def METHOD_NAME(self):
    """
    :return: rms value of background noise
    """
    if self._background_rms is None:
        if self._noise_map is None:
            raise ValueError(
                "rms background value as 'background_rms' not specified!"
            )
        self._background_rms = np.median(self._noise_map)
    return self._background_rms