⚛️ phenomenology

Axion signals from neutron star populations

This paper investigates axion dark matter signals from neutron star populations, finding that while wider-angle searches outside the Galactic Centre lack competitive sensitivity, the collective signal from the Galactic Centre population offers detection potential comparable to that of individual magnetars, justifying continued searches for both.

Original authors: U. Bhura, R. A. Battye, J. I. McDonald, S. Srinivasan

Published 2026-03-16

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CC BY 4.0

Original authors: U. Bhura, R. A. Battye, J. I. McDonald, S. Srinivasan

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

Imagine the universe is filled with invisible, ghostly particles called axions. Scientists suspect these axions make up "Dark Matter," the mysterious stuff that holds galaxies together but refuses to be seen. The big question is: How do we catch them?

This paper is like a detective story where the authors are trying to figure out the best way to hunt for these axion ghosts using Neutron Stars.

The Cast of Characters

Neutron Stars: Think of these as the universe's most extreme lighthouses. They are the crushed cores of dead stars, spinning incredibly fast and surrounded by magnetic fields so strong they could rip a credit card apart from a million miles away.
Axions: The invisible ghosts.
The Magic Trick: The theory is that when an axion flies near a neutron star's super-strong magnetic field, it can magically transform into a radio wave (a photon). If we can detect this specific radio signal, we've found the axion!

The Detective's Dilemma: The "Invisible" Crowd vs. The "Famous" Star

The authors are trying to decide which hunting ground is better:

Option A: The Galactic Center Crowd (The "Invisible" Population)
The center of our galaxy is a crowded neighborhood. The authors propose that there might be thousands of neutron stars hiding there, too close to the black hole to be seen directly.

The Problem: It's like trying to count the number of people in a dark, foggy room where you can't see anyone. You have to guess the number based on how many babies are born in that room (birth rates) and how fast they run away.
The Twist: The authors realized that neutron stars are born with a massive "kick" (like a cannonball shot out of a cannon). Many of these stars might get kicked so hard they fly out of the Galactic Center, leaving the room much emptier than we thought. If the room is empty, the signal is weak.

Option B: The Famous Magnetar (The "Famous" Star)
There is one specific, known neutron star right in the center of the galaxy called the Galactic Center Magnetar.

The Pro: We know exactly where it is and how strong its magnetic field is. It's like hunting for a ghost in a single, well-lit room.
The Con: We don't know exactly how it's oriented (is it facing us or away?). If it's facing away, we might miss the signal.

The Investigation: What Did They Find?

The authors ran complex computer simulations (using a tool called PsrPopPy, which is like a "Star Population Simulator") to see which method works best.

The "Crowd" is Hard to Count: They found that because of the "kicks" at birth, the number of stars in the Galactic Center might be much lower than previous theories suggested. This makes the "Crowd" method risky. If you guess the crowd size wrong, your whole theory falls apart.
The "Famous Star" is a Strong Contender: Even with the uncertainty of its orientation, the single Magnetar might actually be just as good at finding axions as the whole crowd of hidden stars combined.
The "Normal" Stars Outside the Center: They also looked at the thousands of normal neutron stars scattered all over the galaxy. They found that while there are many of them, their signals are too faint and spread out to beat current laboratory experiments. It's like trying to hear a whisper from a crowd of people spread across a whole city; the background noise drowns them out.

The Verdict: Don't Put All Your Eggs in One Basket

The paper concludes with a simple piece of advice for future astronomers: Hunt for both.

Don't just look at the big, famous Magnetar.
Don't just try to count the invisible crowd in the center.
Do both.

Why? Because the uncertainties are so high. The "crowd" might be smaller than we think, making the Magnetar the winner. But if the crowd is huge, the Magnetar might be the underdog. By searching for signals from both the single star and the hidden population, we maximize our chances of catching the axion ghost.

The Bottom Line Analogy

Imagine you are trying to find a lost coin in a park.

Method 1: You assume there are 1,000 people in the park, and you hope one of them dropped a coin. But you can't see the people, and you aren't sure if they are even there.
Method 2: You know for a fact that one specific person, "Mr. Magnetar," is standing right there, and he might have dropped a coin, but he might be hiding it in his pocket.

This paper says: "Don't just search the crowd, and don't just search Mr. Magnetar. Search both, because you don't know which one actually has the coin."

They also suggest that future giant radio telescopes (like the SKA) will be the perfect "metal detectors" to finally hear these faint whispers from the stars.

1. Problem Statement

The detection of axion dark matter via radio signatures from neutron stars (NSs) is a promising indirect search method. Previous studies (e.g., Ref. [51]) proposed using the population of "invisible" neutron stars in the Galactic Centre (GC) to place strong constraints on the axion-photon coupling constant ( $g_{a\gamma\gamma}$ ).

However, this approach suffers from significant astrophysical uncertainties:

Lack of Direct Observations: There are very few observed pulsars in the central 1 pc³ of the Galaxy. Consequently, the total number of NSs ( $N_{GC}$ ) and their statistical properties (magnetic field $B_0$ , period $P$ , inclination $\alpha$ ) are not directly measured but inferred solely from birth rate models.
Stellar Dynamics: Neutron stars are born with high "kick" velocities. The authors argue that stellar dynamics and the Galactic gravitational potential may cause a large fraction of these stars to diffuse away from the GC, potentially reducing the actual population size by orders of magnitude compared to birth rate predictions.
Model Dependence: Relying on inferred populations introduces large systematic errors, making derived constraints on $g_{a\gamma\gamma}$ unreliable.

The paper aims to investigate these uncertainties, explore alternative populations (outside the GC) to circumvent them, and compare the sensitivity of population-based searches against single-object searches (specifically the Galactic Centre Magnetar, GCM).

2. Methodology

A. Signal Modeling and Marginalization

Flux Averaging: Instead of computationally expensive ray-tracing for individual stars, the authors calculate the axion signal by averaging over all stellar orientations ( $\theta, \phi$ ). This allows them to marginalize over unknown parameters ( $B_0, P, \alpha$ ) and treat the population signal as a sum of integrated luminosities ( $L$ ) divided by distance squared.
Absorptive Effects: The model incorporates two suppression mechanisms for strongly magnetized stars:
1. Cyclotron Resonance: Photons are re-absorbed by the plasma. Modeled via an optical depth factor $e^{-\tau}$ .
2. Adiabatic Re-conversion: In high-field regimes, photons may convert back to axions. The authors conservatively excise stars where the phase-space averaged conversion probability $\langle P_{a\gamma} \rangle > 0.1$ to avoid overestimating the signal in the non-perturbative regime.

B. Population Modeling Strategies

The authors compare three distinct components:

"Normal" Pulsar Population (PsrPopPy): They use the open-source package PsrPopPy, calibrated against the Parkes Multi Beam Survey (PMBS). This provides a robust, observationally normalized model of the NS population across the Galaxy, including the "invisible" population (beaming away from Earth). This avoids the uncertainties of the GC-specific birth rate models.
Galactic Centre (GC) "Invisible" Population: They model the GC population (central 1 pc³) using birth rate arguments but parameterize the total number of stars ( $N_{GC}$ ) as a free variable. They explicitly model stellar diffusion (kinematics) to show how kick velocities reduce the number of stars remaining in the GC potential well.
Galactic Centre Magnetar (GCM): They treat the GCM (PSR J1745–2900) as a single point source, utilizing the latest branching-tree simulations (Ref. [55]) to account for adiabatic suppression.

C. Instrumental Sensitivity Analysis

The authors evaluate the detectability of these signals using:

Existing/Proposed Telescopes: MeerKAT, SKA-low, and HERA.
Bespoke "Strawman" Instrument: A hypothetical horn array designed to maximize sensitivity to the diffuse, wide-angle signal from the general NS population.

3. Key Contributions

Quantification of GC Population Uncertainty:
- The authors demonstrate that stellar diffusion significantly reduces the number of NSs remaining in the GC. While birth rate models suggest $N_{GC} \sim 1600$ , kinematic modeling suggests only $\sim 20$ stars might remain in the central region after 10–30 Myr.
- They highlight that the "invisible" GC population is highly sensitive to the depth of the Galactic potential and the initial kick velocity distribution, introducing an uncertainty of at least an order of magnitude in $N_{GC}$ .
Validation of PsrPopPy for Axion Searches:
- By normalizing the population model to observed pulsars (PMBS), the authors provide a robust lower bound on the axion signal from the wider Galactic population. This circumvents the "blind spot" of the GC.
- They show that the signal from the general population is dominated by stars with $B_0 \sim 10^{13}$ G (for $g_{a\gamma\gamma} \sim 10^{-11}$ GeV $^{-1}$ ) and is less sensitive to the specific uncertainties plaguing the GC region.
Comparison: Population vs. Single Object:
- The study compares the sensitivity of the GC population (variable $N_{GC}$ ) against the GCM.
- Result: The GCM offers a competitive or superior constraint due to its narrow bandwidth and high magnetic field, unless the GC population is extremely large ( $N_{GC} \gtrsim 1000$ ). However, given the diffusion arguments, the population size is likely much lower, making the GCM a more reliable target.
- Crucially, the GCM signal has large uncertainties regarding the viewing angle ( $\theta$ ) and misalignment ( $\alpha$ ), whereas the population signal averages these out.
Frequency and Instrument Constraints:
- Low Frequencies (100 MHz – 1 GHz): Signals from the "normal" population are strongest here. However, current constraints from pulsar polar caps and lab experiments (ADMX) are already stronger than what MeerKAT or SKA-low can achieve in this band.
- High Frequencies (C-band, 4–8 GHz): This is the most promising regime for axion searches because laboratory constraints are weaker. Here, the GCM and the (uncertain) GC population are the primary targets.

4. Results

Signal Strength: The predicted all-sky brightness temperature from the "normal" pulsar population is weak ( $\sim 10^{-2} \mu$ K at 1 GHz), scaling as $g_{a\gamma\gamma}^2$ .
GC Population Limits: For a fiducial $N_{GC} = 1000$ , the GC population constraints are comparable to the GCM. However, if $N_{GC}$ drops to $\sim 20$ (due to diffusion), the population constraint vanishes, and the GCM becomes the dominant probe.
Instrumental Prospects:
- HERA, MeerKAT, SKA-low: These instruments could potentially rule out $g_{a\gamma\gamma} \sim 10^{-11}$ GeV $^{-1}$ in the mass range $0.4 - 4 \mu$ eV. However, these limits are currently not competitive with existing constraints from pulsar polar caps or laboratory haloscopes.
- Future Strategy: To be competitive, future surveys must focus on the C-band (4–8 GHz) where lab constraints are sparse.

5. Significance and Conclusions

Re-evaluation of GC Searches: The paper argues that relying solely on the "invisible" GC population for axion constraints is risky due to the large uncertainties in stellar dynamics and population size. The assumption that $N_{GC} \sim 1000$ may be an overestimate by a factor of 10–50.
Dual-Target Strategy: The authors conclude that future observing strategies (e.g., with the SKA) should simultaneously search for both signals:
1. The GCM (single object), which offers high sensitivity but depends on unknown geometric angles.
2. The GC Population (diffuse), which offers statistical robustness but depends on uncertain population numbers.
Broader Implications: While the "normal" pulsar population outside the GC provides a theoretically clean model, current telescope sensitivities are insufficient to beat existing constraints in the low-frequency regime. The most promising path forward remains high-frequency searches targeting the GC, where the interplay between the Magnetar and the hidden population must be carefully managed.

In summary, the paper provides a rigorous statistical framework for axion searches using NS populations, highlighting that while population models reduce magnetosphere uncertainties, they introduce significant astrophysical uncertainties regarding the number of stars, necessitating a combined approach with single-object targets.