We report the discovery of PSR J0952 - 0607 , a 707-Hz binary millisecond pulsar which is now the fastest-spinning neutron star known in the Galactic field ( i.e. , outside of a globular cluster ) .
PSR J0952 - 0607 was found using LOFAR at a central observing frequency of 135 MHz , well below the 300 MHz to 3 GHz frequencies typically used in pulsar searches .
The discovery is part of an ongoing LOFAR survey targeting unassociated Fermi Large Area Telescope \gamma -ray sources .
PSR J0952 - 0607 is in a 6.42-hr orbit around a very low-mass companion ( M _ { \mathrm { c } } \gtrsim 0.02 M _ { \odot } ) and we identify a strongly variable optical source , modulated at the orbital period of the pulsar , as the binary companion .
The light curve of the companion varies by 1.6 mag from r ^ { \prime } = 22.2 at maximum to r ^ { \prime } > 23.8 , indicating that it is irradiated by the pulsar wind . Swift observations place a 3- \sigma upper limit on the 0.3 - 10 keV X-ray luminosity of L _ { X } < 1.1 \times 10 ^ { 31 } erg s ^ { -1 } ( using the 0.97 kpc distance inferred from the dispersion measure ) .
Though no eclipses of the radio pulsar are observed , the properties of the system classify it as a black widow binary .
The radio pulsed spectrum of PSR J0952 - 0607 , as determined through flux density measurements at 150 and 350 MHz , is extremely steep with \alpha \sim - 3 ( where S \propto \nu ^ { \alpha } ) .
We discuss the growing evidence that the fastest-spinning radio pulsars have exceptionally steep radio spectra , as well as the prospects for finding more sources like PSR J0952 - 0607 .