The X-ray Emission and Population of Highly Magnetized Neutron Stars

Over the past few decades, advances in X-ray and gamma-ray astronomy have greatly expanded our knowledge of the neutron-star family. One important recent discovery has been that of the magnetars, isolated neutron stars whose radiation and occasional bursting activity is thought to be powered by their very high magnetic fields (10^14-10^15 G as inferred from timing), unlike ordinary pulsars that are powered by their rotational energy. There do, however, exist rotation-powered pulsars with inferred magnetic fields that approach those of the magnetars ( 10^13 G). These two groups might therefore be expected to show some similarities in their properties or behaviour. Careful study of both the high-magnetic-field pulsars and magnetars, then, may help us to understand magnetar physics and determine their relations and connections with the rest of the pulsar population. In Chapter 3, I present the results of two XMM-Newton observations of the high-magnetic-field radio pulsar PSR J1734-3333. We successfully detect the X-ray counterpart of the pulsar. Its spectrum its well to a blackbody with temperature 300 +- 60 eV, and its bolometric luminosity is Lbb = 2.0+2.2-0.7 x 10^32 erg/s, or 0.4% of its spin-down power, for a distance of 6.1 kpc. We detect no X-ray pulsations from the source, setting a 1 sigma upper limit on the pulsed fraction of 60% in the 0.5-3 keV band. We compare PSR J1734-3333 to other rotation-powered pulsars of similar age and find that it is significantly hotter, supporting the hypothesis that the magnetic field affects the observed thermal properties of pulsars. We also tabulate the properties of this and all other known high-B radio pulsars with measured thermal X-ray luminosities or luminosity upper limits, and speculate on a possible correlation between LX and B. In Chapter 4, I present an analysis of the extended emission around the magnetar 1E 1547.0-5408. Based on four XMM-Newton observations taken with the source in various stages from outburst to quiescence, we find that the extended emission flux is highly variable and strongly correlated with the flux of the magnetar. From this result, as well as spectral and energetic considerations, we conclude that the extended emission is dominated by a dust-scattering halo and not a pulsar wind nebula (PWN), as has been previously argued. We obtain an upper limit on the 2-10 keV flux of a possible PWN of 4.7 x 10^14 erg/s/cm2, three times less than the previously claimed value. We do, however, find strong evidence for X-ray emission from a supernova remnant surrounding the pulsar, as previously reported. Finally, I present a study of the magnetar population as a whole in Chapter 5, with a catalog of the 26 currently known magnetars and magnetar candidates. Tables are provided of astrometric and timing data for all catalog sources, as well as of their observed radiative properties, particularly the spectral parameters of the quiescent X-ray emission. We show histograms of the spatial and timing properties of the magnetars and compare them with the known pulsar population. We measure the scale height of magnetars to be in the range of 20-31 pc, assuming they are exponentially distributed. This range is smaller than that measured for OB stars, providing evidence that magnetars are born from the most massive O stars. From the same fits, we find that the Sun lies 13-22 pc above the Galactic plane, consistent with previous measurements. We confirm previously identified correlations between quiescent X-ray luminosity, L_X, and magnetic field, B, as well as X-ray spectral power-law indexes, Gamma and B, and show evidence for an excluded region in a plot of L_X versus Gamma. We observe that while there is a clear correlation between the hard and soft X-ray fluxes in magnetars, the radio-detected magnetars all have low, soft X-ray flux, suggesting, if anything, that the two bands are anti-correlated.