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Spatially resolved spectral properties of M87* on event horizon scales

This paper presents the first spatially resolved spectral-index map of M87* on event-horizon scales using simultaneous 3.5 mm and 1.3 mm VLBI observations, revealing a radial gradient that transitions from positive to negative values and providing new constraints on the emission mechanisms and jet launching processes in the black hole's immediate vicinity.

Original authors: Shan-Shan Zhao, Ru-Sen Lu, Rocco Lico, Yuh Tsunetoe, Sijia Peng

Published 2026-08-26

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CC BY 4.0

Original authors: Shan-Shan Zhao, Ru-Sen Lu, Rocco Lico, Yuh Tsunetoe, Sijia Peng

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the heart of the giant elliptical galaxy M87 lies a cosmic heavyweight: a supermassive black hole containing the mass of billions of suns. This object, known as M87*, is a prime laboratory for understanding how gravity behaves at its most extreme. Because the black hole is so massive and relatively close to Earth, it appears large enough in the sky for our most powerful telescopes to resolve details on the scale of its event horizon—the point of no return where even light cannot escape. For years, astronomers have captured images of the glowing ring of hot gas swirling around this darkness, but those images have mostly shown us where the light is, not what the light is made of or how it changes as it moves away from the center. To truly understand the physics of this environment, scientists need to know how the color of the light shifts across the image, a property called the spectral index, which reveals whether the gas is thick and opaque or thin and transparent.

A team of researchers has now mapped this color shift for the first time with high precision, creating a detailed portrait of the gas dynamics right next to the black hole. Using data collected in 2018 by two different networks of radio telescopes—one observing at a wavelength of 3.5 millimeters and the other at 1.3 millimeters—the team combined their views to see the same region of space through two different lenses. By aligning these images perfectly and comparing the brightness of every single point in the picture, they calculated how the spectrum of the light changes from the center of the ring outward. The result is a clear, spatially resolved map that shows the gas is not uniform; instead, it follows a distinct pattern that tells a story about how the black hole feeds and how it launches its powerful jets of particles.

The map reveals a smooth, ring-like structure where the properties of the gas change in a predictable way as you move away from the black hole. Close to the center, inside the bright ring seen at the shorter wavelength, the light has a "positive" spectral character, meaning the gas is thick and opaque, trapping the radiation within. As you move outward, the spectral character shifts, becoming "negative" and indicating that the gas is becoming thinner and more transparent. This transition happens at a specific distance, roughly 30 microarcseconds from the center, which coincides with the size of the ring seen at the longer wavelength. The researchers found that the gas becomes increasingly transparent as you move away from the black hole, a behavior consistent with the idea that higher-frequency light can escape from closer to the event horizon, while lower-frequency light can only escape from regions further out where the gas has thinned enough to let it pass.

This finding provides a crucial new test for theories about how black holes interact with their surroundings. Theoretical models predict that the gas swirling around a black hole should be hot, thick, and geometrically thick, and that the way it glows should change depending on how close you are to the event horizon. The observed pattern—a gentle rise in spectral properties near the center followed by a steady decline further out—matches these predictions remarkably well, even when accounting for the limitations of current telescope resolution. It suggests that the bright ring seen in images is not just a random collection of glowing gas, but a specific boundary where the physical nature of the plasma changes. The data also helps rule out the idea that the entire region is uniformly transparent or uniformly thick; instead, it confirms a layered structure where the innermost regions are dominated by thick, trapped radiation that gradually gives way to thinner, free-streaming light.

While the map shows a clear overall trend, the researchers also noted a small, localized area of brighter spectral activity along the northwest side of the ring, which aligns with the base of the jet of particles shooting out from the black hole. Although this feature is intriguing and might indicate a change in the gas conditions where the jet begins, the team cautions that the current data is not yet sharp enough to confirm if this is a permanent feature or a temporary fluctuation. Future observations with even more powerful telescopes will be needed to settle this question. For now, the study stands as the first time scientists have successfully measured these spectral changes across the event horizon, offering a new way to probe the invisible physics of black holes. By combining these new measurements with older data from lower frequencies, astronomers are beginning to build a continuous picture of the black hole's environment, stretching from the immediate edge of the event horizon all the way out to the vast, parsec-scale jets that stretch across the galaxy.

Technical Summary: Spatially Resolved Spectral Properties of M87 on Event Horizon Scales*

Problem and Motivation
The supermassive black hole (SMBH) at the center of M87 (M87*) serves as a primary laboratory for testing general relativity and black hole astrophysics. While previous Event Horizon Telescope (EHT) and Global Millimeter VLBI Array (GMVA) observations have successfully imaged the morphology and polarization of M87* at 1.3 mm (230 GHz) and 3.5 mm (86 GHz), the spatially resolved spectral properties of the emission near the event horizon remain largely unconstrained. Characterizing the spectral index ( $\alpha$ , where $S_\nu \propto \nu^\alpha$ ) on horizon scales is critical for diagnosing the physical conditions of the plasma (magnetic fields, optical depth, electron energy distribution) and for isolating frequency-independent components, such as the photon ring, to enable more stringent tests of general relativity.

Methodology
This study analyzes nearly simultaneous high-resolution images of M87* obtained in April 2018:

Data Sources: 3.5 mm (86 GHz) images from the GMVA (including ALMA and the Greenland Telescope) and 1.3 mm (230 GHz) images from the EHT.
Image Processing: To ensure pixel-by-pixel comparability, the authors performed three key steps:
1. Alignment: Images were registered to the black hole center, defined by the most probable ring center, using phase cross-correlation. Core-shift effects were neglected as emission at both frequencies is dominated by the accretion flow.
2. Resampling: Both images were resampled to a common angular pixel scale of 0.2 $\mu$ as.
3. Blurring: Images were convolved with a common circular beam of 30 $\mu$ as to mitigate resolution-induced spectral artifacts.
Spectral Index Calculation: The spectral index was calculated pixel-by-pixel using the flux densities at the two frequencies. To characterize radial trends and reduce sensitivity to azimuthal asymmetries or day-scale variability, the authors also derived spectral indices from:
- Annular Integration: Flux densities integrated within concentric annuli (width 2 $\mu$ as) centered on the black hole.
- Directional Profiles: Flux densities integrated along strips aligned with the jet axis (PA=288 $^\circ$ ) and perpendicular to it (PA=18 $^\circ$ ).
Uncertainty Estimation: Errors were estimated accounting for systematic uncertainties ( $\sim$ 10%) and thermal noise, yielding a typical spectral index uncertainty of $\sigma_\alpha \sim 0.2$ .

Key Results
The analysis yields the first spatially resolved spectral-index map within the compact region ( $\le$ 100 $\mu$ as) of M87*:

Morphology: The spectral index map exhibits a ring-like morphology consistent with total intensity images. The inner region (inside the bright ring) shows inverted (positive) spectral indices, while regions outside the ring show progressively steeper (negative) spectra.
Radial Gradient: A robust radial gradient is detected within $\sim$ $\sim$ 50 $\mu$ $μ$ as:
- Inner Region ( $\lesssim 20 \mu$ as): The spectral index is positive ( $\alpha \approx 1$ ), indicating optically thick synchrotron emission. A marginal enhancement is observed near $\sim$ 15 $\mu$ as.
- Transition: The spectral index decreases steadily with radius, crossing zero ( $\alpha \approx 0$ ) near $\sim$ 30 $\mu$ as, which coincides with the characteristic radius of the 3.5 mm ring.
- Outer Region: Beyond the 3.5 mm ring, the spectrum continues to steepen, becoming negative.
Directional Consistency: Profiles along the jet axis and transverse directions reproduce the overall radial behavior, confirming that the trend is intrinsic rather than an artifact of image reconstruction.
Integrated Spectrum: The integrated spectral index within the compact region ( $<100 \mu$ as) is $\alpha \simeq -0.04$ , consistent with a nearly flat spectrum and previous spatially unresolved multi-wavelength measurements.

Significance and Claims
The paper claims that these results provide new observational constraints on the physics of the inner accretion flow and jet launching in M87*:

Physical Interpretation: The observed radial evolution (positive to negative $\alpha$ ) is interpreted as a transition from optically thick plasma dominating the inner region to progressively optically thinner synchrotron emission at larger radii. This is consistent with frequency-dependent synchrotron opacity, where higher-frequency (1.3 mm) emission becomes optically thin closer to the black hole than lower-frequency (3.5 mm) emission.
Theoretical Context: The measured radial gradient ( $d\alpha/dr \sim -0.11 \mu$ as $^{-1}$ in the declining region) is comparable to predictions from General Relativistic Magnetohydrodynamic (GRMHD) simulations (specifically thermal-electron SANE models). The authors note that while theoretical models predict a sharp spectral spike at the photon ring, realistic observational resolution smooths this feature, leaving the robust radial decline as the primary detectable signature.
Diagnostic Utility: The spatially resolved spectral index profile, including the location of spectral transitions and the radial gradient, serves as a powerful diagnostic to distinguish between accretion scenarios (e.g., MAD vs. SANE), constrain electron energy distributions, and estimate physical parameters like black hole spin and magnetic field strength.
Scope: The findings offer the first observational evidence of a systematic radial spectral transition on horizon scales in M87*, bridging the gap between horizon-scale accretion flow physics and parsec-scale jet structures.

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