One of the cooler and least talked about phenomena in astronomy is the light echo. Any time a momentary bright something occurs (supernova, stellar flare, etc…), a packet of light is sent rocketing through space like a “Hallo!” vocally screamed out from a canyon floor. And when that light hits something – dust, gas, ices – it get echoed just as sound gets echoed when it hits a canyon wall. Astronomers using the X-ray observatories XMM-Newton and now Swift have discovered that distant gamma-ray bursts are able to create light echos in clouds of material in our own galaxy and others.
The most famous light echo is V838 Mon, a star that had a complex brightening and fading back in 2002 that isn’t fully understood. As the light from this event has moved through space, it has progressively illuminated larger shells of material around the star. While the light itself traces out a nice spherical shell, the material it is illuminating is oddly distributed, creating a fabulous opportunity to take pretty pictures with Hubble. Most light echos, however, are much more boring and appear are perfect arcs or perfect circles created when a shell of light passes through a cloud of gas. For instance, light echos from supernova that occurred 400-600 years ago have been found in the Large Magellanic Cloud.
In each of these cases, the light echos are created by a spherical object emitting a spherical shell of photons that travel through space. When these photons hit something, they scatter off of it and back to us, effectively illuminating the object.
Another way to create a light echo is to send a beam of photons into a thin cloud. As the photons hit particles in the thin cloud they scatter. The photons that get scattered through the cloud may travel a short distance before getting scattered back toward the observer. These scattered and then rescattered photons can make it appear that a disk of material is illuminated in the cloud.
Now, if instead of shining a beam of photons into the cloud, you send a tight packet in the form of a burst (like a gamma ray burst), those photons will hit the cloud and some will get reflected and then re-reflected such that it appears there is a growing halo of light in the cloud. As the halo grows, it gets fainter as the photons fill a larger and larger volume, in the same way that a growing smoke ring gets more and more diffuse as it grows and its particles fill a larger and larger volume. With the gamma ray bursts, these light rings are created by the bright X-ray photons that follow after the initial burst of gamma rays. It is possible to watch the X-Rays brighten and fad and then watch the same brightening and fading in a growing halo of illuminated material.
In a paper by G. Vianello, A. Tiengo, and S. Mereghetti, two gamma-ray burst related light halos observed using the Swift space observatory were discussed. In a neat application of geometry, they were able to use the expanding halo to measure the distance to one of the clouds. Light conveniently travels at a set speed. As the light moves through the cloud, illuminating new shells of material, the apparent angular size of the illuminated halo grows. How big the halo is after 5 seconds, 30 seconds, or 200 seconds is strictly a function of the distance the cloud (which admittedly is changing with time, but that can be ignored if the cloud is close enough). In short, nearby clouds will have larger halos after 50 seconds, farther clouds will have a smaller appearing halos, and it is possible to employ some not straight forward math (stupid expanding universe ruins a simple equation again) to sort out distances.
So, this brings us to why this is useful. When a gamma ray burst pierces through an intervening cloud of material and creates a light halo, we can map out the location of the gas. In some cases, these diffuse gas clouds may be extremely distant and otherwise invisible. Because of scattering we not only see a neat ring, but we can discover and measure the distance too. We know there is a fair amount of invisible stuff out there and this is definitely a great way to measure it out.