🔭 astrophysics

Impacts of radiative cooling on the images of a black hole shadow and extended jets in two-temperature GRMHD simulations

This study utilizes two-temperature GRMHD simulations with general relativistic radiative transfer to demonstrate that incorporating radiative cooling significantly lowers electron temperatures in the inner accretion disk, resulting in dimmer disks, brighter and more extended jets, and reduced total flux for the M87* black hole, with these effects potentially resolvable by next-generation Event Horizon Telescope arrays.

Original authors: Mingyuan Zhang, Yosuke Mizuno, Indu K. Dihingia, Christian M. Fromm, Ziri Younsi, Hai Yang, Alejandro Cruz-Osorio

Published 2026-05-18

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Mingyuan Zhang, Yosuke Mizuno, Indu K. Dihingia, Christian M. Fromm, Ziri Younsi, Hai Yang, Alejandro Cruz-Osorio

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

Imagine a supermassive black hole as a cosmic whirlpool, sucking in gas and dust from its surroundings. This swirling material forms a hot, glowing disk, and sometimes shoots out powerful beams of energy called jets. For years, scientists have tried to take a "selfie" of this phenomenon using the Event Horizon Telescope (EHT), which successfully captured the famous shadow of the black hole in the galaxy M87.

However, there's a problem with how we simulate these images. Think of the gas around the black hole as a pot of soup. In the past, scientists assumed that if you heated the soup, the whole pot got hot evenly, and the heat stayed there. But in reality, the soup can also lose heat to the air (radiative cooling). If you don't account for the soup cooling down, your recipe for the "cosmic soup" is wrong, and the picture you paint of the black hole won't match reality.

This paper is like a chef deciding to add a new ingredient to the recipe: cooling. The authors ran supercomputer simulations to see what happens when they let the gas around the black hole actually lose heat, rather than keeping it artificially hot.

Here is what they found, using simple analogies:

1. The "Cooling" Effect on the Disk

Imagine the accretion disk as a busy highway of gas particles.

Without Cooling: The gas particles are like overheated engines, blazing hot and glowing brightly.
With Cooling: The authors found that when they let the gas cool down, the inner part of the highway (closest to the black hole) became significantly colder, like a car engine that has been turned off and is losing heat.
The Result: Because this inner disk is cooler, it glows less brightly. It's like turning down the dimmer switch on a lightbulb.

2. The Jets Get a Spotlight

Here is the most surprising part. Even though the main "disk" got dimmer, the jets (the beams shooting out from the poles) actually became brighter and more extended.

Analogy: Imagine a stage with a spotlight. If you dim the main stage lights (the disk), the audience's eyes are drawn more to the actors standing in the wings (the jets). Because the disk isn't blindingly bright anymore, the faint, extended structures of the jets become much more visible and stand out against the dark background.
The Paper's Claim: Radiative cooling makes the disk dimmer but the jets more extended and brighter relative to the disk.

3. The Total Light Output Drops

Even though the jets got a bit brighter, the total amount of light coming from the whole system decreased.

Analogy: It's like a campfire. If you let the fire burn out a bit (cooling), the flames might look different, but the total heat and light you feel from the fire is less than before. The paper found that for any given amount of gas falling in, the system with cooling produces less total energy than the system without it.

4. The "Flicker" Becomes Smoother

Black holes aren't static; they flicker and change brightness over time.

Analogy: Think of a lightbulb that is flickering wildly. The authors found that as more gas falls into the black hole (higher accretion rate), the flickering slows down and becomes smoother, like a steady LED light.
The Source: They discovered that this flickering mostly comes from the "midplane" (the flat, central part of the disk), not the jets. Whether the gas is cooling or not, the center is the main driver of the changes.

5. The "Recipe" Matters

The paper also tested two different ways the gas gets heated: Turbulence (like stirring the soup violently) and Magnetic Reconnection (like snapping rubber bands).

They found that when you add cooling, the difference between these two heating methods becomes more noticeable. In some cases, the magnetic "snapping" method produced more total light than the "stirring" method, but only when cooling was taken into account.

The Bottom Line

The authors conclude that to get a true picture of what a black hole looks like, we can't just assume the gas stays hot forever. We have to let it cool down.

The New Picture: A cooler, dimmer central ring, but with jets that stretch out further and shine brighter by comparison.
Future Hope: While the current Event Horizon Telescope might not be sensitive enough to see these subtle changes clearly yet, the authors suggest that the next generation of telescopes (like the ngEHT) will be powerful enough to spot these "cooling" features, helping us understand the true physics of these cosmic monsters.

In short: Letting the cosmic gas cool down changes the black hole's face from a blinding, uniform ring to a dimmer center with more visible, extended jets.

Technical Summary: Impacts of Radiative Cooling on the Images of a Black Hole Shadow and Extended Jets in Two-Temperature GRMHD Simulations

Problem Statement
Recent Event Horizon Telescope (EHT) observations of the M87* black hole shadow have revealed a radiative efficiency in the hot accretion flow that suggests radiative cooling is non-negligible. While General Relativistic Magnetohydrodynamic (GRMHD) simulations are standard tools for modeling accretion physics, many existing models neglect radiative cooling or rely on single-fluid approximations with parametric electron temperature prescriptions (e.g., the "R-β" model). Furthermore, previous studies have often assumed thermal electron distribution functions (eDF), potentially ignoring nonthermal processes driven by magnetic reconnection and turbulence. This study addresses the gap in understanding how radiative cooling, when coupled with a physically driven two-temperature model and nonthermal eDFs, alters the electron thermodynamics, disk structure, and the resulting synthetic images of black hole shadows and extended jets.

Methodology
The authors performed General Relativistic Magnetohydrodynamic (GRMHD) simulations of magnetized accretion flows onto a rotating black hole ( $a = 0.9375$ ) using the BHAC code. The simulations employed a two-temperature framework where electron thermodynamics are evolved separately from ions, incorporating:

Heating Mechanisms: Two distinct prescriptions were tested: turbulent heating (Kawazura et al. 2019) and magnetic reconnection heating (Rowan et al. 2017).
Cooling Mechanisms: Radiative cooling (bremsstrahlung, cyclotron-synchrotron, and inverse Compton scattering) and Coulomb interactions between protons and electrons were included as source/sink terms.
Accretion Rates: Simulations were run across a range of mass accretion rates normalized to the Eddington rate: $\dot{m} = (1.0 - 10) \times 10^{-6} \dot{M}_{Edd}$ .
Radiative Transfer: General Relativistic Radiative Transfer (GRRT) calculations were performed using the BHOSS code to generate images at 230 GHz with an inclination of $163^\circ$ .
Electron Distribution: The study utilized a hybrid electron distribution function combining a thermal Maxwell-Jüttner distribution and a variable nonthermal $\kappa$ distribution. The $\kappa$ parameter was defined parametrically based on magnetization ( $\sigma$ ) and plasma beta ( $\beta$ ) using models derived from Particle-in-Cell (PIC) simulations (Ball et al. 2018; Meringolo et al. 2023).

Key Contributions and Results
By comparing models with and without radiative cooling across different accretion rates and heating prescriptions, the study yields the following findings:

Electron Temperature and Density Structure: Radiative cooling sharply decreases the electron temperature in the inner disk ( $r \lesssim 10 r_g$ ) around the equatorial plane, where density is highest. In contrast, the temperature in the jet sheath decreases only slightly. The inclusion of cooling leads to the formation of a thinner disk structure at higher accretion rates due to increased cooling efficiency.
Image Morphology and Flux: Radiative cooling results in a dimmer accretion ring (shadow) and a reduction in the total emitted flux. However, it produces more extended and relatively brighter jet structures. The contribution of the nearside jet to the total image increases in cooling models compared to non-cooling models.
Spectral Energy Distributions (SEDs): The inclusion of cooling shifts the turnover frequency (peak of the SED) to lower frequencies for a given accretion rate, indicating that synchrotron radiation becomes less self-absorbed. At 230 GHz, non-cooling models at higher accretion rates become optically thick, whereas cooling models remain in the optically thin regime.
Time Variability: The study finds that time variability in the 230 GHz light curves primarily originates from the midplane region for both cooling and non-cooling cases. As the mass accretion rate increases, the time variability (modulation index) decreases for both scenarios.
Heating Prescriptions: Under the same accretion rate, reconnection heating models generally produce higher total fluxes than turbulent heating models, both with and without cooling. However, the qualitative impact of radiative cooling (dimmer disk, brighter jets) remains consistent across heating prescriptions.

Significance and Claims
The authors claim that incorporating radiative cooling into two-temperature GRMHD simulations with nonthermal electron distributions is crucial for accurately modeling the black hole shadow and large-scale jet structure of M87*. The study demonstrates that neglecting cooling can lead to incomplete models or inconsistent predictions regarding electron temperature and flux variability.

The paper concludes that while the specific features identified (such as the enhanced visibility of the extended jet and specific flux reductions) are currently below the dynamic range of current EHT observations, they represent testable predictions for next-generation arrays like the ngEHT. The results are presented as generally applicable to other low-luminosity active galactic nuclei (LLAGNs) in the Magnetically Arrested Disk (MAD) state, suggesting that radiative cooling universally leads to cooler disks, brighter jets, reduced total flux, and smoother light curves, independent of black hole mass and distance. The authors maintain a modest stance, noting that their simulations are not yet tuned to perfectly fit M87* data and that future work must more self-consistently integrate cooling terms for nonthermal electrons to fully reproduce broadband spectra.