def new(self, grp_id, branch_id):
        """
        :param grp_id: ordinal of the source group
        :param branch_name: name of the UCERF branch
        :param branch_id: string associated to the branch
        :returns: a new UCERFSource associated to the branch_id
        """
        new = copy.copy(self)
        new.orig = new
        new.src_group_id = grp_id
        new.source_id = branch_id
        new.idx_set = build_idx_set(branch_id, self.start_date)
        with h5py.File(self.source_file, "r") as hdf5:
            new.start = 0
            new.stop = len(hdf5[new.idx_set["mag"]])
        return new
:param grp_id: ordinal of the source group
        :param branch_name: name of the UCERF branch
        :param branch_id: string associated to the branch
        :returns: a new UCERFSource associated to the branch_id