Back in 2001 Niel Brandt, a professor of Astronomy and Astrophysics at Penn State University, and Franz Bauer, then a postdoctoral scholar working with Brandt at Penn State (now at Columbia University), detected a highly radio- and x-ray- luminous object. They were making observations with NASA’s Chandra X-ray Observatory during its first year in orbit when they noticed the source. They then looked for earlier data collected on this object and found it has become brighter with time. Previously, the supernova SN 1987A had been the only known supernova with an x-ray output that increased over time. But this similarity, along with other properties exhibited by the object, were not enough to classify it as a supernova, or as any other type of object for that matter.
“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 multi-wavelength 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.”
A supernova is an exploding star that can become billions of times as bright as the sun before gradually fading from view. At its maximum brightness, the exploded star may even outshine an entire galaxy. The recently identified supernova, named SN 1996cr, is one of the nearest that occurred in the last 25 years. Because this object lies in the nearby galaxy Circinus, it is relatively easy to study and the public archives of many telescopes contain data on this galaxy.
The research team searched through data from 18 different telescopes, both in space and on the ground. In addition, they obtained important new spectral data with the European Southern Observatory’s Very Large Telescope. “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.”
The team, consisting of astronomers from the US, Canada, and South Africa, found that this supernova is one of the brightest ever seen in radio and x-ray. It also displays properties similar to those of 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.” The extreme brightness of the supernova is helping scientists learn about the nature of the material surrounding it.
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. Astronomers may have missed it at the time it occurred because it was visible only in the southern hemisphere, which is not as widely monitored as the northern hemisphere. Of the five nearest supernovae in the last 25 years, SN 1996cr is the only one that was not seen shortly after its explosion.
The team was able to deduce the following account of the supernova’s formation: the parent star cleared out a large cavity around it. When the explosion occurred, the blast wave expanded relatively unimpeded into this cavity. When the blast wave hit the dense material surrounding SN 1996cr, it caused the system to glow brightly in x-ray and radio emission. This material was probably more compact than that surrounding SN 1987A, resulting in brighter emissions.
Astronomers think that both SN 1987A and SN 1996cr show evidence for these pre-explosion clear outs by the parent stars. This suggests that such a process may be common during the death of massive stars, meaning that SN 1987A isn’t as unusual as previously thought. Furthermore, it sheds light on the activities that can take place in massive stars. The relative nearness of SN 1996cr qualifies it as a target for future research in these areas.
TFOT covered NASA’s first real time imaging of a supernova explosion. Scientists used the Swift Satellite in order to see the explosion. Swift’s high capabilities and some alert astronomers, as well as a bit of good luck, made it possible to observe the real time explosion. TFOT also reported on NASA’s Spitzer Space Telescope detection of an unusual ring of material surrounding the magnetic remains of a blasted star. The stellar “corpse,” called SGR 1900+14, belongs to a class of objects known as magnetars, which are the cores of massive stars that blew up in supernova explosions.
Further information on the new research, which will be published in an upcoming issue of The Astrophysical Journal, can be found in the Penn State press release.