Enlarge (credit: ESO/M. Kornmesser )

Quasars initially confused astronomers when they were discovered. First identified as sources of radio-frequency radiation, later observations showed that the objects had optical counterparts that looked like stars. But the spectrum of these ostensible stars showed lots of emissions at wavelengths that didn't seem to correspond to any atoms we knew about.

Eventually, we figured out these were spectral lines of normal atoms but heavily redshifted by immense distances. This means that to appear like stars at these distances, these objects had to be brighter than an entire galaxy. Eventually, we discovered that quasars are the light produced by an actively feeding supermassive black hole at the center of a galaxy.

But finding new examples has remained difficult because, in most images, they continue to look just like stars—you still need to obtain a spectrum and figure out their distance to know you're looking at a quasar. Because of that, there might be some unusual quasars we've ignored because we didn't realize they were quasars. That's the case with an object named J0529−4351, which turned out to be the brightest quasar we've ever observed.

Enlarge / Ions of nitrogen and carbon show up in the spectrum of X-rays from a black hole's accretion disk. (credit: NASA/Chandra )

Even huge stars are not always safe out there. When the orbit of a star three times as massive as our own took the star too close to a hefty black hole, the black hole’s gravity ripped the star’s guts out and scattered them across a cosmic crime scene.

Nearly a decade ago, this tidal disruption event caught scientists’ attention not only because of its enormity but also because the carnage happened “only” 290 million light-years away, which is relatively close to Earth. This event, termed ASASSN-14li, was almost mistaken for a supernova when it was discovered in 2014). While a closer tidal disruption event has been discovered since, ASASSN-14li has continued to draw astronomers because the star involved might be one of the largest, if not the largest, known to have been devoured by a black hole. Now, a new forensic analysis of this event brings more about the stellar victim to light.

Exhibit A

While the proximity of ASASSN-14li and the cause of the star’s death were already known, the research team had to think like cosmic medical examiners to figure out the size of the star. For this, they relied on data from NASA’s Chandra and ESA’s XMM-Newton X-ray telescopes. When a star is ripped apart by the gravitational forces of a black hole, what is left of the star is heated so much by the intensity of those forces that a flare occurs. Flares like this can be observed in X-rays, as well as visible and ultraviolet light.

Enlarge / The jets of material ejected from around black holes can be enormous. (credit: NASA, ESA )

Active galactic nuclei, powered by the supermassive black holes they contain, are the brightest objects in the Universe. The light originates from jets of material hurled out at nearly the speed of light by the environment around the black hole. In most cases, these active galactic nuclei are called quasars. But, in rare instances where one of the jets is oriented directly toward Earth, they're called a blazar and appear brighter.

While the general outline of how a blazar operates has been worked out, several details remain poorly understood, including how the fast-moving material generates so much light. Now, researchers have turned a new space-based observatory called the Imaging X-ray Polarimetry Explorer (IXPE) toward one of the brightest blazars in the sky. The data from it and other observations combined indicate that light is produced when the black hole jets slam into slower-moving materials.

Jets and light

The IXPE specializes in detecting the polarization of high-energy photons—the orientation of the wiggles in the light's electric field. Polarization information can tell us something about the processes that created the photons. For example, photons that originate in a turbulent environment will have an essentially random polarization, while a more structured environment will tend to produce photons with a limited range of polarizations. Light that passes through material or magnetic fields can also have its polarization altered.

Enlarge (credit: Aaron Horowitz/Getty Images)

Three numbers.

Just three numbers—that’s all it takes to completely, unequivocally, 100 percent describe a black hole in general relativity. If I tell you the mass, electric charge, and spin (i.e., angular momentum) of a black hole, we’re done. That’s all we’ll ever know about it and all we’ll ever need to describe its features.

Those three numbers allow us to calculate everything about how a black hole will interact with its environment, how objects around it will respond to it, and how the black hole will evolve in the future.