Astronomers have used the telescopes at the Canary Islands Observatories on La Palma, including the Gran Telescopio Canarias (GTC) and the Nordic Optical Telescope (NOT), to study the evolution of an exceptionally fast and bright cosmic outburst in a small galaxy located 500 million light years away. The outburst, identified as CSS161010, reached its brightest peak in just 4 days and declined to half of this brightness in only 2.5 days, making its discovery and subsequent observations of its evolution particularly challenging.
Astronomers have used the telescopes at the Canary Islands Observatories on La Palma, including the Gran Telescopio Canarias (GTC) and the Nordic Optical Telescope (NOT), to study the evolution of an exceptionally fast and bright cosmic outburst in a small galaxy located 500 million light years away. The outburst, identified as CSS161010, reached its brightest peak in just 4 days and declined to half of this brightness in only 2.5 days, making its discovery and subsequent observations of its evolution particularly challenging
The event was discovered by the Catalina Real-Time Transient Survey, with an earlier detection reported by the All-Sky Automated Survey for SuperNovae. Thanks to the wide field of view and high-cadence imaging capabilities of modern time-domain surveys, astronomers have recently started identifying such rare and rapidly evolving cosmic events.
To date, astronomers have detected only around ten cosmic explosions with this type of brightness and evolution, but their origin is still a complete mystery. However, the research team believes that for the first time, the unprecedented spectral properties of CSS161010 give important clues about its physical origin, and their analysis suggests it was the result of a small black hole engulfing a star. Very broad spectral lines of hydrogen were observed to show high velocities of up to 10% of the speed of light and an unprecedented evolution. Two months after the outburst began, the object's brightness had diminished by a factor of 900 from its peak. Surprisingly, the GTC spectra at this epoch revealed that the entire spectral line profiles were still blueshifted from the rest wavelength — an observation never seen before in any other cosmic explosion
throughout its evolution. This indicates strong outflows of gas and is completely unexpected for any supernova.
"Discovering and analysing these cosmic explosions is particularly challenging due to theirrapidly evolving nature. However, our collaboration's fast response enabled us to obtain high-quality spectra. These data revealed unique properties never observed in any other transient, allowing us to constrain the nature of this extraordinary event," explains Dr Claudia Gutiérrez, a research fellow at the Institut d'Estudis Espacials de Catalunya (IEEC) and the Institute of Space Sciences (ICE-CSIC) in Spain. Dr. Gutiérrez is the lead author of the paper "CSS161010: A Luminous Fast Blue Optical Transient with Broad Blueshifted Hydrogen Lines," recently published in the Astrophysical Journal.
“When we saw the spectra, we didn't know what to say," Dr. Gutiérrez tells. "We had never encountered a hydrogen line profile so blueshifted before. It was both surprising and intriguing, prompting us to investigate potential connections with the galaxy where the event occurred."
The outburst occurred in a tiny galaxy which contains a mass of stars about 400 times lower than in our Milky Way. Therefore, If the galaxy hosts a massive black hole also its mass must be small corresponding to a so-called intermediate mass black hole.
“So far, such black holes have been extremely difficult to identify and astronomers know only a handful of confirmed cases” explains Professor Seppo Mattila, from the University of Turku in Finland, who is one of the lead authors of the paper.
“Identifying and characterizing intermediate-mass black holes is essential for understanding the formation pathways and evolution of black holes. These black holes serve as fundamental building blocks for the supermassive black holes found at the centers of galaxies, such as our Milky Way, and are observed to exist even in the early Universe” Prof. Mattila continues.
Professor Peter Lundqvist from the Stockholm University adds: -”the way the line emission evolves in this object resembles that seen in Active Galactic Nuclei, where supermassive black holes are known to exist. This similarity provides strong evidence that CSS161010 also harbours a black hole, though not very massive. The disruption of a star that came too close to the intermediate-mass black hole revealed the otherwise quiescent black hole. There are likely other such black holes in other dwarf galaxies, and we need to keep track of events like CSS161010 to constrain the properties of these black holes.”
“Upcoming high-cadence surveys will be pivotal in uncovering more of these rare, fast-evolving events,” Dr. Gutiérrez remarked. “Meanwhile, new spectrographs will play a crucial role in characterizing them. We are on the cusp of an era filled with groundbreaking discoveries.”
The study has been published in The Astrophysical Journal
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