Time-of-arrival post-fit residuals for PSR J0248+6021, allowing for linear variations of dispersion measure over time, as in Fig. 9, but showing only data acquired with the BON backend. The 2.0 GHz data (blue squares) are clearly above the 1.4 GHz data (black dots) for the year 2006 and for 2009−2010, but below these data in 2008. We interpret the increasing, then decreasing DM value as evidence that the neutron star is moving through a dense environment.