Like a team of forensic detectives in a television show that could be called “CSI: Milky Way,” a University of Chicago astrophysicist and his associates are piecing together how a mysterious infrared ring got left around a dead star that displays a magnetic field trillions of times more intense than Earth’s.

This image shows a ghostly ring extending seven light-years across around the corpse of a massive star. The collapsed star, called a magnetar, is located at the exact center of this image. NASA’s Spitzer Space Telescope imaged the mysterious ring around magnetar SGR 1900 14 in infrared light. The magnetar itself is not visible in this image, as it has not been detected at infrared wavelengths (it has been seen in X-ray light).

NASA’s Spitzer Space Telescope detected the ring around magnetar SGR 1900+14 at two narrow infrared frequencies in 2005 and 2007. The ringed magnetar is of a type called a soft gamma repeater (SGR) because it repeatedly emits bursts of gamma rays.

“The universe is a big place, and weird things can happen,” said Stephanie Wachter of NASA’s Spitzer Science Center at the California Institute of Technology. “I was flipping through archived Spitzer data of the object, and that’s when I noticed it was surrounded by a ring we’d never seen before.”

Wachter enlisted Vikram Dwarkadas, a Senior Research Associate in Astronomy & Astrophysics at the University of Chicago, to help determine how the ring formed. Wachter, Dwarkadas and five other co-authors present the results of their investigation in the May 29 issue of the journal Nature.

“It’s the first time something like this has ever been seen around a magnetar,” Dwarkadas said. Magnetars come from massive stars that have exploded as a core-collapse supernova. “These stars are at least eight times the mass of the sun, or more massive than that,” he said.

Magnetars interest astrophysicists because of their mysterious and unusual characteristics. When massive stars collapse, they usually form compact objects called neutron stars or black holes. “We have no idea why some neutron stars are magnetars and some are not,” Dwarkadas said.

SGR 1900+14 seems to belong to a nearby cluster of massive stars that resides along the plane of the Milky Way. Since the most massive stars live the shortest lives, the object hints that perhaps only the most massive stars become magnetars.

When Wachter’s team began pondering the origin of the ring, “We thought initially of all the standard explanations,” Dwarkadas said. But the team considered and eliminated several possibilities before concluding that a powerful flare that burst from the magnetar formed the ring, which measures seven light-years across.

“It’s as if the magnetar became a huge flaming torch and obliterated the dust around it, creating a massive cavity,” said co-author Chryssa Kouveliotou, senior astrophysicist at NASA’s Marshall Space Flight Center in Alabama. “Then the stars nearby lit up a ring of fire around the dead star, marking it for eternity.”

Vikram Dwarkadas, Senior Research Associate in Astronomy & Astrophysics at the University of Chicago. Along with colleagues at NASA and elsewhere, Dwarkadas has been studying a strange ring circling a dead star.

A theoretical astrophysicist supported by the National Science Foundation and NASA, Dwarkadas specializes in various phenomena related to supernova remnants and stellar winds. He helped Wachter’s team systematically eliminate several potential causes for the ring.

Was the ring an infrared echo, a mass of dust lit up by a flare moving out from the magnetar? The 2007 Spitzer image showed no discernable change in the ring after two years. “If it hasn’t moved, it hasn’t changed, it can’t be an infrared echo,” Dwarkadas said. “It’s a stationary ring.”

Could the ring be a bubble blown by solar winds emitted from the star before it exploded? Shock waves of a supernova travel at approximately 10,000 miles a second. If the ring was a wind-blown bubble, the supernova shock wave would overtake it somewhere between a few decades to a century or two, at most.

“It would mean that the supernova should have actually gone through and destroyed the ring unless it was very, very recent,” Dwarkadas said. If the ring was a wind-blown bubble that somehow survived the supernova shock wave, “then you’d need a massive bubble,” he said. “We did some calculations and we ran some simulations, and it just didn’t work.”

Wachter’s team next considered whether the ring could be related to the supernova. That possibility also failed to pan out. “If there is a supernova, there would be shocks. You would see X-ray, radio and optical emission. We looked at archival data, and there was no emission at any wavelength except in the Spitzer images,” Dwarkadas said.

The paper’s other co-authors are Jonathan Granot of the University of Hertfordshire, England; Enrico Ramirez-Ruiz of the University of California, Santa Cruz; Sandy Patel of the Optical Sciences Corporation, Huntsville, Ala.; and Don Figer at the Rochester Institute of Technology in New York.

The most recent supernova in our Galaxy has been discovered by tracking the rapid expansion of it is remains. This result, using NASA’s Chandra X-ray Observatory and NRAO’s Very Large Array (VLA), has implications for understanding how often supernovas explode in the Milky Way galaxy.

A composite image of X-ray (orange) and radio (blue) data from NASA’s Chandra X-ray Observatory and the Very Large Array shows the remains of the supernova remnant G1.9 0.3 on the left. By comparing the images that were obtained over 20 years apart, scientists can determine how quickly it is expanding and therefore when its progenitor exploded. G1.9 0.3 was created about 140 years ago, making it the most recent supernova in the Milky Way. It was not detected in optical light because it is near the center of the Galaxy and obscured by gas and dust as seen in the infrared image from the 2MASS telescope to the right.

The supernova explosion happened about 140 years ago, making it the most recent supernova in the Milky Way as measured in Earth’s time frame. Previously, the last known galactic supernova occurred around 1680, based on studying the expansion of its remnant Cassiopeia A.

The recent supernova explosion was not seen in optical light about 140 years ago because it occurred close to the center of the Galaxy, and is embedded in a dense field of gas and dust. This made it about a trillion times fainter, in optical light, than an unobscured supernova. However, the supernova remnant it caused, G1.9+0.3, is now seen in X-ray and radio images.

This artist’s impression shows what the supernova explosion that resulted in the formation of the supernova remnant G1.9 0.3 might have looked like. The expanding debris from the supernova explosion is shown in white, including some interaction with the surrounding gas (green). The crowded environment near the center is shown by diffuse gas (red) and dust (brown) as well as large numbers of stars with different masses and colors.

“We can see some supernova explosions with optical telescopes across half of the Universe, but when they’re in this murk we can miss them in our own cosmic backyard,” said Stephen Reynolds of North Carolina State University, who led the Chandra study. “Fortunately, the expanding gas cloud from the explosion shines brightly in radio waves and X-rays for thousands of years. X-ray and radio telescopes can see through all that obscuration and show us what we’ve been missing.”

Astronomers regularly observe supernovas in other galaxies like ours, and based on those rates, estimate that about three should explode every century in our Milky Way, although these estimates have large margins of error.

“If the supernova rate estimates are correct, there should be the remnants of about 10 supernova explosions that are younger than Cassiopeia A,” said David Green of the University of Cambridge in the United Kingdom, who led the VLA study. “It’s great to finally track one of them down.”

This artist’s impression shows a view looking down on the Milky Way galaxy. The position of the Sun is shown, as are the approximate positions and names (shown in orange) of historical supernovas. These are stellar explosions that are thought to have occurred in the last 2,000 years and may have been seen by early astronomers. The estimated position of the recently discovered G1.9 0.3 is shown in black. Although the distance to this remnant is uncertain, the angle is accurately known. Note that G1.9 0.3 is the only object that is found in the bulge of the galaxy.

The tracking of this source began in 1985 when astronomers, led by Green, used the VLA to identify G1.9+0.3 as the remnant of a supernova explosion near the center of our Galaxy. Based on its small size, it was thought to have resulted from a supernova that exploded about 400 to 1000 years ago.

Twenty two years later, Chandra observations of this object revealed that the remnant had expanded by a surprisingly large amount, about 16% since 1985. This indicates that the supernova remnant is much younger than previously thought.

The young age was confirmed when new radio observations from the VLA were made just within the past several weeks. This “apples to apples” comparison nails the age of the remnant to be about 140 years (less if it has been slowing down), making it the youngest on record in the Milky Way.

Finding such a recent, obscured supernova is a vital first step in making a better estimate of the supernova rate in our Galaxy. Knowing this rate is important because supernovas heat and redistribute large amounts of gas, pump large amounts of heavy elements out into their surroundings, and can trigger the formation of new stars, closing the cycle of stellar death and rebirth. The explosion may also leave behind, in addition to the expanding remnant, a central neutron star or black hole.

In addition to being a record holder for youth, G1.9+0.3 is of considerable interest for other reasons. The high expansion velocities and the extreme particle energies that have been generated are unprecedented and should stimulate deeper studies of this object with Chandra and the VLA.

“No other object in the Galaxy has properties like this,” said Reynolds. “Finding G1.9+0.3 is extremely important for learning more about how some stars explode and what happens in the aftermath.

Scientists can also use it to probe the environment into which it exploded. At perhaps only a few thousand light years from the center of the Galaxy, it appears to be embedded in the dense environment near the Milky Way’s supermassive black hole.

These results will appear in The Astrophysical Journal Letters. NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency’s Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.