X-ray observations of young neutron stars

The extreme physical properties of neutron stars make them efficient emitters at all wavelengths of the electromagnetic spectrum and, traditionally, they have been extensively studied at radio wavelengths. The neutron stars with the highest estimated magnetic fields (so-called "magnetars") have remarkably different characteristics from the rest of the population: they emit no persistent radio emission but show large amounts of high-energy radiation that is thought to be powered by their large magnetic fields. For this thesis we have studied the X-ray emission properties of various types of young neutron stars, discovering unusual characteristics, constraining long-term behaviour and finding associated nebulae. We have observed the neutron stars PSR B0154+61 and PSR J1119-6127, which have high magnetic fields but otherwise emit normal radio emission. For the latter, unusual thermal X-ray emission was discovered that points to the possible effects of a magnetic field on the surface. Also, this source now represents the youngest neutron star from which thermal emission from the surface has been detected. However, we find no evidence for clear magnetar-like characteristics in these sources. The reason for this discrepancy, as yet unclear and a matter of debate, poses a great challenge to our understanding of the evolution of neutron stars and their emission mechanisms. We have also studied the long-term properties of the 'anomalous X-ray pulsar' 4U 0142+61, thought to be a magnetar. We find that changes are present in almost all of its emission characteristics over the last 7 years. The observed changes agree with the general predictions made by the magnetar model of such sources. However, the details of these changes suggest that further work is still needed on the expected emission from these objects. In addition, neutron stars are seen to power extended structures, called pulsar wind nebulae (PWNe), which can radiate large amounts of high-energy emission. Here we also present the discovery of one such nebula around the young, energetic neutron star PSR B1046-58. While both the star and its PWN are possible gamma-ray sources (a very unusual characteristic), we find that their X-ray emission is similar to those seen around other neutron stars with similar ages and energetics. The nebula has a peculiar morphology and we explore various scenarios to account for its properties. The discovery of this nebula presents a new testing site for current theories of PWNe production and emission characteristics.