High-energy radiation from the massive binary system Eta Carinae

Abstract

Context. The most massive binary system Eta Carinae has been recently established as a gamma-ray source by the AGILE and Fermi-LAT detectors. The high energy spectrum of this gamma-ray source is very intriguing. It shows two clear components and a lack of any evidence of variability with the orbital period of the binary system. Aims. We consider different scenarios for the acceleration of particles (both electrons and hadrons) and the production of the high energy radiation in the model of stellar wind collisions within the binary system Eta Carinae with the aim to explain the gamma-ray observations and predict the behaviour of the source at very high gamma-ray energies. Methods. The gamma-ray spectra calculated in terms of the specific models are compared with the observations of Eta Carinae, and the neutrino spectra produced in hadronic models are confronted with the atmospheric neutrino background and the sensitivity of 1 km 2 neutrino telescope. Results. We show that spectral features can be explained in terms of the stellar wind collision model between the winds of the companion stars in which the acceleration of particles occurs on both sides of the double shock structure. The shocks from the Eta Carinae star and the companion star can accelerate particles to different energies depending on the different conditions determined by the parameters of the stars. The lack of strong GeV gamma-ray variability with the period of the binary system can be also understood in terms of such a model. Conclusions. We predict that the gamma-ray emission features at energies above ∼100 GeV will show significant variability (or its lack) depending on the acceleration and interaction scenario of particles accelerated within the binary system. For the hadronic models we predict the expected range of neutrino fluxes from the binary system Eta Carinae. This can be tested through observations with the large-scale neutrino telescopes, which will support or disprove the specific hadronic models.

Gamma-ray follow-up studies onηCarinae

“…The decrease is significant at low-(0.2 to 10 GeV) as well as high-(10 to 300 GeV) energies, albeit showing a different time dependence (see above). In general, the observed flux decrease can be qualitatively understood with predictions in the framework of colliding-wind binary models for high-energy γ-ray emission (Reimer et al 2006;Bednarek & Pabich 2011). As the stars move away from each other, the matter density -as well as the radiation density -decreases in the wind collision region where particle acceleration and subsequent γ-ray emission are thought to occur.…”

“…Both massive stars in the η Car system are expected to produce powerful stellar winds. Massloss rates and terminal wind velocities (as given in the table) are thought to be sufficiently high to form a wind-wind collision zone of shocked, hot gas, wherein particle acceleration (as illustrated by e.g., Eichler & Usov 1993;Dougherty et al 2003) in general and subsequent γ-ray emission in particular (as described by e.g., Reimer et al 2006;Bednarek & Pabich 2011) can occur. The conditions in the wind-wind collision zone depend on the orbital phase of the binary system.…”

A model for the non-thermal emission of the very massive colliding-wind binary HD 93129A

“…(6) prescribed in Antokhin et al (2004). As both shocks, S1 and S2, have different properties because they depend on the conditions of the respective incoming wind (Bednarek & Pabich 2011), we apply an analytic hydrodynamical (HD) model to characterize the values of the relevant thermodynamical quantities for each side of the CD, which implies that there are actually two overlapped emitters at the CD. The WCR is formed by a sum of these 1D-emitters (linear-emitters hereafter) that are symmetrically distributed around the two-star axis in a 3D space: each discrete emission cell is first defined in the XY-plane, and the full 3D structure of the wind interaction zone is then obtained via rotation around the X-axis.…”

The first full orbit ofηCarinae seen byFermi

“…In this, charged particles are accelerated at the shock fronts of an extensive wind-collision region (WCR), which forms where the powerful stellar winds of the high-mass stars collide (Eichler & Usov 1993;Pittard & Dougherty 2006). Traveling alongside the hot, shocked gas in the WCR, the charged particles (leptonic and hadronic in nature) interact with stellar radiation fields, magnetic fields, and the surrounding plasma, subsequently emitting nonthermal radiation via inverse Compton emission, synchrotron emission, relativistic bremsstrahlung, and neutral pion decay (Reimer et al 2006;Bednarek & Pabich 2011). Because the properties of the WCR change drastically with stellar separation, the ensuing nonthermal emission is expected to show strong modulation on orbital timescales for systems with high eccentricity such as η Carinae.…”