A star’s fatal brush with a supermassive black hole can light up a quiet galaxy. When gravity shreds the star, its gas spirals inward and briefly glows. Astronomers call these flares tidal disruption events, or TDEs. They unfold over months or years, letting you watch black holes feed and, at times, fire jets of matter into space.
One event, known as AT2020afhd, has now delivered an unusually clear view of a predicted effect from general relativity. In this case, the inner gas disk and a newborn jet appear to wobble together. Their motion repeats on the same schedule. The result offers rare evidence that a spinning black hole can twist nearby spacetime and force its surroundings to precess in unison.
AT2020afhd lies in the heart of the galaxy LEDA 145386. The system sits at a redshift of 0.027. The Zwicky Transient Facility first noticed it in 2020 as a faint optical flare. Years later, in early January 2024, the source suddenly brightened again. This rebrightening marked the start of a detailed observing campaign across the spectrum.
Optical data showed a blue glow with strong helium and hydrogen emission lines. These features matched those seen when a star is torn apart. The light then faded at a rate expected for falling stellar debris. From this behavior, researchers estimated the black hole’s mass at about five million times that of the Sun.
First theorized by Einstein in 1913 and then mathematically defined by Lense and Thirring in 1918, the observation confirms a general relativity prediction, offering scientists new avenues for studying black hole spin, accretion physics, and jet formation.
Dr. Cosimo Inserra, a Reader in the School of Physics and Astronomy at Cardiff University and one of the paper’s co-authors, told The Brighter Side of News, “Our work offers the strongest evidence so far of Lense–Thirring precession, where a black hole drags spacetime much like a spinning top stirs water in a whirlpool.”
“It confirms a prediction Einstein made more than a century ago and gives us new insight into tidal disruption events, when a star is torn apart by a black hole’s gravity,” he continued.
“Unlike past events with steady radio signals, AT2020afhd showed rapid variations we couldn’t tie to the usual energy sources, strengthening our view of the dragging effect and giving scientists a new way to study black holes,” he concluded.
Space telescopes soon turned toward the flare. The Neil Gehrels Swift Observatory began monitoring in late January 2024. Its instruments detected dramatic swings in x-ray brightness over weeks. The peaks rose more than ten times above the lows. Follow-up data from the NICER observatory confirmed the pattern.
Early on, the x rays were very soft. They came from a hot inner disk whose temperature rose and fell with brightness. This close link suggested that the disk responded directly to changes in how gas flowed inward.
After about seven months, the signal changed. The x-ray light curve settled into a repeating rhythm. Brightness rose and fell every 19.6 days. The swing remained large, often more than an order of magnitude. Statistical tests showed this cycle stood well above random noise.
During this phase, the disk temperature followed the same beat. It warmed at each peak and cooled at each dip. Optical and ultraviolet light, however, showed no matching short-term pattern. Those bands continued a slow decline. This contrast hinted that x rays came from deep near the black hole, while other light formed farther out.
Radio telescopes added a second piece to the puzzle. Just days after the x-ray flare, the Karl G. Jansky Very Large Array detected a compact radio source. Earlier surveys had shown no radio activity from this galaxy. The new signal pointed to a fresh outflow, likely a jet tied to the stellar disruption.
Over the next months, observatories around the world tracked the radio emission. They saw strong changes on timescales of weeks. The brightness sometimes jumped or dropped by more than a factor of four. The radio spectrum also evolved, shifting from thick to thin emission as the outflow expanded.
High-resolution observations confirmed the source stayed compact. The signal did not break into extended pieces. Careful checks ruled out effects from the Milky Way’s interstellar gas. The variations had to be intrinsic to the distant system.
Although the radio data were unevenly sampled, researchers compared them with the x-ray record. A cross-correlation analysis revealed a strong link. The rises and falls in radio and x rays lined up, repeating on the same 19.6-day cycle. This near synchrony pointed to a shared cause.
The most natural explanation comes from a relativistic effect called Lense-Thirring precession. When a black hole spins, it drags spacetime around with it. If incoming gas orbits at a tilt, the inner disk can slowly wobble. Any jet anchored to that disk should wobble as well.
Models of AT2020afhd support this picture. During the first year, the system likely fed above the Eddington limit. The disk stayed thick and extended close to the black hole. As the disk precessed, its projected area changed. At times, parts of the disk also blocked the hottest regions. These effects produced the x-ray cycle.
At the same time, the jet’s direction swept around. When it pointed closer to your line of sight, radio emission brightened due to Doppler boosting. When it swung away, the signal dimmed. Using the observed amplitude, researchers inferred a mildly relativistic jet. Its speed corresponded to a Lorentz factor between 1.2 and 1.6.
Assuming the 19.6-day rhythm reflects the precession period, the team constrained the black hole’s spin. Their analysis favored a modest positive spin. The geometry also suggested a viewing angle near 38 degrees and a precession cone of about 15 degrees.
The close link between disk and jet did not last. After about 250 days, radio variations slowed. Soon after, the x-ray signal dropped sharply. The repeating pattern vanished. Radio and x rays even became anticorrelated. This shift marked a clear break in the system’s behavior.
Later, optical and ultraviolet light brightened again. Sparse data make the cause uncertain. Current simulations predict gradual alignment of tilted disks and jets. They do not yet explain such an abrupt decoupling. AT2020afhd therefore challenges existing models.
The host galaxy itself adds intrigue. Earlier spectra suggest its nucleus changed character over time. Weak x rays appeared even before the 2024 flare. Some features resemble Bowen fluorescence events, yet the strong fading and thermal x rays look more like a classic TDE. Coronal emission lines appeared months later, hinting at delayed ionization effects.
Despite these puzzles, AT2020afhd stands apart. It offers the clearest case yet where a tidal disruption event shows tightly linked, quasi-periodic swings in x rays and radio. You see a black hole system precessing on a timescale you can track within a year.
These findings give researchers a new way to measure black hole spin and geometry. By tracking repeating x-ray cycles, astronomers can flag candidates for rapid radio follow-up.
This strategy may uncover more precessing disks and jets. Over time, a larger sample will sharpen models of how black holes launch and steer outflows.
This work deepens our understanding of galaxy evolution and the role black holes play in shaping their surroundings.
Research findings are available online in the journal Science Advances.
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