Nearby Supernova Discovered, Among the Brightest Ever in X Rays

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 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. The supernova was first singled out in 2001 as a bright, variable object in a Chandra image, but it was not confirmed as a supernova until years later, using data from a total of 18 different telescopes, nearly all of which was from archives. Credit: X-ray (NASA/CXC/Columbia/F.Bauer et al); Optical (NASA/STScI/UMD/A.Wilson et al.)

The discovery of one of the nearest supernovas to Earth to occur in the last 25 years will be revealed in a paper to be published in an upcoming issue of The Astrophysical Journal. The object — an exploding star that briefly shined brighter than billions of stars combined — has now been positively identified as a supernova more than a decade after it first occurred and has been named SN 1996cr. "After almost eight years of puzzling over the remarkable properties of this X-ray source, it is wonderful to know definitively that it is the remnant of a supernova that is exceptionally luminous in X-ray and radio energies," said Niel Brandt, a professor of Astronomy and Astrophysics at Penn State and a member of the research team. "This supernova continues to rise in X-ray and radio luminosity, similar to the behavior of the famous supernova 1987A, but it is approximately 1000 times more powerful."

X-ray and radio energies from the supernova first were detected in 2001 by Brandt and Franz Bauer, then a postdoctoral scholar working with Brandt at Penn State. Bauer, who now is at Columbia University, and Brandt noticed the bright object in the nearby Circinus spiral galaxy when they made observations with NASA's Chandra X-ray Observatory during its first year in orbit. When Bauer and Brandt looked for the same object in data from earlier X-ray observatories, they found that the object had become much brighter over time. Though the source displayed this and other exceptional properties, at the time Bauer and Brandt could not identify it definitively as a supernova or as some other kind of object. Only recently were Bauer, Brandt, and other astronomers on their research team able to confirm this object as a supernova.

"When Franz Bauer and I first detected this bright object in 2001, we correctly guessed that it was a supernova based on its X-ray and radio properties, but ever since then we have been working to determine its nature definitively," Brandt said. "We now have a clear identification of this object as a supernova as a result of our compiling rich multiwavelength data from the vast online archives of many of the world's premier telescopes to show the rise of the supernova to very high luminosities over a long period of time." The extreme brightness of the supernova is helping scientists to learn about the nature of the material surrounding it.

Because this object was found in a nearby galaxy, making it relatively easy to study, the public archives of many telescopes contained abundant data on this galaxy. The team searched through data — nearly all of which were from public archives — from 18 different telescopes, both in space and on the ground. In addition, Bauer, Brandt, and their team obtained important new spectral data with the European Southern Observatory's Very Large Telescope (VLT).

The data show that this supernova, 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 visible only in the southern hemisphere, which is not as widely monitored as the northern hemisphere. Among the five nearest supernovas of the last 25 years, SN 1996cr is the only one that was not seen shortly after its explosion.

SN 1996cr was not detected around the time of its explosion by other major X-ray observatories in orbit, including ROSAT and ASCA. Rather, it wasn't until several years later that the object was detected as a brightening X-ray source by Chandra, which was launched in 1999. 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 one are quite rare and may appear only 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 have found SN 1996cr like we did, and we never could have nailed it without the serendipitous data taken by all of these other telescopes. We've truly entered a new era of 'Internet astronomy'."

The data, combined with theoretical work, has led the team to the following narrative model of the supernova's history: 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.

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. The astronomers also note that the X-ray and radio emission from the earlier SN 1987A object is fainter because the surrounding material likely is less compact.

Not only does the team's work suggest that SN 1987A isn't as unusual as previously thought, but it also reveals more about the tremendous upheavals that massive stars can undergo during their lifetimes. SN 1996cr, at a distance of about 12 million light years, will be a compelling target for future research because it is nearby and because it is so much brighter than a typical supernova.

In addition to Bauer and Brandt, other co-authors on this paper include Vikram Dwarkadas of the University of Chicago, Stefan Immler of NASA's Goddard Space Flight Center, Norbert Bartel of York University in Canada, and Michael Bietenholz of York University in Canada and Hartebeesthoek Radio Observatory in South Africa.

NASA's Marshall Space Flight Center 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, Massachusetts.

CONTACTS

Niel Brandt: (+1)814-865-3509, niel@astro.psu.edu

Barbara Kennedy (PIO): 814-863-4682, science@psu.edu