Gamma-ray production in Be-XPBs Brian van Soelen University of the Free State supervisor P.J. Meintjes

SA SKA 2009 Postgraduate Bursary Conference 2 Project Outline Modelling inverse Compton gamma-ray emission from Be-XPBs Observations of Be stars and Be- XPBs Optical and Infrared Modelling Flux from the Be star Gamma-ray production via inverse Compton scattering, taking into account the Be star Variation through orbital period Aharonian et al., (2005) A&A, 442, 1

SA SKA 2009 Postgraduate Bursary Conference 3 Be X-ray Pulsar Binaries Multi-wavelength objects Radio Pulsar Synchrotron radiation Optical Be star Accretion disc X-ray Accretion on to the pulsar Gamma-ray Pulsar wind can produce gamma-rays through inverse-Compton scattering

SA SKA 2009 Postgraduate Bursary Conference 4 Be X-ray Pulsar Binaries The interaction between the pulsar and the Be star results in a bow shock Pressure balance between the pulsar wind and the Be star wind Synchrotron and IC scattering occurs here Seen in isolated pulsars PSR B (Stappers et al 2003) Taken from Gaensler & Slane (2006), ARA&A, 44, 17

SA SKA 2009 Postgraduate Bursary Conference 5 Clark et al. (2003) A&A, 403, 239 Porter & Rivinuis (2003) PASP, 115, 1153 Be stars Normal B type stars show absorption lines, but Be stars are characterised by emission lines The emission lines are explained by a circumstellar disc As the disc grows and shrinks we see variability in the emission lines H  observations of o Andromedae Bill Pounds

SA SKA 2009 Postgraduate Bursary Conference 6 Be stars The disc also creates an infrared excess E.g. Optical and infrared observations of X Persei Low state fitted with a Kurucz model, high state is fitted using a curve of growth method Previous models of Be-XPBs have ignored the infrared excess Telting et al. (1998) MNRAS, 296, 785

SA SKA 2009 Postgraduate Bursary Conference 7 Gamma-ray Binaries PSR B / SS 2883 Gamma-ray binary detected by HESS Be star & pulsar in a ~3.4 year orbit Eccentricity e = 0.87 Pulse period ~48 ms Non-pulsed radio to gamma-ray emission during periastron passage The non-pulsed radio emission is variable Connected to the variability in the disc Does the variability influence the gamma-rays? Johnston et al., (2005) MNRAS, 358, 1069 Aharonian et al., (2005) A&A, 442, 1

SA SKA 2009 Postgraduate Bursary Conference 8 Modelling: Be stars Model the flux from optical to infrared Separating the contribution from the star from the contribution from the disc Show the variability Since the disc is variable, we need to have simultaneous optical and infrared data. The deeper the infrared observations the better, to give you a better fit Waters (1986) A&A, 162, 121

SA SKA 2009 Postgraduate Bursary Conference 9 Modelling: Be stars Lamers & Waters (1984) Curve of Growth Method Waters (1986) A&A, 162, 121 Density of the disc Ratio of the excess is given by Optical Depth through the disc

SA SKA 2009 Postgraduate Bursary Conference 10 Modelling: Be stars Fit for SS 2883 Data UBV (Westerlund & Garnier,1989) JHK (2MASS) 8.28 &  m (Midcourse Space Experiment Point Source Catalog) Geometry of system implies disc > 24 R star Star Star temperature: 25000K log g: 3.5 Disc n: log X*: R disc : 50 R star (held) T disc : K (held) Theta: 5 ° (held)

SA SKA 2009 Postgraduate Bursary Conference 11 Modelling: Be stars From the flux we can calculate the photon density which is used for the IC scattering calculation This method might not completely separate the disc component from the stellar component The disc might contribute to the optical region Whether or not the disc profile has been cleanly removed is of secondary importance The disc parameters might be wrong As long as the fit accurately predicts the energy spectrum the model still works if the observations are simultaneous

SA SKA 2009 Postgraduate Bursary Conference 12 Modelling: Inverse Compton Scattering As the disc growths and shrinks there is a change in the infrared flux. The change in the number of target photons changes the gamma-ray emission Assume isotropic scattering and ignore geometric effects. Johnston et. al., (1999) MNRAS, 302, 277

SA SKA 2009 Postgraduate Bursary Conference 13 Modelling: Inverse Compton Scattering The number of scatterings are (Blumenthal & Gould, 1970): The flux is dependent on the photon number density, which we model using the curve of growth method where && Klein-Nishina cross-section and

SA SKA 2009 Postgraduate Bursary Conference 14 Modelling: Inverse Compton Scattering Fit for PSR B /SS 2883 Star Star temperature: 25000K log g: 3.5 log m: 0 Disc n: log X*: R disc : 50 R star (held) T disc : K (held) Theta: 5 ° (held) Electron energy γ = α= 2.2 There is a greater change at lower energy gamma-rays

SA SKA 2009 Postgraduate Bursary Conference 15 Modelling: Inverse Compton Scattering PSR B /SS 2883 Star Star temperature: 25000K log g: 3.5 log m: 0 Disc n: log X*: R disc : R star T disc : K Theta: 5 ° Electron energy γ = α= 2.2

SA SKA 2009 Postgraduate Bursary Conference 16 Modelling: Inverse Compton Scattering The density profile has a large influence on the disc: Show the photon energy density for n = 1.5, 2 and 2.5 R disc = 5 – 20 R star Other parameters the same as the previous fit. Thicker discs show more variability Higher peak at nearer infrared Photon number density

SA SKA 2009 Postgraduate Bursary Conference 17 Modelling: Inverse Compton Scattering Extreme example: n = log X * = Theta = ° R disc = 0 – 12 R star Could have large variations because of the disc.

SA SKA 2009 Postgraduate Bursary Conference 18 Modelling: Inverse Compton Scattering Change in IC flux due to orbital motion for PSR B /SS2883 Integrated flux between 20 MeV and 300 GeV for 60 days before and after periastron (LAT on Fermi) Considers change in photon distribution, currently ignores the geometry of the disc and the eclipse which occurs ~ day around periastron

SA SKA 2009 Postgraduate Bursary Conference 19 Future Work Improving the model to take into account: Geometric considerations Change in scattering/observation angle Distance between Be star & pulsar Eclipse Distance to source Emission Volume Density of the Pulsar Wind Fitting to the curves to data Code is still under development Memory and CPU issues means we might need to run it on a cluster Speeding it up do least-squares fitting Aharonian et al., (2005) A&A, 442, 1

SA SKA 2009 Postgraduate Bursary Conference 20 Future Work Be discs models need to be applied to observed sources Observation time at Sutherland 30 Dec 2009 – 5 Jan 2010 Mid-IF Account for angle of the disc Sample of different stars will give us an idea of the allowed states By having simultaneous observations in radio, optical, infrared and Gamma-ray we can better model the system. Waters (1986) A&A, 162, 121

SA SKA 2009 Postgraduate Bursary Conference 21 Conclusion Previous models have ignored the infrared excess when calculating the inverse Compton scattering. We have shown that just changing just the infrared flux, creates variability in the gamma-ray emission This model is applicable to all Be-XPB systems. By having simultaneous observations in radio, optical, infrared and Gamma-ray we can better model the system. Aharonian et al., (2005) A&A, 442, 1

Thank you References Aharonian et al., (2005) A&A, 442, 1 Blumenthal & Gould (1970) Rev. of Modern Physics, 42, 237 Charles & Coe, In: Compact stellar X-ray sources Clark et al. (2003) A&A, 403, 239 Gaensler & Slane (2006), ARA&A, 44, 17 Howells (2002) PhD Thesis Johnston et. al., (1999) MNRAS, 302, 277 Johnston et al., (2005) MNRAS, 358, 1069 Martayan, Baade & Fabregat (2009) IAUS, 256, 349 Porter & Rivinuis (2003) PASP, 115, 1153 Stappers et al., (2003), Science, 299, 1372 Telting et al., (1998) MNRAS, 296, 785 Waters (1986) A&A, 162, 121