(Authors: Phil Marshall)
Strong lens time delays have been demonstrated to be a powerful tool for cosmology. At present, the power of this method of measuring distances in the Universe is limited by the small number of lenses with measured time delays. However, things are about to change: wide-field surveys this decade will increase the numbers of lensed quasars by an order of magnitude; LSST will find even more, and monitor them all. Each one of the thousands of LSST timedelay lenses will have a multi-filter lightcurve of up to a thousand photometric measurements, extracted from observations spaced on average by a few days or so, over seasons of several months in length. We will see the 2 or 4 images of the AGN or supernova source brightening and fading, with time delays between the images that we will need to infer to a mean accuracy of nearly one part in a thousand, if we are to contribute a useful measurement of the accelerating expansion of the Universe. However, we will also see the effects of microlensing and calibration errors - and the data is sparse. How well can we really measure the time delays?
We are trying to answer this question by setting a series of blind data analysis challenges: the LSST DESC SL ``Evil Team'' has produced several thousand realistic simulated lightcurves, and the righteous Good Teams of the astronomy community are in the process of trying to infer the time delaysthat went into them. TDC0 is the first ladder to climb: it contains enough simulated lightcurves to check that the Good code runs on the challenge data, and to let the Good teams know if their algorithms provide enough accuracy to make it worth their while attempting TDC1. So far, in TDC0 we have had 27 entries from 7 independent analysis teams, three of whom have graduated to TDC1. In summer 2014 we will collate the TDC1 submissions, unblind the data, and, together with the Good teams, write a paper on the community's results. As well as assessing time delay measurement methods, we will be able to provide feedback to the LSST project on how our accuracy depends on the survey observing strategy, which is still to be fully optimised.
You can read more about the Strong Lens Time Delay Challenge in the experimental design paper at http://arxiv.org/abs/1310.4830, and you can take part in the challenge at http://timedelaychallenge.org.