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Only a few binary systems with compact objects display TeV emission. Thephysical properties of the companion stars represent basic input to understandthe physical mechanisms behind the particle acceleration, emission, andabsorption processes in these so-called gamma-ray binaries. Here we presenthigh-resolution and high signal-to-noise optical spectra of LS2883, the Be starforming a gamma-ray binary with the young non-accreting pulsar PSR B1259-63,showing it to rotate faster and be significantly earlier and more luminous thanpreviously thought. Analysis of the interstellar lines suggest that the systemis located at the same distance as (and thus is likely a member of) CenOB1.Taking the distance to the association, d=2.3kpc, and a color excess ofE(B-V)=0.85 for LS2883, results in Mv=-4.4. Because of fast rotation, LS2883 isoblate (R_eq=9.7R_sun and R_pole=8.1R_sun) and presents a temperature gradient(T_eq=27500K, log g_eq=3.7; T_pole=34000K, log g_pole=4.1). If the star did notrotate, it would have parameters corresponding to a late O-type star. Weestimate its luminosity at log(L*/L_sun)=4.79, and its mass at M=30M_sun. Themass function then implies an inclination of the binary systemi_orb=23deg,slightly smaller than previous estimates. We discuss theimplications of these new astrophysical parameters of LS2883 for the productionof high energy and very high energy gamma rays in the PSR B1259-63/LS2883gamma-ray binary system. In particular, the stellar properties are veryimportant for prediction of the line-like bulk Comptonization component fromthe unshocked ultra-relativistic pulsar wind.

Astrophysical parameters of LS2883 and implications for the PSR B1259-63 gamma-ray binary