Scientists have only recently encountered this class of X-ray flares, called QPEs, or quasi-periodic eruptions. A system astronomers have nicknamed Ansky is the eighth QPE source discovered, and it produces the most energetic outbursts seen to date. Ansky also sets records in terms of timing and duration, with eruptions every 4.5 days or so that last approximately 1.5 days.
"These QPEs are mysterious and intensely interesting phenomena," said Joheen Chakraborty, a graduate student at the Massachusetts Institute of Technology in Cambridge. "One of the most intriguing aspects is their quasi-periodic nature. We're still developing the methodologies and frameworks we need to understand what causes QPEs, and Ansky's unusual properties are helping us improve those tools."
Ansky's name comes from ZTF19acnskyy, the moniker of a visible-light outburst seen in 2019. It was located in a galaxy about 300 million light-years away in the constellation Virgo. This event was the first indication that something unusual might be happening.
A paper about Ansky, led by Chakraborty, was published Tuesday in The Astrophysical Journal.
A leading theory suggests that QPEs occur in systems where a relatively low-mass object passes through the disk of gas surrounding a supermassive black hole that holds hundreds of thousands to billions of times the Sun's mass.
When the lower-mass object punches through the disk, its passage drives out expanding clouds of hot gas that we observe as QPEs in X-rays.
Scientists think the eruptions' quasi-periodicity occurs because the smaller object's orbit is not perfectly circular and spirals toward the black hole over time. Also, the extreme gravity close to the black hole warps the fabric of space-time, altering the object's orbits so they don't close on themselves with each cycle. Scientists' current understanding suggests the eruptions repeat until the disk disappears or the orbiting object disintegrates, which may take up to a few years.
"Ansky's extreme properties may be due to the nature of the disk around its supermassive black hole," said Lorena Hernandez-Garcia, an astrophysicist at the Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes, the Millennium Institute of Astrophysics, and University of Valparaiso in Chile. "In most QPE systems the supermassive black hole likely shreds a passing star, creating a small disk very close to itself. In Ansky's case, we think the disk is much larger and can involve objects farther away, creating the longer timescales we observe."
Hernandez-Garcia, in addition to being a co-author on Chakraborty's paper, led the study that discovered Ansky's QPEs, which was published in April in Nature Astronomy and used data from NICER, NASA's Neil Gehrels Swift Observatory and Chandra X-ray Observatory, as well as ESA's (European Space Agency's) XMM-Newton space telescope.
NICER's position on the International Space Station allowed it to observe Ansky about 16 times every day from May to July 2024. The frequency of the observations was critical in detecting the X-ray fluctuations that revealed Ansky produces QPEs.
Chakraborty's team used data from NICER and XMM-Newton to map the rapid evolution of the ejected material driving the observed QPEs in unprecedented detail by studying variations in X-ray intensity during the rise and fall of each eruption.
The researchers found that each impact resulted in about a Jupiter's worth of mass reaching expansion velocities around 15% of the speed of light.
The NICER telescope's ability to frequently observe Ansky from the space station and its unique measurement capabilities also made it possible for the team to measure the size and temperature of the roughly spherical bubble of debris as it expanded.
"All NICER's Ansky observations used in these papers were collected after the instrument experienced a 'light leak' in May 2023," said Zaven Arzoumanian, the mission's science lead at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Even though the leak - which was patched in January - affected the telescope's observing strategy, NICER was still able to make vital contributions to time domain astronomy, or the study of changes in the cosmos on timescales we can see."
After the repair, NICER continued observing Ansky to explore how the outbursts have evolved over time. A paper about these results, led by Hernandez-Garcia and co-authored by Chakraborty, is under review.
Observational studies of QPEs like Chakraborty's will also play a key role in preparing the science community for a new era of multimessenger astronomy, which combines measurements using light, elementary particles, and space-time ripples called gravitational waves to better understand objects and events in the universe.
One goal of ESA's future LISA (Laser Interferometer Space Antenna) mission, in which NASA is a partner, is to study extreme mass-ratio inspirals - or systems where a low-mass object orbits a much more massive one, like Ansky. These systems should emit gravitational waves that are not observable with current facilities. Electromagnetic studies of QPEs will help improve models of those systems ahead of LISA's anticipated launch in the mid-2030s.
"We're going to keep observing Ansky for as long as we can," Chakraborty said. "We're still in the infancy of understanding QPEs. It's such an exciting time because there's so much to learn."
Research Report:Rapidly Varying Ionization Features in a Quasi-periodic Eruption: A Homologous Expansion Model for the Spectroscopic Evolution
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