Near the heart of the Milky Way, NASA’s Chandra spotted a bubble of X-rays too bright to be easily explained — 10 times brighter than massive young star clusters — and astronomers believe it may be the still-expanding debris of a supernova that exploded around 300 AD.

Astronomers have found a bright X-ray knot near the center of the Milky Way Galaxy that may be the remains of a star that exploded about seventeen centuries ago. The candidate is located within Sagittarius c, a star-forming region about 26,000 light-years from Earth, and the argument for this is based on data from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton Observatory. The results were published in the Astrophysical Journal by Zhenlin Zhu and Mark Morris of the University of California, Los Angeles, along with Gabriele Ponti of the Italian National Institute of Astrophysics and Ping Zhu of the University of Nanjing; A record of the paper is available via arXiv.

The word for doing heavy lifting is “possible.”

This is a candidate for supernova remnant, not a confirmed candidate, and this distinction shapes how the entire result is read.

What the data actually shows

The evidence is the “point” of X-ray emission, in NASA’s own formulationembedded in a larger bubble of expanding gas around a young, massive star. That bubble, where radiation stripped electrons from hydrogen, is the source of the bright radio known as Sagittarius C. The X-ray bubble is what the team says could be the debris of a massive star that collapsed and exploded.

It’s easy to miss a detail in coverage. The X-ray data here is not new. These data come from five observations made by Chandra between July 2001 and August 2014, approximately 57 hours in total, plus XMM-Newton. What is new is analysis, not vision. This may be the most interesting part of the story: a finding drawn from archived data that has been on record for a decade.

Why does ten times brighter matter?

The most specific number in the paper is the brightness comparison. The X-ray emission from the bubble is 10 times brighter than the X-rays from known large clusters of bright, massive stars. That number is the main reason the team decided to rule out the obvious alternative: that the glow comes from a crowd of exciting young stars and not from an explosion. Based on the available evidence, the authors do not believe that the star cluster explanation is appropriate.

There is a complication they clearly mention, which is in conflict with, rather than against, the supernova reading. The true remnant should show enhanced amounts of elements formed in the explosion, things like iron, oxygen, and silicon. The team searched for that signature but did not find it. Their proposed explanation is that the debris has already mixed with the surrounding gas, which is plausible for older remains.

Non-detection is still non-detection.

Seventeenth century question

If the blob is a supernova remnant, the team estimates it is expanding at about 2 million miles per hour, or approximately 3.2 million kilometers per hour, and is at least about 1,700 years old. Counting backwards from now, “at least 1,700 years” indicates an explosion around 300 AD or earlier. The phrase “at least” is worth sticking to, because it is a minimum rather than a fixed date, and the event could be much older.

This is not the first sign that something has exploded here. Observations on NASA’s retired SOFIA mission have already shown an expanding envelope of gas around Sagittarius C, suggesting a star had exploded in the same place. The X-ray filter fits this prior clue, although fitting a prior hint is not the same as confirmation.

What does confirmation mean?

The reason why this is worth paying attention to is the location. If it holds, this will be one of the closest supernova remnants ever identified to Sagittarius A*, the supermassive black hole at the center of the galaxy. This region is dense with massive stars, strong magnetic fields, and fast-moving gas clouds, all of which make clean detection more difficult, and the remnants confirmed there are particularly useful for studying how stars live and die in the most crowded part of the galaxy.

What will decide the matter is chemistry. Deeper X-ray observations that either find the missing elemental signature, or rule it out, would do more than just measure another brightness. For now, the honest summary is the one used by the researchers themselves: a strong candidate, an old SOFIA hint, and a search for heavy elements came back empty. Whether the point graduates from possible to certain is something to watch.