Stimulated capture of axion dark matter around primordial black holes

summary

The gist

The gist: The study investigates how stimulated capture mechanisms, mediated by quartic self-interactions, can cause axion and axion-like particles to form hydrogenic ground-state clouds around

In short

The study investigates how axions and axion-like particles form dense, hydrogenic clouds around primordial black holes via stimulated capture mediated by quartic self-interactions. It identifies specific conditions where this growth overcomes dilution, derives survival criteria for these clouds, and explores observational signatures like enhanced photon emission in neutron star magnetospheres and modified microlensing.

Key concepts

Stimulated Capture: This mechanism describes how axions are captured around a black hole not just through standard gravity, but through their self-interactions. These interactions lead to Bose enhancement, which boosts the growth rate of the axion cloud beyond what simple capture mechanisms would achieve alone.
Focusing Parameter ($\xi$): This parameter, $\xi \equiv 2\pi/kR_{\star}$, determines whether gravitational focusing enhances incoming waves within a region around the black hole ($r \sim R_{\star}$). If $\xi \gtrsim 1$, this focusing suppresses the stripping process and triggers exponential growth of the bound axion population.
Resonant Conversion: This refers to a specific condition in neutron star magnetospheres where axions can efficiently convert into photons. This resonance occurs when the local plasma frequency matches the axion mass, creating a shell around the neutron star where photon production is maximized.

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This episode discusses

The paper

Stimulated capture of axion dark matter around primordial black holes · Read on arXiv

Nuno P. Branco Joãao G. Rosaa

University of Coimbra · CFisUC

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Stimulated capture of axion dark matter around primordial black holes".

Jocelyn: The gist: The study investigates how stimulated capture mechanisms, mediated by quartic self-interactions, can cause axion and axion-like particles to form hydrogenic ground-state clouds around primordial black holes,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So to recap this paper, "Stimulated capture of axion dark matter around primordial black holes," they are looking at how axions and ALPs interact around these primordial black holes using their quartic self-interactions as the mediator for the capture process.

Jocelyn: The thesis is that they can identify specific parametric regions where this Bose-enhanced growth manages to overcome the cosmological dilution we usually expect from dark matter density decreasing over time.

Subrahmanyan: This means they are deriving analytic criteria specifically for when this hydrogenic ground-state cloud will form efficiently and survive for a long time, which is a major step in understanding axion dark matter substructure.

Vera: They then confront these formation and survival criteria with the current bounds we have on axion-photon couplings, which helps us see if this mechanism is even viable under current experimental limits.

Jocelyn: On the observational side, they discuss what these PBH plus cloud systems mean for us, focusing on resonant axion-to-photon conversion in neutron star magnetospheres and how finite-size effects alter microlensing distortions.

Subrahmanyan: The implications are that these clouds might produce transient line flux densities that exceed what current VLA and ALMA sensitivities can detect in regions where the Galactic encounter rate is observationally relevant.

Conclusion: Vera: So to wrap up, the authors Nuno P. Brancoa Jo˜ao G. Rosaa are studying stimulated capture around primordial black holes and they found that this mechanism can populate long-lived axion and ALP clouds in parameter spaces that seem viable for us to see signals from.

Jocelyn: It's important to remember that this paper suggests stimulated capture is a compelling alternative mechanism compared to the superradiant instability we've studied before for forming these axion clouds.

Subrahmanyan: The main implication is that the predicted peak spectral flux density can reach about thirty-one Jy at Galactic-centre distances if we assume the maximal cloud-mass benchmark and all the necessary exterior conversion cuts are met.

Vera: And they also show that the extended-cloud regime demonstrates that PBH plus cloud systems can naturally lead to departures from the standard point-lens Paczy´nski description in microlensing observations.

Jocelyn: That means we should look for these specific signatures in those astrophysical environments, like neutron stars and microlensing events, to test this model.

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