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

This study presents all available multi-epoch 3.6 and 4.5 μm photometry from Spitzer Space Telescope observations of white dwarf debris discs, including weekly cadence observations of 16 relatively bright systems, and 5 h staring-mode observations for five of these. Significant variability is detected in 85 per cent of discs and across all time-scales probed, from minutes to weeks to years, where the largest flux changes correlate with the longest time baselines, and the infrared excesses persist utterly. While each source is idiosyncratic, the overall results indicate the most variable discs correlate with those that are the brightest (dustiest), and also among those with detected gas, demonstrating both dust and gas are produced via ongoing collisions. There is a correlation between flux and colour changes, where discs tend to appear redder when dimmer and bluer when brighter, consistent with an excess of small dust grains produced in collisions, followed by a gradual return to equilibrium. The overall results are a drastic departure from the predictions of the canonical – geometrically thin optically thick – disc in both flux and colour, but are broadly consistent with collisional evolution based on a simple model. The data presented herein constitute a legacy resource that can inform time-series studies of polluted and dusty white dwarfs, and importantly serve as a basis for future disc modelling, beyond the pioneering canonical framework.