The effect of HII regions on rotation measure of pulsars

We have obtained new rotation measures for 11 pulsars observed with the Effelsberg 100-m radio telescope, in the direction of the Perseus arm. Using a combination of 34 published and the 11 newly measured pulsar rotation measures we study the magnetic field structure towards the Perseus arm. We find that two pulsars towards l similar to 149degrees (Region 1) and four pulsars towards l similar to 113degrees (Region 2) lie behind HII regions which seriously affect the pulsar rotation measures. The rotation measure of PSR J2337+6151 seems to be affected by its passage through the supernova remnant G114.3+0.3. For Region 1, we are able to constrain the random component of the magnetic field to 5.7 muG. For the large-scale component of the Galactic magnetic field we determine a field strength of 1.7 +/- 1.0 muG. This average field is constant on Galactic scales lying within the Galactic longitude range of 85degrees < l < 240degrees and we find no evidence for large scale field reversal upto 5-6 kpc. We highlight the great importance to include the effects of foreground emission in any systematic study.