IoW_20250604 - Gaia

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New transient event class found with Gaia: Extreme Nuclear Transients

Figure 1. Gaia lightcurve of Gaia18cdj as obtained from the Gaia Photometric Science Alerts website. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

Thanks to Gaia and the Zwicky Transient Facility a new class of transient events was found: Extreme Nuclear Transients or ENTs. The most energetic transient events now known! Current data shows these events are powered by massive stars (with a weight of about 3 to 10 solar masses) being ripped apart by a supermassive black hole (SMBH). While their brightness makes them visible at great distances and their flare is visible for a very long time, their occurence is extremely rare. The record holder of them all is Gaia18cdj, emitting 25 times more energy than the most energetic known supernova. That is extremely energetic if you consider a typical supernova shines as bright as 10 billion Suns at its peak.

"We’ve observed stars getting ripped apart as tidal disruption events for over a decade, but these ENTs are different beasts, reaching brightnesses nearly ten times more than what we typically see," said Jason Hinkle. “Not only are ENTs far brighter than normal tidal disruption events, but they remain luminous for years, far surpassing the energy output of even the brightest known supernova explosions.”

In the paper "The most energetic transients: tidal disruption of high mass stars" by Jason Hinkle, this new class of ENTs is defined with three events, of which two were initially obtained through Gaia's Photometric Science Alerts: Gaia16aaw (AT2016dbs) and Gaia18cdj (AT2018fbb). While Gaia brought two of these rare events to the sample, and the most energetic one of the new family, the third ENT was detected by the Zwicky Transient Facility, also monitoring the sky over many years.

Without the high signal-to-noise ratio of Gaia data and the long-term Gaia light curves, these events might have been missed. ENTs slowly rise their luminosity over more than 100 days, then, after reaching peak luminosity, show an equally slow decrease in luminosity while the flare slowly dies out over more than 150 days. The sensitivity of Gaia to slowly-evolving sources was crucial for this study. Many time domain surveys are optimised for transients with shorter time scales, of the order of several weeks. Gaia complements these surveys with its long baseline of 10.5 years, regularly observing 2 billion objects while scanning the entire sky.

"Gaia doesn’t tell you what a transient is, just that something changed in brightness," said Jason Hinkle. "But when I saw these smooth, long-lived flares from the centers of distant galaxies, I knew we were looking at something unusual."

Gaia's Photometric Science Alerts were sent out during the spacecraft's operational life when objects in the sky showed a peculiar dimming or brightening on top of their expected behaviour. With these alerts, many supernova explosions (SNs) and tidal disruption events (TDEs) were found and/or observed, as well as other transients such as microlensing events or active galactic nuclei (AGNs). Now, with this study, a new class of transients is found, the Extreme Nuclear Transients. The broader follow-up that was initiated following the alert sent out by Gaia allowed the team to study them in more detail.

Simon Hodgkin from the Gaia Photometric Science Alerts team at the Institute of Astronomy (University of Cambridge) expressed his joy at the discovery of these extreme nuclear transients. “We built our transient system to enable the discovery of exciting and unexpected events in the daily Gaia data downloads. This team have dug really deep into the haystack of alerts and found a couple of really shiny needles!”.

A multi-year follow-up campaign was launched following the two Gaia alerts, to figure out the nature of these extreme sources. Patience was needed since the events slowly evolve over several years. Confidence was gained when the third event was found by the Zwicky Transient Facility and reported independently by two teams. This showed strong support that Extreme Nuclear Transients are a distinct new class of extreme astrophysical events.

Extreme Nuclear Transient events are extremely rare though, finding them depends on survey missions continuously monitoring the sky. The rate of occurence for an ENT is 10 million times lower than the rate at which a supernova occurs. With upcoming surveys more of these events may be found!

Figure 2. The life of an Extreme Nuclear Transient Event. Credits: University of Hawaii by Nancy Hulbirt.

These ENTs may be the best probes to study the most massive black holes in the universe, particularly those which are otherwise not actively eating gas and dust, the quiescent supermassive black holes. This complements studies of actively accreting supermassive black holes at high redshifts with the James Webb Space Telescope, hence giving us a more complete overview of the supermassive black hole population.

IoW_20250604 - Gaia

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New transient event class found with Gaia: Extreme Nuclear Transients

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