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

Gamma-ray production in Be-XPBs

Brian van SoelenUniversity of the Free State

supervisor

P.J. Meintjes

SA SKA 2009 Postgraduate Bursary Conference2

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


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


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


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


Gamma-ray Binaries

• PSR B1259-63 / 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


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


Modelling: Be stars

• Lamers & Waters (1984) Curve of Growth Method

disc *R /, ( )

,* ,* 1

( )[1 ]2 d

Rdisc qdisc

F B Te q q

F I

Waters (1986) A&A, 162, 121

*( ) ( , , )q f n q X X

/2 ( / )(1 )[ ( , ) ( , )]dischv kTdisc disc discX kT h e g T b T

35 2 3/ 2 2 2* 0 *4.923 10 ( / )discX z T R R

0*

( )n

rr

R

Density of the disc

Ratio of the excess is given by

Optical Depth through the disc


Modelling: Be stars

• Fit for SS 2883• Data

• UBV (Westerlund & Garnier,1989)

• JHK (2MASS)• 8.28 & 12.13 m (Midcourse

Space Experiment Point Source Catalog)

• Geometry of system implies disc > 24 Rstar

• Star• Star temperature: 25000K• log g: 3.5

• Disc• n: 2.2691• log X*: 7.7572• Rdisc: 50 Rstar (held)• Tdisc: 12500 K (held)• Theta: 5 ° (held)


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


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



• 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

2 202

1

2 ( )( ) 1( ) 2 ln (1 2 )(1 ) (1 ) d d

2 1TotaldN r c qn d

N q q q q qdtd q

2

4

mc

1

1(1 )

Eq

E

11 2E

mc

where & &

Klein-Nishina cross-section

min max

( ) 0 elsewhere

AN

and



• Fit for PSR B1259-63 /SS 2883• Star

• Star temperature: 25000K• log g: 3.5• log m: 0

• Disc• n: 2.2691• log X*: 7.7572• Rdisc: 50 Rstar (held)• Tdisc: 12500 K (held)• Theta: 5 ° (held)

• Electron energy• γ = 106 - 107

• α= 2.2• There is a greater change at

lower energy gamma-rays



• PSR B1259-63/SS 2883• Star

• Star temperature: 25000K• log g: 3.5• log m: 0

• Disc• n: 2.2691• log X*: 7.7572• Rdisc: 1 - 50 Rstar• Tdisc: 12500 K• Theta: 5 °

• Electron energy• γ = 106 - 107

• α= 2.2



• The density profile has a large influence on the disc:

• Show the photon energy density for n = 1.5, 2 and 2.5

• Rdisc = 5 – 20 Rstar

• Other parameters the same as the previous fit.

• Thicker discs show more variability • Higher peak at nearer infrared

0*

( )n

rr

R

Photon number density



• Extreme example:• n = 0.498• log X* = 6.259 • Theta = 22.478°• Rdisc = 0 – 12 Rstar

• Could have large variations because of the disc.



• Change in IC flux due to orbital motion for PSR B1259-63/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 ~ 10-20 day around periastron


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



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


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.


Thank you

References

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

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

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