The Instituto de Astrofísica de Andalucía (IAA-CSIC) contributed to this study by providing key data obtained with its ALFOSC spectrograph, installed on the Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory in La Palma.
This information complements data from NASA’s Neil Gehrels Swift Observatory and the ZTF project.
In recent years, the number of facilities capable of monitoring the entire sky and measuring variations in the brightness of stars and other astronomical objects has increased significantly. This has enabled the detection of more cases of objects whose behavior deviates from typical patterns, resulting in unusual phenomena known as exotic nuclear transient events. IAA-CSIC has collaborated on a study, published in Astronomy & Astrophysics, which sheds light on the nature of one of these peculiar events, named AT 2021hdr, whose behavior may be explained as a binary system of supermassive black holes accreting a gas cloud.
AT 2021hdr had previously displayed a stable luminosity until mid-2021, when it began to show strange behavior: its brightness would suddenly increase, creating peaks that varied over time, as if it were "oscillating." These fluctuations were detected by the Zwicky Transient Facility (ZTF) project and NASA's Swift Observatory. "We think that a gas cloud engulfed the black holes. As they orbit each other, the black holes interact with the cloud, perturbing and consuming its gas. This produces an oscillating pattern in the light from the system" explains Lorena Hernández-García, a researcher at the Millennium Institute of Astrophysics (MAS) and the Millennium Nucleus for Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), who led the study.
The contribution of the IAA-CSIC in optical spectroscopy – a technique that allows for the analysis of the light emitted by celestial objects – was crucial for this study. "Thanks to the data collected with IAA’s ALFOSC instrument, we confirmed that this source is located in a Seyfert 1 galaxy, known for its highly active and bright nucleus," notes Josefa Masegosa, a researcher at the Instituto de Astrofísica de Andalucía (IAA-CSIC) who participated in the study. "Although the brightness of this galaxy changes significantly over time, the spectral lines – 'fingerprints' of different chemical elements in its light – remain stable," she adds. This stability allows researchers to rule out that it is a "changing-look" galaxy, which is characterized by drastic changes in appearance and behavior over relatively short periods.
The ALFOSC spectrograph, installed on the Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory in La Palma. Credit: R. Rekola
WHEN ASTROPHYSICS AND ARTIFICIAL INTELLIGENCE GO HAND IN HAND
The AT 2021hdr phenomenon was first detected in March 2021 by ZTF, a project aimed at systematically searching for transient astronomical events. It was identified as a potential point of interest by ALeRCE (Automatic Learning for the Rapid Classification of Events), a software system using artificial intelligence tools to alert the astronomical community about occurrences in the night sky based on the massive data stream collected by survey programs like ZTF.
Hernández-García and her team have been monitoring the AT 2021hdr source using NASA’s Neil Gehrels Swift Observatory since November 2022. This enabled them to determine that the binary system produces ultraviolet and X-ray oscillations at the same intervals in which ZTF observes them in the visible range.
“Although this flare was originally thought to be a supernova, but outbursts in 2022 made us think of other explanations,” said co-author Alejandra Muñoz-Arancibia, an ALeRCE team member and astrophysicist at MAS and the Center for Mathematical Modeling at the University of Chile. “Each subsequent event has helped us refine our model of what’s going on in the system.”
After considering various hypotheses, the research team determined that the light variations recorded by Swift and ZTF are due to the interaction between the binary system and a gas cloud: intense tidal forces generated by the black holes began to dissolve the cloud, leading to intense heating of the gas, causing some of it to be absorbed and other parts to be ejected with each orbit.
A pair of monster black holes swirl in a cloud of gas in this artist’s concept of AT 2021hdr, a recurring outburst studied by NASA’s Neil Gehrels Swift Observatory and the ZTF Transient Facility at Palomar Observatory in California. Credit: NASA/Aurore Simonnet (Sonoma State University)
With the aim of better understanding the system and improving their models, Hernández-García and her team plan to continue observing AT 2021hdr and studying the galaxy in which it resides.
"Continuing to study how these objects vary across different wavelengths is crucial for determining the most appropriate physical models to explain these variations," concludes Isabel Márquez, deputy director of IAA-CSIC and scientific director of the Severo Ochoa project, who is also part of this study.