Astronomers have assembled the most detailed timeline to date of the aftermath of a star being torn apart by a supermassive black hole. Using a network of powerful radio telescopes, researchers tracked the event, known as AT2019dsg, for more than 500 days, capturing the evolution of the material violently ejected into space and providing a unique laboratory for studying the physics of these extreme encounters.
This sustained observational campaign has offered an unprecedented view of the expanding outflow of stellar debris. The data reveal a blast wave of material moving at a fraction of the speed of light, allowing scientists to measure its speed, energy, and evolution. These findings are helping to refine models of tidal disruption events (TDEs) and are challenging previous interpretations, particularly a tantalizing but now questioned association between this event and the detection of a high-energy subatomic particle.
A Long-Term Observational Campaign
The discovery of AT2019dsg in April 2019 by an optical sky survey triggered a rapid response from the astronomical community. While optical and X-ray telescopes captured the initial brilliant flash of the star’s destruction, radio telescopes provided the crucial long-term perspective. The primary instruments for this follow-up were the Karl G. Jansky Very Large Array (VLA) in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. This effort marked the first time a tidal disruption event was successfully detected with ALMA.
The extensive series of observations began 55 days after the initial discovery and continued for 560 days. This long baseline was essential for tracking the physical changes in the outflow as it expanded and cooled. Radio waves from these events are generated by synchrotron emission, which occurs when electrons, accelerated to high speeds by the explosion, spiral around magnetic field lines. By monitoring the changing brightness and spectrum of this radio light over time, researchers could reverse-engineer the properties of the material ejected by the black hole.
Mapping the Stellar Outflow
The comprehensive radio data allowed scientists to paint a detailed picture of the material emanating from the disrupted star. Unlike the tightly focused, ultra-fast jets seen in a small fraction of TDEs, AT2019dsg produced a more common, wide-angled outflow of gas. This material acts like a spherical piston, plowing through the ambient medium that surrounded the black hole before the event.
An Expanding Shell of Gas
As the researchers collected data across a wide range of radio frequencies, they were able to watch the emission’s peak brightness first increase and then fade. The peak occurred around 200 days after the disruption. This evolution is characteristic of an expanding, self-absorbing synchrotron source. Initially, the cloud of debris is so dense that it is opaque to its own radio waves. As it expands, it becomes more transparent, allowing more emission to escape and causing the apparent brightness to increase. Eventually, the expansion causes the cloud to dim as its energy dissipates. By modeling this process, the team could directly calculate the radius and energy of the expanding shell of stellar material.
Sub-Relativistic Speeds
A key finding from the analysis is the speed of the outflow. The data show the material is expanding at approximately 0.07 times the speed of light, or about 21,000 kilometers per second. While incredibly fast by terrestrial standards, this is considered non-relativistic in astrophysical terms. The velocity and the kinetic energy derived from the observations are broadly typical of other non-jetted TDEs and are comparable to the energy released in certain types of supernova explosions, specifically Type Ib/c supernovae.
The Energetics of a Cosmic Meal
The prolonged monitoring revealed how the energy of the event evolved. An equipartition analysis, which assumes a balance between the energy stored in the magnetic fields and the energetic particles, showed that the kinetic energy of the outflow grew significantly during the early phases of observation. It increased by a factor of about five between day 55 and day 200, after which it plateaued at a relatively steady level. This suggests that the central black hole was not continuously powering the outflow over a long period. Instead, it points toward a scenario where the energy was imparted to the material in a single, powerful ejection event around the time the star was first disrupted.
This interpretation favors a model where the observed radio light comes from the interaction of this ejected plasma with the surrounding medium. As the ejecta slams into ambient gas, it creates a powerful shock wave that accelerates particles and generates the observed synchrotron emission. This process is analogous to the formation of a supernova remnant. While a model involving a persistent, jet-like outflow from the black hole’s accretion disk cannot be entirely ruled out, it would require significant fine-tuning to match the observations.
The Neutrino Question
One of the most intriguing aspects of AT2019dsg was its potential connection to a high-energy neutrino detected by the IceCube Neutrino Observatory in Antarctica. TDEs have long been theorized as potential cosmic particle accelerators capable of producing such elusive particles. The initial spatial coincidence made AT2019dsg a prime candidate for the first TDE with an associated neutrino. This would suggest the presence of a relativistic jet—a powerful engine capable of accelerating particles to near the speed of light.
However, the detailed radio follow-up presents a major challenge to this association. The observed outflow is relatively slow and its energy is “ordinary” when compared to other TDEs. To produce the observed radio emission with a truly relativistic jet, the jet would need to be aimed almost perfectly at Earth and have an opening angle of only a few degrees, which is considered physically improbable. The measured properties of the outflow are more consistent with a standard, non-relativistic event, which is not expected to be an efficient neutrino factory. This discrepancy has led the radio astronomy team to express doubts about the claimed association, suggesting the neutrino’s arrival may have been a cosmic coincidence rather than a direct product of this stellar death.
