1. Hard X-ray source sizes Eduard Kontar and Natasha Jeffrey Department of Physics and Astronomy University of Glasgow, UK Genoa RHESSI workshop, September, 2009

2. Primary and Compton scattered flux IdId IuIu Observer Flaring region Observed Flux = Direct + Scattered Downward emitted X- ray photons can be either a)Compton scattered X-rays into observer direction or a)Photoelectrically absorbed

3. Reflected flux Reflected flux can be ~40-45 % of the total flux even for an isotropic source! Reflected flux Observed flux Primary

4. Source sizes How does X-ray albedo affect the source sizes? h, source height X-ray source size a) Source (r) ~ exp(-r 2 /(2Size 2 )) Radial distance from the source centre b) Photon spectrum is power-law Primary source:

5. Isotropic source of photons Isotropic distribution Primary Gaussian source Reflected flux Total observed flux

6. Source sizes “Beamed” distribution “Pan-cake” distribution Note that total Flux downwards = total Flux upwards!

7. Source sizes Size =0.5h Size =2h Size =0.2h Size =5h

8. Source sizes: can we detect albedo ? Size =5h Size =0.2h

9. Source sizes and Compton back-scattering X-ray back-scattering need to be taken into account when X-ray source sizes are discussed X-ray back-scattering component is probably better seen for sources height > source size X-ray source sizes are sensitive to angular distribution of electrons (additional method to detect angular distribution of electrons? )