Hourly radio variability of PDS70c from time-differential photometry

Barbar Ercolano; Gabriel-Dominique Marleau; Hauyu Baobab Liu; Miguel Carcamo; Ondrej Chrenko; Oriana Dominguez-Jamett; Simon Casassus; Yuhiko Aoyama

arxiv: 2604.24991 · v1 · submitted 2026-04-27 · 🌌 astro-ph.EP

Hourly radio variability of PDS70c from time-differential photometry

Simon Casassus , Miguel Carcamo , Oriana Dominguez-Jamett , Yuhiko Aoyama , Gabriel-Dominique Marleau , Ondrej Chrenko , Hauyu Baobab Liu , Barbar Ercolano This is my paper

Pith reviewed 2026-05-07 17:46 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords PDS70cprotoplanetradio variabilityALMAaccretion shockcircumplanetary diskfree-free emissiontime-differential photometry

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The pith

PDS70c exhibits hourly radio flux variability consistent with HI free-free emission from an accretion shock on a circumplanetary disk.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper applies time-differential photometry to ALMA Band 7 data to isolate short-term changes in the point-source radio emission from PDS70c. It reports a 170 percent flux rise over roughly one hour in 2017 observations at 3.7 sigma significance, while 2023 data remains steady within 15 percent on two-hour scales but shows 49 percent dispersion on 20-minute intervals. This pattern of variability that appears on hourly timescales yet averages out over longer periods matches the expected behavior for free-free radiation from hydrogen at an accretion shock on the surface of a circumplanetary disk. The same observations impose a planet-to-environment mass ratio below 10 to the minus 4 if the emission were instead steady thermal radiation from the surrounding material. Such a link between radio variability and accretion physics provides a direct observational handle on the growth of embedded protoplanets.

Core claim

Using visibility alignment, self-calibration, and time-differential photometry on ALMA 343 GHz observations, PDS70c was detected only in one 2017 execution block where its flux density rose by 170 percent with 3.7 sigma significance, while control sources remained stable. The 2023 dataset shows constant flux within a 15 percent scatter over two-hour blocks, yet splitting those blocks into 20-minute intervals yields an intrinsic dispersion of 49 percent significant at 2.6 sigma. The observed hourly variability, which is averaged out on daily timescales, is precisely the signature expected for HI free-free emission originating at an accretion shock on a circumplanetary disk surface; a planet-

What carries the argument

Time-differential photometry performed in the visibility domain after visibility alignment and self-calibration to extract variable point-source flux from PDS70c while subtracting the time-averaged extended disk emission.

If this is right

The radio emission mechanism for PDS70c is HI free-free radiation from an accretion shock rather than steady thermal emission from the circumplanetary environment.
Accretion onto PDS70c proceeds with variations on hourly timescales that are smoothed when averaged over days.
A planet-to-environment mass ratio below 10 to the minus 4 is required if the emission were thermal, to prevent radiative diffusion from erasing the short-term signal.
Time-differential photometry in the visibility domain can separate variable embedded point sources from bright extended disk emission in ALMA data.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same photometry technique could be applied to other ALMA datasets targeting embedded protoplanets to test whether hourly radio variability is a general signature of active accretion.
If the variability is confirmed, radio light curves may become a practical probe of the inner accretion flow around forming planets at spatial scales inaccessible to other wavelengths.
Independent mass estimates for the circumplanetary disk around PDS70c could directly test the reported mass-ratio upper limit.

Load-bearing premise

Residual calibration errors, extended disk emission, or self-calibration artifacts have been fully removed by visibility alignment and time-differential subtraction, leaving only genuine variability from the point source PDS70c.

What would settle it

Additional ALMA observations of PDS70c at 343 GHz with comparable hourly cadence that show no significant flux changes above the thermal noise level would falsify the reported variability.

Figures

Figures reproduced from arXiv: 2604.24991 by Barbar Ercolano, Gabriel-Dominique Marleau, Hauyu Baobab Liu, Miguel Carcamo, Ondrej Chrenko, Oriana Dominguez-Jamett, Simon Casassus, Yuhiko Aoyama.

**Figure 1.** Figure 1: Restored images for IB17, with Briggs parameter view at source ↗

**Figure 2.** Figure 2: Time-differential photometry of PDS 70c. The blue ±1σ error bars (2σ in total) record ∆FB7 = FB7 − ⟨FB7⟩ as a function of Julian day for each EB. The same quantity extracted from the spatial scatter (see text) is shown in thick orange error bars. Each type of marker corresponds to 3 synthetic and constant pointsources, injected at the positions of the insets in view at source ↗

read the original abstract

The radio emission mechanisms from accreting protoplanets, and their variability, link observations and physical properties. We revisit the variability of the ~343GHz (ALMA Band7) flux density from PDS70c (F_B7). 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 yields close to thermal residuals in each execution block (EB) of ALMA observations, allowing the time-differential photometry of point-source in the visibility domain. The variability of PDS70c is checked against synthetic control point sources. In images of the 2017 ALMA dataset, with three ~1h EBs, PDS70c was detected only on 6 Dec. 2017, where F_B7 rose by 228%+-69% (3.3sigma). Time-differential photometry confirms a rise by 170%+-46% (3.7sigma). An application to ~2h 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 in 20min intervals, with a chi2 test significant at 2.6sigma, and an intrinsic dispersion of 49%21%. The radio variability of PDS70c, observed over hours but averaged out on longer timescales, is indeed expected if the signal is due to HI free-free from an accretion shock on a circum-planetary disk surface. A planet-to-environment mass ratio <1E-4 is required to avoid smoothing by radiative diffusion if the signal is due to thermal emission from the environment.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper reports a 3.7-sigma hourly radio flux rise for PDS70c in 2017 ALMA data via differential photometry, but the synthetic controls likely miss position-dependent reduction artifacts for this embedded source.

read the letter

The one thing to know is that this paper claims a 3.7-sigma detection of a 170% rise in PDS70c's 343 GHz flux over roughly an hour in the 2017 ALMA execution blocks, confirmed by time-differential photometry after visibility alignment and self-calibration, while the 2023 data stays flat on hour scales but shows 2.6-sigma scatter on 20-minute bins. If the signal is real, it links short-term radio changes to accretion shocks on a circumplanetary disk, with a planet-to-environment mass ratio below 10^-4 needed to keep thermal emission unsmoothed. This specific hourly variability measurement and the mass-ratio bound are new relative to earlier PDS70 ALMA papers. They do a solid job checking the result against synthetic control sources and running chi-squared tests on the binned fluxes, and the free-free interpretation follows standard physics without forcing new parameters. The soft spot is exactly the one in the stress-test note. An embedded source sits in different uv-sampling and experiences different subtraction residuals than a control placed in a clean field, so epoch-dependent phase or amplitude errors that correlate with the planet's location can survive the differential step and appear as variability. A 170% swing means even 10-20% systematics could dominate, and the abstract gives little on full covariance or data exclusion rules. This is for observers tracking protoplanet radio emission and modelers of late-stage accretion. A reader who cares about ALMA techniques for point sources inside disks would get practical value from the method. The claim is concrete and falsifiable enough to deserve referee time, even if the authors will need to add more residual tests and error details.

Referee Report

2 major / 2 minor

Summary. The manuscript analyzes ALMA Band 7 (~343 GHz) observations of PDS70c from 2017 (three ~1h execution blocks) and 2023 (~2h blocks), claiming detection of hourly-scale radio flux variability via time-differential photometry performed in the visibility domain after per-EB visibility alignment and self-calibration. It reports a 228%±69% (3.3σ) rise in imaged flux on 6 Dec 2017 and a 170%±46% (3.7σ) rise via differential photometry, with 2023 data showing constant flux within ~15% scatter but 2.6σ-significant intrinsic dispersion (49%±21%) when split into 20-min intervals; variability is validated against synthetic control sources and interpreted as consistent with HI free-free emission from an accretion shock on a circumplanetary disk surface, requiring a planet-to-environment mass ratio <10^{-4} to avoid radiative-diffusion smoothing.

Significance. If the reported variability is confirmed as intrinsic point-source emission rather than residual artifact, the result would provide a rare observational link between radio flux changes on hourly timescales and accretion physics around embedded protoplanets, supporting free-free shock models and placing quantitative limits on mass ratios that could be tested with future multi-epoch monitoring. The pipeline's use of visibility-domain differential photometry and synthetic controls represents a reproducible approach to isolating variable embedded sources amid extended disk emission.

major comments (2)

[section on synthetic control sources and time-differential photometry] The validation with synthetic control point sources (described in the section on time-differential photometry and results for the 2017/2023 datasets) places controls at clean locations without extended disk emission. However, PDS70c is embedded within the disk, so its uv-sampling, self-calibration residuals, and time-differential subtraction behavior differ from injected controls; any epoch-dependent phase/amplitude errors correlated with the planet's position would survive the differential step and could produce apparent variability at the level of the claimed 170%±46% change. This directly undermines the central claim that the 3.7σ (2017) and 2.6σ (2023) signals are intrinsic after subtracting the time-averaged extended disk.
[results section on 2023 scatter and chi-squared test] In the results for the 2023 dataset (20-min binning and chi-squared test), the reported 2.6σ significance and 49%±21% intrinsic dispersion rely on binned data and chi-squared statistics, but the manuscript provides limited explicit detail on full error propagation, covariance between bins, or data-exclusion criteria. Given that the 2017 detection rests on only three EBs with one showing the rise, small unaccounted systematics in the differential photometry could shift the significance below the threshold for a robust variability claim.

minor comments (2)

[abstract and results] The abstract states the 2017 rise as both 228%±69% (3.3σ) in images and 170%±46% (3.7σ) in differential photometry without clarifying whether these are independent measurements or derived from the same underlying data; a brief reconciliation in the results section would improve clarity.
[methods] Notation for flux density (F_B7) and error bars is used consistently but could be defined once in the methods with explicit reference to the visibility-domain measurement equation to aid readers unfamiliar with ALMA differential photometry.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed review of our manuscript on the hourly radio variability of PDS70c. The comments raise important points about the validation of our time-differential photometry approach and the statistical details of our analysis. We address each major comment below and describe the revisions we will make to improve the robustness and clarity of the paper.

read point-by-point responses

Referee: The validation with synthetic control point sources (described in the section on time-differential photometry and results for the 2017/2023 datasets) places controls at clean locations without extended disk emission. However, PDS70c is embedded within the disk, so its uv-sampling, self-calibration residuals, and time-differential subtraction behavior differ from injected controls; any epoch-dependent phase/amplitude errors correlated with the planet's position would survive the differential step and could produce apparent variability at the level of the claimed 170%±46% change. This directly undermines the central claim that the 3.7σ (2017) and 2.6σ (2023) signals are intrinsic after subtracting the time-averaged extended disk.

Authors: We appreciate the referee's emphasis on the limitations of our control source placement. The synthetic controls were positioned in clean regions to establish a baseline for the differential photometry pipeline's performance in the absence of extended emission, confirming that the method itself does not generate artificial variability. We acknowledge that the embedded location of PDS70c within the disk means its uv-coverage and potential residual errors differ, and position-correlated systematics could in principle affect the differential measurements. To directly address this, the revised manuscript will include new simulations injecting synthetic point sources at the precise location of PDS70c, using a model of the surrounding disk emission. These tests will quantify any position-dependent effects on the recovered variability and allow us to re-evaluate the significance levels. We maintain that the per-EB self-calibration and time-differential subtraction mitigate much of the extended emission, but agree that these additional position-matched simulations are necessary to strengthen the validation. revision: yes
Referee: In the results for the 2023 dataset (20-min binning and chi-squared test), the reported 2.6σ significance and 49%±21% intrinsic dispersion rely on binned data and chi-squared statistics, but the manuscript provides limited explicit detail on full error propagation, covariance between bins, or data-exclusion criteria. Given that the 2017 detection rests on only three EBs with one showing the rise, small unaccounted systematics in the differential photometry could shift the significance below the threshold for a robust variability claim.

Authors: We agree that additional details on the statistical procedures are required for full transparency. In the revised manuscript, we will expand the methods and results sections to provide explicit descriptions of the error propagation for the binned time-differential photometry, including how per-bin uncertainties are derived from the visibility data and any treatment of covariance between adjacent time bins. We will also document the data-exclusion criteria applied and report the complete chi-squared statistics with degrees of freedom. For the 2017 dataset, we recognize the constraint of only three execution blocks and the fact that the rise appears in one; we will add a dedicated discussion of robustness, incorporating Monte Carlo simulations to assess how plausible systematics could affect the 3.7σ significance. These revisions will clarify the analysis without altering the core findings. revision: yes

Circularity Check

0 steps flagged

No circularity: empirical measurements with standard interpretation

full rationale

The paper reports direct observational results from ALMA visibility data: a 3.7σ flux rise in 2017 PDS70c via time-differential photometry after visibility alignment and self-calibration, plus 2.6σ scatter in 2023 sub-intervals, validated against synthetic controls. These are measurements of flux changes, not derivations. The statement that hourly variability (but not longer-term) is expected for HI free-free from an accretion shock invokes standard free-free physics and a mass-ratio limit from radiative diffusion timescales; neither is obtained by fitting parameters inside the paper's equations nor by reducing to self-citations. No self-definitional loops, fitted inputs renamed as predictions, or ansatzes smuggled via author citations appear in the derivation chain. The central claims remain independent empirical detections.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard radio-astronomy assumptions about calibration and emission mechanisms rather than new free parameters or invented entities.

axioms (2)

domain assumption Self-calibration and visibility alignment produce thermal-noise-limited residuals that do not introduce spurious variability on hourly timescales.
Invoked to justify that detected changes are astrophysical.
domain assumption The observed radio emission is dominated by HI free-free radiation from an accretion shock rather than other mechanisms.
Used to connect the variability timescale to physical expectations.

pith-pipeline@v0.9.0 · 5652 in / 1531 out tokens · 61623 ms · 2026-05-07T17:46:27.654952+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

[1]

2018, ApJ, 866, 84 Benisty, M., Bae, J., Facchini, S., et al

Aoyama, Y ., Ikoma, M., & Tanigawa, T. 2018, ApJ, 866, 84 Benisty, M., Bae, J., Facchini, S., et al. 2021, ApJ, 916, L2 Cárcamo, M., Román, P. E., Casassus, S., Moral, V ., & Rannou, F. R. 2018, Astronomy and Computing, 22, 16 Casassus, S. & Cárcamo, M. 2022, MNRAS, 513, 5790 Casassus, S., Cieza, L., Cárcamo, M., et al. 2023, MNRAS, 526, 1545 Chael, A. A....

work page 2018
[2]

no visalign, apcal

Table A.1 provides a summary of the image properties in the IB17 dataset. We also tabulate the values ofF B7 measured in each EB as well as in the concatenated dataset. The elliptical Gaussian fits were carried out following two strategies: 1- by fit- ting a beam, assuming that the source is unresolved, and 2- by fitting an elliptical Gaussian. Fitting a ...

work page 2017

[1] [1]

2018, ApJ, 866, 84 Benisty, M., Bae, J., Facchini, S., et al

Aoyama, Y ., Ikoma, M., & Tanigawa, T. 2018, ApJ, 866, 84 Benisty, M., Bae, J., Facchini, S., et al. 2021, ApJ, 916, L2 Cárcamo, M., Román, P. E., Casassus, S., Moral, V ., & Rannou, F. R. 2018, Astronomy and Computing, 22, 16 Casassus, S. & Cárcamo, M. 2022, MNRAS, 513, 5790 Casassus, S., Cieza, L., Cárcamo, M., et al. 2023, MNRAS, 526, 1545 Chael, A. A....

work page 2018

[2] [2]

no visalign, apcal

Table A.1 provides a summary of the image properties in the IB17 dataset. We also tabulate the values ofF B7 measured in each EB as well as in the concatenated dataset. The elliptical Gaussian fits were carried out following two strategies: 1- by fit- ting a beam, assuming that the source is unresolved, and 2- by fitting an elliptical Gaussian. Fitting a ...

work page 2017