In a distant galaxy in the constellation Hercules, a gigantic black hole has torn apart a giant star. This is shown by extensive observations with several observatories, reported by an international, DESY-led research team in the journal Physical Review Letters. The cosmic catastrophe produced a glistening “dust echo” in the infrared range after several months. In addition, the neutrino telescope IceCube in Antarctica may have picked up a particle from the shredded star.

The so-called tidal disruption event (TDE) occurred in a galaxy 4.4 billion light years away, in the centre of which resides a black hole 35 million times the mass of our sun. A giant sun ventured too close and was torn to pieces by the tidal forces of the black hole. “The tidal forces arise because the black hole pulls harder on the front side of the star than on the back,” explains the paper's main author, Simeon Reusch from DESY. “This difference initially causes the giant sun to be stretched until it eventually ruptures.”

The stellar debris forms a so-called accretion disk around the black hole similar to the whirlpool of water in a bathtub when you pull the plug. Before the stellar matter finally disappears into oblivion, it circles the black hole faster and faster like the water above the bathtub drain, heating up enormously in the process and starting to glow brightly. “This tidal disruption event may even have been the most luminous transient cosmic phenomenon ever observed,” emphasises co-author Marek Kowalski from DESY. Because of the enormous brightness of the event, the researchers assume that the ruptured star must have been a giant sun so that enough matter could collect and finally glow on the accretion disk.

The intense radiation burned a cavity in the huge dust cloud surrounding the black hole. Within the range of about half a lightyear, the dust evaporated immediately. Beyond that point the radiation heated the dust intensely, which started to glow brightly in the infrared range. Due to the geometry of the light path, this dust echo reached its maximum only about a year after the demise of the giant star. “The dust echo in the infrared range is a key signature of the tidal disruption event,” Reusch reports. “This gave away the nature of this transient object.”

The phenomenon had first been observed with the Zwicky Transient Facility (ZTF) observatory, which specifically looks for such transient events. Astronomers then targeted the celestial position with numerous other instruments at different wavelengths, from radio waves to gamma radiation. Observations with the infrared satellite WISE of the US space agency NASA revealed the light echo about a year after the original eruption.

At the South Pole, the IceCube observatory also caught a high-energy neutrino that could have come from the event. This might have been the second time IceCube has picked up a particle from a shredded star – a success for the fledgling discipline of neutrino astronomy. “Neutrinos give us insights into cosmic objects that are not possible with light and other electromagnetic radiation,” explains Kowalski, head of the neutrino astronomy at DESY. “With electromagnetic radiation, we look at the surface of an object. Neutrinos, however, reach us unhindered from the interior.”

In particular, the combination of observations in the field of electromagnetic radiation with neutrinos enables new insights. The researchers call this multi-messenger astronomy because the messengers in the two fields, photons and neutrinos, are quite different in nature. For example, the measurements with radio waves showed that this phenomenon is a cosmic particle accelerator. The independent measurement of the neutrino supports this observation and points to it being an accelerator of protons (hydrogen nuclei), but not of the much lighter electrons. Further analyses of the measurement data from IceCube should provide more precise information.

The work involved 49 researchers from 30 institutions.

 

Reference:
Candidate Tidal Disruption Event AT2019fdr Coincident with a High-Energy Neutrino; Simeon Reusch et al.; Physical Review Letters, 2022; DOI: 10.1103/PhysRevLett.128.221101