An international team of astronomers has discovered the cosmic equivalent of a regularly gushing geyser: a distant galaxy that erupts roughly every 114 days.
Using data from different space missions, scientists have studied 20 repeated bursts of an event called ASASSN-14ko. These various telescopes and instruments are sensitive to different wavelengths of light. Using them collaboratively, the scientists obtained more detailed images of the outbursts.
We believe that a supermassive black hole in the center of the galaxy creates the explosions as it partially consumes an orbiting giant star, « said Anna Payne, a NASA graduate fellow at the University of Hawaii at Manoa, who presented the findings at the 237th virtual meeting of the American Astronomical Society. An article on the source and these observations, led by Payne, is undergoing scientific review.
Astronomers classify galaxies with unusually bright and variable centers as active galaxies. These objects can produce much more energy than the combined contribution of all their stars, including higher-than-expected levels of visible, ultraviolet, and X-ray light.
Astrophysicists believe that the additional emission comes from near the galaxy’s central supermassive black hole, where a spinning disk of gas and dust collects and heats up due to gravitational and frictional forces. The black hole slowly consumes the material, creating random fluctuations in the light emitted by the disk.
But astronomers are interested in finding active galaxies with flares that occur at regular intervals, which could help them identify and study new phenomena and events.
ASASSN-14ko was first detected on November 14, 2014 by the All-Sky Automated Survey for Supernovae (ASAS-SN), a global network of 20 robotic telescopes based at Ohio State University (OSU) in Columbus. It occurred in ESO 253-3, an active galaxy more than 570 million light-years away in the southern constellation Pictor. At the time, astronomers thought the outburst was most likely a supernova, a one-time event that destroys a star.
Six years later, Payne was examining ASAS-SN data on known active galaxies as part of his thesis work. Looking at the ESO 253-3 light curve, or the graph of its brightness over time, he immediately noticed a series of evenly spaced flashes – a total of 17, all separated by about 114 days. Each flash reaches its maximum brightness in approximately five days, then steadily dims.
Payne and his colleagues predicted that the galaxy would flare up again on May 17, 2020, so they coordinated joint observations with ground and space facilities, including multi-wavelength measurements with Swift. ASASSN-14ko erupted just in time. Since then, the team has predicted and observed subsequent outbreaks on September 7 and December 20.
The researchers also used the TESS data for a detailed look at a previous outbreak. TESS observes swaths of the sky called sectors for about a month at a time. For the first two years of the mission, the cameras collected a full sector image every 30 minutes. These snapshots allowed the team to create an accurate timeline of a flash that began on November 7, 2018, tracking its appearance, increase to maximum brightness, and decrease in great detail.
Using measurements from ASAS-SN, TESS, Swift, and other observatories, including NASA’s NuSTAR and the European Space Agency’s XMM-Newton, Payne and his team gave three possible explanations for the repeated flares.
One scenario involved interactions between the disks of two supermassive black holes orbiting at the center of the galaxy. Recent measurements, also under scientific review, suggest that the galaxy does host two of those objects, but they do not orbit closely enough to explain the frequency of the flares.
The second scenario the team considered was a star passing in an inclined orbit through the disk of a black hole. In that case, scientists would expect to see asymmetrically shaped flares caused when the star disrupts the disk twice, on either side of the black hole. But all the flares in this galaxy have the same shape.
The third scenario, and the one the team believes is most likely, is a partial tidal disruption event.