2023MNRAS.523.4949J

The pulsar B0458+46 was previously believed to have a distance of about 1.3 kpc and to be associated with a nearby supernova remnant, SNR HB9 (G160.9+2.6). We observe the neutral hydrogen (H I) absorption spectrum of PSR B0458+46 by using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), and detect two absorption lines at radial velocities of V_LSR = -7.7 and -28.1 km s^–1. Based on the Galactic rotation curve with a modification factor correcting for the systematic stream in the Galactic anticentre region, we derive the kinematic distance of the farther absorption cloud, which is found to be located $2.7^{+0.9}_{-0.8}$ kpc away, just beyond the Perseus Arm. We also obtain a direct distance estimation of the farther absorption cloud as being $2.3_{-0.7}^{+1.1}$ kpc, based on a comparison of the velocity with the H I emission in the Perseus and Outer Arms that was well-defined by recently measured parallax tracers. As a result, we conclude that PSR B0458+46 should be located beyond the Perseus Arm, with a lower limit for the distance at 2.7 kpc, and therefore not associated with SNR HB9. The doubled distance indicates a deficiency of thermal electrons in the immediate outer Galaxy, with a much less density than current models predict. Additionally, we detect a new high-velocity H I cloud in the direction of this pulsar.