First detection of frequency-dependent, time-variable dispersion measures

Donner J, Verbiest J, Tiburzi C, et al. First detection of frequency-dependent, time-variable dispersion measures. Astronomy & Astrophysics. 2019;624: A22.Context. High-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (DM), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. Correcting for DM variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. This may, however, give incorrect measurements if the probed electron content (and therefore the DM) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies. Aims. We study small-scale density variations in the ionised interstellar medium. These structures may lead to frequency-dependent DMs in pulsar signals. Such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data. Methods. We used high-cadence, low-frequency observations with three stations from the German LOng-Wavelength (GLOW) consortium, which are part of the LOw-Frequency ARray (LOFAR). Specifically, 3.5 yr of weekly observations of PSR J2219+4754 are presented. Results. We present the first detection of frequency-dependent DMs towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured DMs. The observed DM variability is significant and may be caused by extreme scattering events. Potential causes for frequency-dependent DMs are quantified and evaluated. Conclusions. We conclude that frequency dependence of DMs has been reliably detected and is indeed caused by small-scale (up to tens of AUs) but steep density variations in the interstellar electron content. We find that long-term trends in DM variability equally affect DMs measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited

A Gaussian-processes approach to fitting for time-variable spherical solar wind in pulsar timing data

Propagation effects are one of the main sources of noise in high-precision pulsar timing. For pulsars below an ecliptic latitude of 5°, the ionized plasma in the solar wind can introduce dispersive delays of order 100 µs around solar conjunction at an observing frequency of 300 MHz. A common approach to mitigate this assumes a spherical solar wind with a time-constant amplitude. However, this has been shown to be insufficient to describe the solar wind. We present a linear, Gaussian-process piecewise Bayesian approach to fit a spherical solar wind of time-variable amplitude, which has been implemented in the pulsar software RUN_ENTERPRISE. Through simulations, we find that the current EPTA+InPTA data combination is not sensitive to such variations; however, solar wind variations will become important in the near future with the addition of new InPTA data and data collected with the low-frequency LOFAR telescope. We also compare our results for different high-precision timing data sets (EPTA+InPTA, PPTA, and LOFAR) of 3 ms pulsars (J0030+0451, J1022+1001, J2145−0450), and find that the solar-wind amplitudes are generally consistent for any individual pulsar, but they can vary from pulsar to pulsar. Finally, we compare our results with those of an independent method on the same LOFAR data of the three millisecond pulsars. We find that differences between the results of the two methods can be mainly attributed to the modelling of dispersion variations in the interstellar medium, rather than the solar wind modelling

Current results and future prospects from PSR J1757-1854, a highly-relativistic double neutron star binary

Pulsars, rapidly-rotating highly-magnetised neutron stars, can serve as useful laboratories for probing aspects of fundamental physics. Binary pulsars, especially those in tight binary systems with massive, compact companions, are useful in testing different theories of gravity, the current paradigm being General Relativity (GR). Additionally, binary pulsars can also be utilised to explore other areas of fundamental physics, such as the behaviour of matter at ultra-high densities and the neutron star moment of inertia. A standout example is PSR J1757-1854, a 21.5-ms pulsar in a highly-eccentric (e=0.61), 4.4-hr orbit around a neutron star companion. This pulsar exhibits some of the most extreme relativistic parameters ever observed in a binary pulsar, reaching a maximum line-of-sight acceleration of close to 700 m/s/s and displaying among the strongest relativistic effects due to gravitational wave damping. To date, five post-Keplerian parameters have been measured in PSR J1757-1854, allowing for three independent tests of gravity to be conducted (of which GR passes all three) and for the component neutron star masses to be separated. The extreme properties of this system (particularly its high eccentricity) are expected to allow for future measurements of Lense-Thirring precession effects (allow for a measurement of the neutron star moment of inertia) and the relativistic deformation of the orbit, both of which remain almost completely unexplored by other binary systems. Although first discovered by the Parkes Radio Telescope in 2016 as part of the High Time Resolution Universe Southern Galactic Plane survey, it is ongoing observations with the Green Bank Telescope (GBT) which have provided the backbone of PSR J1757-1854’s continuing study. The large-bandwidth, high-precision observations afforded by the GBT played a fundamental role in delivering the science derived from the pulsar so far, and will be critical in allowing it to reach its full scientific potential going forward. In this talk I will provide a progress report on the ongoing timing of the system, including a review of the latest mass measurements and gravity tests, with an emphasis towards the future science which this pulsar will make possible

Pulsars Probe the Low-Frequency Gravitational Sky: Pulsar Timing Arrays Basics and Recent Results

Tiburzi C. Pulsars Probe the Low-Frequency Gravitational Sky: Pulsar Timing Arrays Basics and Recent Results. PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF AUSTRALIA. 2018;35: 14.Pulsar Timing Array experiments exploit the clock-like behaviour of an array of millisecond pulsars, with the goal of detecting low-frequency gravitational waves. Pulsar Timing Array experiments have been in operation over the last decade, led by groups in Europe, Australia, and North America. These experiments use the most sensitive radio telescopes in the world, extremely precise pulsar timing models and sophisticated detection algorithms to increase the sensitivity of Pulsar Timing Arrays. No detection of gravitational waves has been made to date with this technique, but Pulsar Timing Array upper limits already contributed to rule out some models of galaxy formation. Moreover, a new generation of radio telescopes, such as the Five hundred metre Aperture Spherical Telescope and, in particular, the Square Kilometre Array, will offer a significant improvement to the Pulsar Timing Array sensitivity. In this article, we review the basic concepts of Pulsar Timing Array experiments, and discuss the latest results from the established Pulsar Timing Array collaborations

The Statistics of Radio Astronomical Polarimetry: Disjoint, Superposed, and Composite Samples

van Straten W, Tiburzi C. The Statistics of Radio Astronomical Polarimetry: Disjoint, Superposed, and Composite Samples. ASTROPHYSICAL JOURNAL. 2017;835(2): 293.A statistical framework is presented for the study of the orthogonally polarized modes of radio pulsar emission via the covariances between the Stokes parameters. To accommodate the typically heavy-tailed distributions of singlepulse radio flux density, the fourth-order joint cumulants of the electric field are used to describe the superposition of modes with arbitrary probability distributions. The framework is used to consider the distinction between superposed and disjoint modes, with particular attention to the effects of integration over finite samples. If the interval over which the polarization state is estimated is longer than the timescale for switching between two or more disjoint modes of emission, then the modes are unresolved by the instrument. The resulting composite sample mean exhibits properties that have been attributed to mode superposition, such as depolarization. Because the distinction between disjoint modes and a composite sample of unresolved disjoint modes depends on the temporal resolution of the observing instrumentation, the arguments in favor of superposed modes of pulsar emission are revisited, and observational evidence for disjoint modes is described. In principle, the four-dimensional covariance matrix that describes the distribution of sample mean Stokes parameters can be used to distinguish between disjoint modes, superposed modes, and a composite sample of unresolved disjoint modes. More comprehensive and conclusive interpretation of the covariance matrix requires more detailed consideration of various relevant phenomena, including temporally correlated subpulse modulation (e.g., jitter), statistical dependence between modes (e.g., covariant intensities and partial coherence), and multipath propagation effects (e.g., scintillation and scattering)

Constraints on the magnetic field in the Galactic halo from globular cluster pulsars

Abbate F, Possenti A, Tiburzi C, et al. Constraints on the magnetic field in the Galactic halo from globular cluster pulsars. NATURE ASTRONOMY. 2020.The Galactic magnetic field plays an important role in the evolution of the Galaxy, but its small-scale behaviour is still poorly known. It is not known whether the Galactic field permeates the halo of the Galaxy. By observing pulsars in the halo globular cluster 47 Tucanae, we have probed the Galactic magnetic field at arcsecond scales, discovering an unexpected large gradient in the component of the magnetic field parallel to the line of sight. This gradient is aligned with a direction perpendicular to the Galactic disk and could be explained by magnetic fields amplified to some 60 mu G within the globular cluster. Such a scenario supports the existence of a magnetized outflow that extends from the Galactic disk to the halo and interacts with 47 Tucanae. Observations of the Faraday rotation towards pulsars in the halo globular cluster 47 Tucanae have been used to constrain the magnetic field strength in the Galactic halo, finding that it is unexpectedly strong