  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-metre Sky Survey, allowing for sub-arcsecond
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 spin-down 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/XRT 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 $N_{\rm H}=1.35\times10^{21}$ 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.
