New aperture photometry of QSO 0957+561; application to time delay and microlensing

Abstract

Abstract. We present a re-reduction of archival CCD frames of the doubly imaged quasar 0957+561 using a new photometry code. Aperture photometry with corrections for both cross contamination between the quasar images and galaxy contamination is performed on about 2650 R-band images from a five year period (1992)(1993)(1994)(1995)(1996)(1997). From the brightness data a time delay of 424.9 ± 1.2 days is derived using two different statistical techniques. The amount of gravitational microlensing in the quasar light curves is briefly investigated, and we find unambiguous evidence of both long term and short term microlensing. We also note the unusual circumstance regarding time delay estimates for this gravitational lens. Estimates by different observers from different data sets or even with the same data sets give lag estimates differing by typically 8 days, and error bars of only a day or two. This probably indicates several complexities where the result of each estimate depends upon the details of the calculation.

How accurate are the time delay estimates in gravitational lensing?

“…However, SOLA differs from our approach in several (Haarsma et al 1999), c) optical data at g-band (Kundic et al 1997), and d) optical data at r-band (Ovaldsen et al 2003). Kundic et al (1997) -Linear 417 ± 3 -Cross correlation -PRH -Dispersion Oscoz et al (1997) -Cross correlation 427 ± 3 -Dispersion Pijpers (1997) -SOLA 425 ± 17 Pelt et al (1998b) -Dispersion 416.3 ± 1.7 Haarsma et al (1999) -PRH 409 ± 30 -Dispersion Oscoz et al (2001) -Linear 422.6 ± 0.6 -Cross correlation -Dispersion Burud et al (2001) -χ 2 algorithm 423 ± 9 Colley et al (2003) -PRH 417.09 ± 0.07 Ovaldsen et al (2003) -Dispersion 424.9 ± 1.2 -χ 2 algorithm respects: (i) SOLA does a symmetric treatment of the two estimated fluxes (flux A is fixed and flux B is varied to match A and vice versa); (ii) the reported time delay is the mean of the estimated time delays in the two symmetric cases; (iii) a free parameter is used to adjust the relative weighting of the errors in the variance-covariance matrix. We also note that parameter estimation in SOLA is problematic (Larsen & Hansen 1997;Rabello-Soares et al 1999) and this method has been rarely used.…”

“…The χ 2 algorithm (Burud et al 2001;Ovaldsen et al 2003) is a χ 2 -based method similar in spirit to our model in that it also uses a notion of an underlying model curve when fitting the two observed fluxes. However, the underlying model is assumed to be regularly sampled.…”

Computational Intelligence in Astronomy – A Win-Win Situation

“…However, we speculate that the estimate of 417±3 days constitutes an underestimate. For the quasar Q0957+561, the latest reports also give estimates around 420 days by using other data sets [24]. The delay estimates should be robust across the wavelength range since the gravitational lensing theory predicts that the time delay must be the same regardless of the wavelength of observation [31,16,32].…”

“…Moreover, like all astronomical time series measurements, they are also typically irregularly sampled with possibly large observational gaps (missing data) [24,29,23,15]. This is due to practical limitations of observation such as equipment availability, weather conditions, the brightness of the moon, among many other factors [7].…”

“…-We simulated optical observations as in [24]. The data sets were irregularly sampled with three levels of noise and observational gaps of different size.…”