Dipole Anisotropy in Integrated Linearly Polarized Flux Density in NVSS Data

Abstract

We study the dipole anisotropy in integrated linearly polarization flux density in NRAO VLA Sky Survey (NVSS). We extract the anisotropy parameters in the number counts, number counts weighted by polarization observables, i.e. degree of polarization (p) and polarization flux (P). We also determine the anisotropy in the degree of polarization per source and polarization flux per source. We consider data with several different cuts on the flux density, S>10,20,30,40,50 mJy. For studies with degree of polarization we impose the additional cut 0.01<p<1. Similarly for polarization flux we impose, 0.5<P<100 mJy. We find a very significant signal of dipole, both in number counts and P (or p) weighted number counts. For the case of P weighted number counts the significance ranges from 8 \sigma to 5 \sigma for S>20 mJy to S>50 mJy. The corresponding direction parameters are found to be stable with the cut on flux density. The significance is even higher for the case of p weighted number counts. The observed anisotropy is found to be much larger in comparison to the CMBR expectations. We find that polarization observables show a much higher level of anisotropy in comparison to pure number counts or flux weighted number counts.