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Decoding a Black Hole Jet.

VISIBLE LIGHT AND X-RAYS helped astronomers paint a detailed picture of the relativistic jet in V404 Cygni, a black hole-star system. The results appear in Nature Astronomy, published online on October 30th.

Poshak Gandhi (University of Southampton, UK) and colleagues employed NASA's NUSTAR X-ray satellite and the super-fast UltraCam on the William Herschel Telescope in La Palma, Spain, to track emissions from V404 Cygni. The black hole tugs gas away from its stellar partner as they whip around each other, resulting in flares. In June 2015 V404 Cygni underwent the brightest binary outburst in the 21st century.

Astronomers had previously monitored visible light from black hole systems, but they hadn't been able to pinpoint its origin--the photons could arise in the gaseous disk that feeds the black hole, the stellar companion that feeds the disk, or the jets that the black hole-disk system powers.

The addition of X-ray data resolves this ambiguity. By exactly timing incoming X-rays and visible photons, Gandhi and colleagues discovered that visible-light flashes trailed X-ray flares by 0.1 second. So the visible-emitting region has to be in the jet, some 30,000 km (19,000 miles) downstream from its X-ray-emitting origin. The two regions bookend the acceleration and collimation zone, a poorly understood space where the jet's plasma narrows and achieves relativistic speeds.

The results mesh nicely with previous studies of some supermassive black holes, says Alan Marscher (Boston University), but not all of them: The supermassive black hole at the center of the galaxy M87, for example, exhibits a different delay, suggesting that some details remain to be worked out.

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Title Annotation:BLACK HOLES
Author:Young, Monica
Publication:Sky & Telescope
Date:Jan 10, 2018
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