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In June 2015, we were treated to a show. A black hole 7,800 light-years away has awoken from a nap to devour a piece of a star – a messy process that has resulted in an eruption of howling light across the galaxy.
The echoes of this light, shown in a recently released image, allow astronomers to map and understand the invisible space dust that drifts between stars, as described in an article published in 2016.
This object is part of a binary system named V404 Cygni, found in the northern constellation Cygnus. The black hole is what is called a microquasar, surrounded by matter on which it feeds.
This material is siphoned off from the black hole’s binary companion, an early stage red giant; as the two pirouette their extremely close orbital dance, the black hole’s gravitational field strips the outer matter of the red giant.
This process is the cause of the 2015 explosion. When concentrations of matter from the star entered the accretion disk of the black hole, it triggered a series of intense x-ray light pulses, that we detected here on Earth.
These pulses also did something interesting in the space around V404 Cygni. As light propagated through the dust around the system, it resonated from it, producing a series of concentric rings of X-radiation.
It’s not unheard of, but it’s rare. Only three other X-ray light echoes have been detected from flaming stars in the Milky Way galaxy. So, naturally, astronomers took advantage of the echoes produced by V404 Cygni to learn more, not only about the explosion behavior of the black hole, but also the cosmic dust surrounding it.
Composite x-ray and optical image of the rings. (X-rays: NASA / CXC / U. Wisc-Madison / S. Heinz et al .; Optical / IR: Pan-STARRS)
Images from the Chandra X-ray Observatory show eight concentric rings, created by an eruption from the black hole, traveling through the dust between us and the binary. The rings are thin because the flare was short.
The diameters of the rings are also revealing. They tell us the distance between the ring and ourselves, because we know the distance to V404 Cygni and the speed at which light travels. The closest rings have larger diameters, the farthest rings are smaller – consider looking through a tube. The perspective will make the near end of the tube wider than the far end.
The illustration shows in detail how the ring structure is produced. (Univ. Of Wisconsin-Madison / S. Heinz)
The eight rings therefore represent eight separate dust clouds encountered by light as it propagates in space.
From this, we know that the nearest and larger ring is created by a cloud of dust about 3,363 light-years away and the furthest is about 6,934 light-years away.
Finally, the light that echoes the dust can be analyzed to determine the structure and composition of cosmic dust. Some elements absorb certain wavelengths of x-ray light, which means scientists can study the spectrum of x-rays that reach us to see what dust is made of. They found that cosmic dust is probably mostly silica and graphite, and also that it is not uniform in all directions.
This particular study was published in 2016, but continuing to observe light echoes will allow scientists to learn more about the dust that is usually invisible between stars. Additionally, V404 Cygni is rowdy, experiencing an explosion roughly every few decades, so we can expect future eruptions to help us understand how interstellar dust might change over time.
There is also a lot more we can learn from these explosions. For example, the 2015 explosion showed us that the magnetic field of the V404 Cygni black hole was much weaker than expected; and that the black hole is also wobbling, because of the way it trails space-time.
In fact, there have been a lot of articles published about the 2015 explosion of the V404 Cygni. It is the gift of the black hole that continues to give, and will likely continue to do so, well into the future.
Imagine if everyone looked at you like that every time you ate a snack.
The article was published in 2016 in The Journal of Astrophysics.
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