Chandra eyes young supernova remnant in "first light" observation
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Aug. 26, 1999: When the Chandra X-ray Observatory took its "first light" image, it wasn't looking at just another star shining in the darkness. It was watching a foundry distribute its wares to the rest of the galaxy.
The Cas A supernova remnant is a well known thorn in the side of astronomers that keeps raising awkward questions. Why didn't observers see Cas A explode more than 300 years ago? Where is the pulsar or black hole that most supernovae leave behind? Why is Cas A a source of cosmic titanium-44, a radioactive isotope of the metal prized by the aerospace industry? Scientists hope that Chandra will finally provide answers.
The "first light" announcement came a little more than a month after Chandra's July 23, 1999, launch aboard Space Shuttle Columbia.
With it, scientists may have found a pulsar that they long have suspected lurked at the heart of Cas A, but which had eluded detection in radio, optical, and even X-ray wavelengths.
"Last Thursday night we pointed Chandra in the direction of Cas A," said Dr. Harvey Tannanbaum, Director of the Smithsonian Astrophysical Observatory's Chandra X-ray Center in Cambridge, Mass., "and this is what we saw. That's just a beautiful sight."
The image corresponds with earlier X-ray pictures taken by the German Roentgensatellit, or ROSAT, but with incedible amount of detail. A video of the Chandra team watching the Cas A image appear on computer screens shows amazement, awe, and wonder amidst clapping and shouts of joy.
"It works," said Dr. Ed Weiler, director of astrophysics at NASA Headquarters. "It works perfectly. It's meeting all of our objectives. The United States is back in charge of observational X-ray astronomy. We have the most powerful X-ray astronomy telescope in the world, and it's called Chandra."
As the Cas A image appeared, scientists readily spotted a small point source just about a dead center.
"We think we may have found the neutron star" that was expected to be at the center of the supernova remnant, Tannanbaum said. One of the stunning aspects about the picture is that it shows this wealth of detail after 90 minutes of exposure whereas ROSAT - which made dozens of significant contributions to X-ray astronomy - took days to make its image.
An early spectrum of Cas A shows strong lines emitted by silicon, sulfur, argon, calcium, and iron among other elements.
"This is the material that we're made of," said Dr. Martin Weisskopf, the Chandra project scientist at NASA Marshall Space Flight Center. "The silicon that makes up the mirrors of Chandra itself were made in these types of explosions."
Chandra is not through with Cas A. Tannanbaum said that more studies will follow, even as the telescopes calibration and verificaton tests are executed over thenext four weeks, to see if the point source's spectrum, brightness, and time variation correspond with a neutron star.
Finding a pulsar will solve one of the outstanding mysteries about Cas A and "balance the books," Tannanbaum said.
Meanwhile, Weisskopf has nicknamed the object Leon X-1 in honor of Dr. Leon van Speybroeck, the Chandra telescope scientist.
Cassiopeia A - Cas A for short but known by several other names - is the remnant of a star that blew itself apart about 9,400 years ago. It was one of the first objects to be discovered when radio astronomy itself was discovered, and has been studied extensively across the spectrum up through gamma rays.
Chandra now brings to bear new instruments behind the world's finest X-ray telescope, allowing scientists to delve deeper into Cas A's mysteries.
Cas A has been discovered in bits and pieces since early astronomers apparently missed its debut in the mid-1600s. It is relatively close - only about 9,100 light years away - so it should have been bright enough to cause a stir when it appeared in the night sky. Sir John Flamsteed, Britain's Astronomer Royal, may have seen Cas A in 1670. If so, he misidentified it as a star and made no follow-up observations. No other sightings are known (an earlier, unrelated supernova was recorded in Cassiopeia in 1572).
| DISCOVERY AT FIRST LIGHT
"First light" is the equivalent of commissioning a ship. It's been launched, outfitted, checked, and now certified ready for business. It's not the actual first light through the telescope. That started when the main aperture door opened on Aug. 12. The opening was followed by engineering tests and fine tuning of the telescope and its instruments. As part of this orbital verification period, Chandra observed PKS 0637-752, a distant, X-ray source with little background noise or other clutter. Effectively, it was supposed to be an X-ray pinpoint in a black sky. Such an image lets engineers determine what is known as the point-spread function, a measure of how much of the focused radiation falls inside a small circle that defines sharp focus for a telescope. "But nature often has surprises in store," Tannanbaum, cautioned. Instead of a neat little point source, the Chandra team discovered a massive jet spewing from a quasar. Although radio observations show a jet, astrophysicists did not expected to see it because of the ultra-high resolution the radio astronomer achieved by using multiple telescopes. "It's immediately apparent that we're not seeing a dot, that we're not seeing a simple point source," said Tannanbaum. The jet appears to be at least 200,000 light years long, "a very big chunk of space" that could hold our entire galaxy. By contrast, the central source is no larger than our solar system. Understanding how such a massive burst of matter and energy can extend across os much space "is one of the fundamental theoretical and observational problems in front of us," said Weisskopf. With the PSK 0637-752 observation in hand, Chandra was next aimed at Cas A, an extended object with more than its share of mysteries. Cas A may also be pressed into service as a "standard candle" for use in checking the performance of Chandra's instruments over the years. As part of the orbital verification activities, Cas A was centered on each chip in Chandra's instruments for 1,000 to 5,000 seconds to measure each one's relative sensitivity and response to a line source. Cas A emits bright lines over the range 1.8 to 6.5 keV. If suitable, this target will serve as a standard candle for monitoring purposes. |
Because the remnant of the explosion is a very faint nebula, Charles Messier apparently missed it in 1784-86 when he compiled his famous catalog of blurry objects that should not be mistaken for comets, his real quarry.
It finally was noticed the 1930s when Karl Jansky discovered that the Milky Way emits radio noise. Jansky, an engineer at Bell Labs, was determining the source of background radio noise and whether it would interfere with radio transmissions.
He found not only that the Milky Way was a powerful radio emitter, but that sources in deep space emitted as well. His simple maps showed several regions of the sky that were stronger than others. The strongest source was in the region of Cassiopeia and so was named Cas A.
World War II put radio astronomy on hiatus for several years, then yielded new technologies that boosted the field in the 1950s. Cambridge University mapped the skies in detail in radio wavelengths, and designated Cas A as 3C 461 in their third catalog.
The next discovery came from the Uhuru Small Astronomy Explorer launched in 1970. Uhuru's observations led to the designation 3U 2321+58 (in the 3rd Uhuru catalog). The numbers give the position in right ascension (23 hours, 21 minutes) and declination (58 degrees up) from Earth's equatorial plane. Finally, Cas A has also been designated G111.7-2.1 in the galactic coordinate system.
Extensive observations in radio, infrared, visible, and X-ray wavelengths reveal an incomplete shell of expanding gas with compact knots of material at temperatures up to 28 million K (50 million deg. F). Its outer shell is expanding at 800 km/s (about 1.73 million mph!). That's rapid enough that images taken over the years show the gas shell changing structure and cooling down.
| Chandra's "eyes" While we often think of telescopes as seeing stars, they are really just the front end of the operation, the part that collects and focuses radiation into an image. The next step, of course, comprises detectors at the focal plane. Chandra has two sets mounted on a special rail to position them at the focal point in a Science Instrument Module. Both of Chandra's instruments are two-in-one devices, consisting of an imager and a spectrometer. The latter are used when one of two transmission gratings is swung into place behind the telescope mirrors to spread the X-rays into separate energies, somewhat like a prism spreading sunlight into a rainbow. ACIS is the AXAF CCD Imager and Spectrometer (This compound acronym comes from the observatory's original name, the Advanced X-ray Astrophysics Facility). It uses 10 charge-coupled devices (CCDs) similar in principle to those used in home camcorders. The imager, ACIS-I, has a 2x2 array of CCDs to produce images 2,048 pixels across and covering about 16.9 arc-second of sky. That's a little more than half the apparent diameter of the Moon (31 arc-minutes wide). The spectrometer, ACIS-S, has a strip of six CCDs that take the light from an object and slice it into 12,288 discrete "colors." HRC is the High-Resolution Camera which uses microchannel plates, a completely different detection method. The microchannels are microscopic tubes carrying a high electrical charge. An X-ray entering a tube will liberate an electron that bounces off the wall and releases several more and so on until a shower arrives at the bottom where the discharge is interpreted as a measure of the X-ray's energy. The HRC-I (imager) is a single large array 9x9 cm (3.5x3.5 in) with a 31x31 arc-minute field of view. The HRC-S is a single strip, 2x30 cm (0.8x11.8 in) with special metal foils masking some parts to help in spectral analyses. |
The shell has two components, the high-temperature blast wave, still plowing outward through the interstellar medium, and a low-temperature reverse shock traveling inward. On the western edge, Cas A plows into a molecular gas cloud. On the opposite side, a high-speed jet moves through relatively empty space.
In addition to being the youngest supernova remnant in our galaxy, it's also quite extended, about 5 light years wide, more than the distance from the Sun to the nearest star, Alpha Centauri (3 light years). The shell's apparent diameter is 5 arc-minutes, about 1/6th the apparent diameter of the Moon.
Cas A probably started as a Wolf-Rayet star with an initial mass 30 times that of our sun. Being a heavyweight doomed Cas A. It burned hot - with a surface temperature of 50,000 K (90,000 deg. F), more than eight times hotter than the surface of our Sun- and fast. This blew off an intense solar wind that reduced Cas A's mass to about 10 solar masses by the time it was reduced to a suicide diet.
When massive stars expend the hydrogen fuel at their cores, they go on a fiery binge, burning the ash from one round of fusion into still heavier ash to be burned in a shorter, hotter cycle until only iron is left. Fusing iron nuclei into heavier atoms consumes rather than produces energy. Cas A's furnace turned off and the star imploded. Heavier elements were produced by this inward rush and absorbing much of the energy, but not enough to keep the implosion from rebounding and blowing the star apart.
What happened next is a bit of a mystery. While we tend to think of a supernova blowing outward in all directions, it can explode along the poles. A star's magnetic field becomes more compacted and intense as the outer layers collapse. This can control the star's blast so it acts like an armor-piercing charge that forms a high-speed jet.
But that was in the past. All we are left with now is a bright, fragmented shell that is largely the result of Cas A overtaking its own solar wind and then running into the interstellar medium. What may look like empty space to us is actually peppered with pockets and wisps of gas and dust that will become the stuff of new planets in a few billion years.
Supernovas are the process by which the galaxy seeds the growth of new solar systems. We are the nuclear "ash" of stars that blew up billions of years ago and spewed heavier elements from their cores. Cas A continues that process.
Infrared and X-ray spectral studies of Cas A reveal strong emission lines for oxygen, neon, silicon, sulfur, argon, iron, calcium, and even magnesium silicates, the stuff of interstellar dust.
A real oddity is titanium 44 (a radioactive cousin of the prized aerospace metal). Cas A apparently has an unusual abundance of Ti-44 as compared to nickel 56, another product of nucleosynthesis in a star. The Compton Telescope aboard the Compton Gamma Ray Observatory has observed emission lines at 1.156 MeV, corresponding to scandium 44 decaying into calcium 44, the last step in titanium 44's life cycle.
If correct, the Ti-44 is direct probe of nuclear fusion at the heart of the doomed Cas A source, a sample of star stuff from just above the layers that formed the neutron star or black hole at the center of the Cas A remnant.
If there one was left behind. One of the mysteries about Cas A is why no central object has yet been found for Cas A.
Chandra may help answer this and other questions about Cas A. It certainly will provide much finer details about the more than 300 glowing knots of gas that make up the remnant, and help define just what Cas A's contribution will be to the makeup of future worlds.