Chandra image shows a powerful connection in the Crab Nebula

"The gift that keeps on giving" offers a new answer and a new puzzle for astronomers

Sept. 29, 1999: Superlatives were the order of the day as a team of astronomers revealed the best ever X-ray image of the Crab Nebula, the most studied object in the night sky. The picture showed a glowing ring encircling the Crab and apparently serving as the missing connector between the powerhouse at the center and the light show in the outskirts.

Right: The powerhouse lighting up the Crab Nebula is a pulsar too tiny to be resolved even with Chandra's remarkable telescope. It's located at the tip of the yellow arrow where astronomers added a white spot for visibility. The red arrow points to the ring of hot material encircling the pulsar and apparently connecting the Crab to the nebula surrounding it. Credit: NASA and Chandra Science Center.

If you have RealPlayer G2 installed, this will play the entire Space Science Update first broadcast on NASA Select on September 28, 1999. RealPlayer G2 is available for free. Credit: NASA

"We have another treat for you today from Chandra," said Dr. Alan Bunner Science Director of NASA's Structure & Evolution of the Universe program as he started with an understatement. Since its launch on July 10, the Chandra X-ray Observatory has been undergoing a series of checkouts and warm-up exercises. Several of those have produced striking images that intrigue and entice scientists even though the images warrant further study.

Chandra's "first light" images by its Advanced CCD Imaging Spectrometer (ACIS) showed striking details of the Cas A supernova remnant and of a distant quasar. These hinted at marvelous details that could be pried from the rest of the universe. On Tuesday, Chandra delivered again as with an image of the Crab Nebula taken by ACIS and the High-Energy Transmission Grating (HETG).

Right: A wider view of the Crab as seen by Chandra shows the rings of material expanding outwards and two jets spewing from the pulsar's magnetic poles. One jet aims left and curves down. An opposing jet is largely obscured by other material in the area. The reason for the curve, and for the "thumb" blocking part of the view to the right, will require further study. Links to 504x504-pixel, 133KB JPG. Credit: NASA and Chandra Science Center.

"This is another fabulous discovery from Chandra," said Dr. Martin Weisskopf, the Chandra project scientist at NASA's Marshall Space Flight Center. "Since its launch, Chandra has really amazed us as each new calibration has brought us a new result."

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In its image of the Crab, Chandra revealed a ring of hot gas that may be the extension cord connecting the neutron star at the heart of the Crab to the brilliant nebula surrounding it.

"It seems that we are seeing the glowing power conduit that connects the two," said Dr. Mark Hester of the University of Arizona.

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The Crab Nebula is the remnant of a star whose supernova explosion was seen here in 1054. It was seen, recorded, and forgotten until 1758 when French astronomer made it No. 1 on his list of things that look like comets (his real quarry) but are not. Since then, it has become "the most intensely studied object out there," Hester said, because its many features are found in dozens of other celestial bodies.

In visible light, the Crab looks like its namesake. This is partly from the supernova's shock waves running into uneven pockets of gas and dust between stars. But much of its glow is from synchrotron radiation, so-named for manmade particle accelerators where the phenomenon was discovered.

As charged particles spiral back and forth along magnetic field lines, they emit radiation as radio waves, visible light, or even X-rays if the field is strong enough.

Right: More powerful than ConEdison and all the other utility companies put together is the pulsar-as-dynamo at the center of the Crab Nebula. While it looks big compared to New York City, it's far too small to be resolved by any telescopes now in use. While no one knows if a neutron star is dark gray, the sunlight glaring off it probably is real since the star's intense gravity would make it the smoothest object in the universe. Links to 600x533-pixel 362KB jpg. Credit: NASA/Marshall Space Flight Center.

The driving force behind that magnetic field is at ground zero in the Crab: a neutron star about 20 km wide but as massive as our Sun, and spinning at 33 rpm. Its magnetic field was is another remnant that was frozen in when the star that became the Crab imploded.

"The ultimate power source is a spinning magnetized neutron star too small to be seen even by Chandra," said Dr. Malvin Ruderman of Columbia University. He said that it is similar to a dynamo at a power plant. Both rotate at about the same speed, but the Crab clearly is the heavyweight: it is 10,000 times bigger than any manmade dynamo, has a magnetic field 1 billion times stronger, produces voltages a million billion times higher, and power 100 million billion times greater.

"That dynamo is producing power and getting rid of that power at a rate 100 times greater than the total radiation of our sun," Ruderman added. The strength and regularity of the source are such that the Crab is used as a "standard candle" for calibrating X-ray telescopes including Chandra.

Still the sharpest X-ray eye

While the images from Chandra have enthralled the public and scientists alike, some concerns have developed about the health of the Advanced CCD Imaging Spectrometer (ACIS), one of Chandra's four main instruments. (The other is the High Resolution Camera (HRC). Each has imaging and spectroscopy sections. In turn, these observe objects directly or through two spectral gratings that are placed in the light path.) Soon after the first ACIS images were released, news emerged that ACIS was starting to fail. In fact, the Front Illuminated ACIS CCDs (charge coupled-devices) had lost some sensitivity, while the reminder were unaffected. Weisskopf said the Chandra team is starting to understand what happened and has taken steps to halt further damage. "These appear to be highly successful and we can continue with our mission unimpaired," he said. While the final engineering outcome is pending, it appears that the ACIS chips were slightly damaged as Chandra passed through the Van Allen radiation belts. The solution has been to retract the ACIS on the rail it shares with the HRC so ACIS is fully inside the instrument housing during the radiation belt transits. "Since we have done that, we have seen no further degradation," Weisskopf continued. ACIS remains undamaged as an imaging instrument. As a spectrometer, it has lost some of its "color" perception, but that can be compensated by building mosaic images with the backside chips, which have a smaller field of view.

Yet it is not visible to the naked eye because 99 percent of the energy is shed in ways that are not fully understood: through the star's own magnetic field, through electrons and positrons (anti-electrons) "somehow accelerated to speeds that are spectacularly close to the speed of light," and through ions lifted from the surface of the neutron star and also accelerated to near the speed of light. What proportion goes where remains an open question, as does the process behind the nearly 100 percent efficient electron and positron acceleration, something that scientists would like to replicate in their own accelerators.

"The remarkable Chandra pictures may give remarkable clues to how neutron stars shed power," Ruderman said.

"The Crab is the gift that just keeps on giving," Hester added. "It's hard to come up with enough superlatives to describe what this thing is doing."

Hester has studied the Crab in visible light, including images taken by the Hubble Space Telescope that show wisps of materials expanding outward at half the speed of light and "a ring that is dancing." In a profession where astronomers often count themselves lucky to see an object change in the course of their careers, images a few days apart show that the Crab reinvented itself, Hester continued.

The bright areas in the Chandra images correspond with the bright areas in the Hubble images taken in 1996, and appear to be telling part of what is going on inside the heart of the Crab.

While the visible portion of the Crab has been known since the mid-1700s, the pulsar has only been known since 1968 when radio astronomers stumbled across radio pulsating stars. The Crab turned out to be pulsing in visible light and X-rays as well.

The two obviously were linked, but the exact connection was missing. Hester said it was like knowing where Hoover Dam and Las Vegas are located, but not knowing where the power lines run.

"The sharp ring [in Chandra's image] is exactly where it needs to be to tie the pulsar to the rest of the nebula," he said. While he cautions that "we're not going to say we understand it yet. ... We're finally seeing the place where the pulsar and the Crab Nebula couple each other together."

<A HREF="https://webarchive.library.unt.edu/eot2008/20090512230708/http://science.nasa.gov/chandra/quicktime3/chandra_tnl.mov"><IMG SRC="https://webarchive.library.unt.edu/eot2008/20090512230708im_/http://science.nasa.gov/chandra/images/crab.jpg" ALIGN="RIGHT" BORDER="0" WIDTH="160" HEIGHT="120"></A>

Another striking aspect of the image is a sharp edge on the right side of the Crab. Weisskopf assured reporters that it is a real effect and not an artifact of the telescope or the image. When asked whether it might be caused by the Crab's magnetic fields, Hester replied:

Right: A QuickTime movie of a Chandra's-eye view of the Crab nebula. First is a view of Chandra pointing toward the visible light image of the Crab nebula, and then zooming in with the X-ray view. Also available in a larger format. Credit: NASA

"Come back in a year, come back in a couple of years and perhaps we can answer that. ... Science progresses by learning new things, which lead to new puzzles. From those come new questions and the next set of new puzzles."

Peering into the heart of a Crab - The Crab is one of the most beautiful - and most studied - bodies in the skies, and serves as a Rosetta Stone for modern astrophysics.
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Xrayastronomy.com, science news from the Chandra X-Ray Observatory
Chandra X-ray Observatory Center home page, with links to education, news, and technical pages.
Chandra news from Marshall Space Flight Center
Chandra Project Science is managed at NASA/Marshall, has links to individual instruments and the prime contractor.
X-ray astrophysics branch at NASA/Marshall conducts a broad range of research and technology work, as well as supporting the Chandra X-ray Observatory.
Additional images of the Crab Nebula are available from NASA's Astronomy Picture of the Day web site:
High Energy Crab Nebula | Pulsar Powered Crab | M1: Filaments of the Crab Nebula | Exploding Crab Nebula
And from the Students for the Exploration and Development of Space web nebula page.