Astronomers observe a luminous dying star so bright that it's 'terrifying'

"If you take a typical supernova and multiply it a thousand times, we're still not at how bright this is – and supernovas are among the most luminous objects in the sky," Milisavljevic, adding that "this is the most energetic phenomenon I have ever encountered."

Milisavljevic, who is an expert on stellar life cycles also explained that it is incredibly unusual to see a daying star produce such enormous amounts of light.

The object, designated 'ZTF20abrbeie', is affectionately referred to by astronomers as "Scary Barbie". This is due partly to the fact that its random letter designation almost resembles Barbie, and partly because of how anomalous the event is. As Milisavljevic explained, "it’s so much of an outlier; its characteristics are terrifying!”

Scary Barbie forms part of a category known as transients — referring to large objects in space that change noticeably over short periods, rather than millennia.

The "most luminous transient in the universe"

In a new paper, Milisavljevic, graduate student Bhagya Subrayan, and a team analyzed the data and determined that the luminous, long-lived transient is a black hole that is in the process of devouring a star.

They used big data and AI software to analyze massive amounts of sky survey data in order to pinpoint the event, which was first observed in 2020. Though it's extremely bright, the transient is also extremely far away, which accounts for the fact it was not noticed before by astronomers.

According to the researchers, Scary Barbie is orders of magnitude brighter and more energetic than any transient ever observed. Not only that, but it is also lasting a lot longer than the typical transient — most last weeks or months, but Scary Barbie has so far lasted over two years.

Interestingly, the scientists behind the discovery believe the event itself lasted a much shorter time. We have likely been able to see it for over two years due to the law of relativity, which slows down light as it travels across space, making it seem like it lasts for a longer time.

"We think a very supermassive black hole pulled in a star and ripped it apart,” Subrayan explained. "The forces around a black hole, called tidal disruption, pull other objects apart in a process called 'spaghettification.' We think that's what happened but on extreme time scales: The most massive of black holes ripping apart a massive star. The duration is unlike anything we've ever seen before, and it produced the most luminous transient in the universe."

Abstract:

We report multi-wavelength observations and characterization of the ultraluminous transient AT 2021lwx (ZTF20abrbeie; aka ``Barbie'') identified in the alert stream of the Zwicky Transient Facility (ZTF) using a Recommender Engine For Intelligent Transient Tracking (REFITT) filter on the ANTARES alert broker. From a spectroscopically measured redshift of 0.995, we estimate a peak observed pseudo-bolometric luminosity of log (Lmax/[erg/s]) = 45.7 from slowly fading ztf-g and ztf-r light curves spanning over 1000 observer-frame days. The host galaxy is not detected in archival Pan-STARRS observations (g>23.3 mag), implying a lower limit to the outburst amplitude of more than 5 mag relative to the quiescent host galaxy. Optical spectra from Lick and Keck Observatories exhibit strong emission lines with narrow cores from the H Balmer series and ultraviolet semi-forbidden lines of Si III] λ1892, C III] λ1909, and C II] λ2325. Typical nebular lines in AGN spectra from ions such as [O II] and [O III] are not detected. These spectral features, along with the smooth light curve that is unlike most AGN flaring activity, and the luminosity that exceeds any observed or theorized supernova, lead us to conclude that AT 2021lwx is most likely an extreme tidal disruption event (TDE). Modeling of ZTF photometry with MOSFiT suggests that the TDE was between a ≈14M⊙ star and a supermassive black hole of mass MBH∼ 108M⊙. Continued monitoring of the still-evolving light curve along with deep imaging of the field once AT 2021lwx has faded can test this hypothesis and potentially detect the host galaxy.