Planets and Moons  ID: 3959

RXTE Views X-ray Pulsar Occulted by the Moon

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.

Visualization Credits

Ernie Wright (USRA): Lead Animator
Scott Wiessinger (USRA): Producer
Francis Reddy (Syneren Technologies): Writer
Please give credit for this item to:
NASA/Goddard Space Flight Center Scientific Visualization Studio

Science Paper:
Subarcsecond location of IGR J17480-2446 with Rossi XTE

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LRO (Lunar Reconnaissance Orbiter)

Data Used:
Clementine/LIDAR/Lunar topography
Feb 26 to Apr 21 1994
LRO/LOLA/Digital Elevation Map also referred to as: DEM
Aug 2009 to Sep 2011
Note: While we identify the data sets used in these visualizations, we do not store any further details nor the data sets themselves on our site.

This item is part of these series:
The Moon
LRO - Animations
Astrophysics Visualizations

SVS >> Laser Altimeter
SVS >> Lunar
SVS >> Moon
SVS >> Neutron Star
SVS >> X-ray
SVS >> Hyperwall
SVS >> Astrophysics
SVS >> Universe
SVS >> Pulsar
SVS >> Lunar Reconnaissance Orbiter
SVS >> Space
SVS >> Star
SVS >> Lunar Topography
NASA Science >> Planets and Moons