The high-mass binary system PSR B1259-63/LS 2883 is one of only six known gamma-ray binaries,
and the only one where the compact object is known from the direct observation of a pulsed radio signal
to be a 48 ms pulsar. During itâs eccentric 3.4 year orbit, the pulsar moves through the circumstellar
disc of the optical companion, approximately twenty days before and after periastron. This results
in conditions for complicated interactions between the material in the disc, the fast rotating pulsar,
the pulsar wind, and the radiation field from the star and disc. The system has been the object of
multi-wavelength campaigns with telescopes such as the VLT, H.E.S.S. and Fermi. The interaction
between the stellar and pulsar wind results in the formation of a radiating pulsar wind nebula within
the binary system, which has been detected from radio to TeV gamma-ray energies. The spectral energy
distribution is dominated by the emission at gamma-ray energies, classifying this system as a gammaray
binary. The interaction between the stars is greater near periastron where the pulsar passes closest
to the optical companion. Approximately twenty days from periastron the pulsar passes through or
behind the Be starâs circumstellar disc, obscuring the pulsed radio signal. During this period there is
a corresponding increase in the unpulsed emission from the system. The TeV gamma-rays are believed
to be produced by electrons in the pulsar wind which cool via the inverse Compton up-scattering of
stellar photons from the optical companion. The circumstellar disc associated with the Be star produces
an infrared flux below ! 1015 Hz, which is greater than that expected from the blackbody distribution
associated with star, providing additional target photons which could increase the inverse Compton
scattering rate. The scattering of infrared photons can occur in the Thomson limit with its significantly
larger cross-section and should produce GeV energy gamma-rays in the energy range observed by the
Fermi telescope. A curve of growth method is presented to model the infrared free-free and free-bound
emission from the circumstellar disc, taking into account the changing viewing angle as observed from the
pulsar. The curve of growth model is fitted to archive near-infrared and optical data and mid-infrared
data obtained with the Very Large Telescope during January 2011. The effect of this infrared excess on
the inverse Compton scattering rate is considered for an isotropic and anisotropic photon distribution,
considering preâ and postâshock electron distributions. The anisotropic modelling considers the effects
of the changing size and orientation of the circumstellar disc relative to the pulsar, as well as the change
in the inverse Compton scattering angle during the orbit. The inverse Compton scattering rate for three
disc orientations is modelled over a period of approximately 160 days around periastron, including the
disc crossing epochs before and after periastron. The maximum disc contribution is found to occur close
to periastron and not near the discâcrossing where the low infrared flux from the disc, at a radius of
! 45 stellar radii, has a less significant effect. It is found that the inclusion of the infrared flux from
the circumstellar disc can increase the GeV flux from the system by a factor ! 2 near periastron, for
favourable disc orientations. The predicted increase is, however, less than was detected with Fermi
during the 2011 periastron passage. The observations showed a flare which cannot be explained by this,
or any current model.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-05272013-112808 |
| Date | 27 May 2013 |
| Creators | van Soelen, Brian |
| Contributors | Prof PJ Meintjes |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-05272013-112808/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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