Einstein's theory of gravity-the general theory of relativity-is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field. In contrast to almost all alternative theories of gravity, the strong equivalence principle of general relativity requires universality of free fall to apply even to bodies with strong self-gravity. Direct tests of this principle using Solar System bodies are limited by the weak self-gravity of the bodies, and tests using pulsar-white-dwarf binaries have been limited by the weak gravitational pull of the Milky Way. PSR J0337+1715 is a hierarchical system of three stars (a stellar triple system) in which a binary consisting of a millisecond radio pulsar and a white dwarf in a 1.6-day orbit is itself in a 327-day orbit with another white dwarf. This system permits a test that compares how the gravitational pull of the outer white dwarf affects the pulsar, which has strong self-gravity, and the inner white dwarf. Here we report that the accelerations of the pulsar and its nearby white-dwarf companion differ fractionally by no more than 2.6 × 10. For a rough comparison, our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests and a factor of almost a thousand smaller than that obtained from other strong-field tests.

LOFAR Discovery of the Fastest-spinning Millisecond Pulsar in the Galactic FieldBassa, C.G.; Pleunis, Z.; Hessels, J.W.T.; Ferrara, E.C.; Breton, R.P.; Gusinskaia, N.; Kondratiev, V.I.; Sanidas, S.A.; Nieder, L.; Clark, C.J.; Li, T.; van Amesfoort, A.S.; Burnett, T.H.; Camilo, F.; Michelson, P.F.; Ransom, S.M.; Ray, P.S.; Wood, K. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. AbstractWe 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, 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 22.2 at maximum to ¢ > r 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 <Ĺ 1.1 10 X 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 a~-3 (where n µ a S ). 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.

We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9−342058, EXO 1745−248, and IGR J18245−2452) at a luminosity between ∼ 10 36−37 erg s −1. When fitting the Swift X-ray spectra (0.5 -10 keV) in those states with an absorbed power-law model, we found photon indices of Γ ∼ 1, significantly lower than the Γ = 1.5 -2.0 typically seen when such systems are in their so called hard state. For individual sources very hard spectra were already previously identified but here we show for the first time that likely our sources were in a distinct spectral state (i.e., different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e., up-scattering of soft photons due to hot electrons) then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained Γ as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. (2015). We confirm their general results in that also our sample of sources follow the same track as the other neutron star systems, although we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems.