The echo mapping process, also known as reverb mapping, begins when the drive is hot
In the center there is a supermassive blacka hole surrounded by a disk of material called an accretion disk. As the disk becomes brighter, short bursts of visible light appear. The blue arrows show the light from this flare moving away from the black hole, both towards an observer on Earth and towards a huge donut-shaped structure (called a torus) made of dust. The light is absorbed, causing the dust to heat up and emit infrared light. This lightening of the dust is a direct response—or, you might say, an “echo”—of changes occurring in the disk. The red arrows show this light moving away from the galaxy, in the same direction as the initial flash of visible light. Thus, the observer will first see visible light and then (with suitable equipment) infrared. Credit: NASA/Caltech Jet Propulsion Laboratory
Distance from accretion disk to innerparts of the dusty torus can be huge - billions or trillions of kilometers. Even light traveling at a speed of 300,000 km per second can travel this distance in months or years. If astronomers can observe both the initial burst of visible light in the accretion disk and the subsequent infrared brightening of the torus, they can also measure the time it took for light to travel between the two structures. Since light travels at a standard speed, this information gives astronomers an idea of the distance between the disk and the torus.
Scientists can then use the measurementdistances for calculating the luminosity of the disk and theoretically its distance from the Earth. How? The fact is that the temperature in the part of the disk that is closest to the black hole can reach tens of thousands of degrees. It is so high that even atoms are torn apart, and dust particles are able to form. As you move away from the black hole, the temperature of the disk gradually decreases.
Astronomers know that dust forms whenthe temperature drops to about 1,200 degrees Celsius. Thus, the more energy a disk emits, the further away dust will form. As a result, measuring the distance between the accretion disk and the torus gives an idea of the disk's output energy, which is directly proportional to its luminosity.
The idea to use echo mapping for measurementThe distance from Earth to distant galaxies is not new, but a new experiment by scientists is showing significant success in demonstrating its capabilities. This is the largest study of its kind, confirming that a similar measurement works the same in all galaxies, regardless of variables such as the size of the black hole.
However, due to many factors, the authors did notthere is enough distance measurement accuracy. In particular, scientists say they need to learn more about the structure of the inner regions of the dusty torus that surrounds the black hole. This structure can influence exactly what wavelengths of infrared light the dust emits when the light first reaches it.
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