Posts Tagged ‘supernova’

One of the nearest supernovas in the last 25 years has been identified over a decade after it exploded. This result was made possible by combining data from the vast online archives from many of the world’s premier telescopes.

This composite image shows the central regions of the nearby Circinus galaxy, located about 12 million light years away. Data from NASA’s Chandra X-ray Observatory is shown in blue and data from the Hubble Space telescope is shown in yellow (”I-band”), red (hydrogen emission), cyan (”V-band”) and light blue (oxygen emission). The bright, blue source near the lower right hand corner of the image is the supernova SN 1996cr, that has finally been identified over a decade after it exploded. Optical images from the archives of the Anglo-Australian Telescope in Australia show that SN 1996cr exploded between February 28, 1995 and March 15, 1996. Among the five nearest supernovas of the last 25 years, SN 1996cr is the only one that was not seen shortly after the explosion. It may not have been noticed by astronomers at the time because it was only visible in the southern hemisphere, which is not as widely monitored as the northern. The supernova was first singled out in 2001 as a bright, variable object in a Chandra image. Despite some exceptional properties, its nature remained unclear until years later, when scientists were able to confirm this object was a supernova. Clues in data from the European Southern Observatory’s Very Large Telescope led the team to search through data archives from 18 different telescopes, both in space and on the ground, nearly all of which was from archives. This is a remarkable example of the new era of `Internet astronomy’. The Circinus galaxy is a popular target for astronomers because it contains a supermassive black hole that is actively growing, and it shows vigorous star formation. It is also nearby, at only about 4 times the distance of M31. Therefore, the public archives of telescopes contain abundant data on this galaxy.

The supernova was first singled out in 2001 by Franz Bauer, then at Penn State and now at Columbia University, who noticed a bright, variable object in the spiral galaxy Circinus using NASA’s Chandra X-ray Observatory. Though the source displayed some exceptional properties, at the time Bauer and his Penn State colleagues could not confidently identify its nature.

It was not until years later that Bauer and his team were able to confirm this object was a supernova. Clues in a spectrum from the European Southern Observatory’s Very Large Telescope (VLT) led the team to search through data from 18 different telescopes, both in space and on the ground, nearly all of which was from archives. Because this object was found in a nearby galaxy, making it relatively easy to study, the public archives of these telescopes contained abundant data on this galaxy.

The data show that this supernova, dubbed SN 1996cr, is among the brightest supernovas ever seen in radio and X-rays. It also bears many striking similarities to the famous supernova SN 1987A, which occurred in a galaxy only 160,000 light years from Earth.

“This supernova appears to be a wild cousin of SN 1987A,” said Bauer. “These two look alike in many ways, except this newer supernova is intrinsically a thousand times brighter in radio and X-rays.”

Optical images from the archives of the Anglo-Australian Telescope in Australia show that SN 1996cr exploded between February 28, 1995 and March 15, 1996, nearly a decade after SN 1987A. SN 1996cr may not have been noticed by astronomers at the time because it was only visible in the southern hemisphere, which is not as widely monitored as the northern. Among the five nearest supernovas of the last 25 years, it is the only one that was not seen shortly after the explosion.

SN 1996cr was not detected by other major X-ray observatories in orbit – ROSAT and ASCA – around the time of explosion. Rather, it wasn’t until several years later that it was detected as an X-ray source by Chandra (launched in 1999), and has become steadily brighter ever since. Previously, SN 1987A had been the only known supernova with an X-ray output observed to increase over time.

“Supernovas that are close enough to be studied in detail like this are quite rare and may only appear once a decade, so we don’t want to miss such an important opportunity for discovery,” said Bauer. “It’s a bit of a coup to find SN 1996cr like we did, and we could never have nailed it without the serendipitous data taken by all of these telescopes. We’ve truly entered a new era of `Internet astronomy’.”

The data, combined with theoretical work, has led the team to the following model. Before it exploded, the parent star cleared out a large cavity around it, either via a fast wind or an outburst from the star late in its life. Then, the blast wave from the explosion expanded relatively unimpeded into this cavity. Once the blast wave hit the dense material surrounding SN1996cr, the impact caused the system to glow brightly in X-ray and radio emission. The X-ray and radio emission from SN 1987A is fainter because the surrounding material is probably less compact.

Astronomers think that both SN 1987A and SN 1996cr show evidence for these pre-explosion clear-outs by the star doomed to explode. Having two nearby examples suggests that this type of activity could be relatively common during the death of massive stars.

“Not only does our work suggest that SN 1987A isn’t as unusual as previously thought, but it also teaches us more about the tremendous upheavals that massive stars can undergo during their lifetime,” said co-author Vikram Dwarkadas of the University of Chicago.

SN 1996cr, at a distance of about 12 million light years, will be a compelling target for future work because it is nearby and so much brighter than a typical supernova.

These results will appear in an upcoming issue of The Astrophysical Journal. Other co-authors on this paper include Niel Brandt (Penn State), Stefan Immler (NASA Goddard Space Flight Center), Norbert Bartel (York University, Canada), and Michael Bietenholz (York University and Hartebeesthoek Radio Observatory, South Africa).

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.

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.