Pulsars, transients and relativistic astrophysics

Abstract:

We present the results of an intensive multi-epoch radio frequency campaign on the energetic and nearby GRB 171010A with the Karl G. Janksy Very Large Array and Arcminute Microkelvin Imager Large Array. We began observing GRB 171010A a day after its initial detection, and were able to monitor the temporal and spectral evolution of the source over the following weeks. The spectra and their evolution are compared to the canonical theories for broad-band GRB afterglows, with which we find a general agreement. There are, however, a number of features that are challenging to explain with a simple forward shock model, and we discuss possible reasons for these discrepancies. This includes the consideration of the existence of a reverse shock component, potential microphysical parameter evolution, and the effect of scintillation.

Abstract:

We investigate aperiodic X-ray flux variability in accreting highly magnetized neutron stars – X-ray pulsars (XRPs). The X-ray variability is largely determined by mass accretion rate fluctuations at the NS surface, which replicate accretion rate fluctuations at the inner radius of the accretion disc. The variability at the inner radius is due to fluctuations arising all over the disc and propagating inwards under the influence of viscous diffusion. The inner radius varies with mean mass accretion rate and can be estimated from the known magnetic field strength and accretion luminosity of XRPs. Observations of transient XRPs covering several orders of magnitude in luminosity give a unique opportunity to study effects arising due to the changes of the inner disc radius. We investigate the process of viscous diffusion in XRP accretion discs and construct new analytical solutions of the diffusion equation applicable for thin accretion discs truncated both from inside and outside. Our solutions are the most general ones derived in the approximation of Newtonian mechanics. We argue that the break observed at high frequencies in the power density spectra of XRPs corresponds to the minimal time-scale of the dynamo process, which is responsible for the initial fluctuations. Comparing data from the bright X-ray transient A 0535+26 with our model, we conclude that the time-scale of initial variability in the accretion disc is a few times longer than the local Keplerian time-scale.