Accretion stream mapping with 'genetically modified fireflies'

Abstract

We apply an eclipse mapping technique using 'genetically modified fireflies' to the eclipse light curves of HU Aqr and EP Dra. The technique makes as few assumptions as possible about the location of accretion stream material, allowing the emission to be located anywhere within the Roche lobe of the white dwarf. We model two consecutive eclipses in the UBVRc band for HU Aqr, and four consecutive 'white'-light eclipses for EP Dra, to determine the changing brightness distribution of stream material. We find firefly distributions which are consistent with accretion through a curtain of material in both HU Aqr and EP Dra, and show that the previously assumed two-part ballistic and magnetic trajectory is a good approximation for polars. Model fits to the colour-band data of HU Aqr indicate that the material confined to the magnetic field lines is brightest, and most of the emission originates from close to the white dwarf. There is evidence for emission from close to a calculated ballistic stream in both HU Aqr and EP Dra. We propose that a change in the stream density causes a change in the location of the bright material in the accretion stream in EP Dra.