Pulsar B1237+25 at 111~MHz: average profile, mode switching, nullings, microstructure

The observations of B1237+25 at a frequency of 111 MHz were analyzed. For the first time in the normal radiation mode a new component in the central region in the average profile was detected. This component is manifested in all modes of pulsar emission: quiet-normal (QN), flare-normal (FN) and in the abnormal mode (AB). The subpulse drift is observed in the QN mode only in the first and last components of the average profile. The normal mode is interrupted by nullings and transitions into the abnormal AB mode. In the AB mode, the structure at the edge of the outer cone is destroyed, the distance between the inner and outer cones is almost doubled, and the distance between the inner cone and the central region is reduced.Analysis of our data has shown that the components of the outer and inner cones of the average profile are formed by an ordinary mode of radio emission (O-mode) and form a single cone radiation of the pulsar. The central components of the average profile (wide and narrow) are formed by an extraordinary mode (X-mode). Estimates of the height of the radiation output from the central region (X-mode) and the cone radiation (O-mode) are obtained: 80~km and 370~km, respectively. A microstructure with a time scale of $\tau_\mu\le0.5$~$\mu s$ has been detected. This time scale corresponds well to the time of the development of a spark discharge in the polar cap. For this value $\tau_\mu$, the height of the vacuum gap should be $h_p\le750$~cm. Based on the steepness of the individual pulse's trailing edge at the longitude of the first component, a limit was obtained on the value of the $\gamma$ factor of the relativistic secondary plasma: $\gamma\ge$260. The dependence of the distance between the components of the outer and inner cone of radiation on the frequency is the same and corresponds to a power law with an exponent of -0.16.