Jul 20, 1999

Chandra has a busy observing schedule

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Chandra has busy observing schedule

Dying magnetars, supernovae, and the future of the universe are on tap for NASA's next Great Observatory

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July 20, 1999: Dying magnetars, supernovae in multicolor coats, everyone's favorite Crab, and the future of the Universe are on the observing schedule for scientists at NASA's Marshall Space Flight Center when the Chandra X-ray Observatory starts observing the heavens later this year.

While NASA/ Marshall managed the Chandra project since the mid-1970s, and played an active role in calibrating its telescope, it also has been preparing to use Chandra as a science tool. Launch of Chandra is now set for Thursday, July 22. Within a few months of launch, Chandra will gradually emerge from verification tests and be turned into an operational observatory. Among the scientists waiting to use it as guest investigators are four at NASA/Marshall.

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Why did the supernova change colors?

Most objects in the sky can be pigeonholed into a few of the hundreds of categories that classify stars, galaxies, and other bodies. Every now and then, you get one that changes its colors - literally - and seems to beg for closer examination. That's the case with a supernova known as SN 1993J.

radio telescope sequence of SN1993
"It started out as a classic Type II supernova," said Dr. Douglas Swartz, "with hydrogen lines in its spectrum. These weakened in a few weeks and helium lines appeared, more like a Type Ib supernova."

In effect, SN 1993J changed its appearance from a supernova caused by an ordinary massive star to a supernova caused by a dense helium stellar core.

Left: A radio telescope view of SN 1993J expanding.

To help resolve the apparent conflict, Swartz will use Chandra, which he also helped calibrate.

Because the supernovae have different origins, they emit light differently as they explode. But SN 1993J is a transition object which had lost most, but not all, of its hydrogen envelope.

"Supernova 1993J is one of the few that has made the transition from one type to another," Swartz explained. Only one other supernova, SN 1987K, has been seen making such a change. With Chandra, Swartz hopes to see X-ray signatures of the chemical makeup of the outrushing material, including the hydrogen that disappeared a few weeks after the blast.

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How hot is the crab?

In early summer of 1054, long before the first Independence Day celebration in the United States, the people of Japan and China witnessed an amazing display of fireworks in the summer sky. 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.

Crab nebula, with HST inset of core
"The Crab Nebula and the star at the center of it are the Rosetta Stone of modern astrophysics," said Dr. Martin Weisskopf, the Chandra 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.

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.

Left: A view of the Crab Nebula, including a detailed view of the core as seen by the Hubble Space Telescope.

"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 are a unique laboratory for probing various physical phenomena. 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.

The high resolution camera aboard Chandra 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 Chandra will be like using a telescope to focus on a single street light in the middle of the city."

Looking for pulsars living in the fast lane

The discovery in 1998 of the first magnetar - a highly magnetized star - also put the spotlight on a small class of stars called Anomalous X-ray Pulsars, or AXPs. While the magnetar discovery involved Soft Gamma Repeaters (SGR), the magnetar theory holds that these objects become AXPs before they fade from the scene altogether.

Dr. Jan van Paradijs, who won the 1997 Rossi Prize for identifying the first optical counterpart for a gamma ray burster, hopes to use Chandra to take a closer look at two AXPs - one third of the known population.

"The reason I'm interested in them is that I suspect they're magnetars," said van Paradijs, an astronomer with the University of Amsterdam and the University of Alabama in Huntsville and working at NASA/Marshall.

Right: An scientist's concept of a magnetar surrounded by hot plasma. The blue lines represent magnetic field lines. credit: Robert Mallozzi/University of Alabama at Huntsville and NASA/MSFC

Short life is the price of being a magnetar. The SGR phase lasts only 10,000 years, and may be followed by the AXP phase for another 10,000 years or so. Until recently, astronomers did not link the SGR and AXP classes. Indeed, the AXP class was formed because a handful of X-ray pulsars did not fit into the normal categories.

"It's a combination of things," van Paradijs said. "They have an awkward spectrum and they also have a very limited period range which says there's something very special going on." Some are associated with relatively recent supernova remnants. All these factors, van Paradijs believes, add up to a strong magnetic field that is aging the pulsar faster than normal.

"With Chandra, we hope to get evidence that they are indeed magnetars," van Paradijs said. Unfortunately, it is not likely that he will be able to hunt for what comes after the AXP phase. After 20,000 years, magnetars are believed to fade away from notice.

Measuring the scale of the Universe

Combining measurements by Chandra with radio telescope observations may help refine our understanding of the age of the universe.

Web Links
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.
STS-93, the Chandra mission, launch status at Kennedy Space Center.

NASA/Marshall scientists will use Chandra to study:
Pulsars in the Fast Lane - Scientists are looking for bizarre, short-lived, powerhouse stars that burst with some of the brightest energy in the universe.
Why did the supernova change colors? - SN 1993J was seen to be one kind of massive explosion, but then seemed to morph into a distinctly different kind.
How hot is the Crab? - NASA's next Great Observatory takes aim at the Crab Nebula pulsar.
Chandra will target the age of the Universe - Astronomers plan to combine radio images of galaxy clusters with new data from the Chandra X-ray Observatory and open a new window on the history of the Universe.
"We could be on the brink of answering some important cosmological questions," says Dr. Marshall Joy, "but we need more data that only Chandra can provide."

Joy and Dr. John Carlstrom, of the University of Chicago, plan to measure the age of the Universe with the little-known Sunyaev-Zeldovich Effect. This happens when microwaves from the cosmic microwave background radiation collide with hot gas between distant galaxies and are scattered at higher energies. The effect is a slight dip in radio intensity in an otherwise smooth background.

"Chandra has the collecting area and the sensitivity to high energy X-rays to make high quality images of the hot cluster atmospheres. If we could get radio maps of the same clusters and measure the size of the Sunyaev-Zeldovich effect, we could begin to do some interesting cosmology."

Joy and Carlstrom plan to combine X-ray data from Chandra with their existing radio images to estimate the absolute distances to a number of galaxy clusters.

radio image of galaxy cluster composite
By combining their distance measurements with optical redshift data they can calculate the Hubble constant for each cluster.

Right: Radio telescope and early X-ray views of an Abell cluster of galaxies one of the observing targets for S-Z effect measurements.

"We can also use Sunyaev-Zeldovich measurements to draw some conclusions about the total mass in the Universe," said Carlstrom. From that can come a better estimate of whether the universe will expand forever, or eventually slow and collapse on itself.

"We're optimistic", said Carlstrom, "because we'll be getting real answers to some of the most important questions in cosmology."

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Author: Dave Dooling
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