An active galactic nucleus in the center of ESO 253-G003, an active galaxy over 570 million light-years away in the southern constellation of Pictor, erupts roughly every 114 days, according to a paper to be published in the Astrophysical Journal.
A supermassive black hole siphons gas off of an orbiting giant star. Image credit: NASA’s Goddard Space Flight Center / Chris Smith, USRA & GESTAR.
ASASSN-14ko was first detected on November 14, 2014, by the All-Sky Automated Survey for Supernovae (ASAS-SN).
At the time, astronomers thought the event in ESO 253-G003 was most likely a supernova, a one-time event that destroys a star.
“ASASSN-14ko is currently our best example of periodic variability in an active galaxy, despite decades of other claims, because the timing of its flares is very consistent over the six years of data,” said Dr. Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center who was not involved in the study.
“This result is a real tour de force of multiwavelength observational astronomy.”
“We think a supermassive black hole at the galaxy’s center creates the bursts as it partially consumes an orbiting giant star,” said lead author Anna Payne, an astronomer at the University of Hawai’i at Mānoa.
Looking at the ESO 253-3 light curve, or the graph of its brightness over time, Payne and colleagues noticed a series of 17 flares, all separated by about 114 days. Each flare reaches its peak brightness in about five days, then steadily dims.
The astronomers predicted that the galaxy would flare again on May 17, 2020, so they coordinated joint observations with ground- and space-based facilities, including multiwavelength measurements with NASA’s Neil Gehrels Swift Observatory. ASASSN-14ko erupted right on schedule.
They have since predicted and observed subsequent flares on September 7 and December 20, 2020.
They also used data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to create a precise timeline of a flare that began on November 7, 2018, tracking its emergence, rise to peak brightness, and decline in great detail.
Using measurements from ASAS-SN, TESS, Swift and other observatories, including NASA’s NuSTAR and ESA’s XMM-Newton observatories, the researchers came up with three possible explanations for the repeating flares.
One scenario involved interactions between the disks of two orbiting supermassive black holes at the center of ESO 253-G003.
Recent measurements suggest the galaxy does indeed host two such objects, but they don’t orbit closely enough to account for the frequency of the flares.
“There is evidence that a second supermassive black hole exists in that galaxy,” said co-author Dr. Chris Kochanek, an astronomer at Ohio State University.
“The galaxy that hosts this object is something of a ‘trainwreck’ consisting of two galaxies in the process of merging into one.”
The second scenario the scientists considered was a star passing on an inclined orbit through a black hole’s disk. In that case, they would expect to see asymmetrically shaped flares caused when the star disturbs the disk twice, on either side of the black hole. But the flares from this galaxy all have the same shape.
The third scenario, and the one the team thinks most likely, is a partial tidal disruption event.
In this case, one of the galaxy’s supermassive black holes, one with about 78 million times the Sun’s mass, partially disrupts an orbiting giant star.
The star’s orbit isn’t circular, and each time it passes closest to the black hole, it bulges outward, shedding mass but not completely breaking apart.
Every encounter strips away an amount of gas equal to about three times the mass of Jupiter.
“We don’t know how long the flares will persist,” the authors said.
“The star can’t lose mass forever, and while we can estimate the amount of mass it loses during each orbit, they don’t know how much it had before the disruptions began.”
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Anna V. Payne et al. 2020. ASASSN-14ko is a Periodic Nuclear Transient in ESO 253-G003. ApJ, in press; arXiv: 2009.03321
