Over the last few months, we witnessed a supernova about 22 million light-years away: SN 2024ggi. We thought, how can it get better than this?

A supernova is a pretty spectacular event, but a galaxy’s core awakening… we can only commiserate with the creatures inhabiting that galaxy… if there are any (left).

Over four years since December 2019, galaxy SDSS1335+0728 has continued to grow brighter, displaying variations unlike any previously seen in a galaxy. That’s astrophysicists’ slang for “we’ve never seen anything like it before”. This dramatic increase in brightness led a team to investigate the galaxy in question, using data from several space and ground-based observatories, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT). Not only did it increase in brightness in the visible spectra, but also ultraviolet, optical, and infrared wavelengths. In February 2024 it began emitting X-rays. When a galaxy emits X-rays, it generally indicates the presence of high-energy processes and phenomena within the galaxy. It can be either from Active Galactic Nuclei (AGN) or X-ray binaries, supernova remnants, hot gas in galaxy clusters or starburst regions.

Activation of a Massive Black Hole

The scientists who wrote the paper think it is the awakening of the galaxy’s supermassive black hole.

“Imagine you’ve been observing a distant galaxy for years, and it always seemed calm and inactive,” explains Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study. “Suddenly, its core starts showing dramatic changes in brightness, unlike any typical events we’ve seen before.”

This significant brightening classifies SDSS1335+0728 as having an ‘active galactic nucleus’ (AGN) — a bright, compact region powered by a supermassive black hole.

What does it mean: “a black hole awakens?”

When astronomers refer to a supermassive black hole’s “awakening,” they typically mean that the black hole, which was previously in a relatively dormant or inactive state, begins to exhibit significant activity. This activity is usually characterized by the black hole starting to accrete or pull in large amounts of matter from its surroundings. This process leads to several observable phenomena:

1. Increased Luminosity: As the black hole accretes matter, the infalling material forms an accretion disk around the black hole. The friction and gravitational forces in the accretion disk cause the material to heat up and emit radiation, often making the black hole and its surroundings much brighter across various wavelengths, including ultraviolet, optical, and X-rays.

2. Emission of High-Energy Radiation: The accretion process can produce significant amounts of high-energy radiation, including X-rays and gamma rays. This radiation is generated as the matter in the accretion disk becomes extremely hot and energetic.

3. Formation of Jets: Some active black holes can produce relativistic jets, which are narrow beams of charged particles that are ejected at nearly the speed of light from the regions close to the black hole. These jets can extend far beyond the host galaxy and are detectable in radio and sometimes optical and X-ray wavelengths.

4. Spectral Changes: The awakening of a black hole often leads to changes in the spectral lines of the host galaxy. These changes can include the broadening of emission lines and the appearance of new lines indicative of high-energy processes near the black hole.

5. Active Galactic Nucleus (AGN): The black hole becomes the central engine of an active galactic nucleus (AGN). AGNs are extremely luminous regions at the centre of some galaxies, powered by the accretion of matter onto the supermassive black hole. This activity can outshine the rest of the galaxy and is often used to classify galaxies into different types of AGNs, such as quasars, Seyfert galaxies, and blazars.

The awakening of a supermassive black hole is a significant event in astronomy because it provides insights into the processes of black hole growth and the evolution of galaxies. It also offers a unique opportunity to study high-energy astrophysical phenomena in real time.

Observational Findings in SDSS1335+0728

• UV and Optical Brightness: The galaxy’s UV flux has increased by a factor of four since 2004, and its optical flux has also shown stochastic variations for more than 1,550 days, decreasing slightly from its peak.

• Infrared Emissions: Mid-infrared flux has risen more than two times since June 2022, and the galaxy’s WISE colour has become redder, indicating significant changes.

• X-ray Emissions: Detected for the first time in February 2024, the galaxy’s X-ray emissions suggest the presence of a forming AGN corona.

Implications and Future Observations

The activation of SDSS1335+0728’s massive black hole provides valuable insights into how black holes grow and evolve.

“This process has never been observed before,” says co-author Lorena Hernández García. If confirmed, this event will mark the first real-time observation of a massive black hole’s activation. Continuous observations are crucial to confirm the exact nature of this phenomenon and rule out alternative explanations such as a slow tidal disruption event or a new class of nuclear transients.

Further Research

The study, presented in the paper “SDSS1335+0728: The awakening of a ∼ 10⁶ M⊙ black hole” published in Astronomy & Astrophysics, highlights the necessity of future observations to understand this unique event. Instruments like the upcoming Extremely Large Telescope (ELT) will play a key role in this ongoing research.

We simply hope they’ll find a better name for it, like SiD or something like that.

Collaborative Effort

This discovery results from collaborative efforts by astronomers from various institutions, showing the importance of international cooperation in astronomical research. The European Southern Observatory (ESO) continues to support scientists worldwide in uncovering the Universe’s secrets for the benefit of all.

References

This summary is based on the research article “SDSS1335+0728: The awakening of a ∼ 10⁶ M⊙ black hole” published in Astronomy & Astrophysics and available for detailed review here.