The black hole–part of a binary system called V404 Cygni, located about 7,800 light-years away from Earth– is actively pulling material away from a companion star — with about half the mass of the Sun — into a disk around the invisible object creating acolossal set of rings, has been captured using NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory. The giant rings, which glow in X-rays, so astronomers refer to these systems as “X-ray binaries,” have revealed new information about dust located in our Galaxy.
On June 5 2015, Swift discovered a burst of X-rays from V404 Cygni. The burst created the high-energy rings from a phenomenon known as light echoes shown in the NASA image below. Instead of sound waves bouncing off a canyon wall, the light echoes around V404 Cygni were produced when a burst of X-rays from the black hole system bounced off of dust clouds between V404 Cygni and Earth. Cosmic dust is not like household dust but is more like smoke, and consists of tiny, solid particles.
“It’s a Mystery”
“In 2015, we did some study of the dust composition itself, finding it best fits a model with separate dust populations of carbon and silicate grains, Harvard’s Randall Smith told The Daily Galaxy. “This is a bit hard to understand, since we’d expect that the grains would evolve inside clouds, merging the silicate and graphite grains together. However, we see no evidence for it, and in fact models that include such things don’t fit as well. It’s a mystery.
“NASA’s new image shows the power of multiwavelength images – the optical light from the stars reveals information about the surface of the dust grains, while the X-rays probe the entire grain, showing both where the grains are in the Galaxy and what they’re made of,” continued Smith. “With more data like this, we hope to understand just how dust cycles through stars and clouds in the Galaxy, forming planets and stars that go on to expel more dust.”
The Chandra Data
“One thing that really surprised me about the Chandra data was just how precisely we were able to pinpoint the distance to the different clouds,” wrote astronomer Sebastian Heinz at the the University of Wisconsin in Madison, in an email to The Daily Galaxy. “The spatial resolution really allowed us to measure distances down to percent level, which, in astronomy, is very rare. And the resolution also allowed us to identify even very, faint dust clouds—all 8 of them. The combination of Chandra with the data from the Neil Gehrels Swift Observatory,” Heinz continued, “allowed us to constrain the composition and nature of the dust grains. While this method does not allow molecular spectroscopy, we used it to determine that the dust itself seems to be composed of fairly standard silicate and graphite dust, rather than more complex grain composition with ice mantles, for example.”
In a new composite image shown at the top of the page, X-rays from Chandra (light blue) have been combined with optical data from the Pan-STARRS telescope on Hawaii that show the stars in the field of view. The image contains eight separate concentric rings. Each ring is created by X-rays from V404 Cygni flares observed in 2015 that reflect off different dust clouds. (An artist’s illustration explains how the rings seen by Chandra and Swift were produced. To simplify the graphic, the illustration shows only four rings instead of eight.)
The team analyzed 50 Swift observations made in 2015 between June 30 and August 25. Chandra observed the system on July 11 and 25. It was such a bright event that the operators of Chandra purposely placed V404 Cygni in between the detectors so that another bright burst would not damage the instrument.
Rings Tell a Story
The rings tell astronomers not only about the black hole’s behavior, but also about the landscape between V404 Cygni and Earth. For example, the diameter of the rings in X-rays reveals the distances to the intervening dust clouds the light ricocheted off. If the cloud is closer to Earth, the ring appears to be larger and vice versa. The light echoes appear as narrow rings rather than wide rings or haloes because the X-ray burst lasted only a relatively short period of time.
The researchers also used the rings to probe the properties of the dust clouds themselves. The authors compared the X-ray spectra — that is, the brightness of X-rays over a range of wavelengths — to computer models of dust with different compositions. Different compositions of dust will result in different amounts of the lower energy X-rays being absorbed and prevented from being detected with Chandra. This is a similar principle to how different parts of our body or our luggage absorb different amounts of X-rays, giving information about their structure and composition.
The team determined that the dust most likely contains mixtures of graphite and silicate grains. In addition, by analyzing the inner rings with Chandra, they found that the densities of the dust clouds changes are not uniform in all directions. Previous studies have assumed that they did not.
This result is related to a similar finding of the X-ray binary Circinus X-1, which contains a neutron star rather than a black hole, published in a paper in the June 20, 2015, issue of The Astrophysical Journal, titled, “Lord of the Rings: A Kinematic Distance to Circinus X-1 from a Giant X-Ray Light Echo” (preprint). This study was also led by Sebastian Heinz.
The V404 Cygni results were led by the same astronomer, Sebastian Heinz of the University of Wisconsin in Madison. This paper was published in the July 1, 2016 issue of The Astrophysical Journal (preprint).
Image credit: X-ray: NASA/CXC/U.Wisc-Madison/S. Heinz et al.; Optical/IR: Pan-STARRS