Are we talking about the camera now?
Xperia arc does support auto flash. Just semi-press the camera button and it will light up if needed.
For ray however you need to activate it manually before taking a photo.
I don't think there's any app for this unfortunately.
If you're looking for a flashlight/torch there are apps available but you need to test them to see if they work.
Do a search on 'flashlight' in Market and you'll find a lot. This one for example:
https://market.android.com/details?id=com.devuni.flashlight&feature=search_result#?t=W251bGwsMSwxLDE...
