Winds that charge away from supermassive black holes at a fraction of the speed of light have long been mysterious and even contentious. Now, new evidence sheds light on their origins.
Astronomers used to be skeptical of UFOs. No, not those UFOs — ultra-fast outflows, the name for plasma winds streaming away from black holes at 10-20% the speed of light. (Gotta love astronomers and their acronyms.) Now Michael Parker (University of Cambridge, UK) and colleagues have reported strong evidence of a supermassive black hole’s speedy wind in the March 2nd Nature.
Although these UFOs have nothing to do with aliens, they’re still pretty awesome. These incredible winds have the potential to shape the growth of both the black hole and its host galaxy.
UFOs were long met with skepticism because they’re so difficult to detect. These winds allegedly launch from the bright gaseous disk that surrounds a feeding black hole, and because the gas the winds carry away is so thin and hot, it’s invisible except in relief. That is, we only know UFOs exist because escaping X-ray photons generated very near the black hole’s event horizon pass through them on the way out of the system. Along the way, some of the X-ray photons strip the gas’s few heavy atoms of their electrons. The remaining photons then carry the chemical traces of these highly ionized elements to our space-based detectors.
For a long time these chemical traces, appearing as absorption lines in accreting black holes’ X-ray spectra, hovered at the edge of detectability. This lack of clarity made a number of astronomers uneasy about interpreting the spectral lines as evidence for ultra-fast outflows. The new study takes a big step toward eroding that skepticism.
Observing a UFO
Around the black hole at the center of galaxy IRAS 13224–3809, Parker and colleagues found perhaps the strongest evidence yet for a UFO, thanks to a number of absorption spectral lines detected by two instruments on the European XMM-Newton X-ray space telescope.
Even though the two instruments are sensitive to slightly different ranges of photon energies, the absorption lines they each detect show a striking similarity: the lines are all shifted to higher energies (AKA, blueshifted) by the same amount. That means that the same outflow created all the chemical fingerprints — and that outflow is streaming away from the black hole at 23.6% the speed of light (160 million mph).
“A large fraction of the people who are usually most skeptical of outflow detections are my coauthors on the paper,” Parker says. “It's definitely the most robust and complete detection of a UFO.”
Also, Parker adds, X-ray observations have consistently spotted the wind signature from this source over a period of five years, unlike a lot of other sources where wind signatures are detected once and then never seen again.
Where Do Ultra-Fast Outflows Come From?
What’s more, Parker’s team may have found clues to UFOs’ origin. (And no, it’s not aliens.)
The glowing gas nearest the black hole at the core of IRAS 13224–3809 flickers quickly and dramatically. When Parker’s team compared the strength of the wind signatures during times of high and low X-ray luminosity, they found that the strength of the wind signature wanes considerably during brighter periods, sometimes over the span of only hours.
The astronomers surmise that, as the gas feeding the black hole pumps out more X-rays, the photons heat the wind. And as the wind gets hotter, its elements become ionized. Eventually, it can’t absorb any more X-rays, so the wind signatures in the X-ray spectrum disappear.
The wind’s fast response time to the X-ray variability tells us that it can’t exist too much farther out than the X-ray source itself, which lies very near the black hole. A very rough estimate based on the time delay tells us that the wind originates a few astronomical units (a.u., the distance between Earth and the Sun) from the million-solar-mass black hole.
There’s a lot more to be learned about UFOs, especially their role in carrying black hole feedback into the galactic host. Future X-ray instruments as well as multi-wavelength observations will no doubt be part of that effort, paper coauthor Chris Reynolds says. “All that information will be crucial to understanding how these outflows are connected to galaxy formation.”