Enormous rings round a black gap – with that?


August 5, 2021

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NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory recorded a spectacular series of rings around a black hole. The x-rays of the giant rings have revealed new information about dust in our galaxy, on a principle similar to the x-rays used in doctors’ offices and airports.

The black hole is part of a binary star system called V404 Cygni, which is located about 7,800 light years from Earth. The black hole actively pulls material away from a companion star – with about half the mass of the sun – into a disk around the invisible object. This material glows in X-rays, so astronomers refer to these systems as “X-ray binaries”.

On June 5, 2015, Swift detected an explosion of X-rays from the V404 Cygni. The burst created the high energy rings from a phenomenon known as light echoes. Instead of sound waves bouncing off a canyon wall, the light echoes were generated around V404 Cygni when an eruption of X-rays from the black hole system ricocheted off dust clouds between V404 Cygni and Earth. Cosmic dust is not like household dust, but more like smoke and is made up of tiny, solid particles.

In a new composite image, X-rays from Chandra (light blue) have been combined with optical data from the Pan-STARRS telescope in Hawaii, showing 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 and reflected from various clouds of dust. (An artist’s illustration explains how the rings seen by Chandra and Swift were made. To simplify the graphics, the illustration shows only four rings instead of eight.)

The team analyzed 50 Swift observations between June 30 and August 25, 2015. Chandra observed the system on July 11th and 25th. It was such a bright event that Chandra’s operators purposely placed V404 Cygni between the detectors so that another bright burst would not damage the instrument.

The rings tell astronomers not only about the behavior of the black hole, but also about the landscape between V404 Cygni and Earth. For example, the diameter of the rings in the X-ray image reveals the distance to the clouds of dust in between, which the light bounces off. If the cloud is closer to the earth, the ring appears larger and vice versa. The light echoes appear as narrow rings rather than wide rings or halos because the X-ray beam lasted only a relatively short period of time.

The researchers also used the rings to study the properties of the dust clouds themselves. The authors compared the X-ray spectra – i.e. the brightness of X-rays over a range of wavelengths – with computer models of dust with different compositions. Different dust compositions mean that different amounts of the low-energy X-rays are absorbed and, with Chandra, prevented from being detected. This is a similar principle to how different parts of the body or our luggage absorb different amounts of X-rays and provide information about their structure and composition.

The team found that the dust most likely contains mixtures of graphite and silicate grains. By analyzing the inner rings with Chandra, they also found that the density of dust cloud changes is not uniform in all directions. Previous studies suggested this was not the case.

This finding is related to a similar finding from the Circinus X-1 x-ray binary system, which contains a neutron star rather than a black hole, that was reported in an article in the June 20, 2015 issue of the Astrophysical Journal entitled “Lord of the Rings : A kinematic distance to the Circinus X-1 from a huge X-ray light echo ”(preprint). This study was also led by Sebastian Heinz.

The V404 Cygni results were directed by the same astronomer, Sebastian Heinz of the University of Wisconsin at Madison. This paper was published in the July 1, 2016 issue of the Astrophysical Journal (Preprint). The study co-authors are Lia Corrales (University of Michigan); Randall Smith (Center for Astrophysics | Harvard & Smithsonian); Niel Brandt (Pennsylvania State University); Peter Jonker (Dutch Institute for Space Research); Richard Plotkin (University of Nevada, Reno) and Joey Neilson (University of Villanova).

NASA’s Marshall Space Flight Center administers the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science from Cambridge, Massachusetts and flight operations from Burlington, Massachusetts.

Photo credit: Roentgen: NASA / CXC / U.Wisc-Madison / S. Heinz et al .; Optical / IR: Pan-STARRS

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