Polar Mounds on Strangeon Stars: the Neutrino Emission from Ultraluminous X-ray Pulsars

Ultraluminous X-ray pulsars (ULXPs) serve as unique astrophysical laboratories, offering critical insights into accretion physics under extreme conditions, such as strong magnetic fields and super-Eddington accretion rates. Additionally, the nature of pulsars, i.e., the equation of state of supranuclear matter, is still a matter of intense debate, basing on either conventional neutron stars or strange stars. In this work, in order to differentiate the conjectured states of matter, we investigate accretion columns of ULXPs based on the strangeon-star (SS) model, focusing on the thermal mound at the column base. Accounting for Coulomb and strangeness barriers of SSs, we find that the mound can reach $0.7-0.95,\rm km$ in height with temperatures above $10^9, \rm K$, enabling substantial neutrino emission via electron-positron annihilation. At low accretion rates ($< 10^{20}, \rm g,s^{-1}$), photons dominate the luminosity, while at higher rates ($> 10^{21}, \rm g, s^{-1}$), photon trapping makes neutrino emission the main cooling channel, with total luminosity exceeding photon emission, which saturates near $10^{41}, \rm erg,s^{-1}$. Even though the predicted neutrino flux from the nearest system, Swift J0243.6$+$6124, lies well below the diffuse MeV background–implying that detectable emission would require substantially closer or more luminous sources–these results demonstrate the key role of the thermal mound and SS properties in accretion, providing a foundation for future ULXP studies and suggesting that neutrino observations could, in principle, offer a novel probe of SSs and extreme supranuclear matter.

💡 Research Summary

The paper investigates the thermal mound that forms at the base of the accretion column in ultraluminous X‑ray pulsars (ULXPs) under the hypothesis that the compact object is a strangeon star (SS) rather than a conventional neutron star. Strangeon stars are self‑bound objects composed of localized clusters of up, down, and strange quarks (strangeons). Because of both a Coulomb barrier and a “strangeness barrier” at the surface, matter accreting onto an SS piles up just above a sharp density discontinuity, giving the mound a very low effective heat capacity. Consequently, for a given mass accretion rate the mound reaches temperatures well above 10⁹ K, far hotter than in neutron‑star models.

The authors adopt a one‑dimensional, steady‑state description of the column, solving the coupled mass, momentum, and energy conservation equations (Eqs. 1‑2) with appropriate boundary conditions: a free‑fall density at the top of the polar cone and a balance between gas pressure and magnetic pressure at the stellar surface. The photon diffusion length r_D, set by Thomson opacity and the accretion rate, controls how far matter can fall before its kinetic energy is radiated away. Numerical integration yields profiles of radiation energy density, mass density, pressure, velocity, optical depth, and temperature as functions of height for several accretion rates.

Key quantitative results are:

• The thermal mound height H_mound lies between 0.7 km and 0.95 km, an order of magnitude larger than in neutron‑star calculations.
• The base temperature exceeds 10⁹ K for all considered accretion rates (10²⁰–10²² g s⁻¹). At these temperatures, electron‑positron pair creation is prolific and pair annihilation (e⁻+e⁺→ν+ν̄) dominates neutrino production, with an emissivity Q_ν≈4×10²⁴ (T/10¹⁰ K)⁹ erg cm⁻³ s⁻¹.
• For low accretion rates (<10²⁰ g s⁻¹) photon emission carries most of the liberated energy; the photon luminosity L_γ scales with ṁ and saturates near 10⁴¹ erg s⁻¹ because magnetic pressure caps the temperature at the mound base.
• At higher rates (>10²¹ g s⁻¹) photon trapping becomes severe. The advected energy is deposited deeper in the mound, and neutrino cooling overtakes photon cooling. The total luminosity then exceeds the photon component, reaching ≳10⁴² erg s⁻¹ for ṁ≈10²² g s⁻¹.

The authors compute the expected neutrino flux from the nearest ULXP, Swift J0243.6+6124 (distance ≈7 kpc, ṁ≈2×10²⁰ g s⁻¹). The predicted flux, Φ_ν≈10⁻⁹ cm⁻² s⁻¹, lies well below the measured diffuse MeV neutrino background (≈10⁻⁶ cm⁻² s⁻¹) and is far beyond the sensitivity of current detectors such as IceCube or Super‑Kamiokande. Nevertheless, the analysis shows that a ULXP within ≲1 kpc or with an X‑ray luminosity ≥10⁴² erg s⁻¹ would produce a detectable MeV neutrino signal, offering a novel probe of the strangeon‑star equation of state.

In the discussion, the authors emphasize that the presence of the Coulomb and strangeness barriers fundamentally alters the thermal structure of the column base, making the mound a powerful neutrino radiator. This contrasts with neutron‑star models where the larger heat capacity keeps temperatures lower and neutrino losses negligible. They argue that future multi‑messenger observations—simultaneous X‑ray timing/spectroscopy and MeV neutrino detection—could discriminate between SS and NS interiors. Suggested extensions include three‑dimensional magnetohydrodynamic simulations to capture non‑axisymmetric mound geometry, time‑dependent accretion variability, and detailed neutrino spectral modeling.

In conclusion, the paper provides a self‑consistent framework linking the exotic equation of state of strangeon matter to observable high‑energy phenomena in ULXPs. It demonstrates that, under super‑Eddington accretion, the thermal mound on a strangeon star can become an efficient neutrino factory, potentially opening a new window onto the physics of ultra‑dense matter.