Simulated image of the supermassive black hole in M87 seen at multiple frequencies. (Image credit: EHT, D. Pesce, A. Chael, CC BY 4.0)
Astronomers with the Event Horizon Telescope have developed a new way to observe the radio sky at multiple frequencies, and it means we will soon be able to capture color images of supermassive black holes.
Color is an interesting thing. In physics, we can say the color of light is defined by its frequency or wavelength. The longer the wavelength, or the lower the frequency, the more toward the red end of the spectrum light is. Move toward the blue end, and the wavelengths get shorter and the frequencies higher. Each frequency or wavelength has its own unique color.
Of course, we don’t see it that way. Our eyes see color with three different types of cones in our retina, sensitive to red, green, and blue light frequencies. Our minds then use this data to create a color image. Digital cameras work similarly. They have sensors that capture red, green, and blue light. Your computer screen then uses red, green, and blue pixels, which tricks our brain into seeing a color image.
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While we can’t see radio light, radio telescopes can see colors, known as bands. A detector can capture a narrow range of frequencies, known as a frequency band, which is similar to the way optical detectors capture colors. By observing the radio sky at different frequency bands, astronomers can create a “color” image.
But this is not without its problems. Most radio telescopes can only observe one band at a time. So astronomers have to observe an object multiple times at different bands to create a color image. For many objects, this is perfectly fine, but for fast-changing objects or objects with a small apparent size, it doesn’t work. The image can change so quickly that you can’t layer images together. Imagine if your phone camera took a tenth of a second to capture each color of an image. It would be fine for a landscape photo or selfie, but for an action shot the different images wouldn’t line up.
This is where this new method comes in. The team used a method known as frequency phase transfer (FPT) to overcome atmospheric distortions of radio light. By observing the radio sky at the 3mm wavelength, the team can track how the atmosphere distorts light. This is similar to the way optical telescopes use a laser to track atmospheric changes. The team demonstrated how they can observe the sky at both a 3mm and 1mm wavelength at the same time and use that to correct and sharpen the image gathered by the 1mm wavelength. By correcting for atmospheric distortion in this way, radio astronomers could capture successive images at different radio bands, then correct them all to create a high-resolution color image.
This method is still in its early stages, and this latest study is just a demonstration of the technique. But it proves the method can work. So future projects such as the next-generation EHT (ngEHT) and the Black Hole Explorer (BHEX) will be able to build on this method. And that means we will be able to see black holes live and in color.
The original version of this article was published on Universe Today.
