LOFAR Discovery of the Fastest-spinning Millisecond Pulsar in the Galactic Field
Burnett, T. H.; Camilo, F.; Michelson, P. F.; Ransom, S. M.; Ray, P. S.; Ferrara, E. C.; Clark, C. J.; Nieder, L.; Breton, R. P.; Wood, K.; Bassa, C. G.; Hessels, J. W. T.; Kondratiev, V. I.; Sanidas, S.; Li, T.; Gusinskaia, N. V.; Pleunis, Z.; van Amesfoort, A. S.
Netherlands, Canada, United States, United Kingdom, Russia, Germany, China, Spain, South Africa
Abstract
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}}≳ 0.02 {M}⊙ ), 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 0.3-10 {keV} X-ray luminosity of {L}X< 1.1× {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 α ∼ -3 (where S\propto {ν }α ). 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.