An 18-year Fermi-LAT stacking limit on GeV $γ$-ray emission from particle-accelerating colliding-wind binaries
Abstract
The wind-collision regions of massive colliding-wind binaries (CWBs) accelerate relativistic particles, as shown by their non-thermal radio synchrotron and, in Car, hard X-ray emission. Whether CWBs emit GeV rays as a population is unknown: only 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 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 -ray sources leaves a clean, mutually isolated sample of 6, whose stack is consistent with the control-field null (): no evidence for collective GeV emission. Retaining those systems instead yields a spurious excess from chance catalogue coincidences, a caveat for Galactic-plane stacking. The resulting 95% limit on the mean per-source flux, , implies a -ray production efficiency , about two orders of magnitude below 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, 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