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Abstract

Only a few binary systems with compact objects display TeV emission. The physical properties of the companion stars represent basic input for understanding the physical mechanisms behind the particle acceleration, emission, and absorption processes in these so-called gamma-ray binaries. Here we present high-resolution and high signalto- noise optical spectra of LS 2883, the Be star forming 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 than previously thought. Analysis of the interstellar lines suggests that the system is located at the same distance as (and thus is likely a member of) Cen OB1. Taking the distance to the association, d = 2.3 kpc, and a color excess of E(B −V ) = 0.85 for LS 2883 results in MV ≈ −4.4. Because of fast rotation, LS 2883 is oblate (Req 9.7 R and Rpole 8.1 R) and presents a temperature gradient (Teq ≈ 27,500 K, log geq = 3.7; Tpole ≈ 34,000 K, log gpole = 4.1). If the star did not rotate, it would have parameters corresponding to a late O-type star. We estimate its luminosity at log(L∗/L) 4.79 and its mass at M∗ ≈ 30 M. The mass function then implies an inclination of the binary system iorb ≈ 23◦, slightly smaller than previous estimates. We discuss the implications of these new astrophysical parameters of LS 2883 for the production of high-energy and very high-energy gamma rays in the PSR B1259–63/ LS 2883 gamma-ray binary system. In particular, the stellar properties are very important for prediction of the line-like bulk Comptonization component from the unshocked ultrarelativistic pulsar wind.