I, under the supervision of prof. dr. Leon Koopmans, and with dr. Vibor Jelic and dr. Vishhambhar Pandey, work on quantifying polarisation leakage in LOFAR observations caused by various systematic errors, to facilitate the detection of the EoR signal, the cosmological 21-cm signal coming from the Epoch of Reionization.
The EoR signal will provide an excellent probe of structure formation and physical cosmology. The difficulty in detecting it stems from the fact that, it is heavily contaminated by foregrounds and instrumental systematics, e.g. Galactic foreground (diffuse synchrotron and free-free emission) and extragalactic foreground (radio galaxies and galaxy clusters), Earth's ionosphere, Radio Frequency Interference (RFI) and the errors related to the telescope itself. We can hope to detect the EoR signal only after removing all the foregrounds and correcting all the errors, which is done step by step. First, the extragalactic foregrounds, mostly point sources, are removed. Then, we are left with the Galactic diffuse foreground which is subtracted by availing ourself of the (assumed) dichotomy that, the diffuse foreground does not fluctuate as a function of frequency, while the EoR signal does. However, the polarized foreground emission is not a smooth function of frequency. If some polarised foreground emission is leaked into the total intensity, the foreground might also fluctuate along frequency direction, mimicing the EoR signal, and giving rise to the possibility of removing some parts of the expected signal during the foregound subtraction process. So quantifying the amount of foreground polarization leakage is a crucial step in the process of detecting the EoR signal.
We aim to quantify the amount of leakage caused by different types of systematic errors in the LOFAR observations of the EoR signal. Antenna gain errors, and instrumental cross-talk between the receivers of the observing stations are Direction Independent Errors (DIE) as they are same towards all directions of the sky. On the other hand, errors in determining the exact response of a station, i.e. its primary beam, and the cross-polarization between the two perpendicular components of a beam give rise to Direction Dependent Errors (DDE). The DDEs can cause significant polarisation leakage, and the DIEs, although negligible, cannot be ignored. By quantifying the individual leakages caused by these effects we are not only facilitating the EoR detection, but also understanding the Galactic diffuse emission at low frequencies that is still largely unexplored.
We want to accomplish this objective by simulating the LOFAR observations of the 3C196 field, one of the target fields of the LOFAR EoR key science project. First, we build a sky model of this 100 square degrees field, taking data from WENSS catalog and recent LOFAR observations of the 3C196 source itself. A mock observation of this sky is simulated using the standard LOFAR calibration and simulation tool, BBS. Note that in this simulation we do not have anything except the extragalactic point sources, and thermal noise, and also we assume the G-Jones and E-Jones matrices in the RIME (radio interferometry measurement equation) to be just identity matrices. Second, we introduce some errors to our instrument by manipulating the G-Jones and sky model (to give DIEs) and the E-Jones (DDEs). Third, we calibrate the ideal mock observation assuming the erroneous instrument and sky model. Fourth and finally, we compare the erroneously calibrated visibilities with the ideal visibilities in all four Stokes parameters and measure the amount of leakage among the Stokes visibilities.
Results Abstract of the master thesis
Some cool-core non-major merging galaxy clusters host diffuse amorphous radio sources in their central regions (r=100—300 kpc) named Radio Mini-halos (MH). MHs are characterized by steep synchrotron spectra. Their diffuse radio emission surrounds a bright radio source associated to the brightest cluster galaxy (BCG). The corresponding radio emitting particles cannot be connected to the central radio galaxy in terms of particle diffusion. It has been proposed that they could result from a relic population of relativistic electrons re-accelerated by MHD turbulence, necessary energetics being supplied by the cool-core region. Later the MHs in two clusters, namely MS1455.0+2232 and RX J1720.1+2638, were found to be confined within the region delimited by the cold fronts (CF), i.e. edges of dense and cold structures in the thermal intracluster gas observed in X-rays. These MHs have also been found to be spatially correlated with the X-ray spiral structure created by gas sloshing at the cluster centre. Gas sloshing is one of the possible mechanism behind the formation of the CFs. Being a turbulent mechanism, it has been suggested that gas sloshing should also be responsible for the re-acceleration of radio emitting electrons. To examine this possibility we analysed Chandra X-ray data of six clusters (including the aforementioned ones) that host MHs and correlated them with the corresponding radio data. We found that, beside R1720 and MS1455, the MHs and the CFs in R1504, R1347 and A1835 are also spatially correlated. This is consistent with the hypothesis that the electrons responsible for MH emission could be re- accelerated by sloshing induced MHD turbulence.
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Abstract of the bachelor thesis
We realized the importance of SKA (Square Kilometer Array) as a feasible tool for unveiling the mystery of the Universe. So we tried to calculate the precise error margins of the cosmological parameters that SKA will give us. As far as we know Fisher4Cast is an efficient tool to constrain the error margins in an astrophysical survey. But we did not get enough time to use this tool efficiently. So we studied a very important paper by Yi Mao et al. to understand the constraints. We learned that, for future experiments, marginalizing over nuisance parameters may provide almost as tight constraints on the cosmology as if 21 cm tomography measured the matter power spectrum directly. Before studying about the constraining process we studied the basic physics of Early Universe, Reionization era, Dark Ages and 21-cm signal. We have written a review on the physics and observational constraints promised by the future telescopes in this thesis report.