Activity in white dwarf debris discs I: Spitzer legacy reveals variability incompatible with the canonical model

Acknowledgements: The authors thank the anonymous referee for a careful reading of the manuscript. This work is based on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. AS acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 101020057).