def _spectrum(self, photon_energy):
        """Compute differential IC spectrum for energies in ``photon_energy``.

        Compute IC spectrum using IC cross-section for isotropic interaction
        with a blackbody photon spectrum following Khangulyan, Aharonian, and
        Kelner 2014, ApJ 783, 100 (`arXiv:1310.7971
        <http://www.arxiv.org/abs/1310.7971>`_).

        Parameters
        ----------
        photon_energy : :class:`~astropy.units.Quantity` instance
            Photon energy array.
        """
        outspecene = _validate_ene(photon_energy)

        self.specic = []

        for seed in self.seed_photon_fields:
            # Call actual computation, detached to allow changes in subclasses
            self.specic.append(
                self._calc_specic(seed, outspecene).to("1/(s eV)")
            )

        return np.sum(u.Quantity(self.specic), axis=0)
Compute differential IC spectrum for energies in ``photon_energy``.

        Compute IC spectrum using IC cross-section for isotropic interaction
        with a blackbody photon spectrum following Khangulyan, Aharonian, and
        Kelner 2014, ApJ 783, 100 (`arXiv:1310.7971
        <http://www.arxiv.org/abs/1310.7971>`_).

        Parameters
        ----------
        photon_energy : :class:`~astropy.units.Quantity` instance
            Photon energy array.