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Polarization leakage in epoch of reionization windows - I. Low Frequency Array observations of the 3C196 field

Asad, K.M.B.; Koopmans, L.V.E.; Jelić, Vibor; Pandey, V.N.; Ghosh, A.; Abdalla, F.B.; Bernardi, G.; Brentjens, M.A.; de Bruyn, A.G.; Bus, S.; Ciardi, B.; Chapman, E.; Daiboo, S.; Fernandez, E.R.; Harker, G.; Iliev, I.T.; Jensen, H.; Martinez- Rubi, O.; Mellema, G.; Mevius, M.; Offringa, A.R.; Patil, A.H.; Schaye, J.; Thomas, R.M.; van der Tol, S.; Vedantham, H.K.; Yatawatta, S.; Zaroubi, S. (2015) Polarization leakage in epoch of reionization windows - I. Low Frequency Array observations of the 3C196 field. Monthly Notices of the Royal Astronomical Society, 451 (4). pp. 3709-3727. ISSN 0035-8711

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Abstract

Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of Low Frequency Array (LOFAR) and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134-166 MHz, within the central 4° of the field the (Q, U) → I leakage power is lower than the EoR signal at k < 0.3 Mpc-1. The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2 and 0.3 per cent, both of which could be utilized in the removal of leakage. Moreover, we could define an `EoR window' in terms of the polarization leakage in the cylindrical power spectrum above the point spread function (PSF)- induced wedge and below k∥ ˜ 0.5 Mpc-1, and the window extended up to k∥ ˜ 1 Mpc-1 at all k⊥ when 70 per cent of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of ≲ 4° in the future arrays like Square Kilometre Array will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02-1 Mpc-1.

Item Type: Article
Additional Information: This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record K. M. B Asad et.al., 2015. Polarization leakage in epoch of reionization windows - I. Low Frequency Array observations of the 3C196 field. Monthly Notices of the Royal Astronomical Society, 451 (4): 3709-3727 is available online at: http://mnras.oxfordjournals.org/content/451/4/3709.abstract
Uncontrolled Keywords: polarization; instrumentation: polarimeters; intergalactic medium; cosmo-logy: observations; dark ages; reionization; first stars
Subjects: NATURAL SCIENCES > Physics
NATURAL SCIENCES > Physics > Astronomy and Astrophysics
Divisions: Division of Experimental Physics
Depositing User: Vibor Jelić
Date Deposited: 02 Jul 2015 13:03
Last Modified: 02 Jul 2015 13:03
URI: http://fulir.irb.hr/id/eprint/2011
DOI: 10.1093/mnras/stv1107

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