NEWARK, Sept. 4— A Federal judge today granted a convicted
arsonist's request to be freed from jail, ruling that
prosecutors had failed to present exculpatory evidence to the
grand jury that indicted him.  The judge, Clarkson Fisher of
District Court, ordered that the man, Julio Vargas of Hoboken,
be freed from the Hudson County Jail, where he has been held
since Feb. 22.  He also ordered the indictment dismissed.  One
court official called the granting of the writ of habeus corpus
''very rare.'' He noted that hundreds of state and Federal
prisoners filed such writs each year in an attempt to win
freedom.  Judge Fisher ruled that Hudson County prosecutors had
violated Mr. Vargas's right to due process by failing to tell
the grand jury that a witness who implicated him had later
recanted his story. That witness was one of four men indicted
with Mr. Vargas.  The witness later entered a plea bargain and
testified against Mr. Vargas at his trial. The witness confessed
to conspiring with Mr. Vargas to enlist the t