In written English you write it how you say it, because there is no separate rule for whether to use “a” or “an” when writing.
There is no right or wrong, it is solely determined by pronunciation so you will see both written, even within the same publication.
The “a historic” form is now more prevalent, manly due to the spread of R.P. and diminishment of regional accents. Although this page thinks it could be an Americanism, with a graph showing it only became the dominant spelling in the 1980s:
http://www.betterwritingskills.com/tip-w005.html
But where I come from “a historic” looks bloody daft. You would have to pronounce that as if it were two words!