[BAA-ebulletin 00861] Light variations from a black hole!

[Rick]

[BAA-ebulletin 00861] Light variations from a black hole!
(c) 2015 British Astronomical Association    http://www.britastro.org/

The variable star V404 Cyg has just gone into outburst but there is more to it than that as V404 Cyg is an X-ray nova and a low mass X-ray binary (LMXB) with a black hole component.  It also had outbursts in 1938, 1956, possibly in 1979 and in 1989. It has has been intensively monitored since then including by X-ray satellites.

However, perhaps the most interesting thing about V404 Cyg is that observers, including myself and Nick James using CCD cameras, have reported light changes of almost 2.5 magnitudes in less than 25 minutes!  This was from mag 13.0 to mag 15.0.  Most observers with moderate telescopes should be able to see an object of mag 13 visually and will then see this object disappear as it plunges down to mag 15 or so.  However, there is every chance that V404 Cyg may yet increase in brightness and the rapid fluctuations should become easier to see.  CCD observers should have no difficulty in recording these variations.

It is amazing to realise the light is coming from close to a black hole situated at the centre of this binary system.

Finder charts can be obtained from the AAVSO Variable Star Plotter at http://www.aavso.org/vsp where your specific orientation and field of view can be entered to suit.

I must also mention our friend R CrB which after promising to return to normal brightness of around mag 6, only reached mag 7 and has now plunged to below mag 12 again.  So, if you missed seeing it in your binoculars this time, who knows when it will recover again to become visible in such instruments?

If you do make any observations of either R CrB or V404 Cyg, please report then to the Variable Star Section.

Roger Pickard (roger.pickard at sky com)
19/06/2015