NICER / ISS Science Nugget
for April 2, 2020

NICER watches as the Moon blocks the Crab pulsar

On March 31st, NICER executed an observation of the Crab Nebula and Pulsar just as the Moon occulted them. The Crab (Figure 1) is one of the brightest X-ray sources in the sky. Located in the constellation Taurus, it is the remnant of a supernova that was observed on the Earth in the year 1054. It consists of a wispy nebula about 3 arcminutes across and a compact unresolved neutron star at its center. The neutron star pulses (rotates) with a period of about 33 msec. Opportunities to observe lunar occultations of the Crab occur in seasons only every 8-9 years. As the ISS orbits the Earth, the Moon's parallax swings it across the Crab. Predicting these occultations is an exercise in precision celestial mechanics.

X-ray image showing the Crab pulsar and emission from the surrounding nebula

Figure 1: The image of the Crab by NASA's Chandra X-ray observatory reveals fine detail of the Crab that NICER’s field of view and angular resolution cannot resolve. The structure includes a nebula that extends about 3 arcminutes across with a pulsar located near the center.

These occultations offer an opportunity to highly resolve in one dimension the nebula/pulsar system. In addition, the Crab is a standard calibration source for NICER. These observations allow us to understand the structure of the nebula and to disentangle the pulsar emission from the diffuse emission of the nebula.

In Figure 2, we show NICER data of this occultation. The extent of the Crab and the rate of the parallax motion of the Moon yield a transition that lasts about 20 seconds. As the Moon crosses the central pulsar, which represents approximately 10% of the Crab's total brightness, there is a sudden drop in the observed flux. NICER can also detect the pulses of the Crab pulsar. In the same data set, we see the pulses disappear as the Moon occults it. In Figure 2, the power density spectrum showing the Crab pulses appears to brighten as the Moon's limb gets closer to occulting it. This is because the Moon has already blocked much of the nebular background by the time it occults the pulsar itself.

NICER observation of the Moon's occultation of the Crab pulsar

Figure 2: Top Panel: The measured X-ray intensity by NICER of the Crab during the occultation reveals a transition which is about 20 seconds long. Near the center of this transition (dashed line) the Crab pulsar is blocked as indicated by a sudden drop in flux. Bottom Panel: The power density spectrum (PDS) measured by NICER reveals the pulses of the Crab at about 30 Hz. As the pulsar gets blocked, we see the pulse signal disappear.

In about 1 month, NICER will attempt a second observation of a Crab/Moon occultation.

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