The centers of galaxies with actively feeding supermassive black holes are already astounding environments. Now, a team of researchers led by a graduate student from the University of Hawaiʻi Institute for Astronomy (IfA) has found an even more interesting oddball. Feeding black holes typically increase and decrease in brightness similar to the Kilauea volcano, becoming more or less active over time in unpredictable ways. However, the newly discovered black hole is more like Old Faithful geyser at Yellowstone National Park, erupting repeatedly at predictable times.
Astronomers classify galaxies with unusually bright and variable centers as active galaxies. The active centers can produce much more energy than the combined contribution of all the stars in the host galaxy. The excess energy can be seen at visible, ultraviolet, and X-ray wavelengths. Astrophysicists think the extra emission comes from near the galaxy’s central supermassive black hole, where a swirling disk of gas and dust accumulates and heats up because of gravitational and frictional forces. The black hole slowly consumes the material, which usually creates low-level, random changes in the disk’s emitted light.
However, a repeatedly flaring galaxy was first detected in 2014 by the All Sky Automated Survey for Supernovae (ASAS-SN) 14-cm telescope at Haleakalā Observatory, Maui. Headquartered at Ohio State University (OSU), ASAS-SN scans the entire sky every night using telescopes around the world, looking for exploding stars, stellar flares, and comets. The galaxy, ESO 253-3, is a known active galaxy over 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.
The periodic flares then went undetected until 2020 when NASA Graduate Fellow Anna Payne at the IfA discovered them in the ASAS-SN data as part of her doctoral thesis studying active galactic nuclei (AGN). She found that the center of the galaxy ESO 253-3 periodically emits a bright flare, every 114 days. Periodic flares in active galaxies have been sought previously but never before been detected this unambiguously and found to occur so often.
“This is the first clear example of recurring multiwavelength flares from a galaxy’s core that happen this frequently,” said Payne.
An international group of astronomers, led by the UH team, used several other observatories to further reveal the complexity of these flares. Their study combined data from ASAS-SN, UH’s Asteroid Terrestrial-impact Last Alert System, NASA’s Neil Gehrels Swift Observatory, NASA’s Transiting Exoplanet Survey Satellite, the Las Cumbres Observatory (including telescopes at Haleakalā Observatory), and amateur astronomers from around the world. Payne’s analysis found that the flares happen not only in the visible wavelengths, but also at higher energy X-ray and ultraviolet wavelengths.
An accompanying study by fellow UH Ph.D. student and DOE Computational Science Graduate Fellow Michael Tucker used measurements from the Very Large Telescope in Chile to uncover that the host galaxy surrounding ASASSN-14ko adds even more complexity to the puzzle. Tucker found that the surrounding galaxy has intricate gas motions resulting from the merger of two separate galaxies. “Galaxy mergers are rare but incredible events occurring throughout cosmic history. When these mergers happen, they stir up the orbits of stars inside the galaxies and really make a mess of the place.”
What is causing ASASSN-14ko? The best explanation is that ASASSN-14ko is the first known case of a star being slowly ripped apart by the supermassive black hole at the center of ESO 253-3. Astronomers call these tidal disruption events (TDEs), where the gravitational forces create intense tides that break the star apart into a stream of gas. The trailing part of the stream escapes the system, while the leading part swings back around the black hole. Astronomers see bright flares from these events when the stripped gas strikes the black hole’s accretion disk.
A handful of single events have been seen, but astronomers have never seen one repeat. In a typical TDE, the entire star is destroyed in a single close pass by the black hole. However, if the star keeps enough distance from the black hole it will survive the first pass and produce a partial TDE.
“If a giant star with a puffy envelope wanders close, but not too close, on a very elongated orbit, then the black hole can steal some of the outer material without ripping apart the entire star.” says IfA Astronomer Benjamin Shappee, who is thesis advisor to both Payne and Tucker. “In that case, the giant star will just keep returning again and again until the star is exhausted.” In the case of ASASSN-14ko, every encounter strips away an amount of gas equal to about three times the mass of Jupiter.
Because these flares are predictable, Payne and her team can plan observations in advance. They are now examining observations of the most recent flare, which happened just before Christmas 2020, using Keck Observatory on Muanakae, the Hubble space telescope, TESS, 3 NASA X-ray telescopes, and a number of other ground-based observatories. This team includes a Hawaiʻi Student/Teacher Astronomy Research (HI STAR) high school student from Maui who will be analyzing additional data on new flares, taken at Las Cumbres Observatory as part of their state science fair project.
Payne predicts that the next flares will happen in April and August this year, giving more opportunities to study this unique extragalactic system. “Unlike singular TDEs, ASASSN-14ko is extremely forgiving. It allows us to plan for future outbursts, knowing when they will occur, which enables us to get data on these fascinating events that is simply not possible for their one-and-done cousins. ASASSN-14ko promises to be the gift that keeps on giving.”
ASAS-SN is supported by Las Cumbres Observatory and funded in part by the Gordon and Betty Moore Foundation, the National Science Foundation, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at Ohio State, the Chinese Academy of Sciences South American Center for Astronomy and the Villum Fonden in Denmark.
Founded in 1967, the Institute for Astronomy at the University of Hawaiʻi at Mānoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakalā and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaiʻi.