HEAPOW: Black Hole Bull's Eye (2015 Jul 13)
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
http://asterisk.apod.com/viewtopic.php?t=34912
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