Astronomers got a bit of a shock when an eighth magnitude nova appeared in the constellation of Cygnus. That’s bright enough for a pair of binoculars to show an additional star (but nowhere near naked eye visibility). The surprises continued when it was discovered the event occurred at the position of the twelfth-fourteenth magnitude variable star V407 Cyg. The surprise was we knew it was variable, but not that variable.
V407 Cyg is a symbiotic variable containing an M class red giant in a binary system with a white dwarf star – the remains of a previous red giant that has expelled its outer shell. The remaining giant sends material out via the usual stellar winds process and the white dwarf ionises the material, creating a glowing nebula. Some of the material also accretes onto the white dwarf. Variability can come from changing densities within the nebula, pulsations of the already variable red giant or the creation of an accretion disc around the white dwarf. If material crashes down onto the surface of the white dwarf after some event in the disc, this can cause a nova-like flare up.
Before the 11th of March, this particular star had confined its variability to the pulsations of the red giant. Then it suddenly went one hundred times brighter. Not only that but the absoption spectrum of the system (signatures of cooler materials absorbing light from the system) was overwhelmed by the blue continuum of the white dwarf. The emission spectrum showed the nova like outburst as well as the ionised stellar winds, a spectrum unlike the type of variable this star was known to be and more like a star such as RS Ophiuchi. The sudden brightening was also another factor in common – when stars like V407 Cyg (Mira Variables) normally go, they exhibit a slow rather than sudden rise in brightness.
A final surprise was unearthed when Fermi, the space based gamma ray telescope, spotted the star. This would imply the emission of gamma rays from the nova, a feature never seen in symbiotic variables before. This is possibly due to the interaction of shocked and ionised material and the magnetic fields in the system.
It is likely V407 Cyg will prove to be an interesting study in terms of how its light varies from here on in. The accumulation of a dusty nebula and the interaction of a fresh nova system with it should produce a complex and interesting light curve well within the reach of moderate telescopes in amateur hands.