Presentation #102.18 in the session AGN Posters.
The cross correlation function (CCF) is an indispensable tool for measuring the time lag between different wavelengths in AGN light curves. The lag tells us the light-travel time, and hence gives us an estimate of the physical size of the broad line region (BLR) surrounding the supermassive black hole. Here we present a new functional form for measuring the lag that offers advantages over the venerable CCF. To be clear, this is not a new or improved way to compute the standard CCF, nor a way to handle gaps (interpolation) or handle noisy data to get uncertainties (Monte Carlo). This is a different function that does not use the CCF. Rather, it uses the concept of a physical resonance, and initial tests show that it offers higher resolution than the standard CCF and also provides uncertainties in the lag estimate. It is also very simple to compute. The method appears to work well for the reverberation mapping case, where the AGN light curves typically contain high levels of red-noise (i.e. random-walk type behavior). Several examples are shown comparing the standard CCF and this resonance-based method. This new version of the correlation function shows considerable promise not just for reverberation mapping, but for many cases in astrophysics where a lag or shift needs to be determined (e.g. gravitational-lensing multiple-image lags, radial velocity Doppler shifts, image registration, etc.).