Astrophysics: Black hole switches on the radiator
The observers were actually looking for a kilonova. Instead, they discovered signals from a black hole that could quickly devour large quantities of matter.
Ten billion light years away, a supermassive black hole called J221951-484240 has undergone a remarkable transformation: In one of the "most dramatic events" of its kind that astronomers have observed so far, the black hole lit up extremely brightly, as Samantha Oates of the University of Birmingham and her team report in the Monthly Notices of the Royal Astronomical Society. It is one of the most luminous processes, known as transients, known to research to date, the team writes.
The team discovered the event in September 2019 while searching for the electromagnetic light of a gravitational wave event. The team used various instruments on board the Neil Gehrels Swift Space Telescope to search for a kilonova, the merger of a neutron star with another neutron star or black hole. A kilonova usually appears blue, then fades and turns red over a period of a few days. Instead, they found something even more unusual: J221951. The luminous event appeared blue, but then neither changed colour nor faded as quickly as a kilonova.
Oates and co then continued to observe J221951-484240 with the Hubble and ESO's Very Large Telescope, among others, to find out more. Spectral data from Hubble, for example, showed that it could not be a gravitational wave event that ultimately originated 500 million light years away. Instead, the group traced it back to J221951-484240 at a much greater distance.
According to the measurements, the trigger is a supermassive black hole that fed very quickly on the surrounding matter. A galaxy was observed in this area before the discovery of J221951. In addition, the position of J221951 coincides with the centre of the galaxy, where a massive black hole would normally be located. The black hole began to glow very quickly after it had been quiet for some time - retrospectively ten months before the actual discovery, as the data analysis revealed. The ultraviolet spectrum shows absorption features that indicate material that was pushed outwards by an enormous release of energy.
Two mechanisms are possible: either the event is due to a so-called tidal disruption, when a star approaches the black hole in the centre of its galaxy and is torn apart as a result. Or it was caused by a galactic nucleus that changed from a dormant to an active state. J221951 would then be the signal that a dormant black hole has begun to feed on material from an accretion disc that had approached close enough. Further observations should help to clarify the actual cause.
Spectrum of Science
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Cover picture: M. Kornmesser / Eso
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