[Paper Review] Possible evidence of QCD axion stars in HSC and OGLE microlensing events

Sunao Sugiyama, Masahiro Takada+1 more|arXiv (Cornell University)|Aug 6, 2021

Astronomy and Astrophysical Research4 citations

TL;DR

This paper proposes that ultra-short-timescale microlensing events detected by the Subaru Hyper Suprime-Cam (HSC) and OGLE surveys may be caused by Earth-mass axion stars, a form of compact dark matter. Using microlensing light curves, the authors infer that axion stars could constitute ~27% of dark matter, with axion masses in the range 10⁻⁹–10⁻⁶ eV, consistent with QCD axion models and cosmological constraints.

ABSTRACT

Dark matter in the form of axions is expected to form miniclusters, and their dense regions can harbor compact axion stars. Such axion stars could be discovered by microlensing events. In particular, some candidate events reported by Subaru HSC and OGLE can be explained simultaneously if the axion stars with masses of the order of the Earth mass make up about $\sim27^{+7}_{013}$ percent of dark matter. For QCD axions, this corresponds to the axion mass in the range $10^{-9}-10^{-6}$ eV, which is consistent with the experimental constraints, as well as the cosmological anthropic window of parameters.

Motivation & Objective

• To investigate whether ultra-short-timescale microlensing events in HSC and OGLE data can be explained by axion stars.
• To constrain the mass fraction of dark matter in the form of axion stars using observed microlensing light curves.
• To derive bounds on axion parameters (mass and coupling) consistent with microlensing data and existing experimental constraints.
• To assess the compatibility of axion star formation with cosmological and particle physics constraints, including the strong CP problem and anthropic bounds.

Proposed method

• Modeling axion stars as stable, compact objects with masses ~10⁻⁶ M☉ and radii ~10² km, based on the effective potential of QCD axions.
• Using the microlensing magnification formula to simulate light curves for axion stars as point-like lenses, assuming a monochromatic mass function.
• Applying Bayesian inference to combine HSC and OGLE microlensing event data, marginalizing over lensing magnification and mass to derive posterior distributions for axion star mass and abundance.
• Converting the inferred axion star mass distribution into constraints on axion mass and coupling using the relation M_AS ∝ m_a⁻¹ f_a, assuming the KSVZ model for axion-photon coupling.
• Comparing the microlensing constraints with existing bounds from terrestrial experiments and optical magnetometers, highlighting complementarity.
• Assessing the robustness of results under variations in mass function shape and axion model parameters (e.g., E/N in axion-photon coupling).

Experimental results

Research questions

• RQ1 Can the ultra-short-timescale microlensing events observed in HSC and OGLE data be explained by axion stars with Earth-mass scales?
• RQ2 What fraction of dark matter is consistent with being in the form of axion stars, given the observed microlensing events?
• RQ3 What range of axion masses and couplings are compatible with the microlensing data and existing experimental constraints?
• RQ4 How do the microlensing constraints on axion stars compare with those from terrestrial experiments and optical magnetometers?
• RQ5 Is the observed consistency between HSC and OGLE data indicative of a single axion star population, or could an extended mass function reconcile both datasets?

Key findings

• The HSC and OGLE microlensing events are consistent with axion stars constituting ~27⁺⁷₋₁₃% of the total dark matter density.
• For QCD axions, the inferred axion mass range is 10⁻⁹–10⁻⁶ eV, which lies within the experimentally allowed region and is compatible with the cosmological anthropic window.
• The microlensing constraints are complementary to terrestrial experiments, which probe higher axion masses (e.g., GNOME sensitivity >10⁻⁴ eV), while microlensing probes the lower end of the axion mass spectrum.
• The observed HSC and OGLE events are consistent with a single axion star population if the mass function is monochromatic, but an extended mass function could simultaneously explain both datasets by probing different mass ranges.
• Axion stars with masses near Earth mass (10⁻⁶ M☉) and radii smaller than Earth’s are consistent with the observed microlensing timescales and magnitudes.
• The degeneracy direction in the (m_a, g_aγγ) plane from microlensing is orthogonal to that from direct detection experiments, enhancing the potential for cross-validation.