The recent discovery of a radio-emitting neutron star with an ultralong spin period of 76 s, PSR J0901-4046, raises a fundamental question on how such a slowly rotating magnetized object can be active in the radio band. A canonical magnetic field of $1.3\times {10}^{14}\mathrm{G}$ estimated from the pulsar period and its time derivative is wholly insufficient for PSR J0901-4046 to operate. Consideration of a magnetic inclination angle of 10° estimated from the pulse width gives a higher magnetic field of $1.5\times {10}^{15}\mathrm{G}$, which is still an order of magnitude lower than the necessary minimum of $2.5\times {10}^{16}\mathrm{G}$ following from the death line for radio pulsars with magnetic fields exceeding the critical value $4.4\times {10}^{13}\mathrm{G}$. We show that if the observed microstructure of single pulses reflects relativistic beaming, the inferred surface magnetic field appears to be $3.2\times {10}^{16}\mathrm{G}$, and without this assumption it is no less than $2.7\times {10}^{16}\mathrm{G}$, which explains the existence of radio emission from PSR J0901-4046. This estimation makes PSR J0901-4046 a radio pulsar with the strongest magnetic field known and is a sign that PSR J0901-4046 slows down not by magnetic-dipole radiation, but rather by an electric current of 56 MA, when rotational energy is expended in accelerating charged particles over the polar cap.

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• Radio, microwave, & sub-mm astronomy
• Neutron stars & pulsars
• Astrophysical electromagnetic fields