Gravitationally lensed LOFAR sources

UoM administered thesis: Master of Science by Research

  • Authors:
  • Deepika Venkattu


Gravitational lensing has been an interesting phenomenon for a long time now. Since sources at high redshifts are lensed by an intervening galaxy, lensing is a good probe for both extragalactic sources and the cosmology of the Universe through which the lensed radiation propagates. LOFAR is an interferometric array operating in an unexplored low radio frequency regime since 2012. While it has already produced a lot of new science, its further potential lies in its extended baselines in the form of international stations spread across Europe. Data from these stations are available; however calibration has posed a huge problem due to ionospheric variations over these baseline lengths and the lack of suitable calibrators for these baselines. Handling large volumes of data was also a concern. In the past few years, LOFAR and its Long-Baseline (LB) working group have worked on this to produce a set of pipelines to process LB data. This thesis deals with LOFAR data of two gravitationally-lensed sources. We process the first source, 0957+561 using the LB pipeline that was available in February 2019 and partially with AIPS to obtain a radio map centered at 138 MHz and obtain the flux-density ratio between the two lensed images of the quasar, which is a typical observable in quasar lensing. We also discuss the effect of any potential propagation effects (scattering and free-free absorption) that can affect the radio waves as they traverse the Interstellar Medium. This gives a useful limit on these effects (a lower limit on scattering is obtained), especially because lensed sources are known to be slow-varying at low frequencies. We also process the lensing system MG 0751+2716 through the LB pipelines. As the pipelines were ready for testing around this time, the source has been used as part of a larger effort to test how the pipelines process different fields and to identify and solve errors before they can be released for use by the wider scientific community.


Original languageEnglish
Awarding Institution
Award date1 Aug 2020