Identification help
This species is very rare in Austria. During the 2014 spring and summer a massive migration of N. xanthomelas was observed in the North-West of Europe. Austria is not exactly in the Nord-West of Europe, and this photo was taken in 2013 - so, anyway.
In the field its difficult to distinguish between a polychloros and xanthomelas. If you compare wings, like in this comparison photo, the jagged forewing and the blue line on the backwing make it the N. xanthomelas - to the right the you see N. polychloros.
I stumbled upon this in Austria in July 2013 - took it as the N. polychloros. First in 2017 I looked at it again - and was surprised to find the N. xanthomelas.
Please note: Although the flight-period is around July until September, hibernated species could appear in May.
Read more about the massive migration of 2014 here: Nymphalis xanthomelas (Esper, 1781) French with English abstract.