Hourly radio variability of PDS 70c from time-differential photometry

Context. The radio emission mechanisms from accreting protoplanets and their variability link observations and physical properties. Aims. We revisit the variability of the ∼343 GHz (ALMA Band 7) flux density from PDS 70c ( F B7 ). Methods. The subtraction of the extended time-averaged signal may enable the measurement of the flux density from variable and embedded point sources. Visibility alignment and self-calibration yield close to thermal residuals in each execution block (EB) of ALMA observations, thus allowing the time-differential photometry of point-sources in the visibility domain. The variability of PDS 70c was checked against synthetic control point sources. Results. In images of the 2017 ALMA dataset, with three ∼1 h EBs, PDS 70c was detected only on 6 December 2017, where F B7 rose by 228%±69% (3.3 σ ). Time-differential photometry confirms a rise by 170%±46% (3.7 σ ). An application to ∼2 h EBs from the 2023 dataset resulted in constant flux densities, within a scatter of ∼15%. However, F B7 ( t ) shows some scatter when splitting the deep 2023 EBs into 20 min intervals, with a χ 2 test significant at 2.6 σ , and an intrinsic dispersion of 49%±21%. Conclusions. The radio variability of PDS 70c, observed over hours but averaged out on longer timescales, is indeed expected if the signal is due to H I free-free from an accretion shock on a circumplanetary disk surface. A planet-to-environment mass ratio < 10 −4 is required to avoid smoothing by radiative diffusion if the signal is due to thermal emission from the environment.