A lot of historians challenge the traditional assumption that the bombs ended the war. It seems there was an attempt by the military to depose the Emperor and continue to fight. Many feel it was the declaration of war by Russia, not he bombings, that forced Japan to capitulate. In fact the theory is that the Japanese were much more willing to be occupied by US forces than Russian ones.
I say it is a stretch to say "a lot of historians challenge."
The main and central theory is based on Tsuyoshi Hasegawa's book; "Racing the Enemy: Stalin, Truman, and the Surrender of Japan" which I have on my bookshelf.
My interest from Hasegawa's work came from a former co-worker of Japanese ascent and distantly related to Hasegawa. He (co-worker) and I spent quite a bit of time discussing Hasegawa's theory of Japan decisions in surrendering to the USA instead of possibly at some point, surrendering to the Russians. And although the book makes a good argument, I did not come away totally convinced, but then again I am not a historian.
The end result is we may never know as the Japanese decision makers and military people in power at the time, are long gone. However the timing of the first A bomb drops then subsequent Japanese surrender in the days following makes a compelling reason for fear of more A bomb drops. That is not to say though that there may well of been some possible fear of being under Russian control that also hastened the surrender.
There is a fiction book (the name escapes me right now) that plays out in a fiction based on fact scenario of post war Japan and what would of happened if the Japanese indeed, came under Russian control.