York University astronomers have determined the diameter and expansion rate of a supernova hailed by NASA as the first of its kind to be found using the Internet.
The supernova was first detected in 2001 by Franz Bauer of Columbia University who noticed a bright object in the spiral galaxy Circinus using NASA’s Chandra X-ray Observatory. At the time, however, Bauer and his colleagues could not confidently identify its nature.
Not until they mined the vast online public archives containing data from many of the 18 different telescopes in space and on the ground, could Bauer and his team confirm what they had discovered: One of the nearest supernovas in the last 25 years that had exploded a decade earlier.
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Above: A NASA composite image shows SN 1996cr and 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, which 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 taken from archives.
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Its discovery is leading researchers to claim that we have entered a new era of “Internet astronomy.”
The supernova, dubbed SN 1996cr, exploded between Feb. 28, 1995 and March 15, 1996. It 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.
Bauer, principal investigator of the project, enlisted York astronomers Norbert Bartel and Michael Bietenholz to provide a clearer picture of the supernova using their expertise in Very Long Baseline Interferometry (VLBI). VLBI is the simultaneous use of several networked telescopes to detect radio waves and make a very sharp image of a celestial object.
“This supernova is proving to be the strongest radio wave emitter of any supernova we’ve discovered so far, which helps us determine how large the supernova is now, after it has expanded for 10 years,” says Bartel, Distinguished Research Professor in the Department of Physics & Astronomy in York’s Faculty of Science & Engineering.
“It is now 500 times larger than our solar system, and likely also that much larger than any planetary system that may have existed around the star before it exploded. Light now needs seven months to travel from one side of the gigantic shock wave to the other side,” says Bartel.
A supernova is the death of a star, one of the most powerful single events in the universe. When a star explodes, it ejects its mass into space at a velocity of 20,000 km/sec., freeing most of the heavy elements, such as carbon, iron and oxygen, needed to form planets and sustain life in the universe.
Shortly before SN 1996cr exploded, it shed material from its surface into space at a high velocity, creating a vacuum around it before it died, says Bartel. Now, several years after the explosion, that material is being hit by the supernova shock wave, causing the system to glow brightly at X-ray and radio wavelengths.
“We are working to interpret what is presently going on when the shock wave hits the surrounding material that was shed from the surface of the star,” says Bartel.
Astronomers think that both SN 1987A and SN 1996cr show evidence of 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.
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.
The results of the study, "Supernova 1996cr: SN 1987A’s Wild Cousin?", will appear in an upcoming issue of The Astrophysical Journal. It is now available online.
Franz Bauer (Columbia University) is lead author of the study. The co-authors, apart from Bartel and Bietenholz, are Vikram Dwarkadas (University of Chicago), Niel Brandt (Penn State), Stefan Immler (NASA Goddard Space Flight Center) and Stephen Smartt (Queen’s University Belfast).