Universitat de - Departament d'Astronomia i Meteorologia ...

80 Chapter 2. Multiwavelength approach to LS 5039
As we have already discussed, the radio emission originates as synchrotron ra-
diation in a jet-like flow in the presence of a magnetic field. The near infrared and
optical light arises from the high mass companion star. The X-rays probably arise
in the accretion disk around the compact object. Finally, the γ-rays are probably
produced by inverse Compton scattering of the ultraviolet photons of the companion
star by the relativistic electrons present in the jet flow, as we discuss below.
2.8.2 A model based on the γ-ray/radio emission
The movement of matter in the system is proposed to be as follows. The early type
O6.5V((f)) star LS 5039 has a strong wind that accounts for a mass loss rate of
∼ 10 −6 M⊙ yr −1 , according to McSwain & Gies (2002). While leaving the star,
part of this matter will be captured by the intense gravitational field of the nearby
compact object (probably a neutron star). The matter will then fall into the compact
object and form a disk around it, which by viscous heating will emit in X-rays. Part
of the X-ray emission can also be produced in a corona above the disk. A fraction
of the matter forming the disk will be ejected perpendicular to it and in opposite
senses, probably as a result of the Blandford & Payne (1982) mechanism, as seems to
be the case in the quasar M87 (Junor et al. 1999). While flowing away into opposite
jets, the relativistic electrons (matter) will be unavoidably exposed to a huge output
of ultraviolet (UV) photons from the hot optical star. As a consequence, the UV
photons will experience inverse Compton scattering, and γ-rays will be naturally
produced, as detected by EGRET. Later on, the relativistic electrons will produce
synchrotron radio emission. This scenario is schematically reproduced in Fig. 2.23.
It is interesting to use the presence of 5 GHz synchrotron emitting particles far
away from the compact object, ∼ 1000 AU as seen in Sect. 2.4.5, in order to constrain
the jet physical parameters. The starting point of the following calculations will be
the scenario for the proposed EGRET emission of LS 5039 due to IC scattering. We
recall that the energy shift in the IC process may be as high as Eγ ∼ 4γ 2 e EUV, where
the energies of the γ-ray and the stellar UV photon are related through the squared
Lorentz factor of the relativistic electron.
We will adopt here a very simple model of an expanding jet. Cylindrical coor-
dinates, z and r measured parallel and perpendicular to the jet axis, are the best
choice in our case. The jet is assumed to form at a distance z0 from the compact