Press Release

Astronomers see a massive black hole awaken in real time

18 June 2024

In late 2019 the previously unremarkable galaxy SDSS1335+0728 suddenly started shining brighter than ever before. To understand why, astronomers have used data from several space and ground-based observatories, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to track how the galaxy’s brightness has varied. In a study out today, they conclude that they are witnessing changes never seen before in a galaxy — likely the result of the sudden awakening of the massive black hole at its core.

Imagine you’ve been observing a distant galaxy for years, and it always seemed calm and inactive,” says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study accepted for publication in Astronomy & Astrophysics. “Suddenly, its [core] starts showing dramatic changes in brightness, unlike any typical events we've seen before.” This is what happened to SDSS1335+0728, which is now classified as having an ‘active galactic nucleus’ (AGN) — a bright compact region powered by a massive black hole — after it brightened dramatically in December 2019 [1].

Some phenomena, like supernova explosions or tidal disruption events — when a star gets too close to a black hole and is torn apart — can make galaxies suddenly light up. But these brightness variations typically last only a few dozen or, at most, a few hundreds of days. SDSS1335+0728 is still growing brighter today, more than four years after it was first seen to ‘switch on’. Moreover, the variations detected in the galaxy, which is located 300 million light-years away in the constellation Virgo, are unlike any seen before, pointing astronomers towards a different explanation.

The team tried to understand these brightness variations using a combination of archival data and new observations from several facilities, including the X-shooter instrument on ESO’s VLT in Chile’s Atacama Desert [2]. Comparing the data taken before and after December 2019, they found that SDSS1335+0728 is now radiating much more light at ultraviolet, optical, and infrared wavelengths. The galaxy also started emitting X-rays in February 2024. “This behaviour is unprecedented,” says Sánchez Sáez, who is also affiliated with the Millennium Institute of Astrophysics (MAS) in Chile.

The most tangible option to explain this phenomenon is that we are seeing how the [core] of the galaxy is beginning to show (...) activity,” says co-author Lorena Hernández García, from MAS and the University of Valparaíso in Chile. “If so, this would be the first time that we see the activation of a massive black hole in real time.

Massive black holes — with masses over one hundred thousand times that of our Sun — exist at the centre of most galaxies, including the Milky Way. “These giant monsters usually are sleeping and not directly visible,” explains co-author Claudio Ricci, from the Diego Portales University, also in Chile. “In the case of SDSS1335+0728, we were able to observe the awakening of the massive black hole, [which] suddenly started to feast on gas available in its surroundings, becoming very bright.

[This] process (...) has never been observed before,” Hernández García says. Previous studies reported inactive galaxies becoming active after several years, but this is the first time the process itself — the awakening of the black hole — has been observed in real time. Ricci, who is also affiliated with the Kavli Institute for Astronomy and Astrophysics at Peking University, China, adds: “This is something that could happen also to our own Sgr A*, the massive black hole (...) located at the centre of our galaxy," but it is unclear how likely this is to happen. 

Follow-up observations are still needed to rule out alternative explanations. Another possibility is that we are seeing an unusually slow tidal disruption event, or even a new phenomenon. If it is in fact a tidal disruption event, this would be the longest and faintest such event ever observed. “Regardless of the nature of the variations, [this galaxy] provides valuable information on how black holes grow and evolve,” Sánchez Sáez says. “We expect that instruments like [MUSE on the VLT or those on the upcoming Extremely Large Telescope (ELT)] will be key in understanding [why the galaxy is brightening].”  


[1] The SDSS1335+0728 galaxy’s unusual brightness variations were detected by the Zwicky Transient Facility (ZTF) telescope in the US. Following that, the Chilean-led Automatic Learning for the Rapid Classification of Events (ALeRCE) broker classified SDSS1335+0728 as an active galactic nucleus.

[2] The team collected archival data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX), the Two Micron All Sky Survey (2MASS), the Sloan Digital Sky Survey (SDSS), and the eROSITA instrument on IKI and DLR’s Spektr-RG space observatory. Besides ESO’s VLT, the follow-up observations were conducted with the Southern Astrophysical Research Telescope (SOAR), the W. M. Keck Observatory, and NASA’s Neil Gehrels Swift Observatory and Chandra X-ray Observatory.

More information

This research was presented in a paper entitled “SDSS1335+0728: The awakening of a ∼ 106M⊙ black hole” published in Astronomy & Astrophysics (

The team is composed of P. Sánchez-Sáez (European Southern Observatory, Garching, Germany [ESO] and Millenium Institute of Astrophysics, Chile [MAS]), L. Hernández-García (MAS and Instituto de Física y Astronomía, Universidad de Valparaíso, Chile [IFA-UV]), S. Bernal (IFA-UV and Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes, Chile [TITANS]), A. Bayo (ESO), G. Calistro Rivera (ESO and German Space Agency [DLR]), F. E. Bauer (Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Chile; Centro de Astroingeniería, Pontificia Universidad Católica de Chile, Chile; MAS; and Space Science Institute, USA), C. Ricci (Instituto de Estudios Astrofísicos, Universidad Diego Portales, Chile [UDP] and Kavli Institute for Astronomy and Astrophysics, China), A. Merloni (Max-Planck-Institut für Extraterrestrische Physik, Germany [MPE]), M. J. Graham (California Institute of Technology, USA), R. Cartier (Gemini Observatory, NSF National Optical-Infrared Astronomy Research Laboratory, Chile, and UDP), P. Arévalo (IFA-UV and TITANS), R.J. Assel (UDP), A. Concas (ESO and INAF - Osservatorio Astrofisico di Arcetri, Italy), D. Homan (Leibniz-Institut für Astrophysik Potsdam, Germany [AIP]), M. Krumpe (AIP), P. Lira (Departamento de Astronomía, Universidad de Chile, Chile [UChile], and TITANS), A. Malyali (MPE), M. L. Martínez-Aldama (Astronomy Department, Universidad de Concepción, Chile), A. M. Muñoz Arancibia (MAS and Center for Mathematical Modeling, University of Chile, Chile [CMM-UChile]), A. Rau (MPE), G. Bruni (INAF - Institute for Space Astrophysics and Planetology, Italy), F. Förster (Data and Artificial Intelligence Initiative, University of Chile, Chile; MAS; CMM-UChile; and UChile), M. Pavez-Herrera (MAS), D. Tubín-Arenas (AIP), and M. Brightman (Cahill Center for Astrophysics, California Institute of Technology, USA).

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society. 



Paula Sánchez Sáez
European Southern Observatory (ESO)
Garching bei München, Germany
Tel: +49 89 3200 6580

Lorena Hernández García
Millennium Institute of Astrophysics (MAS)
Santiago, Chile

Claudio Ricci
Diego Portales University
Santiago, Chile

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 00

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About the Release

Release No.:eso2409
Type:Local Universe : Galaxy : Activity : AGN
Local Universe : Galaxy : Component : Central Black Hole
Facility:Very Large Telescope


This artist’s impression shows a ring of dark yellow clouds surrounding a brighter yellow region, with an especially bright white dot at its centre, on a dark background. Bright yellow, red, white, and blue stars dot the image.
Artist’s impression: the galaxy SDSS1335+0728 lighting up
This image is made up of two artist’s impressions. The top image shows a wispy spiral structure around a yellow central region, all surrounded by hazy red clouds and on a dark reddish background. The one at the bottom shows a similar structure, but the central region is now brighter with a white central point. The material surrounding it has become a ring of dark red clouds.
Artist’s impression: the black hole at the centre of the galaxy SDSS1335+0728 awakens


Zooming into the galaxy SDSS1335+0728 and its newly awakened black hole
Zooming into the galaxy SDSS1335+0728 and its newly awakened black hole
Artist’s animation of the black hole at the centre of SDSS1335+0728 awakening in real time
Artist’s animation of the black hole at the centre of SDSS1335+0728 awakening in real time