Letter to the Editor

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

¹ Departamento de Astronomía, Universidad de Chile, Santiago, Chile
² Data Observatory Foundation, Eliodoro Yáñez 2990 Providencia, Santiago, Chile
³ University of Santiago of Chile (USACH), Faculty of Engineering, Computer Engineering Department, Santiago, Chile
⁴ School of Physics and Astronomy, Sun Yat-sen University, Guangdong 519082, People’s Republic of China
⁵ Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Charles University, Faculty of Math and Physics, Astronomical Institute, V Holešovičkách 747/2, 180 00 Prague 8, Czech Republic
⁸ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City, 80424, Taiwan
⁹ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 February 2026
Accepted: 18 April 2026

Abstract

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% (3.3σ). Time-differential photometry confirms a rise by 170%±46% (3.7σ). 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 χ² test significant at 2.6σ, 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.