"The gift that keeps on giving" offers a new answer and a new puzzle for astronomers
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"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 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 its High Resolution Camera debuted with an image of the Crab Nebula.
"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."
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.
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.
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 powerplant. Both rotate at about the same speed, but Chandra clearly is the heavweight: 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.
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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 CCDs (charge coupled-devices) on the front of ACIS had lost some sensitivity, while the CCDs on the back 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 front side 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."
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:
"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."