English

An 18-year Fermi-LAT stacking limit on GeV $γ$-ray emission from particle-accelerating colliding-wind binaries

High Energy Astrophysical Phenomena 2026-07-06 v1

Abstract

The wind-collision regions of massive colliding-wind binaries (CWBs) accelerate relativistic particles, as shown by their non-thermal radio synchrotron and, in η\eta Car, hard X-ray emission. Whether CWBs emit GeV γ\gammarays as a population is unknown: only η\eta Car is unambiguously detected by the Fermi Large Area Telescope (LAT). We analyze 17.8 yr of Fermi-LAT data above 1 GeV (where the sharp point-spread function controls plane confusion) at the 61 confirmed (list A) particle-accelerating CWBs (PACWBs) of De Becker & Raucq, using a two-dimensional (photon index ×\times flux) likelihood scan against 200 control fields matched in latitude, local source density, and diffuse intensity. Removing systems whose lines of sight coincide with bright catalogue γ\gamma-ray sources leaves a clean, mutually isolated sample of 6, whose stack is consistent with the control-field null (p=0.83p=0.83): no evidence for collective GeV emission. Retaining those systems instead yields a spurious 3.5σ\gtrsim3.5\sigma excess from chance catalogue coincidences, a caveat for Galactic-plane stacking. The resulting 95% limit on the mean per-source flux, F(>1GeV)1.1×1011 phcm2s1(Γ=2)F(>1\,\mathrm{GeV})\lesssim1.1\times10^{-11}~\mathrm{ph\,cm^{-2}\,s^{-1}} (\Gamma=2), implies a γ\gamma-ray production efficiency η=Lγ/Lwind4×106(d/kpc)2\eta=L_\gamma/L_{\rm wind}\lesssim4\times10^{-6}\,(d/\mathrm{kpc})^2, about two orders of magnitude below η\eta Car at the sample's median distance. A representative single-zone model then bounds the electron acceleration efficiency in CWB wind-collision regions unless their magnetic field is comparable to or above the magnetic-photon equipartition value. Among the high-latitude systems accessible to a clean stack, η\eta Car thus appears to be a singular object rather than the brightest member of an emerging population.

Keywords

Cite

@article{arxiv.2607.05102,
  title  = {An 18-year Fermi-LAT stacking limit on GeV $γ$-ray emission from particle-accelerating colliding-wind binaries},
  author = {Youngwan Son},
  journal= {arXiv preprint arXiv:2607.05102},
  year   = {2026}
}

Comments

Prepared for submission to JCAP