LOFAR Discovery of the Fastest-spinning Millisecond Pulsar in the Galactic Field

We report the discovery of PSR J0952−0607, a 707 Hz binary millisecond pulsar that 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 γ-ray sources. PSR J0952−0607 is in a 6.42 hr orbit around a very low-mass companion ($M_c\gtrsim 0.02$ $M_{\odot }$), 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 3σ upper limit on the $0.3-10keV$ 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.