Estimation of time delays from light curves of gravitationally lensed quasars
Date
2011-07-19
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Abstract
Kauge kvasari ja maise vaatleja vahelise vaatekiire lähedale jääv galaktika toimib gravitatsiooniläätsena ja võib tekitada kvasarist mitmikkujutisi. Valgus (või muu elektromagnetkiirgus) saab seega tulla vaatlejani eri teid pidi. Eri teid mööda kulgemine (aga ka vastavalt erinev viivitus Shapiro efekti tõttu läätse gravitatsiooniväljas) tekitab Maalt vaadeldud kvasari mitmikkujutiste heleduskõverate vahele ajanihke.
Eri kujutiste vaheliste ajanihete abil saab määrata Hubble konstanti H0, aga selle usaldusväärseks hindamiseks on tarvis kasutada paljude läätsesüsteemide ajanihkeid. Teades H0 väärtust, saab omakorda määrata läätsgalaktikate masse ja galaktikate masside jaotust. Ka see eeldab suure hulga ajanihete mõõtmist. Me peame hindama suurt arvu ajanihkeid ka siis, kui tahame nende abil avastada uusi gravitatsiooniläätsi massilise fotomeetria andmetest. Ajas nihutatud heleduskõveraid saab mõõdetud ajanihke abil kokku nihutada. Sedasi saame allikast parema vaatlusrea kui vaid üksikuid kujutisi uurides.
Töö eesmärk oli koostada ajanihete mõõtmise meetod, mis sobiks suure arvu ajanihete automaatseks mõõtmiseks, s.t. ei nõuaks kasutaja poolset sekkumist ja subjektiivsete parameetrite sisestamist. Kuna läätse tekitatud mitmikkujutised võivad vaatlustehnika piiratuse tõttu jääda lahutamata, seati teiseks eesmärgiks koostada algoritmid ka kokku sulanud kujutiste tarbeks. Töö käigus selgitati välja tuntud lähenemiste puudused ja püüti neid vältida.
Vanade tuntud ideede heade külgede ja uute täienduste kombineerimise tulemusena valmis automaatne ajanihete mõõtmise meetod lahutatud kujutiste jaoks, mis testiti reaalsete vaatlusandmetega. Leitud ajanihked on kooskõlas varasemate vähem automaatsete mõõtmistulemustega. Koostati ka kaks uut algoritmi juhtumiteks, kus üks või mõlemad heleduskõverad koosnevad kahe lahutamata jäänud kujutise summast. Viimased kaks algoritmi testiti simuleeritud andmetega, sest piisavalt häid vaatlusridu lahutamata kujutistega polnud veel saada.
If an intervening galaxy is located close to the line of sight between a distant quasar and an observer, multiple images of the quasar may be formed because of the gravitational lensing effect. As the path lengths (and also the lags due the Shapiro effect) are different for separated images, the time delays between light curves arise. The time delays between the gravitationally lensed images can be used to fix the Hubble parameter H0, but to suppress various types of errors, we need to use a large number of gravitational lenses with measured time delays. Knowing the H0, we can use time delay gravitational lenses for measuring masses of galaxies and estimating the distribution of galaxy masses in the Universe. Moreover we can estimate the time delays between (somehow selected) nearby images in massive photometry programs and detect new lens systems this way. We can also shift the light curves of a given source by the estimated time delay to get a combined time series with a better sampling, which in turn, can be used for studying the source itself. We had the following goals for the study. To measure a large number of time delays, there is need for an automatic time delay estimation method that does not need any user set parameters. As longer time series can be often obtained with smaller telescopes in not very good seeing conditions, there is also need for time delay estimation methods from blended light curves. As the result of the study, an automatic time delay estimation method was developed and evaluated with different sets of observational data. The new approach connects the good ideas from previous well known methods with some new improvements avoiding the previous weaknesses. The estimated time delays are consistent with earlier results obtained with not so automatic methods. Two special algorithms for blended images were built and tested with artificial data. Many problems were removed from the methodology that could lead to wrong time delay estimations.
If an intervening galaxy is located close to the line of sight between a distant quasar and an observer, multiple images of the quasar may be formed because of the gravitational lensing effect. As the path lengths (and also the lags due the Shapiro effect) are different for separated images, the time delays between light curves arise. The time delays between the gravitationally lensed images can be used to fix the Hubble parameter H0, but to suppress various types of errors, we need to use a large number of gravitational lenses with measured time delays. Knowing the H0, we can use time delay gravitational lenses for measuring masses of galaxies and estimating the distribution of galaxy masses in the Universe. Moreover we can estimate the time delays between (somehow selected) nearby images in massive photometry programs and detect new lens systems this way. We can also shift the light curves of a given source by the estimated time delay to get a combined time series with a better sampling, which in turn, can be used for studying the source itself. We had the following goals for the study. To measure a large number of time delays, there is need for an automatic time delay estimation method that does not need any user set parameters. As longer time series can be often obtained with smaller telescopes in not very good seeing conditions, there is also need for time delay estimation methods from blended light curves. As the result of the study, an automatic time delay estimation method was developed and evaluated with different sets of observational data. The new approach connects the good ideas from previous well known methods with some new improvements avoiding the previous weaknesses. The estimated time delays are consistent with earlier results obtained with not so automatic methods. Two special algorithms for blended images were built and tested with artificial data. Many problems were removed from the methodology that could lead to wrong time delay estimations.
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Keywords
galaktikad, astronoomilised vaatlused, gravitatsiooniläätsed, mõõtmismeetodid, galaxies, astronomical observations, gravitational lenses, measurement methods