Fresh from its first image of the black hole at the center of our galaxy, the Event Horizon Telescope is ready to take the next step by capturing films of gas flowing turbulently over a black hole.
The two images of the black hole 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 rotating as gas orbits its surface or event horizon, but the images still don’t really show this rotation.
Thus, scientists dream of films produced by repeatedly imaging the black holes over months and years. The researchers hope these films will show how black hole accretion disks evolve as gas flows through them, and how magnetic fields inside the disk become tangled and twisted as they are pulled around the disks. black holes.
There have already been attempts to make a film. “We tried this with data from 2017,” Katie Bouman, a computer scientist at the California Institute of Technology, said at the NSF press conference Thursday, May 12, alluding to the 2017 observational campaign that produced the data behind the images. of the two black holes.
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“We developed algorithms that allowed us to make movies, and applied them to data,” she added. “We saw that while there was something interesting there, the data we have right now doesn’t limit this movie enough to say something we’re really confident about.”
Scientists therefore need more data before a video is feasible, but capturing that data takes a long 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 can turn observations on and off. This capability will allow scientists to use free time on telescopes over a long period of time, rather than a week or two observing campaign.
Vincent Fish, an astrophysicist at the Massachusetts Institute of Technology’s Haystack Observatory, describes the approach as nimble observation. “You make your observations, then [the telescopes] can go back and do their other science the rest of the time,” Fish said at the NSF press 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 described by Bouman.
Milky Way versus M87: Event Horizon Telescope Photos Show 2 Very Different Monster Black Holes
The first movie star will be the black hole M87, an elliptical galaxy at the heart of the Virgo cluster of galaxies, 54.5 million light-years away. Earth. Despite its great distance, this black hole actually appears in the sky at a similar size to Sagittarius A* because it is much larger. The ring of gas imaged around Sagittarius A* could fit inside the orbit of Mercurywhose radius is about 36 million miles (58 million kilometers) while the M87 black hole could easily encompass the orbits of all the planets in the solar system.
The size of the M87 black hole actually helps in making movies. Since Sagittarius A* is so much smaller, the changes happen much faster as gas whips the black hole — too quickly for sporadic EHT observation 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 majestic pace.
Agile observation has other advantages. Sometimes black holes experience an explosion when they tear apart an asteroid or gas cloud that has come too close. Observing such bursts requires rapid monitoring, which the EHT has not been able to do so far, given the logistics of scheduling time on the telescopes and setting up the necessary equipment. With nimble observation, the EHT will be able to track with the flick of a switch if astronomers spot an explosion in M87 or even Sagittarius A*.
“That’s huge to be able to capture near-term flaring,” Ryan Hickox, an astrophysicist at Dartmouth College, told Space.com.
While we shouldn’t be expecting any Sagittarius A* movies anytime soon, there’s plenty more to watch in the meantime. The EHT has already measured the level of gas disc light polarization of M87which tells astronomers the strength and direction of the magnetic fields shrouded in the disk, possibly emanating from the black hole itself.
“Our next step will be to create polarized images of Sagittarius A*, so we can see the magnetic fields near the black hole and see how they are being dragged. [around] by the black hole itself,” Michael Johnson, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, said at the NSF press conference.
Another step will refine the EHT’s view of black holes. Seven observatories collaborated to image the M87 black hole; with the addition of the South Pole Telescope, eight observatories participated in Sagittarius A* imaging.
The Event Horizon Telescope works using very long baseline interferometry, a technique that combines 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, the baselines connecting the observatories may increase in number and length. Lengthening baselines increases resolution, allowing scientists to see finer details. Meanwhile, increasing the number of baselines increases the sensitivity of the EHT and its number of viewing angles. This factor is displayed in the image of Sagittarius A*, which appears uneven: these bright spots are not hotspots, but rather mark regions where the viewing angles of multiple pairs of telescopes coincided, resulting in a stronger signal.
Three new telescopes have been added to the EHT since the M87 and Sagittarius A* imaging campaigns. These are the Greenland Telescope Project, the IRAM NOEMA observatory in the French Alps and the 12-meter Kitt Peak 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. Thus, only 10 telescopes will be able to observe each black hole.
“Adding new stations will help a lot,” Hickox said.
And what about other black holes in other galaxies? Unfortunately, we may have to settle for two black holes for now.
“One of the challenges is that there aren’t really any black holes that have a big enough event horizon, as projected onto the sky, that can be easily imaged with the Event Horizon Telescope,” Hickox said. .
This does not mean that the EHT cannot observe them. The network has already observed jets from some active galaxies, such as the quasar 3C273which is 2.4 billion light-years from Earth and has a central black hole about 880 million solar masses.
These jets can be surprisingly informative, Hickox said. “There are a lot of really interesting structures in these jets that tell us how particles are accelerated around a black hole and how they interact with the environment after being ejected, and how magnetic fields work, and what is the composition of those particles are, and all that stuff, that affects how those jets then influence the very large-scale gas around their galaxy,” he said.
Since the 2020 EHT observing program was canceled because of the COVID-19 pandemic, there is lost time to make up for. However, the hiatus gave scientists the opportunity to process the Sagittarius A* image and develop new technologies and image processing algorithms with which to extract more detail from the images.
We’ve barely scratched the surface of what these two black holes can tell us. Do they spin, and if so, how fast? Where do their magnetic fields come from? Are they consuming gas in small sips or are they grazing gas more gradually? And how do they affect their immediate surroundings in their galaxies?
With the release of the Sagittarius A* image, the answers to some of these questions may be almost at hand.
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