Astronomers are currently fascinated by the outburst of an unusual stellar system in the constellation of Cygnus called V404 Cygni. V404 Cygni went into outburst in mid-June, and as of this writing this ouburst is still ongoing (though apparently waning). V404 Cyg is a binary star system in which a normal, sun-like star is orbited by a black hole. X-rays are produced when the black hole's gravity pulls matter from the normal star; as the matter spirals down onto the black hole, it produces an extremely hot, X-ray emitting accretion disk. This accretion process can be unstable, and if the accretion rate increases dramatically, the black hole can show a bright outpouring of energy over nearly the entire electromagnetic spectrum, including X-rays. The last time V404 Cyg experienced a similar outburst was 1989. The image above shows an X-ray image of V404 Cyg obtained by the X-ray Telescope on the Swift High-Energy satellite observatory during this current outburst. V404 Cygni is the bright point source at the center of the image. The prominent bright X-ray rings surrounding the system are actually X-ray "light echoes". These X-ray echoes are produced by walls of dust in front of the system which are illuminated by the X-rays produced by the outburst. These walls of dust scatter some of the X-rays from the V404 Cygni outburst to us, but because the dust-scattered X-rays travel a longer distance, the scattered X-rays reach us slightly later than light traveling a more direct path. These time delays produce the light echoes which form rings which expand with time.
GMM: X-ray Echoes Create a Black Hole Bull's-Eye
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