Bold claim: Astronomers have unlocked a way to observe the invisible halo around a distant supermassive black hole, offering a window into the extreme physics that govern these cosmic giants. The target is RX J1131, a quasar located about 6 billion light-years away, whose voracious pull on surrounding gas lights up the universe in extraordinary ways.
By harnessing the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, researchers tracked how the quasar’s signal changed over several years. Those tiny, nuanced fluctuations allowed them to measure the size and brightness of the surrounding hot gas—the corona—shielding the black hole’s interior from direct view.
This strategy transforms a far-off quasar into a live, natural laboratory for black-hole physics, providing a rare glimpse of the conditions near one of the cosmos’s most extreme objects.
Overview of RX J1131
Led by Matus Rybak, a senior researcher at Leiden University in the Netherlands, the team explores the hot gas and magnetic fields that envelope actively feeding supermassive black holes. Surrounding RX J1131 is a corona, a cloud of exceptionally hot, tenuous gas just outside the event horizon. In this region, particles reach millions of degrees and emit high-energy X-rays along with lower-energy radio-like radiation.
The quasar shines brilliantly because the black hole is actively consuming matter. As gas and dust spiral inward, they heat up and release enormous energy across the spectrum before crossing the point of no return.
Gravitational lensing as a natural magnifier
Between Earth and RX J1131 lies an intervening galaxy that acts as a gravitational lens. This phenomenon occurs when a foreground mass bends and focuses light from a distant source.
The observations show the quasar’s light split into four distinct images around the lensing galaxy, each tracing a different light path through space. Within the lensing galaxy, individual stars can cause microlensing, a momentary magnifying effect as a lone star interacts with the background light.
As the background source drifts behind different stars, small regions near the black hole become temporarily brighter. The combined effect of strong galaxy-scale lensing and star-by-star microlensing sharpens the view of the corona far beyond what even the most powerful telescopes could ordinarily achieve.
A corona the size of the Solar System
The team revisited older ALMA data alongside new observations. Early in the analysis, Rybak noticed irregular patterns that warranted deeper scrutiny. If the quasar itself caused the brightness variations, all four images would brighten and dim in unison. Instead, each image flickered independently, signaling that the foreground microlenses were focusing on different parts of the source at different times.
Through detailed modeling of these flickers, the researchers concluded that the emission originates from a compact region adjacent to the black hole, radiating millimeter-wavelength light. Based on the microlensing strength, they estimate the emitting zone spans about 50 astronomical units, roughly the distance from the Sun to the outer edge of our solar system’s icy region.
Magnetic fields, corona size, and black-hole physics
This size estimate supports the idea that the corona is a compact region shaped by strong magnetic fields. Earlier theoretical work suggested that long-wavelength radio emission in radio-quiet quasars can arise in this corona rather than from star-forming regions or jets.
The observed millimeter brightness paired with the quasar’s X-ray power aligns with the Gudel-Benz relation, a pattern linking radio and X-ray output in magnetically active stars. Seeing a similar balance in RX J1131 strongly argues for a coronal origin of the long-wavelength emission in this quasar.
A dynamic picture of millimeter light
Millimeter-wavelength emission from radio-quiet quasars was once thought to be relatively steady. Recent X-ray monitoring of RX J1131, however, reveals variability tied to the black hole itself. When correlated with the coronal measurements, this variability helps connect observed changes to the corona’s structure and activity.
Looking ahead
ALMA is expanding its reach to lower radio frequencies where black-hole coronas glow brightest. This expansion will enable microlensing techniques to be applied to many more distant systems, sharpening our map of how these extreme environments are organized and how magnetic fields redistribute energy.
The Vera C. Rubin Observatory is set to image the sky deeply and frequently, likely uncovering thousands of new lensed quasars similar to RX J1131. Even if X-ray mission budgets shrink, combining optical surveys with millimeter observations will keep advancing our understanding of what happens just outside distant black holes.
The study appears in Astronomy & Astrophysics and is accessible here: arXiv:2503.13313.
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