The mysterious causes of the largest cosmic explosion ever detected

Artist impression of a black hole accretion (Image courtesy John A. Paice)
Artist’s impression of a black hole accretion (Image courtesy John A. Paice)

Two articles, one published in the journal “Monthly Notices of the Royal Astronomical Society” and one in “The Astrophysical Journal Letters”, report as many studies on what was defined as the largest cosmic explosion ever detected. Two teams of researchers studied the data collected with various instruments regarding the event cataloged as AT2021lwx offering two different hypotheses for its cause. Both teams believe that a supermassive black hole about 8 billion light-years from Earth caused that explosion but disagree on what triggered it: one team points to a cloud of gas and dust being violently swallowed while the other team points to a tidal disruption event where a star being devoured.

The AT2021lwx event was first detected in 2020 by the Zwicky Transient Facility (ZTF) in California and later by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii. These are two structures that survey the night sky looking for transient objects that change brightness rapidly indicating cosmic events such as supernovae and finding asteroids and comets.

The cataloging as AT2021lwx and the designation ZTF20abrbeie among the additional codes led to the nickname “Scary Barbie” by the researchers led by the American University of Purdue who conducted the study published in “The Astrophysical Journal Letters” for the astonishment felt in ascertaining this event’s extraordinary levels of brightness.

Supernovae and gamma-ray bursts are extremely bright but the duration of the AT2021lwx event is much longer than almost all the events of those types. Transient events can last for fractions of a second such as some gamma-ray bursts to months such as supernovae. Scary Barbie has been lasting for over three years, an anomalous lifespan for a supernova, with a much higher brightness. According to researchers led by the University of Purdue, a supermassive black hole destroyed a star generating an extreme amount of electromagnetic emissions even for a so-called tidal destruction event.

However, another team of researchers led by the British University of Southampton reached different conclusions from the study of the AT2021lwx event. They too studied it with various instruments to obtain observations in different electromagnetic bands which led them to propose the possibility that it was a colossal cloud that was swallowed by a supermassive black hole. According to this hypothesis, the cloud was destroyed by gravity and the fragments were engulfed emitting shock waves, also into the materials surrounding the black hole.

The AT2021lwx event is really out of the ordinary no matter what object was swallowed by the supermassive black hole. Philip Wiseman of the University of Southampton, the lead author of the study that supports the hypothesis of the large gas cloud, explained that such events are extremely rare but are so energetic that they could be crucial in the evolution of galaxies. For this reason, his team intends to conduct simulations to look for new verifications of the cause of the explosion with the hope of finding more of them, also counting on the fact that in the next few years, new instruments will enter service. Meanwhile, monitoring of the AT2021lwx event continues to gain new information, also in hopes of better understanding the activity of truly extreme objects like supermassive black holes.

Artist impression of a tidal disruption event (Image NASA/JPL-Caltech)
Artist impression of a tidal disruption event (Image NASA/JPL-Caltech)

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