Pulsar Spin-Velocity Alignment: Kinematic Ages, Birth Periods and Braking Indices

This paper presents a detailed investigation of the dependence of pulsar spin-velocity alignment, which has been observed for a sample of 58 pulsars, on pulsar age. At first, our study considers only pulsar characteristic ages, resulting in no change in the degree of correlation as a function of age, up to at least 100 Myr. Subsequently, we consider a more reliable estimate of pulsar age, the kinematic age, assuming that pulsars are born near the Galactic plane. We derive kinematic ages for 52 pulsars, based on the measured pulsar proper motions and positions, by modelling the trajectory of the pulsars in a Galactic potential. The sample of 52 pulsar kinematic ages constitutes the largest number of independently estimated pulsar ages to date. Using only the 33 most reliable kinematic ages from our simulations, we revisit the evolution of spin- velocity alignment, this time as a function of kinematic age. We find that the strong correlation seen in young pulsars is completely smeared out for pulsars with kinematic ages above 10 Myr, a length of time beyond which we expect the gravitational pull of the Galaxy to have a significant effect on the directions of pulsar velocities. In the discussion, we investigate the impact of large distance uncertainties on the reliability of the calculated kinematic ages. Furthermore, we present a detailed investigation of the implications of our revised pulsar ages for the braking-index and birth-period distributions. Finally, we discuss the predictions of various SN-kick mechanisms and their compatibility with our results.Comment: 24 pages, 19 figures, MNRAS accepte

WSRT Faraday tomography of the Galactic ISM at \lambda \sim 0.86 m

We investigate the distribution and properties of Faraday rotating and synchrotron emitting regions in the Galactic ISM in the direction of the Galactic anti-centre. We apply Faraday tomography to a radio polarization dataset that we obtained with the WSRT. We developed a new method to calculate a linear fit to periodic data, which we use to determine rotation measures from our polarization angle data. From simulations of a Faraday screen + noise we could determine how compatible the data are with Faraday screens. An unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an unresolved main component in the Faraday depth spectrum. For lines-of-sight with a single unresolved component we demonstrate that a Faraday screen in front of a synchrotron emitting region that contains a turbulent magnetic field component can explain the data.Comment: 5 pages, 5 figures. Accepted for publication as a Letter to the Editor in A&

In Loco Parentis: Holland’s orphan chambers in a European context

Various well-known forms of safeguards for life-cycle risks have been described extensively for the early modern era. In this paper I want to draw attention to one specific and often underrated one, the risk of being orphaned at a young age, and the changing social arrangements covering the consequences. The mortality pattern of European cities translated into high numbers of young orphans. Most of them did not need the physical care provided by orphanages and were taken in by family or friends. However, some of them needed assistance to safeguard assets they had inherited from their parents. All over Western Europe, institutions emerged to act in loco parentis, and supervise guardians and secure the transfer of inheritances. Through our analysis of Holland’s orphan chambers in a European context, we highlight the importance of a hitherto neglected aspect, namely the dynamics of urban growth and migration. In that way, we aim to provide a more profound understanding of the precise functions, and the rise and decline of these remarkable institutions and show their importance as welfare arrangements

Statistical properties of Faraday rotation measure from large-scale magnetic fields in intervening disc galaxies

To constrain the large-scale magnetic field strengths in cosmologically distant galax- ies, we derive the probability distribution function of Faraday rotation measure (RM) when random lines of sight pass through a sample of disc galaxies, with axisymmetric large-scale magnetic fields. We find that the width of the RM distribution of the galaxy sample is directly related to the mean large-scale field strength of the galaxy population, provided the dispersion within the sample is lower than the mean value. In the absence of additional constraints on parameters describing the magneto-ionic medium of the intervening galaxies, and in the situation where RMs produced in the intervening galaxies have already been statistically isolated from other RM contributions along the lines of sight, our simple model of the magneto-ionic medium in disc galaxies suggests that the mean large-scale magnetic field of the population can be measured to within ~ 50% accuracy.Comment: 4 pages, Proceedings of FM8 "New Insights in Extragalactic Magnetic Fields", XXXth General Assembly of the IAU, Vienna, August 20-31, 201

Rotation measure synthesis revisited

We re-formulate rotation measure (RM) synthesis for data sets with discrete frequency channels and an arbitrary channel response function. The most commonly used version of the formalism by Brentjens & De Bruyn assumes a top-hat response function in wavelength squared, while real data sets can often be approximated better with a top-hat in frequency. We simulate mock data sets for various source geometries, using a top-hat response function in frequency, and we compare the quality of the RM spectra that are found with both formalisms. We include the response function of the simulated data to calculate exact RM spectra using our formalism. We show that the formalism by Brentjens & De Bruyn produces accurate results even if depolarization at the lowest frequencies in the observing band is severe. If RMs are large, our formalism reconstructs the emitted signal more accurately, with a higher amplitude and (in most cases) a narrower RM spread function. Our formalism can also detect sources with larger (absolute) RMs for a given sensitivity level of the observations.Comment: Accepted for publication in MNRAS. 6 pages, 4 figure

Polarization signatures of unresolved radio sources

We investigate how the imprint of Faraday rotation on radio spectra can be used to determine the geometry of radio sources and the strength and structure of the surrounding magnetic fields. We model spectra of Stokes Q and U for frequencies between 200 MHz and 10 GHz for Faraday screens with large-scale or small-scale magnetic fields external to the source. These sources can be uniform or 2D Gaussians on the sky with transverse linear gradients in rotation measure (RM), or cylinders or spheroids with an azimuthal magnetic field. At high frequencies, the spectra of all these models can be approximated by the spectrum of a Gaussian source; this is independent of whether the magnetic field is large scale or small scale. A sinc spectrum in polarized flux density is not a unique signature of a volume where synchrotron emission and Faraday rotation are mixed. A turbulent Faraday screen with a large field coherence length produces a spectrum which is similar to the spectrum of a partial coverage model. At low and intermediate frequencies, such a Faraday screen produces a significantly higher polarized signal than Burn's depolarization model, as shown by a random walk model of the polarization vectors. We calculate RM spectra for four frequency windows. Sources are strongly depolarized at low frequencies, but RMs can be determined accurately if the sensitivity of the observations is sufficient. Finally, we show that RM spectra can be used to differentiate between turbulent foreground models and partial coverage models

Radio galaxies and their magnetic fields out to z <= 3

We present polarisation properties at $1.4\,$GHz of two separate extragalactic source populations: passive quiescent galaxies and luminous quasar-like galaxies. We use data from the {\it Wide-Field Infrared Survey Explorer} data to determine the host galaxy population of the polarised extragalactic radio sources. The quiescent galaxies have higher percentage polarisation, smaller radio linear size, and $1.4\,$GHz luminosity of $6\times10^{21}<L_{\rm 1.4}<7\times10^{25}\,$W Hz$^{-1}$, while the quasar-like galaxies have smaller percentage polarisation, larger radio linear size at radio wavelengths, and a $1.4\,$GHz luminosity of $9\times10^{23}<L_{\rm 1.4}<7\times10^{28}\,$W Hz$^{-1}$, suggesting that the environment of the quasar-like galaxies is responsible for the lower percentage polarisation. Our results confirm previous studies that found an inverse correlation between percentage polarisation and total flux density at $1.4\,$GHz. We suggest that the population change between the polarised extragalactic radio sources is the origin of this inverse correlation and suggest a cosmic evolution of the space density of quiescent galaxies. Finally, we find that the extragalactic contributions to the rotation measures (RMs) of the nearby passive galaxies and the distant quasar-like galaxies are different. After accounting for the RM contributions by cosmological large-scale structure and intervening Mg\,{II} absorbers we show that the distribution of intrinsic RMs of the distant quasar-like sources is at most four times as wide as the RM distribution of the nearby quiescent galaxies, if the distribution of intrinsic RMs of the WISE-Star sources itself is at least several rad m$^{-2}$ wide.Comment: 12 pages, 8 figures, accepted for publication into MNRA

An In-Depth Investigation of Faraday Depth Spectrum Using Synthetic Observations of Turbulent MHD Simulations

Basu A, Fletcher A, Mao SA, Burkhart B, Beck R, Schnitzeler D. An In-Depth Investigation of Faraday Depth Spectrum Using Synthetic Observations of Turbulent MHD Simulations. GALAXIES. 2019;7(4): 89.In this paper, we present a detailed analysis of the Faraday depth (FD) spectrum and its clean components obtained through the application of the commonly used technique of Faraday rotation measure synthesis to analyze spectro-polarimetric data. To directly compare the Faraday depth spectrum with physical properties of a magneto-ionic medium, we generated synthetic broad-bandwidth spectro-polarimetric observations from magnetohydrodynamic (MHD) simulations of a transonic, isothermal, compressible turbulent medium. We find that correlated magnetic field structures give rise to a combination of spiky, localized peaks at certain FD values, and broad structures in the FD spectrum. Although most of these spiky FD structures appear narrow, giving an impression of a Faraday thin medium, we show that they arise from strong synchrotron emissivity at that FD. Strong emissivity at a FD can arise because of both strong spatially local polarized synchrotron emissivity at a FD or accumulation of weaker emissions along the distance through a medium that have Faraday depths within half the width of the rotation measure spread function. Such a complex Faraday depth spectrum is a natural consequence of MHD turbulence when the lines of sight pass through a few turbulent cells. This therefore complicates the convention of attributing narrow FD peaks to the presence of a Faraday-rotating medium along the line of sight. Our work shows that it is difficult to extract the FD along a line of sight from the Faraday depth spectrum using standard methods for a turbulent medium in which synchrotron emission and Faraday rotation occur simultaneously