Author(s): Bhagya M. Subrayan, Dan Milisavljevic, Ryan Chornock, Raffaella Margutti, Kate D. Alexander, Vandana Ramakrishnan, Paul C. Duffell, Danielle A. Dickinson, Kyoung-Soo Lee, Dimitrios Giannios, Geoffery Lentner, Mark Linvill, Braden Garretson, Matthew J. Graham, Daniel Stern, Daniel Brethauer, Tien Duong, Wynn Jacobson-Galán, Natalie LeBaron, David Matthews, Huei Sears, and Padma Venkatraman
First Author’s Institution: Department of Physics and Astronomy, Purdue University
Status: Accepted in The Astrophysical Journal Letters [open access]
Supermassive black holes are weird
In addition to all the other reasons for being cool, supermassive black holes (SMBHs) continue to surprise us, even today. Alongside the growing number of discoveries of supernovae and other transients using wide-field optical surveys like the Zwicky Transient Facility (ZTF), we’ve also found many different kinds of transients that we were not anticipating, many of which are spatially co-located with the centers of nearby, previously inactive galaxies. A subset of these have been identified as tidal disruption events (TDEs), which occur when a stray star enters the tidal radius of the SMBH it is orbiting and is shredded, causing an energetic flare that we typically discover in the optical. With only a few hundred TDEs identified, we are just beginning to understand how SMBH accretion works.
In addition to TDEs, a catch-all class of ambiguous nuclear transients (ANTs, e.g., this article) has been identified. It consists of many events that do not meet the traditional definition of a TDE. Their peak brightness (-21 to -26 optical magnitude) exceeds those of TDEs (-17 to 20 mag), and their optical flares can last for two years or more, as opposed to TDEs, which typically only remain observable in the optical for 100s of days. ANTs display optical spectra typical of a hot (10,000K or more) black body with hard X-ray emission. They also display spectral line features not typically seen in TDEs, e.g., Bowen fluorescence has been observed in several ANTs. Importantly, ANTs are distinct from active galactic nuclei (AGN), a class of SMBH that are accreting more material over longer timescales, so their light curves do not display the smooth rise and fall visible in ANTs and instead show stochastic variability associated with the accretion process. Today’s paper reports on a multi-wavelength observing campaign of the most energetic optical transient observed so far AT2021lwx/ZTF20abrbeie/”Barbie.”
Meet “Scary Barbie”
Named for its randomly selected identifier by the ZTF survey, AT2021lwx was discovered on November 10, 2020, and was initially classified as an AGN at z=0.995 nearly two years later on 2022 September 9. The authors of today’s paper used a custom transient-of-interest recommendation pipeline to identify AT2021lwx for further follow-up, of which they obtained seven epochs of optical spectroscopy. In all imaging, AT2021lwx notably does not display a host galaxy of any kind, and the redshift (and thus the inferred distance) is from the intrinsic transient’s spectrum. Given the distance, the peak optical luminosity is a record-breaking 1045.7 erg/second, beating the luminosity of any previously known optical transient!
The spectra do not evolve much over the 14-month time period they were collected. Figure 1 shows these spectra in the upper-left panel, a zoom-in on the highest-quality spectrum in the upper-right, and a comparison with other nuclear transients in the lower panels. The authors highlight that these spectra do not display emission lines that are typically present in AGN spectra like [Ne V] 342.6 nm, [O II] 372.7 nm, [O III] 495.9 nm & 500.7 nm, N II 464.0 nm, and He II 468.6 nm. Using these observations, the authors estimate that the optical flare’s peak luminosity is at least a factor of five greater than the quiescent emission from any pre-existing AGN.
Who invited Scary Barbie?
To determine the origin of AT2021lwx, the authors of today’s paper use a model that predicts the optical light curve from a TDE. Given the energy output, the light curve’s length, and some other simplifying assumptions, the accretion of about half a solar mass of material onto the SMBH within 400 days. Given this extreme accretion rate relative to the population of known AGN, the authors suggest that a TDE could provide the required supply of material to sustain such a significant luminosity for multiple years.
Using the light curve, the authors determine that AT2021lwx could be explained by the disruption of a ~14 solar mass star by a 1.7 x 108 solar mass SMBH. Figure 2 displays the light curves and the evolution of the pseudo-bolometric luminosity, the color, and the best-fitting black-body temperature. Further late-time observations of AT2021lwx are necessary to determine how the optical light curve is fading (either t^-5/3 or t^-4/3), which may help distinguish between models for the event, i.e., a t^-5/3 scaling is typical for the fallback of material from a TDE. However, AT2021lwx’s extreme luminosity exceeds the brightest known confirmed TDEs, e.g., the on-axis jetted TDE AT2022cmc, which has been accompanied by bright radio emission, which is so far absent in AT2021lwx.
An Orphaned Transient
The authors of today’s paper highlight that regardless of the model for the light curve and spectra of AT2021lwx, an essential component is the lack of host galaxy detection. It could be that the galaxy is just below the detection limit of the relatively shallow PanSTARRS survey, but other possibilities exist. The PanSTARRS observations do not exclude a galaxy as massive as 1010 solar masses. Still, such a large galaxy would have stopped star formation an unusually long time ago, or perhaps the galaxy is highly dust-enshrouded so it is not visible in the optical. Further observations of “Scary Barbie” and a multi-wavelength search for its host are needed to determine the origin of this extreme event.
Astrobite edited by Maria Vincent
Featured Image Credit: NASA, ESA, Leah Hustak (STScI)
Discover more from astrobites
Subscribe to get the latest posts to your email.
