Dispersion measure variations in a sample of 168 pulsars
ABSTRACT We analyse dispersion measure (DM) variations in six years of radio observations ofmore than 160 young pulsars, all gamma ray candidates for the Fermi gamma raytelescope mostly located close to the Galactic plane. DMs were t across 256 MHzof bandwidth for observations centred at 1.4 GHz and across three frequencies {0.7 GHz, 1.4 GHz, and 3.1 GHz { where multifrequency observations were available.Changes in dispersion measure, dDM/dt, were calculated using a weighted linear tacross all epochs of available DMs. DM variations were detected at a 3˙level in 11pulsars, four of which were above 5˙: PSRs J0835 4510, J0908 4913, J1824 1945,and J1833 0827. We nd that after 28 years of gradual decline, the DM of PSRJ0835 4510 is now increasing. The magnitude of variations in three of the four (PSRsJ0835 4510, J0908 4913, and J1833 0827) are above what models would predict foran ISM dominated by Kolmogorov turbulence. We attribute this excess as likely dueto the pulsar’s local environment - the supernova remnants near PSRs J0835 4510and J1833 0827 and the pulsar wind nebula around PSR J0908 4913. Upper limitswere determined for all pulsars without detectable values of dDM/dt, most limits werefound to lie above the levels of variations predicted by ISM theory. We nd our resultsto be consistent with scattering estimates from the NE2001 model along these lines ofsight.Key words: pulsars: general | pulsars: individual: PSR J0835 4510 | pulsars:individual: PSR J0908 4913 | pulsars: individual: PSR J1824 1945 | pulsars: in-dividual: PSR J1833 0827 | ISM: structure