def kerr_lightring_velocity(chi):
    """Return the velocity at the Kerr light ring"""
    # If chi > 0.9996, the algorithm cannot solve the function
    if chi >= 0.9996:
        return brentq(kerr_lightring, 0, 0.8, args=(0.9996))
    else:
        return brentq(kerr_lightring, 0, 0.8, args=(chi))
Return the velocity at the Kerr light ring