Smoking Gun Found for Gamma-Ray Burst in Milky Way

A composite Chandra X-ray (blue) and Palomar infrared (red and green) image of the supernova remnant W49B. Scale: Image is 5.7 arcmin per side (Credit: NASA/CXC/SSC/J. Keohane et al.)

Scientists have uncovered evidence that a gamma-ray burst occurred in our own Galaxy only a few thousand years ago. They found this evidence by studying the supernova remnant, W49B, with the Chandra X-ray Observatory and the Palomar 200-inch telescope. W49B is a barrel-shaped nebula located about 35,000 light years from Earth. The new data reveal bright infrared rings, like hoops around a barrel, and intense X-radiation that together indicate that this may be the first remnant of a gamma-ray burst discovered in the Milky Way.

"The nearest known gamma-ray burst to Earth is several million light years away - most are billions of light years distant - so the detection of the remnant of one in our galaxy would be a major breakthrough," said William Reach, an astronomer from the California Institute of Technology, who examined the data.

This Chandra image shows the X-rays from hot iron ions that have been stripped of all but their inner electrons, called K-shell electrons. Scale: Image is 5.7 arcmin per side (Credit: NASA/CXC/SSC/J. Keohane et al.)

"These results provide intriguing evidence that an extremely massive star exploded in two powerful, oppositely directed jets that were rich in iron," said Jonathan Keohane of NASA's Jet Propulsion Laboratory, the lead investigator for this research. "This makes W49B a prime candidate for being the remnant of a gamma ray burst involving a black hole collapsar."

According to the collapsar theory, gamma-ray bursts are produced when a massive star runs out of nuclear fuel and the star's core collapses to form a black hole surrounded by a disk of extremely hot, rapidly rotating, magnetized gas. Much of this gas is pulled into the black hole, but some is flung away in oppositely directed jets of gas traveling at near the speed of light.

An observer aligned with one these jets would see a gamma-ray burst, a blinding flash in which the concentrated power equals that of ten quadrillion Suns for a minute or so. The view perpendicular to the jets is a less astonishing, although nonetheless spectacular supernova explosion. For W49B, the jet is tilted out of the plane of the sky by about 20 degrees.

Palomar Infrared Images of W49B These infrared images, when combined with Chandra X-ray data, help reveal a barrel-shaped supernova remnant consisting of bright infrared rings around a glowing bar of intense X-radiation. The infrared data was observed with the Palomar Hale 200" WIRC (Wide InfraRed Camera). Scale: Image is 5.7 arcmin per side (Credit: Caltech/SSC/J.Rho and T. Jarrett)

Four rings about 25 light years in diameter can be identified in the infrared image. These rings, which are due to warm gas, were presumably flung out by the rapid rotation of the massive star a few hundred thousand years before the star exploded. The rings were pushed outward by a hot wind from the star a few thousand years before it exploded.

Chandra's image and spectral data show that the jets of multimillion-degree-Celsius gas extending along the axis of the barrel are rich in iron and nickel ions, consistent with their being ejected from the center of the star. This distinguishes the explosion from a conventional type II supernova in which most of the Fe and Ni goes into making the neutron star, and the outer part of the star is what is flung out. In contrast, in the collapsar model of gamma ray bursts iron and nickel from the center is ejected along the jet.

At the ends of the barrel, the X-ray emission flares out to make a hot cap. The X-ray cap is surrounded by a flattened cloud of hydrogen molecules detected in the infrared. These features indicate that the shock wave produced by the explosion has encountered a large, dense cloud of gas and dust.

Chandra 3-Color X-ray Image of W49B Chandra X-ray data reveal a barrel-shaped supernova remnant around a glowing bar of intense X-radiation. These X-rays are produced by jets of 15 million degree Celsius gas that is rich in iron and nickel. These features indicate that W49B was produced when the core of a rapidly-rotating massive star collapsed to form a black hole, triggering the ejection of high-energy jets of material. The colors red, green and blue in the images correspond to low, medium and high-energy X-rays, respectively. Scale: Image is 5.7 arcmin per side (Credit: NASA/CXC/SSC/J. Keohane et al.)

The scenario that emerges is one in which a massive star formed from a dense cloud of dust, shone brightly for a few million years while spinning off rings of gas and pushing them away, forming a nearly empty cavity around the star. The star then underwent a collapsar-type supernova explosion that resulted in a gamma-ray burst.

The observations of W49B may help to resolve a problem that has bedeviled the collapsar model for gamma-ray bursts. On the one hand, the model is based on the collapse of a massive star, which is normally formed from a dense cloud. On the other hand, observations of the afterglow of many gamma-ray bursts indicate that the explosion occurred in a low-density gas. Based on the W49B data, the resolution proposed by Keohane and colleagues is that the star had carved out an extensive low-density cavity in which the explosion subsequently occurred.

"This star appears to have exploded inside a bubble it had created," said Keohane. "In a sense, it dug its own grave."