This IS a tough question. As a mother myself I feel for you deeply. I also grew up with an addict for a father. There was a period of time, pretty sure this happened when I was in 4th or 5th grade, when my dad had a horrible relapse. Apparently he left for 8 months! I have always known there was a short time that my dad was gone because he had been using. It was only a couple months ago that my mom and I were talking about it all that I found out he was really gone for 8 months! I don't know why I cannot remember him being gone for so long. I do vaguely remember asking my mom a few times where he was. I do not remember her exact answer but she did not provide me the details I do know that. I want to say she just said that he had to be away for a while.
Has your daughter asked you already where her father is? I can by no means tell you what you should do. But, if my son or daughter hadn't asked yet then I probably wouldn't go out of my way to up and tell them. But if they did ask I am not sure as to how much detail I would provide but you have to say something. One thing I think is important is not to bash her father to her. If you decide to get into the details of why he is gone, then leave it as just that. He made a mistake just like any human in this world and he is now going to serve the consequences. Do not make it about him being a bad person, irresponsible, or that he just doesn't love his family enough. All of the hurt and anger you may feel toward him is valid, but an 8 year old is only impressionable.
I am so sorry what you are going through. I hope I was at least able to help you find some sort of direction. There probably is not a right or wrong thing to do here in this situation. But you do have a choice about HOW you go about it.
God Bless! Remember, one day at a time!happyface