A worldwide collaboration of radio astronomers called the Event Horizon Telescope (EHT) is taking a close look at the atmosphere here on Earth to get a better view of an elusive area of deep space. Thanks to their recent modeling of the past 10 years of global atmospheric and weather data, they can now predict when their nine radio telescopes and arrays scattered around the world are most likely to have the clear view they need to make their extraordinary simultaneous observations.
The scientists’ quarry is the perilous boundary of a black hole, called the event horizon, and the surrounding region of space. Their target is not just any black hole: It’s the hulking, supermassive black hole that lurks at the heart of the Milky Way.
“You have to get all the participating observatories to collectively agree to give the EHT folks time on the sky when they ask for it…and that’s a big deal,” said Scott Paine, an astrophysicist at the Smithsonian Astrophysical Observatory (SAO) in Cambridge, Mass., who also happens to be an atmospheric scientist. “When an observatory commits several days to EHT to observe, we want the EHT to make good use of it, because it represents a significant investment for the observatory.”
Trying to ensure that EHT scientists would make the most of valuable worldwide observing time, Paine advised that they approach the problem scientifically using global atmospheric records. Along with EHT director and SAO astrophysicist Sheperd Doeleman, he spearheaded the creation of a model that predicts the probability of good simultaneous observations at all sites using data gathered by the National Oceanic and Atmospheric Administration (NOAA). Using this new model, the EHT collaboration is coordinating a weeklong observing campaign that will take place this coming April.
It’s not the first time the collaboration will peer at our galaxy’s central black hole, which is known as Sgr A* and weighs in at about 4 million times the mass of our Sun. The inaugural attempt took place in April 2017, and the observers are still crunching the data from that first try.
Even though the collaborators haven’t yet seen the images from that initial look, they geared up to try again, with the expectation of better results. This April and into the future, they hope to achieve the best “seeing” possible for the collection of EHT telescopes and arrays, thanks to their newly developed tools for selecting dates and times of optimal meteorological conditions for the overall observing network.
“We’re trying to make coherent a network the size of the globe, which is incredible when you think about it,” Doeleman told National Geographic. “It’s a heartbreaker if you [plan for] a night and bad weather closes in” or, conversely, if observations are canceled for a night that the weather is clear, he added.
“These tools allow us to determine the ideal observing windows for EHT observations and to assess the suitability and impact of new EHT sites,” said Harvard University undergraduate student Rodrigo Córdova Rosado in a recent presentation of this work. Córdova Rosado, a junior who worked on the project with Paine and Doeleman, presented a poster about this research on 9 January at the 231st meeting of the American Astronomical Society in National Harbor, Md.
A Worldwide Telescope Array
Although a black hole, by definition, does not emit light, gas and dust surrounding the black hole emit copious light as the incredible gravity of the black hole pulls the material onto itself. The brilliant glow, in turn, silhouettes the black hole, an extraordinarily compressed dot of mass, also known as a singularity.
A simulation of light emitted by hot gas as it orbits a black hole, viewed from 45° above the orbital plane. Brightness indicates the intensity of the emitted radio-frequency light. The black hole’s intense gravity bends light emitted from inner parts of the accretion disk around the target’s event horizon, creating the black hole silhouette seen in the center. EHT hopes to observe a snapshot of this activity. Credit: Hotaka Shiokawa
Because of the black hole’s ultracompact size, imaging its immediate environment requires an observing technique called very long baseline interferometry (VLBI). VLBI coordinates observations from multiple radio telescopes around the globe to amplify the light from a target and increase the signal-to-noise ratio of an observation. The wider the physical footprint of the array used in VLBI is, the stronger and clearer the radio signal is. Astronomers have used VLBI to view stars coalescing from giant gas clouds, and they plan to use it to glimpse protoplanets forming in circumstellar disks.
EHT’s nine radio telescopes and arrays at seven observing sites compose the largest VLBI array in the world. Getting onto the observing schedule at any one of the telescopes is very competitive, and negotiating for simultaneous observing time on all nine is even more difficult.
A Two-Pronged Predictive Approach
Deciding when to observe requires solving two problems at once, according to Paine. “There’s the strategic problem,” he said, “that is, which week or two weeks are you going to ask for from the observatories.”
The second is a tactical problem. “Once you’ve got your block of time, and you’re allowed to use a certain number of days within an allocated period, which ones are you going to trigger observations on?” He added, “We’ve been looking at both problems.”
That’s where NOAA comes in. Córdova Rosado tackled the first problem by gathering global weather data from NOAA’s Global Forecast System (GFS) recorded from 2007 to 2017 at approximately 6-hour intervals. Because EHT observes using radio waves, the researchers were primarily interested in records of relative humidity, ozone mixing ratio, cloud water vapor ratios, and temperature at each of the sites because each of those atmospheric conditions affects the quality of observations. Córdova Rosado ran those data through an atmospheric model that Paine had created to calculate how opaque the atmosphere appears at EHT’s observing frequency of 221 GHz, or a wavelength of 1.4 millimeters.
According to Vincent Fish, a research scientist at the Massachusetts Institute of Technology (MIT) Haystack Observatory in Westford, Mass., coordinated, ground-based radio observations of the galactic center thrive at 221 GHz. “At longer observing wavelengths,” he explained in an MIT press release, “the source would be blurred by free electrons…and we wouldn’t have enough resolution to see the predicted black hole shadow. At shorter wavelengths, the Earth’s atmosphere absorbs most of the signal.” Fish was not involved in this research.
EHT Sites Prefer It Dry
Córdova Rosado statistically combined each of the yearly opacity trends to calculate for each day of the year the probability that Sgr A* would have favorable observing conditions simultaneously at all seven sites. The team found that the second and third weeks of April were the best times of year for EHT to observe Sgr A*. The middle of February was a good backup observing window for both the Milky Way’s center and another black hole target. The Northern Hemisphere late spring and summer ranked lowest among possible observing months because of seasonal weather variability.
Some sites, like the South Pole Telescope and the Atacama Large Millimeter/ Submillimeter Array (ALMA) in Chile, offer remarkably stable opacities throughout the year because the areas enjoy consistently low humidity. For more variable Northern Hemisphere sites, the winter months provide the most favorable observing conditions.
Fish commented that “the probability of having really good weather at every site is almost zero.” However, according to Paine, each of the EHT sites may serve a different purpose for each target, either to act as a mission-critical observing location or to enhance the image quality. Which role an observatory plays during a particular observing run depends on the target location and date, he explained. The team may not need perfect conditions at all sites for every observation.
More Telescopes, More Targets
Although climate change has undoubtedly affected the 2007–2017 NOAA meteorological data, it hasn’t significantly influenced the EHT calculations, said Paine. Humidity outweighs temperature as the most important factor for getting clear radio observations, he explained. Although the global average humidity rose slightly over the 10 years of GFS data, he noted, it didn’t go up by enough to alter the team’s predictions.
Paine described the EHT atmospheric model as the first step in creating what he called a “merit function” that he and his colleagues will use to assess the value of conducting observations on a particular day. Continued access to NOAA’s GFS data, he said, will be critical to making the best use of limited observing time.
“[NOAA’s] resources are not only used for weather and climate tasks, but they’re also getting leveraged for things like astronomy,” he said. “We’re fortunate to have this resource for optimizing very expensive astronomical observations.”
—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern
Correction, 6 February 2018: An image caption and a researcher’s statement have been updated to more accurately describe the associated data.
Cartier, K. M. S. (2018), A decade of atmospheric data aids black hole observers, Eos, 99, https://doi.org/10.1029/2018EO092039. Published on 02 February 2018.
Text © 2018. The authors. CC BY-NC-ND 3.0
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