Neutrinos from hidden ultraluminous X-ray sources in the Galaxy

TL;DR

The paper addresses the origin of Galactic high-energy neutrinos by modeling hadronic acceleration in hidden ultraluminous X-ray sources (ULXs) with stellar-mass black holes accreting at super-Eddington rates. Using a one-zone model of magnetic reconnection above the black hole, it computes proton, pion, and muon distributions and derives neutrino emissivities and fluxes, showing that misaligned, X-ray-obscured ULXs can emit detectable neutrino signals via $p\gamma$ interactions with disk photons. In moderate accretion regimes (\dot{m}=10), protons reach PeV energies and yield distinctive TeV-scale neutrino spectra, while hyperaccreting cases (\dot{m}=10^3) are limited to ~100 TeV due to strong cooling. The results imply a potentially significant cumulative contribution from hidden ULXs to the Galactic neutrino background at energies below ~50 TeV, and they highlight the prospects for IceCube-Gen2, KM3NeT, and current IceCube in discovering these sources through multimessenger campaigns.

Abstract

Ultraluminous X-ray sources (ULXs) are point-like sources that exhibit apparent X-ray luminosities exceeding the Eddington limit for stellar-mass compact objects. A widely accepted interpretation is that these systems are X-ray binaries accreting matter possibly at super-Eddington rates. In this regime, photon trapping inflates the accretion disk, making it geometrically and optically thick. Radiation-driven winds launched from the supercritical disk form funnel-shaped walls along the symmetry axis. While the apparent X-ray luminosity can exceed the Eddington limit due to geometrical beaming within this funnel, a misalignment with the observer's line of sight strongly suppresses the X-ray emission, rendering the ULX electromagnetically obscured. This work explores the potential for high-energy neutrino production in black hole-hosting ULXs. We model proton acceleration via magnetic reconnection in the region above the super-accreting black hole. Although electromagnetic emission is efficiently absorbed by the dense wind and radiation fields, neutrinos generated from photomeson interactions can escape. Our model self-consistently accounts for energy losses of pions and muons in this environment. The results indicate that misaligned, electromagnetically obscured Galactic ULXs could produce a neutrino flux detectable by instruments like KM3NeT and IceCube within several years of observation.

Figures (11)

• Figure 1: Schematic diagram of a ULX system. Powerful winds from the supercritical disk form a funnel around the black hole. Inside this funnel, a magnetically dominated region enables particle acceleration via magnetic reconnection.
• Figure 2: Proton acceleration, cooling, and escape rates for the different parameter sets. Left: Scenario with an accretion rate of $\dot{m}=10$ for different densities ($A_1$, $A_2$, and $A_3$). At low energies, $pp$ interactions dominate the cooling, while above $\sim 100$ GeV, $p\gamma$ interactions become the main channel. The maximum proton energy reaches the PeV range. Right: Scenario with $\dot{m} = 1000$ for a range of densities ($B_1$, $B_2$, and $B_3$). As in the left panel, $pp$ interactions dominate at low energies and $p\gamma$ processes take over above $\sim 100$GeV. However, due to the strong cooling, the maximum energy is limited to $\sim 100$TeV.
• Figure 3: Distributions of relativistic protons for scenarios $A_i$ (left) and $B_i$ (right), for different injection spectral indices $\Gamma = 2$, $1.5$, and $1$, corresponding to configurations $i = 1, 2, 3$.
• Figure 4: Pion cooling, decay, and escape rates for the different parameter sets. We show $\pi\gamma$ interactions with disk photons and $\pi p$ interactions, computed for the three matter densities considered in each set $i$. Left: $A_i$ scenarios, where particle decay dominates at low energies, while synchrotron losses dominate above $\sim\mathrm{TeV}$. Right: $B_i$ scenarios, where particle decay dominates at low energies, while $\pi\gamma$ interactions and synchrotron losses dominate above $\sim 10\,\mathrm{TeV}$.
• Figure 5: Muon cooling, decay, and escape rates for the different scenarios. For the IC interaction, we only consider interactions with the X-photon field of the disk, and we neglect interactions with matter. Left: $A_i$ scenarios, where particle decay dominates at low energies, while synchrotron losses become dominant above $\sim 100$ GeV. Right: $B_i$ scenarios, where decay dominates at low energies, while IC and synchrotron losses dominate above $\sim 100$ GeV.