On Oct. 13, 2010, NASA's Rossi X-ray Timing Explorer (RXTE), a satellite in low-Earth orbit, observed a bursting X-ray pulsar as it was eclipsed by the Moon. This provided scientists with an unusual opportunity to calculate the precise position of the pulsar by timing its disappearance and reappearance at the edge of the Moon's disk.
The story began a few days earlier, on Oct. 10, when the European Space Agency's INTEGRAL satellite detected a transient X-ray source in the direction of Terzan 5, a globular star cluster about 25,000 light-years away toward the constellation Sagittarius. This was the start of an extradordinary series of outbursts that ended Nov. 19. The object, dubbed IGR J17480-2446, is classed as a low-mass X-ray binary system, where a neutron star orbits a star much like the Sun and draws a stream of matter from it. As only the second bright X-ray source to be found in Terzan 5, scientists shortened the name of the system to T5X2.
As shown in this animation, ingress (the moment when the pulsar disappeared) occurred on the Moon's eastern limb just above the equator. Egress, 8 minutes 32 seconds later, was near the south pole on the western limb. The timing of ingress and egress depended delicately on the shape of the terrain. In other words, it mattered whether the pulsar passed behind a mountain or a valley. So the calculation relied on the detailed topography measured by both JAXA's Kaguya and NASA's Lunar Reconnaissance Orbiter.
The animation faithfully reproduces the angle of the Sun, the position of RXTE, the position and orientation of the Moon as seen from the satellite, the Moon's topography, and the starry background. RXTE's position was derived from the Goddard Flight Dynamics Facility ephemeris for day 6129 of the satellite's orbit, while the Sun and Moon positions came from JPL's DE421 solar system ephemeris. All of the positions and the viewing direction were transformed into Moon body-fixed coordinates, so that in the animation software, the Moon remained stationary at the origin, while the camera moved and pointed appropriately. The Moon, the stars, the pulsar, and the clock were all rendered separately and layered together.
This graph based on RXTE data illustrates the changing character of X-ray outbursts from a neutron star called T5X2 in October and November 2010. As the persistent X-ray emission rose (upward steps in the plot), the number of bursts increased while their brightness declined. The abrupt dropout on Oct. 13 occurred when the Moon briefly eclipsed the source.
As seen from the Rossi XTE satellite, the Moon passes in front of X-ray pulsar IGR J17480-2446. A wider viewing angle places the Moon and IGR J17480-2446 in context against the celestial sphere. The stars in Sagittarius and Ophiuchus with Bayer designations are labeled.