Cosmic sleuths are attempting to locate one of the estimated one billion neutron stars in the Milky Way–the collapsed core of a doomed supergiant star with the density of atomic nuclei–that was ejected from the blast of a supernova in the Small Magellanic Cloud, a satellite galaxy to our Milky Way 1,700 years ago during the decline of the Roman Empire. The light of a supernova is visible across billions of light years, releasing as much energy in an instant as our sun will produce over its 10-billion-year lifetime. The suspected neutron star was identified in observations with the European Southern Observatory’s Very Large Telescope in Chile, in combination with data from NASA’s Chandra X-ray Observatory.
The Cosmic Corpse
The doomed star, reports the NASA Goddard Space Flight Center, left behind an expanding, gaseous corpse, a supernova remnant named 1E 0102.2-7219 (below), which NASA’s Einstein Observatory first discovered in X-rays. The NASA researchers sifted through archival images taken by Hubble, analyzing visible-light observations made 10 years apart. Based on their estimates, the neutron star must be moving at more than 2 million miles per hour from the center of the explosion to have arrived at its current position.
Is the Object the Surviving Neutron Star?
“That is pretty fast and at the extreme end of how fast we think a neutron star can be moving, even if it got a kick from the supernova explosion,” said John Banovetz, who led the research team hunting for the exotic object along with Danny Milisavljevic, both of Purdue University,. “More recent investigations call into question whether the object is actually the surviving neutron star of the supernova explosion. It is potentially just a compact clump of supernova ejecta that has been lit up, and our results generally support this conclusion.”
Ejecta Clumps –The Ionized Oxygen Trail
“Our study doesn’t solve the mystery, but it gives an estimate of the velocity for the candidate neutron star,” said Banovetz,who measured the velocities of 45 tadpole-shaped, oxygen-rich clumps of ejecta flung by the supernova blast. Ionized oxygen is an excellent tracer because it glows brightest in visible light.
To calculate an accurate explosion age, the astronomers picked the 22 fastest moving ejecta clumps, or knots. The researchers determined that these targets were the least likely to have been slowed down by passage through interstellar material. They then traced the knots’ motion backward until the ejecta coalesced at one point, identifying the explosion site. Once that was known, they could calculate how long it took the speedy knots to travel from the explosion center to their current location.
The researchers’ results differ from previous observations of the supernova’s blast site and age. Earlier studies, for example, arrived at explosion ages of 2,000 and 1,000 years ago. However, Banovetz and Milisavljevic say their analysis is more robust.
“Hubble to the Rescue”
“A prior study compared images taken years apart with two different cameras on Hubble, the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys (ACS),” Milisavljevic said. “But our study compares data taken with the same camera, the ACS, making the comparison much more robust; the knots were much easier to track using the same instrument. It’s a testament to the longevity of Hubble that we could do such a clean comparison of images taken 10 years apart.”
The astronomers also took advantage of the sharp ACS images in selecting which ejecta clumps to analyze. In prior studies, researchers averaged the speed of all of the gaseous debris to calculate an explosion age. However, the ACS data revealed regions where the ejecta slowed down because it was slamming into denser material shed by the star before it exploded as a supernova. Researchers didn’t include those knots in the sample. They needed the ejecta that best reflected their original velocities from the explosion, using them to determine an accurate age estimate of the supernova blast.
Image credits: NASA, ESA, and J. Banovetz and D. Milisavljevic (Purdue University). Image top of page, neutron star, Shutterstock License