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AMPM II. A Lunar-Mass Primordial Black Hole Microlensing Candidate in the Milky Way Halo

This paper reports the discovery of "Phoebe," a lunar-mass microlensing event detected by the DECam survey towards the Large Magellanic Cloud, which is statistically likely to be a Primordial Black Hole residing in the Milky Way's dark matter halo.

Original authors: Renee Key, Edward N. Taylor, Ken C. Freeman, Jeremy Mould, Abhijit Saha, Anais Möller, Timothy M. C. Abbott, Alan R. Duffy

Published 2026-05-20

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CC BY 4.0

Original authors: Renee Key, Edward N. Taylor, Ken C. Freeman, Jeremy Mould, Abhijit Saha, Anais Möller, Timothy M. C. Abbott, Alan R. Duffy

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Hunting for Invisible Ghosts

Imagine the universe is filled with invisible "ghosts" called Dark Matter. Scientists have known these ghosts exist because they have gravity (they pull on things), but they don't shine, so we can't see them with telescopes. One theory suggests that some of these ghosts might be Primordial Black Holes (PBHs)—tiny black holes that formed in the very first split-second after the Big Bang, long before stars existed.

The authors of this paper went on a hunt for these tiny black holes. They didn't look for the black holes themselves; instead, they looked for the shadow they cast on the light of distant stars.

The Method: The "Cosmic Magnifying Glass"

The team used a powerful camera called DECam (mounted on a 4-meter telescope in Chile) to stare at a patch of sky containing the Large Magellanic Cloud (LMC), a small galaxy neighboring our own Milky Way.

They watched thousands of stars for five nights, taking pictures every minute or so. They were looking for a specific trick called Gravitational Microlensing.

The Analogy: Imagine you are looking at a lightbulb in the distance. If a clear glass marble (the lens) passes directly in front of it, the marble bends the light and makes the bulb look briefly brighter.
The Reality: If a tiny, invisible black hole passes in front of a distant star, its gravity bends the star's light, making the star appear to get brighter for a short time. The faster the black hole moves and the smaller it is, the shorter the "blink" of brightness lasts.

The Discovery: "Phoebe"

During their five-night watch, the team spotted exactly one event. They named it Phoebe.

What happened: A single star suddenly got brighter and then faded back down.
How long did it last? Only about one hour.
Why is this special? Most "blinks" caused by planets or stars last for days. A one-hour blink is incredibly fast, suggesting the object causing it is very small and very light.

The Detective Work: Ruling Out Imposters

Before claiming they found a black hole, the team had to play detective to make sure Phoebe wasn't a fake. They checked for several "imposters":

Is it a flare? Sometimes stars (like our Sun) have solar flares that make them brighten suddenly. The team compared Phoebe's light curve to thousands of known flares. Flares usually have a sharp, jagged spike. Phoebe's light curve was smooth and symmetrical, like a perfect hill. Verdict: Not a flare.
Is it a glitch? Could the camera or the atmosphere have caused a false signal? They checked the stars right next to Phoebe. Those neighbors stayed perfectly steady while Phoebe blinked. If it were a camera glitch or bad weather, all the stars would have blinked. Verdict: Not a glitch.
Is it a variable star? Some stars naturally pulse in brightness. But Phoebe didn't pulse; it happened just once and never again. Verdict: Not a pulsing star.

The Mystery: What is Phoebe?

Once they confirmed it was a real microlensing event, they had to guess what Phoebe actually was. They ran computer simulations (using a method called Bayesian modeling) to test three different scenarios:

Scenario A: A Rogue Planet in the Milky Way. Maybe it's a planet that got kicked out of a solar system and is floating alone in our galaxy.
Scenario B: A Rogue Planet in the LMC. Maybe it's a planet floating in the neighboring galaxy.
Scenario C: A Primordial Black Hole in the Dark Halo. Maybe it's a tiny black hole made of dark matter, floating in the invisible halo that surrounds our galaxy.

The Results:

The Mass: In all scenarios, the object is incredibly light—about 3 times the mass of our Moon (or roughly 0.03 times the mass of Earth). This is tiny for a black hole!
The Location: The math showed that Phoebe is most likely located far away from us, deep in the Dark Matter Halo of the Milky Way, rather than in the crowded disk where stars live.
The Probability: The team calculated the odds. It is 100,000 times more likely (5 orders of magnitude) that Phoebe is a dark matter object (a Primordial Black Hole) than a rogue planet.

Why Does This Matter?

If Phoebe is indeed a Primordial Black Hole, it's a huge deal.

Time Travel: It would be one of the oldest objects in the universe, formed before the first stars were born.
Dark Matter Proof: It would prove that at least some of the universe's missing dark matter is made of these tiny black holes.
New Physics: It opens a window into the physics of the very early universe (inflation), helping us understand how the cosmos began.

The Bottom Line

The paper reports the discovery of a mysterious, one-hour "blink" in a distant star's light. After ruling out flares and glitches, the team concludes that the most likely culprit is a tiny, lunar-mass black hole floating in the dark halo of our galaxy. While they can't be 100% certain without finding more examples, the evidence strongly points to this being a piece of the "primordial" dark matter puzzle.

Note: The paper does not claim this has any immediate medical or technological applications; it is purely an astronomical discovery about the nature of the universe.

Technical Summary: AMPM II. — A Lunar-Mass Primordial Black Hole Microlensing Candidate in the Milky Way Halo

Problem and Context
Primordial Black Holes (PBHs) remain a hypothesized candidate for compact dark matter, potentially forming during inflation. While gravitational microlensing is a proven method for detecting compact objects, distinguishing between baryonic lenses (such as Free-Floating Planets or FFPs) and non-baryonic PBHs is challenging because the lensing signal depends primarily on geometry and mass rather than the physical nature of the lens. Previous surveys have identified terrestrial-mass FFPs toward the Galactic Bulge, but the population of compact objects in the dark matter halos of the Milky Way (MW) and Large Magellanic Cloud (LMC) remains poorly constrained. The specific challenge addressed in this work is the identification and characterization of a single, short-duration microlensing event ("Phoebe") detected toward the LMC, determining whether it originates from the stellar populations of the MW/LMC or the intervening dark matter halo.

Methodology
The authors utilized high-cadence optical data from the Dark Energy Camera (DECam) on the Cerro Tololo Blanco 4m telescope, observing a single field in the LMC (Field 51) over five consecutive nights in December 2019. The survey achieved an average cadence of 50 seconds, sensitive to microlensing timescales from minutes to days.

The analysis proceeded through several stages:

Detection and Validation: A single candidate event, "Phoebe," was identified. The authors rigorously validated the signal against potential contaminants, including atmospheric seeing variations, blending with neighboring stars, and stellar variability (flares, pulsations, or eclipsing binaries). Independent photometric reanalysis using AstroPhot confirmed the signal was not an artifact of Point Spread Function (PSF) variations or blending.
Source Characterization: The source star was too faint for Gaia parallax but was characterized using SMASH DR2 photometry and color-temperature relations. The source was assumed to be in the LMC, with an estimated effective temperature of 6500 K and a radius of $2.3 \pm 0.1 R_\odot$ . Finite source effects were incorporated into the modeling.
Bayesian Modeling: The authors employed Markov Chain Monte Carlo (MCMC) sampling within the emcee framework, using MulensModel to fit a finite-source point-lens (FS-PL) model. Three distinct galactic scenarios were tested to define priors for lens distance and velocity:
- MW Stellar: Lens associated with the Milky Way's stellar disk/halo.
- LMC Stellar: Lens associated with the LMC's stellar disk.
- Dark Matter (DM): Lens located within the overlapping dark matter halos of the MW and LMC, modeled with NFW and pseudo-isothermal profiles respectively.
Optical Depth Analysis: To assess the likelihood of the lens belonging to a specific population, the authors calculated the optical depth ( $\tau$ ) for each scenario, accounting for finite source effects and the specific mass range of the candidate.

Key Results
The event "Phoebe" exhibited a symmetric, smooth light curve with an Einstein timescale ( $t_E$ ) of approximately 60 minutes ($0.043$ days), making it one of the fastest and lowest-mass microlensing signals ever detected.

Mass and Distance Estimates:
- MW Stellar Scenario: If the lens is in the Milky Way, it is located at $565^{+661}_{-357}$ pc (thick disk) with a mass of $0.022^{+1.859}_{-0.022} M_\oplus$ .
- LMC Stellar Scenario: If the lens is in the LMC, it is located $700$ pc in front of the source with a mass of $0.100^{+0.192}_{-0.067} M_\oplus$ .
- Dark Matter Scenario: If the lens is in the dark halo, it is located at $18.2^{+24.7}_{-14.1}$ kpc with a mass of $0.026^{+0.194}_{-0.023} M_\oplus$ (approximately 3 lunar masses).
Optical Depth Probability: The optical depth analysis indicates that the lens is 5 orders of magnitude more likely to be part of the Milky Way's dark matter halo than part of the stellar content of the MW or LMC. This probability difference arises from the significantly higher intervening dark matter density along the line of sight compared to the stellar density, even when accounting for the estimated abundance of free-floating planets.
Nature of the Lens: The authors rule out the lens being a stellar remnant black hole (minimum mass $\sim 5 M_\odot$ ) or a standard flare event. While the lens could theoretically be a Free-Floating Planet (FFP), the optical depth comparison strongly favors a non-baryonic origin. Consequently, the authors interpret Phoebe as a candidate PBH.

Significance and Claims
The paper claims the discovery of a microlensing event consistent with a compact object of lunar mass ( $\sim 0.03 M_\oplus$ ) residing in the dark matter halo. The primary significance lies in the statistical likelihood derived from optical depth analysis, which suggests Phoebe is a PBH rather than a planetary object.

The authors note that this detection supports recent findings from Subaru-HSC data toward M31, which identified 12 similar short-duration candidates. They argue that Phoebe, if confirmed as a PBH, would be among the oldest astronomical objects found, offering a window into the physics of inflation. However, the paper maintains a modest stance regarding the definitive nature of the object: with only a single detection from the AMPM survey, the underlying population statistics cannot be fixed. The authors emphasize that while the optical depth analysis favors the dark matter hypothesis, the true nature of the lens remains hidden without further detections across diverse sightlines to disentangle the PBH population from potential FFPs. The work serves as a motivation for future high-cadence surveys (e.g., Roman, Vera C. Rubin Observatory) to characterize the mass distribution of such compact objects.