A Method for Extracting Light Echo Fluxes Using the NN2 Difference Imaging Technique

Light echoes are interesting because of the wealth of information they offer about their progenitors and the reflecting dust. Due to their faint surface brightnesses, difference imaging is necessary to separate most light echoes from the sky background. However, difference images reveal only the relative fluxes between two epochs. Obtaining absolute fluxes for individual epochs has traditionally relied on a single template image that is free of light echoes. Since such an observation is normally unavailable, a light echo-free template must be constructed by a complicated and usually subjective process. Here we present an application of the NN2 method of Barris et al. to extract the relative fluxes of light echoes across a range of epochs directly from a series of difference images. This method requires no privileged image and makes maximal use of the observational data. Statistical methods to estimate the zero-flux level and thus the absolute flux are also presented. The efficacy of the technique is demonstrated by an application to the light echoes around SN 1987A. The resulting images reveal new detail and faint light echo structures. This method can be adapted and applied to other extended variable light sources, such as stellar outflows and supernova remnants.