When stars go supernova they release a huge amount of energy and material. The supernova only stays bright for a few months, but the material can continue to be seen for 100,000 years – this is what we call a supernova remnant, and astronomers have just discovered the largest one yet that is visible in X-rays. The findings are published in Astronomy & Astrophysics.
The eROSITA X-ray Telescope was responsible for these observations. The event – nicknamed Hoinga – is about 90 times larger than the full Moon in the sky. It is the first discovery of this kind for eROSITA and is notable not just for its size but also because of its location. It is not found in the plane of the Milky Way, where most of the stars in our galaxies are.
"We were very surprised that the first supernova remnant popped up straight away," lead author, Werner Becker at the Max Planck Institute for Extraterrestrial Physics, said in a statement. "Moreover, it lies very far off the galactic plane, which is very unusual."
The team discovered that a faint signal of this remnant was actually previously observed in data from 30 years ago. The signal was too faint to suggest something was really there if one wasn’t looking, especially since it wasn’t where we usually expect supernova remnants to be.
A supernova should go off in the Milky Way every 30 to 50 years. Therefore, about 1,200 supernova remnants should be visible from Earth – but only 300 or so have been discovered so far. The discovery of Hoinga suggests that we should start looking beyond the usual places.
The project is a collaboration with Australian researchers. Radio observations were crucial to confirm that this was indeed a supernova remnant. Emissions expected for such a structure were seen in both of these parts of the electromagnetic spectrum.
"We went through archival radio data and it had been sitting there, just waiting to be discovered," co-author Natasha Walker-Hurley, from the Curtin University node of the International Centre for Radio Astronomy Research in Australia, explained. "The radio emission in 10-year-old surveys clearly confirmed that Hoinga is a supernova remnant, so there may be even more of these out there waiting for keen eyes."
This is just the beginning for eROSITA. The complete mission will deliver eight all-sky surveys providing a new and detailed understanding of the universe in X-ray. The team was hoping to find a supernova remnant over the next few years when more data would be collected. This early detection suggests that many more supernova remnants might have been overlooked in a past survey, and they are about to be discovered.