How best to respond to 「ご無沙汰しています」

Question

I just received an email from a good friend in Japan, who has notably over the past few months been instrumental in connecting me to various people, for some research interviews I've been conducting.

The data collection phase of the research has long been completed, but she has just sent me an email with some additional data attached. Given we haven't communicated for some time (as I've been writing up the results of the research, and the phase of research she was helping with is over), her email starts thus:

ヘンリー

ご無沙汰しています。

I'm aware of the meaning of the phrase 「ご無沙汰しています」 -- but how best to respond to it in my reply?

Would a simple 「こちらこそ」 suffice, or would maybe 「私もご無沙汰しています」be better?

I had also considered 「ご無沙汰していますね」 but I don't know if that's got a potentially rude connotation here (i.e. I'm somehow highlighting the lack of communication on her side, rather than agreeing to the plain fact that we've not communicated for a while).

Many thanks! よろしくお願いします。

Answer 1

If you were speaking then こちらこそ alone would be fine, but since it is email, I think it would be best to add ご無沙汰しています。

こちらこそ、ご無沙汰しています。

This is to avoid the disconnect of "こちらこそ about what?"

If she is even a little 目上 or you just want to express added politeness change the います to おります。

こちらこそ、ご無沙汰しております。

Both of these versions can be modified with words like すっかり as in

こちらこそ、すっかりご無沙汰してます。

On the other end of the spectrum, if you want to keep everything more casual,

ご無沙汰していますね

can work too, but I think it is more of a spoken expression.

As you may already know, the more casual you get, the more options you have.
Aside from ご無沙汰していますね you could also say

お久しぶりです。お元気でしたか。

Or even

メールありがとうございます。お元気でしたか？

These casual replies all work if you are certain of the closeness of your relationship.

Many thanks to Chocolateさん who helped me flesh out this answer with many more options