COSMOGRAIL: The COSmological MOnitoring of GRAvItational Lenses

Abstract

Abstract. We use numerical simulations to test a broad range of plausible observational strategies designed to measure the time delay between the images of gravitationally lensed quasars. Artificial quasar light curves are created along with Monte-Carlo simulations in order to determine the best temporal sampling to adopt when monitoring the photometric variations of systems with time delays between 5 and 120 days, i.e., always shorter than the visibility window across the year. Few and realistic assumptions are necessary on the quasar photometric variations (peak-to-peak amplitude and time-scale of the variations) and on the accuracy of the individual photometric points. The output of the simulations is the (statistical) relative error made on the time delay measurement, as a function of 1-the object visibility over the year; 2-the temporal sampling of the light curves; and 3-the time delay. Also investigated is the effect of long term microlensing variations which must be below the 5% level (either intrinsically or by subtraction) if the goal is to measure time delays with an accuracy of 1-2%. However, while microlensing increases the random error on the time delay, it does not significantly increase the systematic error, which is always a factor 5 to 10 smaller than the random error. Finally, it is shown that, when the time delay is comparable to the visibility window of the object, a logarithmic sampling can significantly improve the time delay determination. All results are presented in the form of compact plots to be used to optimize the observational strategy of future monitoring programs.Key words. gravitational lensing -cosmological parameters -cosmology: observations Measuring time delaysMeasuring time delays in gravitationally lensed quasars is difficult, but not as difficult as it first appeared in the late eighties when the first monitoring programs were started. Obtaining regular observing time on telescopes in good sites was (and is still) not easy and the small angular separations between the quasar images require to perform accurate photometry of blended objects, sometimes with several quasar images plus the lensing galaxy within the seeing disk. COSMOGRAILThe COSMOGRAIL project (COSmological MOnitoring of GRAvItational Lenses), started in April 2004, addresses both issues of carrying out photometry of faint blended sources and of obtaining well sampled light curves of lensed quasars.The project involves 5 telescopes: (1) the Swiss 1.2 m Euler telescope located at La Silla, Chile; (2) the Swiss-Belgian 1.2 m Mercator telescope, located in the Canaria islands (La Palma, Spain); (3) the 2 m robotic telescope of the Liverpool University (UK), also located at La Palma; (4) the 1.5 m telescope of Maidanak observatory in Uzbekistan; and (5) the 2 m Himalayan Chandra Telescope (HCT).All 5 telescopes, and others that will join the collaboration, are used in order to follow the photometric variations of most known gravitationally lensed quasars that are suitable for a determination of H 0 . The sample of...

Sn Refsdal: Photometry and Time Delay Measurements of the First Einstein Cross Supernova

“…We used tools from the PyCS (Tewes et al 2013a) 27 and SNPy software packages (Burns et al 2011(Burns et al , 2015. 28 The PyCS program was orginally developed by the Cosmological Monitoring of Gravitational Lenses (COSMOGRAIL) collaboration (Eigenbrod et al 2005) 29 for the measurement of gravitational lensing time delays from single-filter quasar light curve sets. To collapse the multi-band SN Refsdal light curves into a form suitable for use with PyCS, we used a combination of the F125W and F160W observations, which were collected concurrently in almost every epoch, and together have the most complete and well-sampled coverage of the Refsdal light curve.…”

Imprints of the quasar structure in time-delay light curves: Microlensing-aided reverberation mapping

“…To test for this effect, we use PyCS to measure time delays of a set of mock light curves. The measurement of the time delay is a complex problem by itself (see, e.g., Eigenbrod et al 2005;Tewes et al 2013a), which depends on the length of the light curve, on the shape and amplitude of the variability signal, on the time sampling, and presence of observing gaps. Therefore, we will not quantify biases possibly taking place depending of these properties but limit ourselves to simple and well controlled situations.…”

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

“…The COSMOGRAIL project, i.e. the COSmological MOnitoring of GRAvItational Lenses, aims to provide the community with A&A 538, A99 (2012) exquisite lightcurves and accurate time-delay measurements for a sample of more than 20 lensed quasars (Eigenbrod et al 2005;Vuissoz et al 2007Vuissoz et al , 2008Courbin et al 2011). To understand degeneracies affecting lens models, we started to model all systems monitored by COSMOGRAIL as well as those monitored by earlier campaigns in a uniform way.…”