Fermi acceleration along the orbit of η Carinae

Abstract

Context. The η Carinae binary system hosts one of the most massive stars, which features the highest known mass-loss rate. This dense wind encounters the much faster wind expelled by the stellar companion, dissipating mechanical energy in the shock, where particles can be accelerated up to relativistic energies and subsequently produce very-high-energy γ-rays. Aims. We aim at comparing the variability of the γ-ray emission of η Carinae along the binary orbit with the predictions of simulations to establish the nature of the emission and of the seed particles. Methods. We have used data from the Fermi Large Area Telescope obtained during its first seven years of observations and spanning two passages of η Carinae at periastron. We performed the analysis using the new PASS8 pipeline and its improved instrument response function, extracting low and high-energy light curves as well as spectra in different orbital phase bins. We also introduced particle acceleration in hydrodynamic simulations of the system, assuming a dipolar magnetic field generated by the most massive star, and compared the γ-ray observations with the predictions of diffuse shock acceleration in a multi-cell geometry.Results. The main source of the γ-ray emission originates from a position compatible with η Carinae and located within the Homunculus Nebula. Two emission components can be distinguished. The low-energy component cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by a factor less than 2. Short-term variability occurs at periastron. The flux of the high-energy component varies by a factor 3-4 but is different during the two periastrons. The variabilities observed at low energy, including some details of them, and those observed at high energy during the first half of the observations, match the prediction of the simulation, assuming a surface magnetic field of 500 G. The high-energy component and the thermal X-ray emission were weaker than expected around the second periastron suggesting a modification of the wind density in the inner wind collision zone. Conclusions. Diffuse shock acceleration in the complex geometry of the wind collision zone of η Carinae provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. This demonstrates that Fermi acceleration is at work in the wind collisions and that a few percent of the shock mechanical energy are converted into particle acceleration. Further observations are required to understand the periastron-to-periastron variability of the high-energy component and to associate it firmly with hadronic origin. We estimate that η Carinae is a pevatron at periastron and is bright enough to be detected by IceCube after many years of observations. Orbital modulations of the high-energy component can be distinguished from those of photo absorption by the four large size telescopes of the Cherenkov Telescope Array to be placed in the southern hemisphere.

Gamma-ray and X-ray constraints on non-thermal processes inηCarinae

“…In the lower panel the X-ray emission at lower energies as measured by RXTE and Swift for passages culminating with periastron in 1998, 2003, 2009 and 2014 are shown (Corcoran et al 2017). The 300 MeV to 10 GeV γ-ray flux behaves similarly during both passage (as noted by Balbo & Walter (2017)) and shows no distinct disappearance at periastron as seen in the X-ray data.…”

“…Vertical grey shaded areas indicate the phase ranges used in the spectral analysis presented in Section 2.4 around periastron, pre-periastron and off-periastron. The points fromBalbo & Walter (2017) are shown as hollow grey circles. The vertical black dashed line indicates periastron.…”

Overview of non-transient γ-ray binaries and prospects for the Cherenkov Telescope Array

“…Observations match the predictions of the simulation except for the second peak, which is slightly shifted towards earlier phases and has a lower luminosity (see Fig. 6 in Balbo & Walter 2017). The phase difference could be related to the eccentricity (e = 0.9) assumed in the simulation, which is not well constrained by observations (Damineli et al 2000;Corcoran et al 2001), and that has an important effect on the inner shock geometry.…”

“…Unfortunately the number of particles simulated was insufficient to fully predict the probability distribution function of the energetic particles, as well as the consequent spectral energy distribution of radiation. To estimate the non-thermal emission of the system, Balbo & Walter (2017) calculated the maximum energies reached by electrons and hadrons cell-by-cell assuming a dipolar magnetic field at the surface of the main star. The magnetic field is the only additional parameter and can be tuned.…”

The environment of the wind–wind collision region of η Carinae

“…Finally, the wind from the primary star may be powerful enough to stop the secondary wind. In order to investigate which mechanism is consistent with the NuSTAR observations, we compared the observed source flux to the theoretically expected flux given in Balbo & Walter (2017). We used our best-fit model to calculate the intrinsic 8-9 keV continuum emission of the WWC.…”