Seeing is believing: “Observing” simulations of relativistic jets

By Dr. Richard Anantua

Richard Anantua

The high-energy universe is a fascinating place to observe: giant stars explode into supernovae, briefly outshining their own galaxies; pulsars with more mass than our Sun but only twelve miles across spin hundreds of times each second; and supermassive black holes at the centers of galaxies can suck dust and gas into accretion disks and blast this material in plasma form back out in powerful relativistic jets spewed out at close to the speed of light.

These jets are some of the largest single objects in the universe, often stretching hundreds and even thousands of light years across the void. Since the jets are emitted roughly along the rotational axis perpendicular to accretion disks around spinning black holes, they’re often visible shooting out into relatively empty space.

M87 and its jet. (Credit: The Hubble Heritage Team (STScI/AURA and NASA/ESA).)
Streaming out from the center of the giant elliptical galaxy M87 is a black-hole-powered jet of electrons and other subatomic particles traveling at nearly the speed of light. The physics behind such relativistic jet emission is still poorly understood. (Credit: The Hubble Heritage Team (STScI/AURA) and NASA/ESA).)

Huge and readily visible—we must know a lot about these jets, right?

Unfortunately, it’s not that simple.

While jets can emanate from black hole/X-ray binaries and gamma-ray bursts, the jets that output the most energy are born in active galactic nuclei (AGN). An AGN is a maelstrom of gas and dust spiraling in toward a supermassive black hole at the center of a galaxy. As the particles swirl in toward the black hole’s event horizon, they jostle and bump each other and the resulting friction heats the material until it glows across much of the electromagnetic spectrum, from the radio through the infrared and visible light to X-rays and all the way to gamma rays. Add in ubiquitous magnetic fields threading through the disk, and the result is an environment seething with complicated forces that are difficult to resolve. In fact, many of the most basic characteristics of these jets are still unknown.

Three different views of the jet blasting out of quasar 3C 273.  (Credit: Optical: NASA/STScI, X-ray: NASA/CXC, Radio: MERLIN.)
Three different views of the jet blasting out of the core of quasar 3C 273 comparing morphological structure discovered in various regimes of the electromagnetic spectrum. From left to right, the images are optical (HST), X-ray (Chandra), and radio (MERLIN). (Credit: Optical: NASA/STScI, X-ray: NASA/CXC, Radio: MERLIN.)

We believe that the plasma of which some jets are formed is made up of mostly electrons and positrons; however, other jets may be composed primarily of ionic plasma from the disk. We think the jets gain their energy from the interplay of black hole rotation and magnetic fields, and that the magnetic field is also partially responsible for collimating, or narrowing, the jets. But these are hypotheses; even theorists (like KIPAC’s first director and my doctoral advisor, Roger Blandford) can’t yet describe many jet process with certainty.

These jets may be mind-bogglingly big, but they are still very far away—M87, which is one of the best-known of the jet-hosting galaxies and which I used as a test case for my research—is more than 50 million light years from our galaxy. The quasar 3C 279, another test case, is more than five billion light years away. (Quasars comprise a class of exceptionally powerful AGNs with their jets generally pointed almost directly at us.)

Our ability to observe these objects at such vast distances and actually unravel what happens near their chaotic central engines still has a lot of room for improvement, but instruments like the Event Horizon Telescope (EHT), a global network of radio telescopes acting as one giant instrument with an effective diameter of our entire planet, and the Cerenkov Telescope Array  (CTA), which reconstructs images from very high energy incident gamma rays, are closing in on these galactic cores. A major goal of the EHT is to approach the angular resolution required to directly image the shadow around the event horizon of Sagittarius A*, the black hole at the center of our own galaxy. This level of angular resolution is sufficient to give us a good look at the inner jet of M87 as well.

In the meantime, theorists are hard at work creating simulations of what’s happening. The best tools for the job combine magnetohydrodynamics (MHD), which is the study of the magnetic properties of electrically conducting fluids, and Einstein’s General Relativity into something called (not surprisingly): general relativistic magnetohydrodynamics (GRMHD). GRMHD simulations give insight into the complicated relationship between density, pressure and magnetic perturbations, and the distortions of spacetime due to the extreme gravity of the nearby black hole—and thus how the two in combination shape the plasma of the accretion disk. Ultimately, the simulations provide insight as to how a jet is actually formed and accelerated.  

Observations and simulations may be improving, but the ability to compare the two has not kept pace. In response, I have developed a software pipeline to post-process GRMHD simulations, essentially translating the energy associated with the motion of the fluids implied by a simulation to particle acceleration, accounting for properties such as relativistic velocity, dissipation, opacity and polarization in the radiative transfer (how these particles transfer electromagnetic energy among and between them). The pipeline thus “lights up” the simulations, revealing what the observer actually would see, so that we can compare simulation to reality.

The images below are intensity maps based on novel models of synchrotron emissivity (energy loss due to synchrotron radiation). Intensity is measured in Janskys (a unit of spectral flux density) per square milliarcsecond; distances are related to the scale of the black hole via gravitational radius M = GMBH/c2, which can also serve as a time scale equal to the time light takes to travel that distance in units where c=1. The angles θ and ϕ signify the jet’s direction in relation to the point of view of the observer, with θ corresponding to the angle between the jet’s axis and the observer’s line of sight and ϕ corresponding to the angular displacement of an observer flying around the polar axis in the azimuthal direction.

Radiation intensity map from jet simulation. (Credit: Richard Anantua.)                  
A powerful jet viewed at θ=20° from the jet axis. This image was generated from an emissivity function in which the gas pressure is constant along the jet. Intensity of a jet not necessarily associated with a known black hole, as above, is measured in code units that can be converted to physical units for a known black hole (see simulation based on M87, below).  (Credit: Richard Anantua.)

Some very interesting observational consequences arise from relativistic effects like the aberration of angles (a classical analogy is driving through rain: the faster you’re driving, the more steeply the rain seems to be slanted toward you), which can make jets appear to be traveling faster than light.

Intensity map generated by jet simulation. (Credit: Richard Anantua.)    
Intensity map at observer time TObs = 2000M of jet viewed at 20° using a current-density-based synchrotron emission prescription (with absorption) at 43 GHz. The bottom edge of the jet appears brighter than the top edge, possibly indicating large pitch angle (angle from the observer’s horizontal) for a helical jet magnetic field. This simulation is based on the M87 jet. (Credit: Richard Anantua.)

The image above and the following animation demonstrate a phenomenon that could be confirmed by the EHT: the bilateral symmetry of the jet is apparently broken.

One explanation is derived from synchrotron radiation emission and absorption cast in terms of effective magnetic field (which can differ across two edges of the jet if the magnetic field variation is roughly helical). The violation of bilateral symmetry should appear greatest where the pitch angle is large, such as in inner regions that will be seen by EHT around 240 GHz.

Eleven-frame intensity map movie from TObs = 2000M - 2560M of jet viewed at 20° using a current-density-based synchrotron emission prescription, now optically thin at 243 GHz. The intensity bright spot appears closer to the core than in the 43 GHz image above. This simulation is based on the M87 jet. (Credit: Richard Anantua.)

Also, comparing the bright red spot in the simulated movie, which at 243 GHz is expected to be optically thin (emitted by material that is not opaque to the radiation itself), with the corresponding spot in the 43 GHz (optically thick) simulated intensity map suggests there may be an effect in the jet analogous to an already-observed phenomenon in which an AGN’s core shifts inward, closer to the black hole, as the observed frequency increases.

The analysis package we have written can visualize simulations across the electromagnetic spectrum, letting us look at jets in several wavelengths of radiation, and could also provide insight into some very puzzling observations where the gamma-ray intensity from some quasars varies on a timescale of mere minutes!

In addition, the methodology used to create these visualizations is widely applicable to a variety of phenomena, such as how emission changes from the regions very close to the black hole to tens of thousands of gravitational radii away.

There are several other theoretical effects treated in my thesis, such as inverse Compton radiation, in which photons scattering off the relativistic electrons near the jet plasma steal kinetic energy (resulting in emission of extremely energetic radiation, seen as gamma-rays by, e.g., the Fermi Gamma-ray Space Telescope)—as well as fine-tuning the somewhat generic models used. But by the time the EHT can show us black hole event horizons, we’ll be able to compare theory with observation—which is what science is all about.

And more insight into what's happening at the hearts of these far-distant behemoths is some the most fascinating science that we can do from the comfort of our own home planet.

The author would like to thank his collaborators, including Jonathan McKinney and Alexander Tchekovskoy.

Read more related KIPAC blog posts

Extreme galaxies flinging out the highest energy photons

Where have all the magnetic fields disappeared to? A new class of blazar flares

Where are they now? An Interview with KIPAC alum Justin Vandenbroucke [which discusses CTA which would also make observations of these kinds of systems]

Read more related information

Toward Multi-wavelength Observations of Relativistic Jets From General Relativistic Magnetodynamic Simulations (2016) (Doctoral dissertation; requires Adobe Reader)

Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June

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