def get_spectral_index(src, egy):
    """Compute the local spectral index of a source."""
    delta = 1E-5
    f0 = src.spectrum()(pyLike.dArg(egy * (1 - delta)))
    f1 = src.spectrum()(pyLike.dArg(egy * (1 + delta)))

    if f0 > 0 and f1 > 0:
        gamma = np.log10(f0 / f1) / np.log10((1 - delta) / (1 + delta))
    else:
        gamma = np.nan

    return gamma