovered that what was thought to be one X-ray source is really two sources separated by 2.7 arcseconds.
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