code: def _get_tau(self, C, mag):
          
          if mag < 6.5:
              return C["tau1"]
          elif mag < 7.:
              return C["tau1"] + (C["tau2"] - C["tau1"]) * ((mag - 6.5) / 0.5)
          else:
              return C["tau2"]
docs: Returns magnitude dependent inter-event standard deviation (tau)
          (equation 14)