I ask this question because in pali, "to let go (nibbāna)", in this context, is known by monk. It is not just a target. The (ariya) monk thinking of nibbāna in his concentration, in this context.
I just wanna make sure that english translation is right, because when I translate that english sentence, by my terrible english, it look like "to let go " and "object" means "target", not "something is known by monk".
In the pāli: ārammaṇa means "something is known by consciousness", "something is thought by consciousness", "external sense-fields". It doesn't means target/object.
"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to let go, attains concentration, attains singleness of mind. http://www.accesstoinsight.org/tipitaka/sn/sn48/sn48.010.than.html
Edited:
It should to be:
"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making "to let go" as his mind's focus, attains concentration, attains singleness of mind.