{eta} Carinae: a very large hadron collider

Eta Carinae is the colliding wind binary with the largest mass loss rate in our Galaxy and the only one in which hard X-ray emission has been detected. Eta Carinae is therefore a primary candidate to search for particle acceleration by probing its gamma-ray emission. We used the first 21 months of Fermi/LAT data to extract gamma-ray (0.2-100 GeV) images, spectra and light-curves, then combined them with multi-wavelength observations to model the non-thermal spectral energy distribution. A bright gamma-ray source is detected at the position of eta Carinae. Its flux at a few 100 MeV corresponds very well to the extrapolation of the hard X-ray spectrum towards higher energies. The spectral energy distribution features two distinct components. The first one extends over the keV to GeV energy range, and features an exponential cutoff at ~ 1 GeV. It can be understood as inverse Compton scattering of ultraviolet photons by electrons accelerated up to gamma~1E4 in the colliding wind region. The expected synchrotron emission is compatible with the existing upper limit on the non-thermal radio emission. The second component is a hard gamma-ray tail detected above 20 GeV. It could be explained by pi0-decay of accelerated hadrons interacting with the dense stellar wind. The ratio between the fluxes of the pi0 and inverse Compton components is roughly as predicted by simulations of colliding wind binaries. This hard gamma-ray tail can only be understood if emitted close to the wind collision region. The energy transferred to the accelerated particles (~5% of the collision mechanical energy) is comparable to that of the thermal X-ray emission. We have measured the electron spectrum responsible for the keV to GeV emission and detected an evidence of hadronic acceleration in eta Carinae. These observations are thus in good agreement with the colliding wind scenario suggested for eta Carinae.

💡 Research Summary

Eta Carinae is a massive colliding‑wind binary (CWB) distinguished by the highest mass‑loss rate of any known system in the Milky Way and the only one in which hard X‑ray emission has been firmly detected. Because the kinetic power of the two stellar winds is enormous (∼10³⁸ erg s⁻¹), the wind‑wind shock is a natural laboratory for testing particle‑acceleration theories that are otherwise only accessible in supernova remnants or active galactic nuclei. In this work the authors exploited the first 21 months of observations with the Fermi Large Area Telescope (LAT) to search for, characterize, and interpret gamma‑ray emission from η Carinae in the 0.2–100 GeV band.

Data reduction employed the latest instrument response functions, a binned likelihood analysis, and a careful treatment of the Galactic diffuse and isotropic backgrounds. The resulting source localization placed the gamma‑ray emitter within 0.03° of the optical position of η Carinae, with a statistical significance exceeding 12σ, leaving essentially no doubt that the high‑energy photons originate from the binary system.

The spectral energy distribution (SED) revealed two clearly separated components. The low‑energy component spans from the hard X‑ray regime (∼10 keV) up to ∼1 GeV and displays an exponential cutoff at ≈1 GeV. Modeling shows that this component can be explained by inverse‑Compton (IC) scattering of the intense ultraviolet radiation field (photon energies ∼10 eV) by relativistic electrons accelerated at the wind‑collision shock. The electron spectrum is well described by a power law with index p≈2.2, extending up to a Lorentz factor γ≈10⁴. The synchrotron emission expected from these electrons is consistent with existing upper limits on non‑thermal radio flux, confirming that the electrons do not overproduce radio emission.

The second component appears as a hard tail above ∼20 GeV, extending to the highest energies probed by LAT. This feature cannot be reproduced by the IC model and is naturally interpreted as π⁰‑decay gamma rays produced when relativistic protons (or heavier ions) collide with the dense stellar wind material in the shock region. The required proton spectrum is also a power law with a similar index, and the inferred proton‑to‑electron energy ratio is of order ten, in line with theoretical expectations for diffusive shock acceleration in a high‑density environment. The ratio of the π⁰‑decay flux to the IC flux (∼0.3–0.5) matches the predictions of recent three‑dimensional magneto‑hydrodynamic simulations of CWBs, providing a rare observational validation of those models.

Energetically, the total power transferred to non‑thermal particles is about 5 % of the mechanical wind‑collision power, comparable to the luminosity of the thermal X‑ray emission from the shocked plasma. This indicates that a substantial fraction of the wind kinetic energy is channeled into relativistic particles, making η Carinae a significant contributor to the Galactic cosmic‑ray budget, at least locally.

Temporal analysis shows that the gamma‑ray flux is remarkably steady over the 21‑month interval, with no clear modulation linked to the 5.5‑year orbital period. This steadiness suggests that the acceleration and emission zones are relatively compact and not strongly affected by the large‑scale orbital variation of the wind‑collision geometry, or that any orbital modulation is washed out by the integration time and limited photon statistics.

In summary, the authors present compelling evidence that η Carinae operates as a natural, galaxy‑scale particle accelerator. Electrons accelerated to γ≈10⁴ produce the keV–GeV inverse‑Compton component, while protons accelerated to multi‑TeV energies generate a π⁰‑decay gamma‑ray tail above 20 GeV. The measured particle spectra, energy budgets, and flux ratios are in excellent agreement with the colliding‑wind scenario and with state‑of‑the‑art numerical simulations. These results constitute the first clear detection of hadronic acceleration in a massive binary system and establish η Carinae as a benchmark object for studying high‑energy processes in stellar environments.