So, in a previous post on Hanukkah and Purim, I noticed some rather interesting claims that I said I would need to investigate. While writing that post, I did an internet search for the "pagan origins of Purim" just to be thorough, not expecting to find anything. I did however, find quite a bit. I also did some searching for evidence both for and against the historical accuracy of Esther. I found this document (a PDF file) which goes into detail about the timing of events according to the records of the period that we have available and how they line-up with the book of Esther. Before we go any further, I do want to point out that the historical accuracy of the book of Esther has absolutely no bearing on the historical accuracy, or the authority, of the rest of the Bible.
The book of Esther is written as a historical document. While it may be the case that the book is intended to be historical fiction, I believe it's more likely the case that it's intended to be an actual historical document, meaning that the document being proven false would prove it to be a fraud. Now, one of the arguments to support the claim that the book is historical fraud is known as an "argument from absence". The argument from absence is that if we don't find any evidence in favor of a claim, then that is evidence against the claim. This is a fallacious argument in most cases. The rare exception being concerning what is or is not sin. If it's not contrary to God's Law, then it is, by definition, not sin.
So far, I have not found any credible historical evidence contradicting the book of Esther. But neither is there any definitive evidence in support of the book. There is circumstantial evidence in favor of the historical accuracy of the book. And there's also circumstantial evidence against the historical accuracy of the book (eg: the names of the two main characters). Some arguments I've read in defense of Esther, and explaining the lack of extra-biblical evidence is this: that these events were embarrassing to the Persians, so they either (a) did not record these events, or (b) they expunged the record of those events.
Another argument used in defense of Esther is that there were too many witnesses that could contradict the narrative, and that there are details that only a government official would know. This however, ignores the argument that the book was written long after the events it claims to record; long after all the witnesses had died and long after the fall of the Persian empire when nobody would be able to verify the details.
It should be noted that for these types of claims, there is typically a chain of authentication that exists that refutes such claims. For example, some claim that Torah was written during the Babylonian exile or shortly after. But this fails to take into account the historical documentation referring back to Torah that existed long before the exile ever happened. This historical documentation produces a chain of authentication that goes back almost 1,000 years before the exile, which is about the time of Joshua, thus refuting the claim. That and the fact that there are manuscripts of the Torah that date back to before the exile.
Unfortunately, no such chain of authentication exists for the book of Esther. And I have not been able to find any evidence either supporting or refuting this claim. All the evidence supporting Esther is circumstantial, and all the evidence against Esther is circumstantial. What about the pagan origins of many of the practices of Purim? Those practices aren't recorded as being part of the original celebration. Sure, it may be an indication that the holiday is a sanitized pagan holiday, but it could also be that the day has been paganized through thousands of years of tradition. What about the trend of the biblical accounts eventually being proven correct? This has not happened with the book of Esther, unlike everything else in the Bible. Esther told us exactly where to look for the evidence and the evidence was not found there.
I cannot give a definitive answer to the question of Esther's authenticity. But in my opinion, it is probably authentic.