def Powerlaw(x, amp, index, xp):
    """
    Differential spectrum of simple Powerlaw normlized at xp
    :param x: Energies where to calculate differential spectrum
    :param amp: Amplitude of pl
    :param index: Index of pl
    :param xp: Normalization energy
    :return: differential spectrum of a Powerlaw at energies x
    """
    return amp*(x/xp)**index