Beyond the Horizon: Exploring Black Holes, Accretion Disks, and Plasma Physics

Check out Elliot’s Black Hole Simulation on Github.

Black holes and specifically the behaviors of plasma physics in the the surrounding accretion disc are two of the most intriguing and complex fields in astrophysics and cosmology. Lets dive deep into understanding the intricate interactions between black holes and their surrounding plasma disc environments.

What is a Black Hole?

Black Holes are the most massive, compact, and enigmatic forces in the universe. Their gravity creates a vacuum that captures all of matter, even light can’t escape its shadow! Albert Einstein’s general and special relativity predicted them (even though Einstein famously rejected the hypothesis), and Karl Schwartzschild’s equations characterized them, but the first discovery of a black hole wasn’t until Betty Louise Turtle and Paul Murdin discovered Cygnus X-1 in 1971.

Black Hole Formation

Black Holes form when massive stars collapse at the end of their life cycle. Once a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses (M_☉) may form by absorbing other stars and merging with other black holes, or via direct collapse of gas clouds. It is believed that these collapsing gas clouds create galaxies, with black holes at their center.

Types of Black Holes

Stellar Mass Black Holes

Since 1971, ~100 stellar mass black holes have been discovered (3-12 times the size of the sun), however, theoretical projections indicate that stellar-mass black holes are extraordinarily numerous. Roughly 1 out of every 1000 stars formed in the Milky Way is massive enough to end up as a black hole. Given that our galaxy contains on the order of 10^11 stars (100 billion), there should be on the order of 10^8 (100 million) of stellar sized black holes in the Milky Way.

(NASA estimates range from 10 million up to 1 billion stellar black holes in our galaxy, reflecting some uncertainty)

Intermediate Mass Black Holes

There are not very many intermediate sized black holes discovered as of yet. In the entire universe, we have about 100 candidates and these are mostly from inferencing and X-Ray emissions.

However, very recently, this number got a significant boost: in 2025, a team using the DESI survey (Dark Energy Spectroscopic Instrument) reported discovering 300 new intermediate-mass black hole candidates in dwarf galaxies. This is the largest collection of potential IMBHs to date, effectively tripling the candidate pool.^[1]

These objects have masses from roughly a few hundred to 100,000 times the mass of the Sun. It’s long been theorized that IMBHs should exist in significant numbers. They could form as the remnants of the first generation of stars (Population III stars in the early universe) or by mergers of stellar black holes over time science.nasa.gov. These medium black holes are thought to be the “seeds” that can merge and grow into supermassive black holes phys.org. However, because IMBHs are difficult to detect, it’s uncertain how many there truly are.

Supermassive Black Holes

Supermassive black holes (SMBHs) sit at the centers of galaxies and can range from a few hundred-thousand to tens of billions of solar masses. These gargantuan objects are easier to detect when they’re active (as quasars or active galactic nuclei), and we now believe they are a common feature of each galaxy. Virtually every large galaxy surveyed appears to host a supermassive black hole in its core^[2].

In our own Milky Way, the central black hole Sagittarius A** (Sgr A*) is about 4 million solar masses, which is relatively small in comparison to many other Super Massive Black Holes. Other galaxies have even larger central black holes (for instance, M87’s black hole is ~2–3 billion M☉, and some quasars host ~10–20 billion M☉ monsters). Because of their prevalence, the number of confirmed supermassive black holes is essentially “one per galaxy” for massive galaxies. There are thousands of SMBHs actually catalogued or imaged through various surveys – for example, astronomers have compiled databases of active galactic nuclei (quasars), which count over a million actively accreting SMBHs detectable via their luminous emissions

Even in the relatively nearby universe, tens of thousands of galaxies’ central black holes have been studied via stellar motions or radio/gamma-ray emissions. In short, any time we observe a galaxy with a bulge, we can infer a supermassive black hole is present, even if it’s not flaring brightly.

Candidates and Hidden SMBHs: Not all supermassive black holes are actively accreting or easy to spot. Many are “dormant” – like Sgr A* which is currently faint – and can be harder to detect directly. However, advanced techniques are revealing even the hidden ones. A recent 2024 study using infrared and X-ray data uncovered hundreds of previously obscured supermassive black holes that were hidden by dust in their galaxies. The study estimates that perhaps 35–50% of SMBHs in the universe are obscured from our direct view by gas/dust torii.

These would count as candidates until confirmed. Nonetheless, their very existence is implied by galaxy demographics. In total, the number of candidate SMBHs is on par with the number of known galaxies – meaning millions to billions of SMBH candidates (one at each galaxy center) that we have not “seen” in detail yet, but assume to be there due to the galaxy’s properties.

Estimated Total Population: Given that most galaxies host a supermassive black hole, the cosmic tally of SMBHs is enormous. Astronomers believe there are “billions or even trillions” of supermassive black holes in the observable universe. One rough estimate is on the order of 10^11–10^12 SMBHs, assuming each sizable galaxy (including many dwarf galaxies) has one. This number is lower than the stellar-bh count but is still huge.

It means that supermassive black holes are common on large scales – every galaxy cluster contains dozens or hundreds of them (one per member galaxy), and the cosmic web is dotted with these giants at galaxy centers. The distribution of SMBHs follows the distribution of galaxies: they trace the large-scale structure of the universe, often found in dense regions (clusters) where many galaxies (and thus many SMBHs) are concentrated. The masses of SMBHs also correlate with their host galaxies – bigger galaxies have bigger black holes.

For more on black hole classifications, check out NASA’s Black Hole Types resource.

Plasma Dynamics in Accretion Disks

One focal point of research has been the behavior of plasma within accretion disks around black holes.

Sagittarius A Magnetic Spectral Photograph from EHT

The Event Horizon Telescope (EHT) has given us direct horizon-scale views of supermassive black holes.

These disks are composed of incredibly hot, ionized gas spiraling toward the event horizon. Through advanced computational simulations, we’ve explored the turbulent magnetic fields generated within these disks and how they influence plasma flow, heat dissipation, and the emission of powerful jets.

These EHT findings provide unprecedented insight into plasma dynamics in the immediate vicinity of SMBHs. The bright rings suggest that orbital motion and probably turbulence dominate the inner accretion flow, as the plasma orbits at relativistic speeds and rapidly varies (Sgr A* in particular showed day-to-day variability during imaging, consistent with a dynamical, turbulent flow). The polarized images confirm that magnetic pressures are significant – strong enough to affect plasma motion and perhaps launch outflows. Overall, the EHT has opened a new window to study how accreting plasma behaves under extreme gravity, supporting models where magnetized turbulence (driven by magneto-rotational instability) transports angular momentum and feeds the black hole.

Relativistic Jets and Magnetic Fields

Another critical element to understand in the environment of a black hole involves the study of relativistic jets—immense streams of plasma expelled at nearly the speed of light by the magnetic fields of the black hole. By examining magnetohydrodynamic (MHD) models, we’ve uncovered new insights into how magnetic fields twist and channel energy, resulting in these spectacular cosmic structures.

One of the long-standing questions in astrophysics is how supermassive black holes launch relativistic jets of plasma. Recent observations are now directly linking jets to the inner accretion flow and magnetic fields of the black hole. In 2023, astronomers achieved the first image of a black hole’s jet and shadow together in a single frame. Using a global array of radio telescopes (GMVA + ALMA + Greenland Telescope) at 3.5 mm, they observed M87* and saw the base of the jet connected to the glowing ring of accreting material.^[6]

Another breakthrough is in understanding the role of magnetic fields in launching jets. Observations of the radio galaxy 3C 84 (Perseus A) by the EHT in 2018 have resolved the base of its jet and measured the polarization there. The result, reported in 2024, is that the magnetic field in the core of 3C 84 is strong and well-ordered, pointing to a scenario where magnetic forces dominate gravity in launching the jet.

In this “cosmic tug-of-war,” the magnetic field lines anchored in the disk and twisted by the black hole’s spin can fling out matter that wasn’t swallowed, overcoming the black hole’s gravitational pull. This supports models like the Blandford–Znajek mechanism, in which rotational energy and magnetic fields extract energy to power the jet. The EHT’s ability to detect linear polarization in M87* and 3C 84’s cores provides direct evidence that organized fields thread the plasma and are the driving engine of the jets.^[10] These observations strengthen the picture that jets are magnetically launched from the inner accretion flow, rather than being purely hydrodynamic or radiation-driven outflows.

The ErgoSphere of a Black Hole

In astrophysics, the ergosphere is a region located outside a rotating black hole‘s outer event horizon. Its name was proposed by Remo Ruffini and John Archibald Wheeler during the Les Houches lectures in 1971 and is derived from Ancient Greek ἔργον (ergon) ‘work’. It received this name because it is theoretically possible to extract energy and mass from this region. The ergosphere touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator. A black hole with modest angular momentum has an ergosphere with a shape approximated by an oblate spheroid, while faster spins produce a more pumpkin-shaped ergosphere. The equatorial (maximal) radius of an ergosphere is the Schwarzschild radius, the radius of a non-rotating black hole. The polar (minimal) radius is also the polar (minimal) radius of the event horizon which can be as little as half the Schwarzschild radius for a maximally rotating black hole.^[2]

Magnetorotational Instability (MRI)

At the Milky Way’s center, Sgr A* offers a close-up view of SMBH high-energy variability. While Sgr A* is comparatively dim, it sporadically produces X-ray and infrared flares. Observations over the past couple of years have shown that Sgr A* is extraordinarily variable, essentially “always bubbling” with activity. In fact, a 2023–2024 monitoring campaign with the JWST and other telescopes revealed a constant stream of flares with no periods of rest. Some flares are tiny “flickers” lasting only seconds, while others are blindingly bright eruptions occuring daily, and even longer, fainter episodes that persist for months. This behavior suggests the accretion flow is highly dynamic and never reaches a steady state. Astronomers suspect that magnetic reconnection events in the turbulent inner disk (analogous to solar flares on the Sun, but far more energetic) are responsible for these flares. Essentially, the plasma around Sgr A* is likely turbulent, with magnetic fields tangling and breaking to release bursts of energy. This research emphasizes how turbulence and variability are intrinsic to low-luminosity black hole accretion. These rapid flares have been observed in X-rays (with Chandra, XMM) and now in IR (with JWST), showing that high-energy processes are continuous even in a weakly accreting SMBH.

Active galactic nuclei (AGN) with powerful jets can emit even higher-energy radiation, up to gamma rays. A notable example is M87*: it has been observed flaring in TeV gamma rays, indicating extreme particle acceleration in or near the jet. The source of such gamma-ray flares – whether the black hole’s immediate vicinity or farther out in the jet – is an ongoing research topic. Multi-wavelength campaigns coordinated with the EHT are helping pinpoint the origin. For instance, by comparing EHT images and gamma-ray lightcurves, scientists can test if the brightest region of the accretion flow (the “ring”) correlates with gamma activity.

Phenomena Near Supermassive Black Holes

One breakthrough in 2022 came from NASA’s new IXPE (Imaging X-ray Polarimetry Explorer), which provided insight into how high-energy particles are accelerated in jets. IXPE observed a blazar (an AGN with a jet pointed at Earth) — specifically Markarian 501 — and measured the polarization of its X-ray emissions.

The polarization signature turned out to fit a scenario where a strong shock wave in the jet is the main particle accelerator. As charged particles in the jet plasma slam into the shock, they gain energy and produce X-rays.

Downstream of the shock, the magnetic field becomes more turbulent and chaotic, and particles lose energy, emitting lower-frequency light (optical, radio).

This stratified emission — high-energy X-rays from the ordered shock region, lower energies from the turbulent wake — was a long-sought “smoking gun” for shock acceleration.

The observations effectively solved a 40-year-old mystery of how jet particles attain such extreme energies. It’s now clear that internal shocks (moving faster than the local sound speed, like sonic booms in the jet) can tap the kinetic energy of the flow and convert it into cosmic-ray energy and high-energy photons.

This does not rule out magnetic reconnection or turbulence further out in jets, but it shows that at least in some blazars, a relatively localized shock front is key. The polarization measurements even drew an analogy: the flow of particles becomes more turbulent after the shock, “like how the flow of water becomes more turbulent after it encounters a waterfall”.

These studies and observations teach us two primary things about the environments surrounding black holes:

(1) Chaotic variability very close to the black hole (flares from disk/corona activity) and

(2) Structured particle acceleration in jets (shock-driven acceleration, followed by turbulent cooling zones).

X-ray and gamma-ray telescopes, combined with polarimetry and time monitoring, have allowed us to probe the extreme physics of plasma under gravity and magnetic forces near SMBHs.

Bridging Theory and Observation

Magnetic Fields as the Engine: A unifying theme is that magnetic fields play a central role. The EHT’s polarized images and the resolved jet-base studies confirm that strong magnetic fields thread the plasma around SMBHs. These fields can both mediate accretion (through magnetically-induced viscosity/turbulence) and extract energy to launch jets.

The finding that magnetic pressure can overwhelm gravity at the jet base. empirically supports theories like the Magnetically Arrested Disk (MAD) scenario, where the accumulation of magnetic flux near the black hole regulates accretion and drives episodic jets. In essence, we now see that black hole accretion is not a purely gravitational free-fall – it’s a magnetically channeled flow.

Turbulent Accretion Flows: The relentless flaring of Sgr A* and the rapid variability seen in other sources underscore that accretion flows are highly turbulent and dynamic. This turbulence, likely caused by magnetic instabilities, is what enables matter to lose angular momentum and plunge inward.

Observationally, turbulence manifests in the flickering X-ray/IR light curves (a sort of “noise” with no steady signal) and now in the spatial irregularities in jets (like Cen A’s disturbed jet). This reinforces the idea that theoretical models must account for time-varying, chaotic behavior rather than a smooth steady-state flow. Simulations of black hole accretion (GRMHD simulations) that include magnetic turbulence have been largely consistent with the EHT images and variability, lending confidence that we’re on the right track.

Jet Formation and Collimation: Seeing M87’s jet emerge from the ring of infalling gas gives physical reality to decades of theory.

It appears that as gas spirals in, a fraction is diverted into the jet – likely along magnetic field lines that act like a conduit. The connection between the accretion disk and the jet collimation is now visually established. Furthermore, the similarity of polarized structures in M87 and Sgr A* hints that even when a jet isn’t obvious (as in Sgr A*), the same magnetic framework exists and a tiny jet or wind outflow may be present. Jets carry away angular momentum and energy; thus, their presence (or even the possibility of their presence) is integral to the accretion process itself, not just an afterthought.

Particle Acceleration & Emission: High-energy observations (Chandra, IXPE, etc.) have pinpointed mechanisms of energy conversion in SMBH systems.

We now have direct evidence that shock waves within jets accelerate particles to relativistic energies. This helps explain the spectral energy distributions of blazars and radio galaxies – from radio up to TeV gamma rays – as a combination of fast-moving electrons in strong fields (synchrotron radiation) and possibly inverse-Compton upscattering of photons. It also tells us that the structure of magnetic fields (ordered vs turbulent) along the jet is crucial in shaping what kind of light we see. Similarly, the flares near the black hole likely involve magnetic reconnection events, converting magnetic energy into particle energy and heat. These processes are all part of the grand puzzle of black hole accretion physics: how gravitational energy (from infalling matter) and rotational energy (of the black hole or disk) get partitioned into radiation, kinetic energy of jets, and thermal energy of the plasma.

Additional Learnings

Syncotron radiation

Synchrotron radiation (also known as magnetobremsstrahlung) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (a ⊥ v). It is produced artificially in some types of particle accelerators or naturally by fast electrons moving through magnetic fields. The radiation produced in this way has a characteristic polarization, and the frequencies generated can range over a large portion of the electromagnetic spectrum.^[1]

Hawking radiation

Hawking radiation is black body radiation released outside a black hole‘s event horizon due to quantum effects according to a model developed by Stephen Hawking in 1974.^[1] The radiation was not predicted by previous models which assumed that once electromagnetic radiation is inside the event horizon, it cannot escape. Hawking radiation is predicted to be extremely faint and is many orders of magnitude below the current best telescopes‘ detecting ability.

Blandford–Znajek process

The Blandford–Znajek process is a mechanism for the extraction of energy from a rotating black hole,^[1][2] introduced by Roger Blandford and Roman Znajek in 1977.^[3] This mechanism is the most preferred description of how astrophysical jets are formed around spinning supermassive black holes. This is one of the mechanisms that power quasars, or rapidly accreting supermassive black holes.^[4] Generally speaking, it was demonstrated that the power output of the accretion disk is significantly larger than the power output extracted directly from the hole, through its ergosphere.^[5][6] Hence, the presence (or not) of a poloidal magnetic field around the black hole is not determinant in its overall power output. It was also suggested that the mechanism plays a crucial role as a central engine for a gamma-ray burst.^[7]

Penrose Process

The Penrose process (also called Penrose mechanism) is theorised by Sir Roger Penrose as a means whereby energy can be extracted from a rotating black hole.^[1]^[2]^[3] The process takes advantage of the ergosphere – a region of spacetime around the black hole dragged by its rotation faster than the speed of light, meaning that from the point of view of an outside observer any matter inside is forced to move in the direction of the rotation of the black hole.^[4]

This is a simulation I made to illustrate the Penrose process:

In the process, a working body falls into the ergosphere (black region). At its lowest point (red dot) the body fires a propellant backwards; however, to a faraway observer both seem to continue to move forward due to frame-dragging (albeit at different speeds). The propellant, being slowed, falls (thin gray line) to the event horizon of the black hole (black disk). The remains of the body, being sped up, fly away (thin black line) with an excess of energy (that more than offsets the loss of the propellant and the energy used to shoot it).

Black Hole and Plasma Physics FAQs

1. What Is a Black Hole and How Does It Form?

A black hole is an extremely dense object whose gravity is so strong that not even light can escape. They typically form when a massive star exhausts its nuclear fuel and collapses under its own gravity, creating a compact region known as the event horizon.

2. Why Do Black Holes Have Accretion Disks?

When gas, dust, and other matter spiral into a black hole, they form an accretion disk. This disk is made up of hot, ionized plasma moving at high speeds. The disk’s rotation and magnetic fields cause intense heating and radiation emissions.

3. How Are Stellar, Intermediate, and Supermassive Black Holes Different?

Stellar-Mass Black Holes: A few to ~20 times the mass of our Sun, formed from collapsing massive stars.
Intermediate-Mass Black Holes (IMBHs): Hundreds to ~100,000 solar masses, relatively rare and often inferred from X-ray emissions.
Supermassive Black Holes (SMBHs): Millions to billions of solar masses, found at the center of nearly all large galaxies (e.g., Sagittarius A* in the Milky Way).

4. What Are Relativistic Jets and How Do They Form?

Relativistic jets are powerful streams of plasma launched from the regions near a black hole. They likely form via strong magnetic fields that twist and channel energy outward, expelling matter at nearly the speed of light. This process is often explained by mechanisms like the Blandford–Znajek process.

5. What Is the Penrose Process, and Does It Extract Energy From a Black Hole?

The Penrose process describes how energy can be extracted from the rotating region around a black hole called the ergosphere. By splitting particles in this zone, some mass-energy can escape with more energy than it started, effectively tapping into the black hole’s rotational energy.

6. Can We Detect Hawking Radiation?

Hawking radiation is theoretically emitted by black holes due to quantum effects. However, it’s so faint that current telescopes cannot directly observe it. Most black holes’ Hawking radiation is overshadowed by the more intense radiation from accretion disks and jets.

7. Do All Galaxies Host a Supermassive Black Hole?

Observations suggest that most major galaxies contain a supermassive black hole at their core. Even dwarf galaxies are now thought to host intermediate-mass or smaller SMBHs, indicating these massive objects may be more common than previously realized.

Conlusion

The mysteries of black holes are limitless, these massive objects have so much to teach us about the cosmos and ourselves as we illuminate the dramatic and powerful interactions between these cosmic giants and their plasma-rich environments. From the accretion rings of plasma surround the shadowed sphere to the plasma jet emission structures, the dynamics of these almost mystical cosmic objects show us that there is still so much about the universe to endeavor to understand. Stay curious!