Comet’s X-Ray Glow Surprises Astronomers

Comet Hyakutake, which recently made a close pass by planet Earth on its way to the sun, has developed a bizarre crescent-shaped X-ray glow, scientists at the NASA Goddard Space Flight Center and the Max Planck Institute in Germany said Thursday.

This is the first time X-rays have ever been seen coming from a comet, which is essentially an ice-packed ball of soot dropping into the solar system from cold storage in space. Since X-rays normally crackle from extremely hot, high-energy sources such as exploding stars, scientists are at a loss to explain what they saw.

“A comet is not a blazing object. A comet is a dirty ice ball. I’d no more expect X-rays from a comet than I’d expect X-rays from the ice cubes in my Frigidaire,” said Stephen Maran, an astronomer with the Goddard center in Greenbelt, Md.

The X-ray glow extends about 30,000 miles.

The decision to look for X-rays in Hyakutake with the German satellite ROSAT was almost a lark, said Goddard astrophysicist Robert Petre. Since the comet was so close and so bright, scientists felt it was an opportunity they couldn’t pass up. But no one really expected to see X-rays, especially of such brightness.

“Comets have been observed for thousands of years, but no one anticipated this,” Petre said.

In fact, scientists who wanted to look for X-rays from comet Shoemaker-Levy’s fantastic smash-up on Jupiter two years ago were turned down, said UCLA astrophysicist Matt Malkan. “The [science] committee thought there was too little chance,” he said. X-rays are hot; comets are cool. “How was a comet going to make X-rays? And that was the collision of the century. This one’s just sitting there.”

Hyakutake has taken astronomers by surprise ever since the “stealth comet”--as Griffith Observatory’s Ed Krupp calls it--was spotted by an amateur Japanese astronomer in January. It was far brighter than anyone expected, then dimmed far faster than anticipated.

Then last week it started to shed pea-sized chunks, creating a stream of mini-comets in its wake.

“This has been a busy and active comet,” said Stephen Edberg of the Jet Propulsion Laboratory in Pasadena.

It is possible that the ejected chunks of comet material are somehow connected to the X-ray glow, said Goddard astrophysicist Carey M. Lisse, especially because the X-ray flashes have varied erratically in brightness. “They might be some sort of explosive event. That’s unlikely, but not ruled out,” he said.

The crescent-shaped glow sits on the side of the comet that faces the sun. That suggests to some researchers that sunlight is pumping up oxygen atoms in the comet’s atmosphere and causing them to fluoresce, emitting X-rays.

However, the crescent shape is puzzling. It suggests that the radiation from the sun isn’t penetrating very deep inside the cloud of gas that surrounds the comet’s rocky core, but only reflecting off the surface, like sunlight reflecting from the hard surface of the moon.

“You would expect to see right through to the center,” said Lisse. “But the X-rays [from the sun] are being stopped, which is very surprising.”

A competing theory suggests that the comet is plowing through the solar wind like a boat through water, and building up a shock wave that emits the X-rays. The solar wind is a stream of electrified particles sweeping off the sun at hundreds of miles per second. The comet is “moving fast enough to convert things it hits into X-rays,” Lisse said.

The problem with this theory is that the solar wind is very tenuous--and probably doesn’t pack enough punch to produce the bright X-ray glow.

“There’s no obvious, easy answer to this one,” Petre said.

Maran suggests that a clue might come when the comet passes behind the sun and emerges from the other side. If the glow is caused by a shock wave, then the crescent shape should still be visible on the forward side of the comet as it moves away from the sun; if it’s caused by fluorescence, the crescent should still be facing the sun.

Still, viewing the comet when it’s so far from the Earth will be difficult, Petre said. The observation was tricky even when the comet was close to Earth because the ROSAT telescope had to snap a low-speed exposure of a rapidly moving object. Initially, the image scientists got was a blur, but within 24 hours, they had figured out how to subtract the comet’s motion from the image, bringing it into focus.

The astronomers don’t know yet what the significance of their finding might be. “I wish I knew,” Petre said. “All the models that we had previously were out the window, so we’ve had to start from scratch.”