Shock geometry and inverse Compton emission from the wind of a binary pulsar
Lewis Ball, Jennifer Dodd, PASA, 18 (1), in press.
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Introduction

PSR B1259-63 is one of only three known radio pulsars which have a main sequence star binary companion. Such systems provide a unique environment for inverse Compton scattering because of the presence of an enormous density of low energy (optical) photons which serve as targets for electrons and positrons in the pulsar wind. The modest spin-down luminosity, and relatively high distance to two of these systems together imply that they are unlikely to be detectable sources of inverse Compton emission. In contrast, PSR B1259-63 is a galactic radio pulsar with a high spin-down luminosity ( $L_{\rm p}= 8.3\times 10^{28}\;$W) and is relatively nearby (1.5 kpc). If just 0.1% of the wind luminosity is scattered into hard $\gamma$-ray photons the resulting flux should be detectable using current $\gamma$-ray telescopes.

PSR B1259-63 is in a highly eccentric orbit ($e\sim 0.87$) around SS2883, a B2e star of radius $R_*\sim 6 R_\odot$ and luminosity $L_*\sim 8.8\times 10^3\,L_\odot$ [Johnston et al. 1992, 1994, 1996]. At periastron the pulsar is only $23R_*\approx 10^{11}\,{\rm m}$ from its companion. Around periastron there are $3.4\times 10^{11}\,\rm cm^{-3}$ Be-star photons at the pulsar, corresponding to an energy density of $U_{\rm rad}\sim 6\times 10^{11}\,\rm eV\,cm^{-3}$. This is some 11 orders of magnitude larger than the typical background target density available for inverse Compton scattering by the winds of isolated pulsars.

The Crab is presently the only pulsar for which observations place any constraints on the physical parameters of a pulsar wind. Observations of the PSR B1259-63 system aimed at detecting hard $\gamma$-ray inverse Compton emission are planned during 2000. If successful they should provide the first direct probe of the freely-expanding region of the wind of any rotation-powered pulsar, and serve to constrain wind models and parameters.

In the absence of an efficient deceleration process the relativistic wind of a pulsar will expand freely until it attains pressure balance with the surrounding medium. If the wind is still supersonic at the point of pressure balance it will be bounded by a termination shock. Kirk, Ball & Skjæraasen [1999] investigated the $\gamma$-ray emission from PSR B1259-63 resulting from inverse Compton scattering by the shocked pulsar wind, downstream of the termination shock, where the pulsar wind electrons and positrons have been accelerated and isotropised as the radial flow of the wind is disrupted. The effects of the cooling of the shocked wind by inverse Compton scattering were included by Tavani & Arons [1997] in their model for the synchrotron emission from the system. Ball & Kirk [2000a] considered the effects of inverse Compton scattering by the unshocked pulsar wind, upstream of the termination shock. They calculated the deceleration of the wind that occurs as a result of `inverse Compton drag' as energy and momentum are transferred from the wind to the scattered photons. In particular it was shown that the inverse Compton losses were unlikely to contain the wind of PSR B1259-63 before it attains the radius at which pressure balance with the Be-star outflow is likely to occur. Ball & Kirk [2000a] then calculated the emission from the freely-expanding wind subject to the assumption that the wind was not terminated by pressure balance.

In this paper we recalculate the inverse Compton emission from the unshocked wind of
PSR B1259-63 including the effects of termination of the radially-expanding pulsar wind as a result of pressure balance with the Be-star wind. The relative strengths of the pulsar and Be-star winds are unknown. If the Be-star wind dominates then the termination shock will be close to, and wrapped around, the pulsar. This will have the effect of decreasing the emission from the unshocked portion of the wind, and the effects vary over the binary orbit of the system. We hope that inverse Compton $\gamma$-rays will be detected from PSR B1259-63 in the near future, providing data that can be compared with such models.

The calculation of the geometry of the termination shock is outlined in §2. In §3 we present $\gamma$-ray spectra and light curves from the terminated wind, and compare them with those calculated for the unterminated wind by Ball & Kirk [2000a]. Our conclusions are presented in §4.

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