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arxiv: 2606.19442 · v1 · pith:FY6LQTTYnew · submitted 2026-06-17 · 🌌 astro-ph.GA · astro-ph.CO· astro-ph.HE· astro-ph.SR· gr-qc· hep-ph

Eppur non si trovano Vol. 3: Phoebe -- a Mirage of a Primordial Black Hole

Andrzej Udalski , Przemek Mr\'oz This is my paper

Pith reviewed 2026-06-26 20:13 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.COastro-ph.HEastro-ph.SRgr-qchep-ph
keywords primordial black holesmicrolensingvariable starsLarge Magellanic CloudDECam observationsdark matterphotometric variability
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The pith

Reanalysis of DECam data shows Phoebe is a variable star with multiple brightenings, not a primordial black hole microlensing event.

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

The paper re-examines public DECam observations of the star Phoebe in the Large Magellanic Cloud, adding 2020 and 2021 data points. It identifies at least three distinct low-amplitude brightenings plus long-term changes in mean magnitude. These patterns match ordinary variable stars rather than a single short-timescale microlensing event. The authors conclude the reported brightening was misidentified and does not indicate a lunar-mass primordial black hole. This removes a claimed detection that appeared to conflict with prior microlensing surveys excluding substantial PBH dark matter.

Core claim

The object underwent at least three distinct, low-amplitude brightenings (one of which was misinterpreted as a short-timescale microlensing event) in addition to long-term variations of its mean magnitude. These characteristics indicate that Phoebe is an ordinary variable star rather than a microlensing event. This finding resolves the apparent tension with the results from earlier microlensing experiments that rule out the hypothesis that a substantial fraction of dark matter is composed of lunar- and planetary-mass PBHs.

What carries the argument

Independent re-analysis of publicly available DECam observations of Phoebe, incorporating extra 2020-2021 epochs to reveal repeated brightenings and mean-magnitude changes.

If this is right

  • No evidence remains from Phoebe for lunar-mass primordial black holes as dark matter.
  • Earlier surveys that already excluded substantial PBH fractions stay consistent with all data.
  • Claims of short-timescale PBH microlensing must be checked against multi-year baselines before acceptance.
  • The reported tension between this candidate and prior microlensing limits is eliminated.

Where Pith is reading between the lines

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

  • Similar multi-epoch rechecks could be applied to other short-timescale microlensing candidates to test for hidden variability.
  • Long-baseline monitoring of LMC stars may be needed to separate intrinsic variables from true lensing events in future searches.
  • This case shows that sparse sampling can turn repeated low-amplitude stellar variations into apparent single transients.

Load-bearing premise

The additional 2020-2021 DECam data points are free of systematic calibration errors or artifacts that could produce or hide multiple brightenings.

What would settle it

A new photometric campaign that measures only one isolated brightening event whose light curve fits a single microlensing model without residuals, or that detects strict periodicity in the variations.

Figures

Figures reproduced from arXiv: 2606.19442 by Andrzej Udalski, Przemek Mr\'oz.

Figure 1
Figure 1. Figure 1: Comparison between the OGLE (upper panel) and DECam (lower panel) [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: 30′′ ×30′′ cutout of the reference image around Phoebe (PH). The posi￾tions of Phoebe and the two comparison stars (C1, C2) are indicated by tick marks. North is up, and East is to the left. 8833 8834 8835 8836 8837 8838 Time (HJD - 2450000) 21.0 21.1 21.2 21.3 21.4 21.5 21.6 VR M a g nit u d e 2019 Dec 15/16 2019 Dec 16/17 2019 Dec 17/18 2019 Dec 18/19 2019 Dec 19/20 Phoebe 2019 Dec 15/16 2019 Dec 16/17 2… view at source ↗
Figure 3
Figure 3. Figure 3: Light curves of Phoebe and the two comparison stars from program [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Light curves of Phoebe and the two comparison stars from program [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Light curves of Phoebe and the two comparison stars from program [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
read the original abstract

Recent preprints by Key et al. reported the discovery of a short-lived brightening of a star (nicknamed "Phoebe") located in the Large Magellanic Cloud that was interpreted as a short-timescale gravitational microlensing event produced by a lunar-mass primordial black hole (PBH) in the Milky Way dark matter halo. Here, we present an independent re-analysis of the publicly available DECam observations of this object, incorporating additional data from 2020 and 2021. The object underwent at least three distinct, low-amplitude brightenings (one of which was misinterpreted as a short-timescale microlensing event) in addition to long-term variations of its mean magnitude. These characteristics indicate that Phoebe is an ordinary variable star rather than a microlensing event. This finding resolves the apparent tension with the results from earlier microlensing experiments that rule out the hypothesis that a substantial fraction of dark matter is composed of lunar- and planetary-mass PBHs.

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.

Referee Report

1 major / 2 minor

Summary. The manuscript re-analyzes publicly available DECam photometry of the star Phoebe in the LMC, previously interpreted by Key et al. as a short-timescale microlensing event produced by a lunar-mass primordial black hole. Incorporating additional 2020–2021 epochs, the authors identify at least three distinct low-amplitude brightenings plus long-term changes in mean magnitude and conclude that Phoebe is an ordinary variable star, thereby removing an apparent tension with earlier microlensing surveys that constrain the PBH dark-matter fraction at these masses.

Significance. If the reported brightenings are shown to be astrophysical rather than photometric artifacts, the result would reinforce existing upper limits on lunar- and planetary-mass PBHs as dark matter by demonstrating that at least one claimed event is stellar variability. The work’s use of public data for an independent check is a positive feature for reproducibility.

major comments (1)
  1. [Section 3] Section 3 (re-reduced light curve): the manuscript presents the 2020–2021 DECam points and the resulting multiple-peak morphology without an independent cross-calibration against overlapping surveys, a covariance analysis of the added epochs, or injection-recovery tests that quantify the false-positive rate for low-amplitude brightenings under the actual observing conditions. Because the central claim that Phoebe is a variable star (rather than a microlensing event) rests on these additional brightenings being real, the absence of these validation steps is load-bearing.
minor comments (2)
  1. [Abstract / Section 3] The abstract and Section 3 would benefit from an explicit statement of the total number of new epochs, their photometric precision, and the precise time baseline covered by the 2020–2021 data.
  2. [Figures] Figure captions should include the filter(s) used and any applied zero-point offsets so that the long-term mean-magnitude variations can be assessed quantitatively.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and recommendation of major revision. We address the major comment below and will update the manuscript to incorporate additional validation steps.

read point-by-point responses

  1. Referee: [Section 3] Section 3 (re-reduced light curve): the manuscript presents the 2020–2021 DECam points and the resulting multiple-peak morphology without an independent cross-calibration against overlapping surveys, a covariance analysis of the added epochs, or injection-recovery tests that quantify the false-positive rate for low-amplitude brightenings under the actual observing conditions. Because the central claim that Phoebe is a variable star (rather than a microlensing event) rests on these additional brightenings being real, the absence of these validation steps is load-bearing.

    Authors: We agree that the absence of these specific validation steps weakens the robustness of the claim that the additional brightenings are astrophysical. In the revised manuscript we will add an injection-recovery test using the actual DECam observing conditions and noise properties to quantify the false-positive rate for low-amplitude brightenings. We will also include a covariance analysis of the 2020–2021 epochs. For cross-calibration, we will explicitly demonstrate consistency between our re-reduction and the original Key et al. photometry on the overlapping epochs and note the lack of independent overlapping surveys at the required depth and cadence; this limitation will be stated clearly. These changes directly address the load-bearing concern. revision: yes

Circularity Check

0 steps flagged

No circularity: direct observational comparison of light-curve morphology

full rationale

The paper's central claim rests on re-reduced DECam photometry showing multiple distinct brightenings plus long-term magnitude changes, which are then compared to standard expectations for microlensing (single symmetric peak) versus stellar variability (multiple asymmetric events). No equations, fitted parameters, or self-citations are invoked to derive the conclusion; the result is the direct morphological evidence itself. The analysis is self-contained against external benchmarks of light-curve classification and does not reduce any prediction to its inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard domain assumptions about the expected light-curve shapes of microlensing versus intrinsic stellar variability; no free parameters, new entities, or ad-hoc axioms are introduced in the abstract.

axioms (1)
  • domain assumption Standard microlensing light-curve models and stellar variability templates are sufficient to distinguish the two phenomena for this object.
    The classification of the brightenings as stellar variability rather than microlensing depends on these established templates.

pith-pipeline@v0.9.1-grok · 5721 in / 1231 out tokens · 25803 ms · 2026-06-26T20:13:35.980653+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. The swallowed spike: the formation of light primordial black hole structures around heavy seeds

    astro-ph.CO 2026-06 unverdicted novelty 7.0

    Light PBHs around heavy primordial seeds form significantly less dense inner cores than particle DM because no studied torque mechanism prevents capture.

Reference graph

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