Rare explosion may have created Milky Way's youngest black hole

Posted by Thomas Anderson

New data from NASA's Chandra X-ray Observatory indicates that a highly distorted supernova remnant may contain the most recent black hole formed in our Milky Way galaxy.

Indeed, the remnant appears to be the product of a rare explosion in which matter is ejected at high speeds along the poles of a rotating star. Dubbed W49B, the stellar phenomenon is approximately a thousand years old as seen from Earth and located about 26,000 light-years away. 

"W49B is the first of its kind to be discovered in the galaxy," explained Laura Lopez, who led the study at the Massachusetts Institute of Technology (MIT). "It appears its parent star ended its life in a way that most others don't."

Typically, when a massive star runs out of fuel, the central region of the star collapses, triggering a chain of events that quickly culminate in a supernova explosion. Most of these explosions are generally symmetrical, with the stellar material blasting away more or less evenly in all directions.

However, in the case of the W49B supernova, material near the poles of the doomed rotating star was ejected at a much higher speed than material emanating from its equator. Indeed, jets shooting away from the star's poles primarily shaped the supernova explosion and its aftermath.

The remnant now glows brightly in X-rays and other wavelengths, offering the evidence for a peculiar explosion. By tracing the distribution and amounts of different elements in the stellar debris field, researchers were able to compare the Chandra data to theoretical models of how a star explodes.

For example, they found iron in only half of the remnant while other elements such as sulfur and silicon were spread throughout - essentially matching predictions for an asymmetric explosion.

"In addition to its unusual signature of elements, W49B also is much more elongated and elliptical than most other remnants," said co-author Enrico Ramirez-Ruiz of the University of California at Santa Cruz. "This is seen in X-rays and several other wavelengths and points to an unusual demise for this star."

Because supernova explosions are not well understood, astronomers are interested in studying extreme cases like the one that produced W49B. Plus, the relative proximity of W49B also it extremely useful for detailed study.

The researchers examined what sort of compact object the supernova explosion left behind. Most of the time, massive stars that collapse into supernovas leave a dense, spinning core called a neutron star.

Astronomers are often able to detect neutron stars through their X-ray or radio pulses, although sometimes an X-ray source is seen without pulsations. A careful search of the Chandra data revealed no evidence for a neutron star; meaning the lack of such evidence implies a black hole may have formed.

"It's a bit circumstantial, but we have intriguing evidence the W49B supernova also created a black hole," noted co-author Daniel Castro, also of MIT. "If that is the case, we have a rare opportunity to study a supernova responsible for creating a young black hole."

Supernova explosions driven by jets like the one in W49B have been linked to gamma-ray bursts (GRBs) in other objects. GRBs, which have been seen only in distant galaxies, also are thought to mark the birth of a black hole.

There is currently no evidence the W49B supernova produced a GRB, but it may have properties - including being jet-driven and possibly forming a black hole - that overlap with those of a GRB.