In a dark corner of the Milky Way, a natural metronome is at work. Every 8196 seconds, about 2 hours and 16 minutes, X-ray light flares on the surface of a neutron star. The star EP J171159.4-333253 pulls gas from a neighboring star, and this gas, falling inward, accumulates on its surface. When the layer becomes dense and hot enough, it explodes like a pot of overheated porridge, and telescopes catch the flash.
This system has another quirk: the companion star periodically blocks the neutron star. Such eclipses are visible in both X-rays and visible light. From their duration, astronomers calculated the orbit: a full revolution takes 6 hours and 29 minutes, and the eclipse itself lasts about 20 minutes and 45 seconds. The orbital inclination turned out to be 73–75 degrees: we are viewing the system almost edge-on.
These data help understand how matter flows between stars in such close pairs. In the future, such “clocks” could become a precise tool for testing physics in strong gravitational fields.
The orbital period of the system is 6.48 hours. During this time, the neutron star completes a full orbit around the common center of mass, and about three thermonuclear bursts occur on its surface.