için by mistake is written twice. I corrected the text above.
bunu (this one) is the accusative case of bu. İt is smillar to sen-seni, o-onu, şu-şunu

"bu konuyu" > bunu

bunu as a pronoun represents "bu konuyu" in the sentence.
YOu could say:
"Çok teşekkürler Erdinç bunu araştırdığın için."
or
"Çok teşekkürler Erdinç bu konuyu araştırdığın için."

Examples:
1. Kalemi ver. > pass me the pencil.
Onu ver.

2. Bu çok güzel. Bunu çok sevdim.
Here bunu can represent any object the speaker is referring to. It could be :
bu arabayı, bu rengi, bu şarkıyı, ...

3.
-Hangi oyuncağı beğendin?
-Bunu beğendim. (Bu oyuncağı beğendim).

Sorry Erdinç, I have read over your explanation many times and totally do not understand it

I am really really struggling with the idea of applying noun states to pronouns!!! I just cannot get my head around when you use one state and when you use another and I can find any rules that describe this.......I am beginning to lose sleep over pronoun states

You must have read the basics here already, but I post the link anyway: http://www.turkishclass.com/grammar_nounStates.htm

With that in mind, you can apply some logic in a lot of cases. When in doubt, just check the dictionary. I am sure there is not one person studying Turkish who knows all the states that are used with certain verbs by heart.

I saw a post by you with part of a survey of pronouns in different cases. Study those, and that's it. Just try to imagine that you use the equivalent of a noun. When you use dative with a certain noun, then you have to use the dative form of a pronoun as well.

For ex.:
I look at the cat - Kediye bakıyorum
I look at it - Ona bakıyorum

I like wine - Şaraptan hoşlanıyorum
I like it - Ondan hoşlanıyorum

I eat the pear - Armutu yiyorum
I eat it - Onu yiyorum

So, because the verb araştırmak can only be applied to nouns in the accusative state, we would conjugate it as follows:

Beni araştırıyorum
Seni araştırıyorsun
Onu araştırıyor

Is that right???