On the bridge of Arcole, Muiron, one of Napoleon's aides de camp, was killed.

What was his rank ? The sources seem to be in doubt.

An example for capitaine in a dictionary:

https://books.google.de/books?id=dsVBAAAAcAAJ&pg=PA419&dq=muiron

Some books give him the rank of colonel:

https://books.google.de/books?id=T8vjAAAAMAAJ

as does a famous poet:

https://books.google.de/books?id=ywJKAAAAcAAJ

and a description of the Arc de Triomphe:

https://books.google.de/books?id=vCCd8aD23aYC&pg=PA5&dq=muiron

Google Books yields 1540 results for the "colonel muiron" search, and but 231 results for "capitaine muiron".

What are the reliable sources about his rank and what do they say ?

General Bonaparte himself called him "citoyen Muiron", which doesn't help much:

https://books.google.de/books?id=cjWvptNjZoMC&pg=PA208

On 26 October 1796, Muiron was still chef de bataillon, which rules out the capitaine, but doesn't confirm the colonel.

https://books.google.de/books?id=cjWvptNjZoMC&pg=PA77&dq=muiron