Astron. Astrophys. 336, 829-839 (1998)

Microlensing on different timescales in the lightcurves of QSO 0957+561 A,B

J. Pelt ^{1, 5}, R. Schild ^{2, 5}, S. Refsdal ^{3, 5} and R. Stabell <sup>4, 5

1</sup> Tartu Observatory, EE-2444 Tõravere, Estonia
² Harvard-Smithsonian Center for Astrophysics, MS-19 60 Garden Street, Cambridge, MA 02138, USA
³ Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg-Bergedorf, Germany
⁴ Institute of Theoretical Astrophysics, P.O. Box 1029, Blindern, N-0315 Oslo, Norway
⁵ Centre for Advanced Study, Drammensveien 78, N-0271, Oslo, Norway

Received 20 February 1998 / Accepted 26 May 1998

Abstract

A statistical analysis of an extended data set on a time basis of 17 years of optical observations of the gravitationally lensed quasar 0957+561 is presented. We use all available photometry to estimate the time delay between A and B images as days. Using this refined value we investigate basic statistical properties of the difference curve between the A and the time shifted B image. We find a significant variability of 0.25 mag on a time scale of 5 years, which we interpret as microlensing by compact objects in the lensing galaxy.

We show that masses down to about may explain this microlensing event. Solar mass objects alone can more or less be excluded. However, a distribution of compact objects with a small fraction () in solar mass objects, seems to be possible. The best constrained parameter is the radius of the source which roughly should be (within a factor of 3).

With the present noise level an additional variability due to the microlensing on shorter time scales (days to months) is difficult to prove with a reasonable significance. All steps of the analysis are carried out by using simple statistical tests and with as few a priori assumptions as possible.

Key words: methods: statistical methods: data analysis quasars: individual: 0957+561A,B gravitational lensing

Send offprint requests to: R. Stabell⁴

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Contents

• 1. Introduction
  • 1.1. Notation
• 2. Data sets
  • 2.1. Stated and estimated precision
  • 2.2. Merging of data sets
• 3. Time delay
  • 3.1. Dispersion spectra
  • 3.2. Filling of the gaps can be misleading!
  • 3.3. Basic iteration
  • 3.4. ?
  • 3.5. Microlensing curve
  • 3.6. Analysis of residuals
• 4. Short time scale variability
  • 4.1. Cross dispersion between data sets
  • 4.2. Dispersion of
  • 4.3. Fast variability in SR data set
    • 4.3.1. Statistical characterization of the excess variability
• 5. Discussion
• 6. Conclusions
• Acknowledgements
• Appendix A: smoothing techniques
• Appendix B: cross-dispersion estimation
• Appendix C: significance estimation for the trends
• References

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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