LOFAR Discovery of a 23.5 s Radio Pulsar

We present the discovery of PSR J0250+5854, a radio pulsar with a spin period of 23.5 s. This is the slowest-spinning radio pulsar known. PSR J0250+5854 was discovered by the LOFAR Tied-Array All-Sky Survey (LOTAAS), an all-northern-sky survey for pulsars and fast transients at a central observing frequency of 135 MHz. We subsequently detected pulsations from the pulsar in the interferometric images of the LOFAR Two-meter Sky Survey, allowing for subarcsecond localization. This, along with a pre-discovery detection 2 years prior, allowed us to measure the spin-period derivative to be $\dot{P}=2.7\times {10}^{-14}$ s s−1. The observed spin period derivative of PSR J0250+5854 indicates a surface magnetic field strength, characteristic age, and spindown luminosity of $2.6\times {10}^{13}$ G, 13.7 Myr, and $8.2\times {10}^{28}$ erg s−1, respectively, for a dipolar magnetic field configuration. This also places the pulsar beyond the conventional pulsar death line, where radio emission is expected to cease. The spin period of PSR J0250+5854 is similar to those of the high-energy-emitting magnetars and X-ray dim isolated neutron stars (XDINSs). However, the pulsar was not detected by the Swift/X-Ray Telescope in the energy band of 0.3–10 keV, placing a bolometric luminosity limit of $1.5\times {10}^{32}$ erg s−1 for an assumed ${N}_{{\rm{H}}}=1.35\times {10}^{21}$ cm−2 and a temperature of 85 eV (typical of XDINSs). We discuss the implications of the discovery for models of the pulsar death line as well as the prospect of finding more similarly long-period pulsars, including the advantages provided by LOTAAS for this.