Distance of PSR B0458+46 indicated by FAST H i absorption observations

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-hundr ed-meter Apertur e Spherical radio Telescope ( FAST ), and detect two absorption lines at radial velocities of VLSR = -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 (+ 1 . 1)( -0 . 7) kpc, based on a comparison of the velocity with the HI 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 HI cloud in the direction of this pulsar.