Search for neutrino emission from LHAASO observed Microquasar with IceCube 10-year data

TL;DR

This work tests the hadronic origin of ultra-high-energy gamma rays from five LHAASO-identified microquasars by searching for neutrino emission in 10 years of IceCube data, using both single-source and stacking analyses across multiple spectral hypotheses. The analysis finds no significant neutrino signal and sets upper limits that constrain the hadronic contribution to the observed gamma-ray flux, with stacking implying microquasars account for only a small fraction of the Galactic plane diffuse neutrino flux. The results are interpreted with source-specific hadronic models and compared to Galactic-plane neutrino predictions, informing the likely need for other source classes to explain the diffuse flux. The paper also outlines how next-generation detectors like IceCube-Gen2 and HUNT could decisively test these microquasars as Galactic PeVatrons, highlighting the importance of multi-messenger and future observatory capabilities for Galactic cosmic-ray acceleration studies.

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has detected ultra-high-energy (UHE; E>100 TeV) gamma-ray emission from five microquasars, suggesting their potential as Galactic PeV cosmic-ray accelerators. At these energies, the Klein-Nishima effect strongly suppresses leptonic processes, making neutrinos observation a crucial test for hadronic acceleration. We present a search for neutrino emission from these LHAASO-identified Microquasars using ten years of IceCube muon-track data. No significant neutrino signal was found in either single-source or stacking analyses. Our stacking result further shows that the studied microquasars population can only account for a small fraction of the diffuse neutrino flux along the Galactic Plane. Finally, we demonstrate that new-generation neutrino telescopes, such as HUN, will have the sensitivity to probe harmonic emission from these candidate PeVatrons.

Figures (3)

• Figure 1: Constraints on the hadronic gamma-ray flux for individual microquasars. In each panel, the black points show the gamma-ray flux measured by LHAASO, with 1$\sigma$ statistical error bars. The blue lines represent the $90\%$ C.L. upper limits on the hadronic flux, derived assuming a power-law spectrum with the LHAASO best-fit index (solid), and benchmark indices of $\Gamma=2.0$ (dashed) and $\Gamma=3.0$ (dotted). The red curves represent constraints from source-specific, physically motivated models. Specifically, panel (d) compares two models for SS 433, one based on the LHAASO molecular cloud interaction scenario (red dashed line) and the other based on the "wind+jet" model of Carpio:2025arz (red solid line). For V4641 Sgr in panel (e), the red dashed and solid lines correspond to the lepto-hadronic (XRISM region) and purely hadronic (HAWC region) scenarios from Carpio:2025arz, respectively. The limit for GRS 1915+105 in panel (f) is derived from the purely hadronic model from the same work. Finally, in panel (h) for Cygnus X-1, the red curve is the limit derived using an ECPL spectral shape, with the ECPL model fit itself shown as the gray dashed line.
• Figure 2: The 90$\%$ C.L. upper limits on the stacked, all-flavor neutrino flux from the galactic microquasar populations. The limits are shown for three different power-law spectral index assumptions: $\Gamma=2.0$ (solid line), $\Gamma=2.5$ (dashed line), and $\Gamma=3.0$ (dotted line). The constraints are compared to the galactic plane neutrino flux for each Galactic plane models, which are the $\pi^0$ (purple), $\rm{KRA}_\gamma^5$ (orange) and $\rm{KRA}_\gamma^{50}$ (light blue) while solid lines and shaded regions indicates the best-fitting and $1\sigma$ uncertainties. The number above each line displays the fraction can contribute to the galactic plane neutrino flux fitted by $\pi^0$ template.
• Figure 3: Left: Expected neutrino event rates from microquasars, calculated for IceCube-Gen2 (blue solid circles) and HUNT (red open squares). The rates are based on the neutrino flux converted from the LHAASO gamma-ray measurements. Middle: Cumulative statistical significance $\rm{S_{LM}~(\sigma)}$ for a neutrino signal ($E_\nu > 10 \rm TeV$) as a function of observation time (in years) for IceCube-Gen2. Right: Same as the middle panel, but for the HUNT detector.