A New Measure for Weak Lensing Flexion

We study a possibility to use the octopole moment of gravitationally lensed images as a direct measure of the third-order weak gravitational lensing effect, or the gravitational flexion. It turns out that there is a natural relation between flexion and certain combinations of octopole/higher-multipole moments which we call the Higher Order Lensing Image’s Characteristics (HOLICs). This will allow one to measure directly flexion from observable octopole and higher-multipole moments of background images. We show based on simulated observations how the use of HOLICs can improve the accuracy and resolution of a reconstructed mass map, in which we assume Gaussian uncertainties in the shape measurements estimated using deep i-band data of blank fields observed with Suprime-Cam on the Subaru telescope. Subject headings: cosmology: theory — dark matter — galaxies: clusters: general — gravitational lensing Astronomical Institute, Tohoku University, Sendai 980-8578, Japan Institute of Astronomy and Astrophysics, Academia Sinica, P. O. Box 23-141, Taipei 106, Taiwan, Republic of China