Precision Orbital Dynamics from Interstellar Scintillation Arcs for PSR J0437–4715

Abstract

Intensity scintillations of radio pulsars are known to originate from interference between waves scattered by the electron density irregularities of interstellar plasma, often leading to parabolic arcs in the two-dimensional power spectrum of the recorded dynamic spectrum. The degree of arc curvature depends on the distance to the scattering plasma and its transverse velocity with respect to the line of sight. We report the observation of annual and orbital variations in the curvature of scintillation arcs over a period of 16 yr for the bright millisecond pulsar, PSR J0437−4715. These variations are the signature of the relative transverse motions of Earth, the pulsar, and the scattering medium, which we model to obtain precise measurements of parameters of the pulsar’s binary orbit and the scattering medium itself. We observe two clear scintillation arcs in most of our >5000 observations, and we show that they originate from scattering by thin screens located at distances D 1 = 89.8 ± 0.4 pc and D 2 = 124 ± 3 pc from Earth. The best-fit scattering model we derive for the brightest arc yields the pulsar’s orbital inclination angle, i = 137.°1 ± 0.°3, and longitude of ascending node, Ω = 206.°3 ± 0.°4. Using scintillation arcs for precise astrometry and orbital dynamics can be superior to modeling variations in the diffractive scintillation timescale, because the arc curvature is independent of variations in the level of turbulence of interstellar plasma. This technique can be used in combination with pulsar timing to determine the full three-dimensional orbital geometries of binary pulsars and provides parameters essential for testing theories of gravity and constraining neutron star masses.

Probing the local interstellar medium with scintillometry of the bright pulsar B1133+16

“…The high-quality secondary spectra from our observations allow for precise curvature measurements for the scintillation arcs. Our analysis of the secondary spectra arc curvatures made use of the scintools 4 Python module (described in Reardon 2018Reardon , 2020. This tool set has recently been used in measuring the orbital parameters of PSR J0437−4715 via the variations in arc curvatures modulated by the pulsar orbit and the Earth orbit (Reardon et al 2020), providing an independent analysis to those of pulsar timing measurements (Perera et al 2019).…”

“…Our analysis of the secondary spectra arc curvatures made use of the scintools 4 Python module (described in Reardon 2018Reardon , 2020. This tool set has recently been used in measuring the orbital parameters of PSR J0437−4715 via the variations in arc curvatures modulated by the pulsar orbit and the Earth orbit (Reardon et al 2020), providing an independent analysis to those of pulsar timing measurements (Perera et al 2019). It has recently been suggested that particular scattering screen morphologies will cause the apex of the parabolas in the secondary spectrum to be offset from the origin (Shi 2021).…”

“…Cordes et al 1985, Goldreich & Sridhar 2006, and radio astronomy of pulsars has allowed for the number density of the ionized plasma of the ISM to be directly probed and turbulence to be inferred along different lines of sight (e.g. Turner et al 2021, Alam et al 2021, Levin et al 2016, Armstrong et al 1981, Armstrong et al 1995, Reardon et al 2020.…”

“…As the Earth moves across this interference pattern, the observed flux of the pulsar is seen to vary as a function of time and observing frequency. Studies of the scintillation properties of pulsars have proved to be a very sensitive tool for probing the ISM, studying binary pulsars (Lyne 1984, and see recent work by Reardon et al 2020), and identifying structures within the ISM. Stinebring et al (2001) were the first to demonstrate that the twodimensional Fourier transform of a dynamic spectrum produces scintillation arcs in the 'secondary spectrum' which are interpreted as arising from discrete scattering screens along the line of sight to the pulsar.…”

“…The curvatures 1 of these arcs are highly dependent on the velocity as well as the geometry of scattering screens, and studying their variation with time has allowed very precise measurements of scattering regions (e.g. Simard 2019, Reardon et al 2020.…”