Researchers from Stanford University and Harvard suggest the event could be the first-ever X-ray burster identified in the Large Magellanic Cloud (LMC), an unusual flare from a magnetar-one of the universe's most enigmatic objects-or an entirely new and previously unknown cosmic phenomenon.

"Have you ever flipped through old photo albums and suddenly found something fascinating hidden in the background of a picture, no one had ever noticed before? Now imagine doing that on a cosmic scale," said lead author Steven Dillmann, a PhD student at Stanford University.

"Using a novel machine learning approach, we looked back through over 20 years of archived observations from NASA's Chandra X-ray Observatory and discovered a remarkable, powerful X-ray flash from an unknown object outside our own galaxy that had gone unnoticed for years within the vast Chandra archive-a true needle in the haystack event."

The flash, which occurred on May 15, 2020, was detected serendipitously while Chandra was observing the remnants of a supernova in the LMC, a satellite galaxy of the Milky Way. The intense X-ray burst lasted mere seconds and was initially overlooked, remaining stored in Chandra's extensive data archives.

Traditional methods failed to detect this fleeting event, but the newly developed machine learning model revealed the phenomenon, categorized as an extragalactic fast X-ray transient (FXT). The team designated it XRT 200515, named for its detection date.

"The cosmic flash is particularly interesting because of its unusual characteristics that are different to any of the other extragalactic FXTs that have previously been detected by Chandra," explained Dillmann. "It produced an incredibly energetic initial burst that lasted for only 10s, whereas others lasted for minutes or hours. This was followed by a longer, less energetic afterglow lasting for a few minutes."

Since neither Chandra nor any other telescope has recorded this object before or after the event, its nature remains uncertain. One hypothesis suggests it could be the first X-ray burster discovered in the LMC. These systems consist of a neutron star and a companion star, where the neutron star's strong gravitational pull draws in gas from its companion. When enough gas accumulates, it triggers a thermonuclear explosion, producing a sudden burst of X-rays.

Alternatively, the team speculates that XRT 200515 may be a rare giant flare from a distant magnetar, an ultra-magnetic neutron star capable of generating immense bursts of gamma rays. If confirmed, this would be the first observation of a magnetar flare at such X-ray energy levels.

A third possibility is that the event represents a previously unclassified type of cosmic explosion, offering new insights into astrophysical phenomena.

"This discovery reminds us that space is dynamic and ever-changing, with exciting phenomena occurring constantly," said Dillmann. "It also demonstrates the value of using artificial intelligence for scientific discovery in archived astronomical data-there might be countless other discoveries waiting to be found in observations we've already made."

The researchers are now refining their machine learning methods to search for evidence of planets beyond the Milky Way, expanding upon prior work led by co-author Rosanne di Stefano, who previously identified the first potential extragalactic planet candidate.

Research Report:Representation learning for time-domain high-energy astrophysics: Discovery of extragalactic fast X-ray transient XRT 200515

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