View Full Version : X-Ray echoes from a black hole with a companion star: V404 Cygni

2015-Jul-15, 03:29 PM
Black Hole Bull's Eye (http://heasarc.gsfc.nasa.gov/docs/objects/heapow/archive/compact_objects/V404Cyg_swift.html)
Time lapse (http://svs.gsfc.nasa.gov/vis/a010000/a011900/a011948/rings_1080.gif) showing how the rings expand with time.

2015-Jul-19, 12:04 AM
That's quite something! I find it odd that they say "V404 Cyg is a binary star system in which a normal, sun-like star (http://www2.lowell.edu/users/jch/workshop/drs/drs-p1.html) is orbited by a black hole (http://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html)." I daresay this would normally be viewed the other way around!

I'm not sure I follow the mechanism here, though. It starts out OK:

"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."

I get that.

"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 (http://www.skyandtelescope.com/astronomy-news/witnessing-light-echoes-from-a-neutron-star-0630201567/), 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 (http://svs.gsfc.nasa.gov/vis/a010000/a011900/a011948/rings_1080.gif)."

I can see the dust causes a time delay, but how does that produce the observed rings, unless the x-ray outbursts were intermittent in the first place?

2015-Jul-19, 12:51 AM
Inhomogeneous dust distributions also give rings. Compared to the light-travel timescale of an outburst, the dust may in be narrow enough regions to act like sheets; a sheet tilted to the plane of the sky makes an off-center eclipse, such as was seen in the light echo from supernova 1987A.