BOSTON (AP) -- Federal prosecutors say a request by former Boston mobster James "Whitey" Bulger to remove his trial judge is "frivolous and unsubstantiated."

Their motion filed late Monday also calls "absurd" a claim by Bulger's attorney that the one-time FBI informant shouldn't be prosecuted for his role in 19 murders because the government promised him immunity years ago.

Bulger's attorney J.W. Carney Jr. filed a motion last month to remove Judge Richard Stearns because he was a top federal prosecutor when Bulger allegedly was committing crimes with impunity. Carney argued that Stearns would try to shield his former colleagues and could not be impartial.

Prosecutors said those arguments are baseless and should be dismissed without a hearing.

Bulger was captured in California last year after 16 years on the run. His trial is set for March 2013.