Scientists investigate unusual X-ray burst


Around lunchtime on March 31, physics graduate student Don Smith was surprised to find three e-mail alerts from the software that monitors data streaming in from the orbiting Rossi X-ray Timing Explorer (RXTE) satellite. After double-checking, he put the word out to the world: the RXTE had detected a bright new X-ray source in the sky that appeared rapidly out of nowhere.

Twenty-four hours later, Professor of Physics Walter H.G. Lewin dared hope that this object--named XTE J0421+560 to indicate its position--was the new black hole for which he had been waiting.

Nature had not been kind to him and his 21 collaborators from seven countries. In 1995, NASA had granted them first dibs on X-ray data from the next black hole X-ray outburst. Even though black hole transients, as the temporary X-ray outbursts are called, occur on average once a year, the years since 1995 had passed without a sign.

Then, rather than outshining most other X-ray sources in the sky for several weeks, as previous black hole transients have been known to do, this mysterious April Fool's Day stellar entity lost half its intensity in less than 12 hours. Only 2 percent of it was left four days after its initial rise.

The goal of Professor Lewin, graduate students Derek Fox and Jefferson Kommers and Professor Lewin's collaborators is to further our understanding of black holes, whose existence has not been proven conclusively since they were first theorized in 1939.

Black holes, which Professor Lewin describes as "mind-boggling even for insiders," can be formed when gravity causes the inner cores of very massive stars that have consumed their nuclear fuel to collapse. This star "death" is accompanied by a supernova explosion that outshines the combined lights of tens of billions of stars for several weeks.

Although there are probably a hundred thousand black holes in our galaxy, they are only observable when they are part of a binary system in which they drain matter from the donor. This in turn can lead to strong X-ray emission.

A handful of these systems are permanent X-ray sources that can always be detected. Half a dozen are transients, in which X-ray emission is suddenly turned on. Because the mechanism that causes black hole transients is not well understood, Professor Lewin and his large team of collaborators are gathering as much information as they can in the hope that they will get new insight into this mechanism.

"We were hoping to use the RXTE's new instruments of unprecedented sensitivity to study a black hole for many months, and see things that had not been seen before," said Professor Lewin, who has been studying sources of X-rays for more than 30 years. "That's not going to happen because the outburst lasted only a few days. We expected this transient to behave decently like its predecessors. Instead, its behavior is very unusual."

NEUTRON STAR?

There is little doubt that J0421+560 is a star in the constellation Camelopardalis, the Giraffe. CI Cam for short, the star is about 10 degrees northeast of the easily visible Capella, one of the brightest stars in the sky. The companion of CI Cam, from which the X-rays originate, must be either a neutron star or a black hole.

"It became clear that the X-ray intensity was going back down almost as fast as it had risen," said Ron Remillard, a principal research scientist in the Center for Space Research who played a major role in making the All Sky Monitor (ASM) on the RXTE work. "No X-ray transient had ever shown us such a rapid rise and fast decay, so we knew that we were on to something special."

"It had several characteristics of previous black holes, except it faded away in days instead of weeks or even months," said Richard Rothschild of the University of California at San Diego, who was in charge of building one of three instruments on board the RXTE that is sensitive to very high-energy X-rays. Did something happen to shut J0421+560 down prematurely, or was it inherently different? Figuring that out, he said, "is the task before us."

It's hard to speculate at this time whether it is a black hole, Professor Lewin said. "There is some strong evidence in favor, but none is conclusive. We cannot be sure until the mass of the compact object in this binary system has been measured, and that may take years."

Nevertheless, J0421+560 is the first possible new black hole detected by the RXTE since its launch in December 1995, when, for the first time, scientists gained almost complete coverage of X-ray sources in the sky. RXTE is named for Bruno B. Rossi, an MIT pioneerin the field of X-ray astronomy.

"RXTE has a very large area detector that will allow us to look with high precision at how the X-ray flux may vary at very high frequencies up to thousands of oscillations a second," Professor Lewin said.

Despite its unusual behavior so far, he holds out hope that there may be a new burst of X-rays within a few weeks. If not, there may besomething even more interesting afoot.

"Things you don't understand are exciting," he said. "This kind of science is like a dark room in which you are trying to find a door knob. I cannot guarantee that anything completely new will come out of all this. We are stepping on unexplored terrain. In this business, you always want to expand your horizons and uncover new territory."

SEARCHING THE SKY

From the time that the first X-ray transient was detected in April 1967, scientists' search for these ephemeral X-rays was like a search for a needle in a haystack.

Detection of X-rays was limited to five-minute rocket flights. In October 1967, Professor Lewin was the first to catch an X-ray source,although not a transient one, varying before his eyes during a seven-hour balloon observation from Australia. "Now, 40 years later, no one can conceive of X-ray sources that aren't variable, but transient outbursts in which a new source suddenly appears are quite rare," he said.

In an X-ray binary system, two stars revolve around each other. One is a "normal" star like our sun, burning nuclear fuel. In a storm of X-rays and temperatures reaching millions of degrees, it transfers matter to its compact companion, which can be either a neutron star or a black hole.

Matter in the form of hydrogen and helium plasma moves from the donor star to its compact companion, a process known as accretion.Plasma from the donor star forms what is called an accretion disk--a pancake-like structure that pours a spiraling avalanche of matter onto the compact star.

This mass transfer, as it is called, generates a huge outpouring of X-rays. The matter reaches velocities close to that of the speed of light and produces unimaginable amounts of energy. "If you threw a marshmallow onto a neutron star, the energy released on impact would be similar to that of the atom bomb dropped on Hiroshima," Professor Lewin said.

In the case of the transient X-ray sources, when the mass transfer falls below a certain level, the source shuts itself off and the X-ray emission ceases. Another avalanche--with its signature of strong X-ray emission--may not occur for another 50 years.

According to Jeff McClintock of Harvard University's Center for Astrophysics, it usually takes weeks to months for an accretion disc to "rain out" all its matter onto the compact object. Professor Lewin surmises that in this case, the compact object may instead have been smothered by accretion from CI Cam.

"Maybe it's gone off the air in X-rays, so to speak, because it's been choked to death by accretion. If that is the case, it may become a bright X-ray source again in a few weeks," he said. "We're keeping our fingers crossed."

A version of this article appeared in MIT Tech Talk on April 15, 1998.


Topics: Space, astronomy and planetary science

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