Pulsar J1909+0749 and supernova remnant G42.1-0.2. A case of a pulsar that has exited the shell of its associated supernova remnant

The ATNF pulsar catalogue currently features over 4300 Milky Way pulsars, but only 63 of them are possibly associated with a supernova remnant (SNR) arising from the same progenitor supernova explosion. Even rarer is the scenario in which the pulsar has escaped the physical extent of its corresponding SNR. We aim to identify the serendipitously discovered shell-type object G42.1-0.2 as an SNR. The known pulsar (PSR) J1909+0749 lies well outside the shell of SNR G42.1-0.2, but it has a cometary tail pointing towards the centre of the SNR; because of this positional coincidence, we propose that the SNR and the pulsar originated from the same supernova explosion. By measuring the flux densities of the shell G42.1-0.2 from ∼200 MHz to ∼6 GHz, we established the non-thermal nature of its emission. With the characteristic age of the pulsar, its distance from the Sun estimated from the dispersion measure (DM), and the angular separation between the pulsar and the centre of the shell, we inferred the evolutionary state of the SNR and the transverse velocity of the pulsar. Using proper motion measurements, we compared the SNR--pulsar direction with the velocity of the pulsar. The radio spectral index of the shell is -0.36 ± 0.08, confirming that the emission is primarily non-thermal synchrotron. The proper-motion measurements are not well constrained, but the amplitude and the direction are consistent with the interpretation that the SNR and the pulsar are associated. If the characteristic age and the DM-based distance represent the true age and the true distance of the pulsar and the SNR, the standard evolution equations suggest that the SNR must still be in the Sedov--Taylor phase and evolving in a medium with a density of about 1.5 cm -3 $. In addition, as a result of the PSR--SNR association, the transverse velocity of the pulsar PSR,J1909+0749 is estimated to be $∼1460 ^ km,s -1 , ranking it among the highest transverse velocities observed for pulsars in the Milky Way.