Strong Lens
Time Delay Challenge

Testing accuracy on thousands of simulated lenses - blind.

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Welcome to the Strong Lens Time Delay Challenge!

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 time delay 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 have been trying to infer the time delays that went into them. TDC0 is the first ladder to climb: it contains enough simulated lightcurves to check that your code runs on the challenge data, and to let you know if your algorithms provide enough accuracy to make it worth your while attempting TDC1. So far, in TDC0 we have had 46 entries from 16 independent analysis teams, 7 of whom have graduated to TDC1.

On July 1, 2014, the TDC1 challenge came to a close. We are now analysing the TDC1 submissions, with the data unblinded to the Good Teams, and all together are writing 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.

However, the TDC0 data is still available, and the TDC1 data will be made public in late 2014. So, if you're still interested in the time delay measurement problem, you can send us your TDC0 time delay estimates (and 68% uncertainties) and we'll be happy to let you know how you did.

Downloads Read the Paper How to Submit