from the sinusoidal ToA variations due to ultralight dark matter, one needs to include it in a future study to quantitatively assess the impact.
We forecasted the PTA sensitivity in the FAST/SKA era while accounting for re-alistic noise levels. We found that observing the ten best PPTA pulsars for ten years would constrain the density of FDM below 0.05GeV cm−3 for m . 10−23eV, about 10% of measured total dark matter density. At m ≈2×10−23eV, our projected limit is around 0.4GeV cm−3; for higher boson masses, the upper limits increase as ∼ m3. Abovem≈10−22eV, the projected limits are more than one order of magnitude above the local dark matter density. To place interesting limits in this mass range, an am-bitious timing program in which hundreds of pulsars timed with daily cadence and high precision (.20ns) for more than a decade is required. Finally, we point out that high-quality pulsars in the vicinity of the Galactic Center will be ideal tools to test the fuzzy dark matter hypothesis.
Chapter 7
Concluding remarks and future plans
Contents
7.1 Studying the magnetised ISM with pulsars . . . 125 7.1.1 Future plans . . . 126 7.2 Studying dark matter with pulsars . . . 127 7.2.1 Future plans . . . 128
The primary scientic motivation of this thesis is the investigation of eects which inuence the propagation of pulsar signals, in order to probe the properties of the intervening medium and its constituents. We mainly focus our analysis on probing the turbulence in the magnetised ionised ISM using Faraday rotation of pulsars, and dark matter in the Galaxy using a high-precision pulsar timing technique. We investigate how the recent, highly sensitive data can constrain (or measure!) the aforementioned propagation eects that haven't been studied extensively before. The most signicant accomplishments of the work in this thesis as well as potential improvements and future plans are summarised below.
7.1 Studying the magnetised ISM with pulsars
Pulsars are known to be a powerful probe of the magnetoionic phase of the ISM in the Milky Way. Specically in this thesis we used Faraday rotation of linearly polarised radiation from pulsars to study the diuse magnetic eld in the Milky Way. Since ISM eects are more prominent at longer wavelengths, we have conducted the pulsar observations with the low-frequency radio interferometer LOFAR. The RMs of pulsars were obtained with the novel BGLSP technique, which provides reliable estimates of the uncertainties on the observed RMs, and is described in Chapter 3. It was found that detected RM variations are dominated by the Faraday rotation taking place in the Earth's ionosphere, which is ve to six orders of magnitude greater than what we expect from the ISM. In order to compensate for the ionospheric RM we have used a conventional thin-layer ionospheric approximation. The Earth's magnetic eld was reconstructed with so-called geomagnetic eld models produced using magnetic eld measurements from satellites and approximately 200 operating magnetic observatories
around the globe. The electron density in the ionosphere was modelled using a se-lection of global free-electron density maps, produced by dierent scientic groups via various numerical techniques using GPS data. In Chapter4we compared dierent ge-omagnetic models and free-electron density maps, and investigated the reconstruction quality of ionospheric Faraday rotation using several months of LOFAR observations of selected pulsars. We found that on average the UQRG and JPLG maps perform better than other ionospheric maps, while the performance of dierent geomagnetic models is indistinguishable with our current sensitivity. After subtraction of the de-terministic systematics that left residual unmodelled ionospheric eects, the corrected RM measurements have a precision of ∼ 0.06−0.07 rad m−2. This number is approx-imately an order of magnitude higher than the uncertainties of the observed RMs for the considered pulsars and therefore denes our sensitivity to any sort of astrophysical RM variation.
One of the promising signals of interest are RM variations caused by turbulent ISM structures between the pulsar and the observer. In Chapter 5 we made an eort to measure these RM variations using ∼3-year long LOFAR observations of pulsars.
However, no astrophysically reliable signal has been found. We set an upper limit on the amplitude of any magnetic eld uctuations. Our best result, obtained with J0826+2637, is already below the value derived in Haverkorn et al. (2008), however still four times higher that the expected value observed in Minter & Spangler (1996).
With a set of simulations we have shown that for a reliable detection we need ∼ 20 years of regular monitoring of pulsars with LOFAR.
7.1.1 Future plans
In this section we summarise which steps should be undertaken in order to improve our sensitivity to RM variations of pulsars. These ideas are important for future initiatives as well as for current actions.
Regular monitoring of pulsars with low-frequency interferometers
Searching for RM variations in pulsar signals is an on-going study. Ger-man LOFAR stations are now observing more than 100 pulsars bi-weekly at 150 MHz. Such continuing monitoring campaigns of pulsars at low frequencies provide a valuable basis for investigating long-term RM changes. Besides the RM variations caused by the turbulent ISM, the same data set can be used to probe other RM signals of interest, such as those caused by the Solar wind (Oberoi & Lonsdale, 2012), ESEs (Fiedler et al., 1987; Cognard et al., 1993) and reconnection sheets in the ISM (Pen & Levin, 2014). Along with the RMs, the DMs of pulsars can be used to determine the electron density towards the pulsar, providing additional insight into the physical structures and processes taking place between the source and the observer. By simultaneously measuring the RMs and DMs of pulsars, one can probe the possible correlations of these quantities and increase the reliability of the results.
Improving ionospheric modelling As mentioned, the inuence of the ionosphere
7.2. Studying dark matter with pulsars 127