An intervening galaxy bends and amplifies the light from the background quasar RX J1131, producing four distinct images (shown in pink). Small flickers in these images have allowed astronomers, for the first time, to directly measure the size of the black hole's superheated “corona,” revealing that it extends roughly as far as our solar system. Credit: X-rays: NASA/CXC/Univ of Michigan/RCReis et al; Optics: NASA/STScI
Ever tried stacking two magnifying glasses and getting a sharper view than expected? That’s what the Universe just did for us. Welcome, Free AstroScience readers, and today we’ll unpack how a cosmic coincidence gave us the sharpest look yet at the super-hot corona around a supermassive black hole. If you care about how black holes feed, grow, and launch jets—and how we even measure such tiny things across billions of light-years—stick with us to the end for a clean, intuitive picture you can keep in your head.
This story was written for you by FreeAstroScience.com, where we explain complex ideas in simple words—and remind you to never switch off your mind. The sleep of reason breeds monsters.
What did astronomers actually see, and why is it a big deal?
They saw the corona—a compact, magnetized cloud of ultra-hot electrons hovering above the accretion disk—around the lensed quasar RXJ1131-1231. Thanks to a rare alignment, two gravitational lensing effects lined up like a “double zoom”:
- Strong lensing by a foreground galaxy made four images of the background quasar.
- Microlensing by individual stars in that same galaxy then flickered and boosted different tiny zones within the quasar’s core.
The result? A geometric size constraint on the corona at millimeter wavelengths—something no current telescope can get by itself. In plain numbers, the corona’s half-light radius is ≤ 50 AU (about the width of our Solar System). That’s ≤ 2.4 × 10⁻⁴ pc, or < ~46 gravitational radii for this black hole.
A popular report in Focus captured the essence: the team effectively used a cosmic double zoom to read the quasar’s central “halo,” estimating a size ~50 AU and highlighting that RX J1131 spins fast—“more than half the speed of light”—based on earlier work on its spin.
How did the “double zoom” work, step by step?
The aha moment: microlensing isn’t noise—it’s a ruler
Imagine the foreground galaxy as a fun-house mirror. It splits and magnifies the quasar. Then, stars inside that galaxy act like tiny moving magnifying glasses, each briefly brightening patches of the quasar’s core on AU scales. By watching how the flux ratios of the lensed images change over time at millimeter wavelengths, we can infer how small the emitting region must be.
- ALMA observed at 2.1 mm (Band 4) in 2015 and 2020. Because the quasar is at z = 0.658, those observations probe the rest-frame ~1.3 mm emission of the core.
- Between 2015-07-19 and 2020-03-16/17, key flux ratios (A/B, A/C, C/B) shifted by factors of ~2–5, but not on hour-to-day timescales. That pattern fits stellar microlensing of a very compact source, not intrinsic flares or lens-galaxy motion.
- Modeling the microlensing maps yields R1/2 ≤ 50 AU (95% upper limit). Even with conservative assumptions, it stays ≲ 90 Rg. That’s too small for dust in a torus or star-forming gas. It points to the corona.
Focus explains the same idea in plain terms: a galaxy at ~4 billion light-years acts as the first lens; stars inside it produce a second, smaller zoom—together letting astronomers read the corona’s “glitter” at high detail.
Where does the millimeter light come from?
At mm wavelengths, the corona likely glows via synchrotron radiation from relativistic electrons in the black hole’s magnetic field. The observed mm vs. X-ray luminosities follow the well-known Güdel–Benz relation (roughly Lmm ≈ 10⁻⁴ LX), a hallmark of coronal emission. That’s a powerful cross-check that we’re really seeing the corona, not dust or jets.
Using the size limit and the mm luminosity, the team estimated a magnetic field B ≲ 1.3–1.5 G in the corona—remarkably close to independent expectations for radio-quiet AGN coronae.
What does this tell us about black hole neighborhoods?
It’s a clean picture, and it fits:
- The X-ray corona is very compact.
- The mm-wave corona is slightly larger, but still tens of AU.
- The UV/optical accretion disk is larger still.
Those size differences explain why X-ray flux ratios in lensed images can vary more strongly than optical/mm: smaller regions are more microlensed. In RXJ1131, the mm variability in 2015→2020 matched the microlensing story, while day-scale checks showed no huge jumps, arguing against rapid intrinsic flares as the main cause.
Why should you care beyond “cool space stuff”?
Because it changes how we measure galaxies.
Standard SED fitting often treats mm emission as dust (star formation) plus maybe a weak jet. But here, the corona can dominate the mm band and vary by ~×3–4 across a few years. That swings the inferred far-IR luminosity and star-formation rate by ≈ 2× if you don’t account for the coronal piece. This matters for cosmic star-formation histories and for any lensed quasar where compact components get extra magnification.
Can we put numbers in one place?
Yes—bookmark this.
| Quantity | Value | Notes / Source |
|---|---|---|
| Quasar | RXJ1131-1231 | Strongly lensed, radio-quiet quasar |
| Redshift (source) | z = 0.658 | Rest-frame 1.3 mm probed by ALMA at 2.1 mm |
| Lens redshift | z = 0.295 | Foreground early-type galaxy |
| Observations | 2015-07-19; 2020-03-16/17 | ALMA Band 4, flux-ratio shifts detected |
| Corona size | R1/2 ≤ 50 AU (≤ 2.4×10⁻⁴ pc) | 95% upper limit from microlensing :contentReference[oaicite:10]{index=10} |
| Magnetic field | B ≲ 1.3–1.5 G | From synchrotron size–luminosity scaling :contentReference[oaicite:11]{index=11} |
| “Double zoom” picture | Strong lens + microlensing | Explainer and context :contentReference[oaicite:12]{index=12} |
How do the key formulas look (in human-readable HTML)?
Microlensing Einstein radius (in the source plane):
Optically thick synchrotron size–luminosity–B relation:
Where L39 = (νLν / 10³⁹ erg s⁻¹), ν is rest-frame GHz, and B in Gauss. These are the workhorses behind the ≤ 50 AU constraint and the B ≲ 1–2 G estimate.
What’s next—and what could stop us?
- Multiband monitoring is key. The mm and X-ray flickers don’t always align; that encodes the 3-D geometry of the corona and disk.
- Bigger samples: The Vera C. Rubin Observatory will find many more lensed quasars with measurable microlensing. That’s a treasure map for corona physics.
- X-ray observatories matter. Reports warn of budget pressure on Chandra. Losing that capability would blunt our multi-wavelength view of black hole cores.
Quick timeline (for context)
| When | What | Why it matters |
|---|---|---|
| 2003–2020 | Optical COSMOGRAIL monitoring | Baseline flux ratios; huge 2009 A-image magnification |
| 2015-07-19 | ALMA Band 4 | Flux ratios very different vs optical; strong microlensing signature |
| 2020-03-16/17 | ALMA Band 4 | Flux ratios shifted by factors of ~2–5 since 2015 |
| 2025-03-20 | Peer-review manuscript (A&A) | First geometric mm-corona size limit ≤ 50 AU; B ≲ 1.5 G |
FAQs we’d ask if we were you
“Could this be a jet instead of a corona?”
A powerful jet would look different in spectral slope, size, and variability pattern. The compactness and mm–X-ray scaling match a corona in a radio-quiet quasar.
“Is 50 AU small or big for a black hole?”
For a supermassive black hole, 50 AU is tiny—tens of gravitational radii—yet still bigger than the hard X-ray core. It’s exactly the scale where magnetic fields sculpt inflow and outflow.
“What’s with the ‘half-light radius’?”
It’s the radius enclosing half the light. Easy to interpret, robust against model quirks, and the standard in microlensing size work.
Conclusion: two lenses, one clear lesson
A galaxy and its stars acted like stacked magnifiers, letting us measure a black hole’s corona half a Universe away. The ≤ 50 AU size, the mm–X-ray link, and the ~1–2 G field strength give us a crisp sketch of where magnetism, light, and gravity wrestle for control. As we widen the sample with new surveys and keep X-ray eyes on the sky, we’ll turn chance alignments into a repeatable tool for black-hole weather.
Come back to FreeAstroScience.com for more clear, human-first explanations. Keep your mind awake; the sleep of reason breeds monsters.
Post a Comment