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

The wind-collision regions of massive colliding-wind binaries (CWBs) accelerate relativistic particles, as shown by their non-thermal radio synchrotron and, in eta Car, hard X-ray emission. Whether CWBs emit GeV gamma rays as a population is unknown: only 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 gamma-ray sources leaves a clean, mutually isolated sample of 6, whose stack is consistent with the control-field null (p=0.83): no evidence for collective GeV emission. Retaining those systems instead yields a spurious 3.5 sigma excess from chance catalogue coincidences, a caveat for Galactic-plane stacking. The resulting 95% limit on the mean per-source flux, F(>1, GeV) 1.1 times10-11 ph,cm-2,s-1 (=2), implies a gamma-ray production efficiency eta=L gamma/L wind 4 times10-6,(d/kpc) squared, about two orders of magnitude below 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, eta Car thus appears to be a singular object rather than the brightest member of an emerging population.