The event horizon is where a black hole lives up to its name. Past this boundary, nothing—not even light—can escape the black hole's gravitational clutches. The utter lightlessness within this bounded region of space, however, is belied by the stupendously luminous environment that can exist right outside it. There, at the edge of the black hole's maw, matter violently swirls around and emits high-energy radiation. And when the black hole itself is monstrously large, as is the case of the supermassive black holes found in the cores of most galaxies, the upshot is one of the brightest objects in the universe, known in the jargon as an active galactic nucleus.
Understanding these extreme environs around supermassive black holes is the goal for Dan Wilkins, a senior member of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University. As a research scientist, astronomer, and astrophysicist at, Wilkins wields X-ray telescopes and computer simulations to piece together what is happening where cosmic gas and other matter go out in a blaze of glory. Specifically, Wilkins gathers information from the x-rays that reverberate or "echo" off of the material closest to the black hole.
"My research makes use of the X-rays that are produced just outside the black hole's event horizon. They shine down onto the disk of gas that is falling into the black hole, essentially illuminating the material in its final moments before it plunges into the black hole."
Wilkins and colleagues rely on a fleet of space-based X-ray observatories to gather data. These telescopes include NASA's Chandra X-ray Observatory and the European Space Agency's (ESA) XMM-Newton, both of which launched in 1999, as well as NASA's NuSTAR, the spring chicken of the group, having launched a decade ago. The observatories measure variations in the X-ray emissions echoing off of the inspiraling matter. Wilkins has pioneered techniques that combine these measurements with computer simulations and theoretical work to create 3D maps of the raucous scene just outside the event horizon, helping establish how active galactic nuclei are powered.
What will really deepen understanding, though, is a next-generation instrument, called Athena, that is presently in the works. "Athena will be transformational!" Wilkins exults.
Now undergoing detailed technical reviews by ESA and with a ballparked launch year of 2035, Athena will boast an X-ray collecting area around ten times the size of XMM-Newton's, the presently largest X-ray telescope. A bigger collecting area means more X-ray photons, or particles, and lots of them. "The key to getting a detailed picture of the universe is to collect as much light as you possibly can," says Wilkins.
As a high-energy form of light, though, X-rays pose special challenges. As we all know from trips to the dentist's and other medical scans, X-rays readily pass through many materials, including the reflective coatings on mirrors used in optical astronomy. X-ray telescopes accordingly must be designed for X-rays to glint off mirrors at glancing angles toward a detector, an instrumentation challenge that had long made X-ray astronomy a notoriously difficult endeavor. "The old joke in X-ray astronomy is that in the old days, our telescopes collected so few photons that we could give each one a name!" says Wilkins.
Athena will ward off any such photonic naming attempts courtesy of its ample collecting capacity. "This will give us much more detailed measurements of the X-rays that reflect and echo off of the material closest to the black hole," says Wilkins. "Athena will essentially let us zoom in closer than ever before on the gas in its final moments plunging into the black hole to really understand how this infalling material releases so much energy and powers the bright light sources that we see."
As an additional bonus, a larger-area telescope means that far more data can be obtained over an equivalent observing run as current telescopes. Thus, what are now quality single images with XMM-Newton can be strung together into multiple images in sequence. "We will be able to transform the still frame picture we have into a dynamic movie of what is happening around black holes," says Wilkins.
Athena will also sport a new type of X-ray spectrometer, an instrument that measures the wavelength—or essentially the "color"—of each X-ray photon that Athena receives. Spectrometers provide astronomers with a means of identifying chemical elements via the telltale emission and absorption lines they imprint across various wavelengths. As a result, Athena will enable astronomers to learn the composition and dynamics of the gas encirlcing the black hole. "We will be able to piece together the most detailed picture of the gas falling into the black hole, the source of the bright X-ray emission, and powerful winds that are launched outwards into the surroundings," says Wilkins.
The science with Athena won't end there. Besides zoomed-in views of supermassive black holes in the local universe, the instrument will also let astronomers examine colossal black holes at greater distances than ever before. That is significant because actively feeding central black holes impact starforming throughout their host galaxies through radiation and charged particle outflows. As the black holes gobble up more matter and increase in mass and size, they in turn influence the evolution of entire galaxies.
"The growth of a galaxy is deeply entwined with the growth of the black hole in its center," says Wilkins. "By observing the first black holes in the distant universe, we will be able to learn about how supermassive black holes grow so quickly and the role they play in the growth of the galaxies as we see them today."
Sharing the thrill of scientific investigation and discovery is important to Wilkins. In addition to appearing in television shows and offering radio interviews, Wilkins has presented to a wide variety of audiences through public astronomy lectures, live planetarium shows, and stargazing evenings. He has also given guided tours and demonstrations of historical telescopes. Wilkins even has a regular gig aboard the transatlantic ocean liner Queen Mary 2, the flagship of the Cunard Line, a British cruise line, and the home of the largest planetarium at sea.
Right on campus at Stanford, Wilkins founded the KIPAC Public Lecture series called "Discover Our Universe" in 2017. Many KIPAC faculty, postdocs, and graduate students, along with visiting scientists, have given talks on a range of astrophysical topics. The series transitioned to live-streaming in 2020 because of the pandemic and is now moving to a hybrid approach offering both in-person and online attendance.
"I’m passionate about science communication and sharing the wonders of our Universe with as broad an audience as possible," says Wilkins.
Here's to Wilkins carrying on that tradition on through the Athena era and beyond.