Gamma-ray astronomy

Abstract:

In most pulsars, the circularly polarized component, Stokes $V$, is weak in the average pulse profiles. By forming the average profile of $|V|$ from single pulses we can distinguish between pulsars where $V$ is weak in the individual pulses and those where large $V$ of variable handedness is observed from one pulse to the other. We show how $|V|$ profiles depend on the signal-to-noise ratio of $V$ in the single pulses and demonstrate that it is possible to simulate the observed, broad distributions of $V$ by assuming a model where $|V|$ is distributed around a mean value and the handedness of $V$ is permitted to change randomly. The $|V|$ enhanced profiles of 13 pulsars are shown, 5 observed at 1.41 GHz and 8 observed at 4.85 GHz, to complement the set in Karastergiou et al. (2003b). It is argued that the degree of circular polarization in the single pulses is related to the orthogonal polarization mode phenomenon and not to the classification of the pulse components as cone or core.

Abstract:

We have accumulated multiwavelength (X-ray, optical, radio) lightcurves for the eight black hole X-ray binaries which have been observed to enter a supposed 'soft X-ray transient' outburst, but remained in the low/hard state throughout the outburst. Comparison of the lightcurve morphologies, spectral behaviour, properties of the quasi-periodic oscillations and the radio jet provides the first study of such objects as a sub-class of X-ray transients. However, rather than assuming that these hard state X-ray transients are different from the 'canonical' soft X-ray transient, we prefer to consider the possibility that new analysis of both soft and hard state X-ray transients in a spectral context will provide a model capable of explaining the outburst mechanisms of (almost) all black hole X-ray binaries. © 2003 Elsevier B.V. All rights reserved.