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 η Car, hard X-ray emission. Whether CWBs emit GeV γrays as a population is unknown: only η 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 γ-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 ≳3.5σ excess from chance catalogue coincidences, a caveat for Galactic-plane stacking. The resulting 95% limit on the mean per-source flux, F(>1 GeV)≲1.1×10−11 ph cm−2 s−1 (Γ=2), implies a γ-ray production efficiency η=Lγ/Lwind≲4×10−6 (d/kpc)2, about two orders of magnitude below η 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 Car thus appears to be a singular object rather than the brightest member of an emerging population.
Versions
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BibTeX
@misc{son202618year,
title = {{An 18-year Fermi-LAT stacking limit on GeV $γ$-ray emission from particle-accelerating colliding-wind binaries}},
author = {Youngwan Son},
howpublished = {arXiv},
year = {2026},
month = jul,
doi = {10.48550/arxiv.2607.05102},
eprint = {2607.05102},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.05102},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - An 18-year Fermi-LAT stacking limit on GeV γ-ray emission from particle-accelerating colliding-wind binaries
AU - Youngwan Son
PY - 2026
DA - 2026/07/06
PB - arXiv
DO - 10.48550/arxiv.2607.05102
UR - https://arxiv.org/abs/2607.05102
AB - The wind-collision regions of massive colliding-wind binaries (CWBs) accelerate relativistic particles, as shown by their non-thermal radio synchrotron and, in η Car, hard X-ray emission. Whether CWBs emit GeV γrays as a population is unknown: only η 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 γ-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 ≳3.5σ excess from chance catalogue coincidences, a caveat for Galactic-plane stacking. The resulting 95% limit on the mean per-source flux, F(>1 GeV)≲1.1×10^−11 ph cm^−2 s^−1 (Γ=2), implies a γ-ray production efficiency η=L_γ/L_wind≲4×10^−6 (d/kpc)^2, about two orders of magnitude below η 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 Car thus appears to be a singular object rather than the brightest member of an emerging population.
ER -