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

Light echoes are interesting because of the wealth of information they offerabout their progenitors and the reflecting dust. Due to their faint surfacebrightnesses, difference imaging is necessary to separate most light echoesfrom the sky background. However, difference images reveal only the relativefluxes between two epochs. Obtaining absolute fluxes for individual epochs hastraditionally relied on a single template image that is free of light echoes.Since such an observation is normally unavailable, a light echo-free templatemust be constructed by a complicated and usually subjective process. Here wepresent an application of the NN2 method of Barris et al. to extract therelative fluxes of light echoes across a range of epochs directly from a seriesof difference images. This method requires no privileged image and makesmaximal use of the observational data. Statistical methods to estimate thezero-flux level and thus the absolute flux are also presented. The efficacy ofthe technique is demonstrated by an application to the light echoes around SN1987A. The resulting images reveal new detail and faint light echo structures.This method can be adapted and applied to other extended variable lightsources, such as stellar outflows and supernova remnants.Comment: To be published in PASP 118 (Oct. 2006