The high-energy emission from HD~93129A near periastron

Abstract

We conducted an observational campaign towards one of the most massive and luminous colliding wind binaries in the Galaxy, HD~93129A, close to its periastron passage in 2018. During this time the source was predicted to be in its maximum of high-energy emission. Here we present our data analysis from the X-ray satellites \textit{Chandra} and \textit{NuSTAR} and the ${\hat{I}}^3$-ray satellite \textit{AGILE}. High-energy emission coincident with HD~93129A was detected in the X-ray band up to $\sim$18~keV, whereas in the ${\hat{I}}^3$-ray band only upper limits were obtained. We interpret the derived fluxes using a non-thermal radiative model for the wind-collision region. We establish a conservative upper limit for the fraction of the wind kinetic power that is converted into relativistic electron acceleration, $f_{\mathrm{NT},\mathrm{e}}<0.02$. In addition, we set a lower limit for the magnetic field in the wind-collision region as $B_{\mathrm{WCR}}>0.3$~G. We also argue a putative interpretation of the emission from which we estimate $f_{\mathrm{NT},\mathrm{e}}\approx 0.006$ and $B_{\mathrm{WCR}}\approx 0.5$~G. We conclude that multi-wavelength, dedicated observing campaigns during carefully selected epochs are a powerful tool for characterising the relativistic particle content and magnetic field intensity in colliding wind binaries.

11 pages, 10 figures, accepted for publication in MNRAS