Fresh off revealing its first image of the black hole at the center of our galaxy, the Event Horizon Telescope is ready to take its next steps by capturing movies of gas flowing turbulently onto a black hole.
The two black hole images the Event Horizon Telescope (EHT) has produced so far — that of Sagittarius A* in the Milky Way and that of the black hole at the center of the galaxy M87 — are snapshots in time. Black holes are constantly churning as gas orbits around it surface, or event horizon, but still images don't really show this churn.
So scientists dream of movies produced by repeatedly imaging the black holes over months and years. Researchers hope such movies will show the evolution of black hole accretion disks as gas flows onto them and how the magnetic fields within the disk become tangled and wound up as they are dragged around the black holes.
There have already been attempts to make a movie. "We did try this with the 2017 data," Katie Bouman, a computer scientist at the California Institute of Technology, said during the NSF news conference on Thursday (May 12), alluding to the 2017 observing run that produced the data behind the images of both black holes.
Sagittarius A* in pictures: The 1st photo of the Milky Way's monster black hole explained in images
"We developed algorithms that allowed us to make movies, and applied these to the data," she added. "We saw that although there was something interesting there, the data that we currently have doesn't constrain that movie enough in order to say something that we're really confident about."
So scientists need more data before a video is feasible, but capturing that data takes a lot of time, and the telescopes that make up the EHT project have other observing programs to complete.
To meet the challenge, engineers are implementing technical improvements so that by 2024, EHT astronomers will be able to switch observations on and off. That capability will allow scientists to make use of free time on the telescopes over a long period, rather than an observing campaign lasting a week or two.
Vincent Fish, an astrophysicist at the Massachusetts Institute of Technology's Haystack Observatory, describes the approach as agile observing. "You make your observations, and then [the telescopes] can go back and do their other science the rest of the time," Fish said during the NSF news conference.
Although these agile observations will begin in 2024, EHT scientists will need a few years to process the data into a movie using the imaging techniques Bouman described.
Milky Way vs M87: Event Horizon Telescope photos show 2 very different monster black holes
The first movie star will be the black hole in M87, an elliptical galaxy at the heart of the Virgo galaxy cluster, 54.5 million light-years away from Earth. Despite its great distance, that black hole actually appears on the sky at a similar size to Sagittarius A* because it is much larger. The gas ring imaged around Sagittarius A* could fit inside the orbit of Mercury, the radius of which is about 36 million miles (58 million kilometers) while the black hole in M87 could easily encompass the orbits of all the planets in the solar system.
The M87 black hole's sheer size actually helps when it comes to making movies. Because Sagittarius A* is much smaller, changes occur much more quickly as gas whips around the black hole — too quickly for sporadic observing by the EHT to track. Because M87's black hole is so huge, changes in its gas ring take weeks or months to become apparent, allowing movies to be captured at a more stately pace.
Agile observing has other benefits. Occasionally, black holes experience an outburst as they tear apart an asteroid or a gas cloud that has wandered too close. Observing such outbursts requires rapid follow-up, which the EHT has so far been unable to do, given the logistics of arranging time on the telescopes and setting up the necessary equipment. With agile observing, the EHT will be able to follow up with the flick of a switch should astronomers spot an outburst in M87 or even on Sagittarius A*.
"That's huge for being able to pick up short-term flaring," Ryan Hickox, an astrophysicist at Dartmouth College, told Space.com.
Although we shouldn't expect any movies of Sagittarius A* anytime soon, there's plenty more to observe there in the meantime. The EHT has already measured the level of polarization in the light from M87's gas disk, which tells astronomers about the strength and direction of magnetic fields wrapped up in the disk, possibly emanating from the black hole itself.
"Our next step will be to make polarized images of Sagittarius A*, so that we can see the magnetic fields near the black hole and see how they're dragged [around] by the black hole itself," Michael Johnson, an astrophysicist at the Harvard–Smithsonian Center for Astrophysics, said during the NSF news conference.
Another step will sharpen the EHT's view of black holes. Seven observatories collaborated to image M87's black hole; with the addition of the South Pole Telescope, eight observatories took part in imaging Sagittarius A*.
The Event Horizon Telescope operates through Very Long Baseline Interferometry, a technique that pairs off telescopes. The distance between the telescopes, which scientists call the 'baseline,' is equivalent to the aperture of a normal telescope.
If more telescopes can join the EHT project, then the baselines linking observatories can increase in number and length. Lengthening the baselines increases the resolution, allowing scientists to see smaller details. Meanwhile, increasing the number of baselines increases the EHT's sensitivity and its number of viewing angles. This factor is on display in the image of Sagittarius A*, which appears spotty: Those bright spots are not hot spots, but rather mark regions where viewing angles of more of the telescope pairs coincided, resulting in a stronger signal.
Three new telescopes have been added to the EHT since the M87 and Sagittarius A* imaging runs. These are the Greenland Telescope Project, the IRAM NOEMA Observatory in the French Alps and the Kitt Peak 12-meter telescope in Arizona. Because the Greenland Telescope Project is so far north, it can only observe M87 and not Sagittarius A*; on the other hand, the South Pole Telescope cannot see M87. So only 10 telescopes will be able to observe each black hole.
"Adding new stations will help a lot," Hickox said.
And what of other black holes in other galaxies? Unfortunately, we may have to settle for just two black holes for now.
"One of the challenges is that there's not really any black holes that have a large enough event horizon, as projected on the sky, that can be easily imaged with the Event Horizon Telescope," Hickox said.
That doesn't mean that the EHT cannot observe them. The network has already observed the jets of some active galaxies, such as the quasar 3C273, which is 2.4 billion light-years away from Earth and has a central black hole with about 880 million solar masses.
Those jets can be surprisingly informative, Hickox said. "There's lots of really interesting structure in those jets that tells us about how particles get accelerated around a black hole and how they interact with the environment after they've been ejected, and how the magnetic fields work, and what the composition of those particles are, and all that stuff, which affects how those jets then influence gas on very large scales around their galaxy," he said.
Given that the EHT's 2020 observing program was canceled because of the COVID-19 pandemic, there's lost time to be made up. However, the pause gave scientists the chance to process the image of Sagittarius A* and to develop new technology and image-processing algorithms with which to tease more details out of the images.
We've barely scratched the surface of what these two black holes can tell us. Are they spinning, and if so, then how fast? Where do their magnetic fields come from? Do they consume gas in sudden gulps or do they graze on the gas more gradually? And how do they affect their immediate environment in their galaxies?
With the release of the Sagittarius A* image, the answers to some of these questions could be almost at our fingertips.
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