A Detailed Study of Mode Changing and Modulation of PSR B1237+25 with FAST

PSR B1237+25, whose mean pulse profile has five components, is a well-known star to study pulsar emission geometries. We conducted mode changing and modulation analysis on this pulsar using FAST data at 1.25 GHz with a bandwidth of 400 MHz. We observed and identified three emission modes of this pulsar: a quiet normal mode that has little or no core activity with distinctive 2.8-period subpulse modulation on its outer cone, a flare normal mode in which the core is highly active and an abnormal mode in which the core is active and the last component is weak. We found that the core activity cuts off the position angle traverse in flare normal mode and leads to a position angle jumping in abnormal mode. We also found that there exists a quasi-periodical modulation on the outer conal components. Such modulation shows an irregular wave-like pattern, and has a weak correlation with the core component. We discuss the likely origin of such a modulation, and argue that this modulation can be interpreted as precession of the emission cones around the magnetic axis.