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Astronomers wind the clock on the expanding remnants of a nearby exploding star. Using NASA’s Hubble Space Telescope, they traced the rapid shrapnel of the explosion to calculate a more accurate estimate of the location and time of the stellar detonation.
The victim is a star that exploded a long time ago in the Small Magellanic Cloud, a satellite galaxy in our Milky Way. The doomed star left behind an expanding gaseous corpse, a supernova remnant named 1E 0102.2-7219, which NASA’s Einstein Observatory first discovered on x-rays. As detectives, the researchers scrutinized archival footage taken by Hubble, analyzing visible light observations made 10 years apart.
The research team, led by John Banovetz and Danny Milisavljevic of Purdue University in West Lafayette, Indiana, measured the velocities of 45 oxygen-rich tadpole-shaped ejecta clusters projected by the supernova explosion. Ionized oxygen is an excellent tracer because it shines the most in visible light.
To calculate an accurate explosion age, astronomers chose the 22 fastest ejecta clusters, or nodes. The researchers determined that these targets were the least likely to have been slowed down by passage through interstellar material. They then traced the movement of the nodes backwards until the ejecta coalesced into a point, identifying the site of the explosion. Once this was known, they could calculate how long it took for fast nodes to get from the center of the explosion to their current location.
According to their estimate, the light from the explosion arrived on Earth 1,700 years ago, during the decline of the Roman Empire. However, the supernova would have been visible only to the inhabitants of the southern hemisphere of the Earth. Unfortunately, there is no known record of this titanic event.
The researchers’ results differ from previous observations at the site and age of the supernova explosion. Previous studies, for example, arrived at explosion ages of 2000 to 1000 years. However, Banovetz and Milisavljevic say their analysis is more robust.
“An earlier study compared images taken years apart with two different cameras on Hubble, the Wide Field Planetary Camera 2 and the Advanced Survey Camera (ACS),” Milisavljevic said. “But our study compares data taken with the same camera, ACS, which makes the comparison much more robust; the knots were much easier to follow with the same instrument. It’s a testament to Hubble’s longevity that we were able to make such a crisp comparison of images taken 10 years apart. “
Astronomers also took advantage of the sharp ACS images to select which ejecta clusters to analyze. In previous studies, researchers averaged the velocity of all gaseous debris to calculate an explosion age. However, ACS data revealed areas where the ejecta slowed down because they slammed into denser material thrown by the star before it exploded into a supernova. The researchers did not include these nodes in the sample. They needed the ejecta that best reflected their original explosion speeds, using them to determine an accurate estimate of the age of the supernova explosion.
Hubble also clocked the speed of a suspected neutron star – the crushed nucleus of the doomed star – which was ejected from the explosion. Based on their estimates, the neutron star must travel more than 2 million miles per hour from the center of the explosion to arrive at its current location. The suspected neutron star was identified in observations with the Very Large Telescope at the European Southern Observatory in Chile, in combination with data from NASA’s Chandra X-ray Observatory.
“It’s pretty fast and at the extreme of the speed at which we think a neutron star can move, even though it was kicked by the supernova explosion,” Banovetz said. “More recent investigations question whether the object is really the surviving neutron star of the supernova explosion. It is potentially simply a compact cluster of supernova ejecta that has been ignited, and our results generally support this conclusion. “
So the neutron star hunt may still be in progress. “Our study does not solve the mystery, but it does give an estimate of the velocity of the candidate neutron star,” Banovetz said.
Banovetz will present the team’s findings on Jan. 14 at the American Astronomical Society’s winter meeting.
The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts science operations at Hubble. STScI is operated for NASA by the Association of Universities for Astronomical Research in Washington, DC
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