How Hot Is The Crab? NASA's Next Great Observatory Takes Aim At The Crab Nebula Pulsar

The Crab Nebula, as the display came to be known, was an exploding new supernova so bright it was visible in the daytime sky for nearly a month. Soon after, it faded to a level where it would not be rediscovered until newly invented telescopes spotted it in the 18th century.

Countless observations later, the Crab is still the source of some of the most intriguing questions in the field of astronomy. Dr. Martin Weisskopf, an astronomer at NASA's Marshall Space Flight Center, is among those asking the key questions. Weisskopf plans to use the High Resolution Camera aboard the Advanced X-Ray Astrophysics Facility (AXAF), scheduled for launch in December, 1998, to examine the surface temperature of the neutron star at the center of the Crab Nebula.

"The Crab Nebula and the star at the center of it are the Rosetta Stone of modern astrophysics," said Weisskopf, who is also the AXAF project scientist. The neutron star at the center is known as the Crab Pulsar. It is classified as a pulsar because of its flashing nature - it sends bursts of energy out 33 times a second with reliability rivaling that of our most dependable clocks and watches.

The life and times of the Crab

The Crab Pulsar was once a star 8-12 times as massive as our the Sun. About 5,000 B.C., the star used the last of its nuclear fuel, collapsed and emitted a brilliant display of gases which expanded out to create the nebula witnessed on Earth 6,000 years later when the first flash of light arrived. At the center of the nebula remained a neutron star only 12.5 miles in diameter (20 kilometers) with a magnetic field a trillion times stronger than the Earth's. (A pinhead of neutron star material weighs about as much as a World War II battleship.)

The magnetic field is so strong that it causes most of the light and radiation the neutron star emits to be concentrated into cones of emission, like beams from a lighthouse. In fact, the key to a pulsar is the combination of the extraordinary magnetic field and the rotation of a neutron star. If the neutron star is spinning, like the Earth rotates on its axis, and if the Earth happens to lie in the path of the beams, we see a pulse of light each time a beam sweeps across the Earth.

Aside from being the most observed of all pulsars, the Crab Pulsar is also believed to be the youngest of more than 700 known to astronomers.

"Since it is the youngest, it's also the hottest," explained Weisskopf, "and X-rays offer the best way to observe it at these temperatures." Neutron stars cool as they age and the temperature offers evidence of the physical activity occurring inside the star.

Taking its temperature

"Neutron stars are a unique laboratory for probing various physical phenomena," said Weisskopf. "Of interest here is the thermal evolution of the stars." The physical activity in the star's superfluid interior, under a crystalline neutron crust, is impossible to recreate in any laboratory on Earth, so scientists have been working up theories based on observations of the Crab Pulsar and other neutron stars. Different theories predict different temperature ranges for such stars.

The high resolution camera aboard AXAF will help Weisskopf and other scientists test the theories by giving them a better reading of the temperature on the surface of the Crab Pulsar.

"The more resolution the better," said Weisskopf. "Right now we're looking at the glow of activity near the center of the nebula as you might see the glow of city lights from a distance. Examining the pulsar in the center using AXAF will be like using a telescope to focus on a single street light in the middle of the city." For the Crab, the single light is a 12.5-mile-wide star in the middle of a 7-light-year-wide nebula.

Two-for-one imaging

AXAF's High Resolution Camera (HRC) will provide X-ray images that approach the rich detail of the Hubble Space Telescope's Wide field Camera. The HRC actually is two cameras in one, an imager to make pictures of X-ray sources and a spectrometer to take pictures of their "colors."

The HRC works a little bit like night-vision scopes: a weak signal is amplified by letting it strike a surface that is electrically charged almost to the point of discharging. The incoming radiation provides the extra kick and a shower of electrons is released, measured, and reconstructed into an image.

In the HRC, this is done by two microchannel plates, thin sheets of lead oxide glass with microscopic holes at an angle to intercept X-rays which release an ever growing shower of electrons. The shower emerges from the backside of the second plate and strikes an array of fine wires connected to amplifiers that measure the position and energy of the shower. From this, the image can be reconstructed.

The HRC Imager will see a section of sky 31x31 arc-minutes in size, just a little larger than the apparent diameter of the Moon, and have a resolution of 0.5 arc-second, close to Hubble's 0.1 arc-second resolution. HRC Imager pictures will be about 3,700x3,700 pixels on a side, making some of the most richly detailed images ever in X-rays. The HRC Spectroscopy Detector is arranged in a strip. When either of the two gratings behind the mirrors is swung into position, it spreads the X-rays in a spectrum across the detector much as a prism spreads visible light.

(By comparison, the Einstein HEAO-2 observatory, also developed by NASA/Marshall, had a High-Resolution Imaging camera with a 25x25 arc-minute field of view, smaller than AXAF's, and a resolution of 2 arc-seconds, about 1/4th as fine as AXAF's. Images were about 750x750 pixels in size.)

One of the important features of the HRC is its speed. Its time resolution is 0.000016 second, the equivalent of taking 62,500 pictures a second, letting Weisskopf capture images of the Crab when it is "on" or "off."

Complicating the task is the fact that the star is a pulsar, meaning that the X-ray readings must be calibrated with the pulsing.

"We need to pick the X-rays at off-pulse times out of the data," said Weisskopf.

In addition to providing information on the Crab Pulsar and its neutron star, the HRC will provide pictures of other discrete structures within the nebula. High-resolution spectroscopy of interstellar material and high-resolution spectroscopy of the nebula itself are also part of the mission plan.

Although the Crab is the most studied area of the sky outside our solar system, it still seems to generate questions as fast as it generates radiation pulses.