Stellar x-ray polarimetry from the Ariel-V satellite.

X-ray polarisation observations from the Ariel-V satellite are described. Notably, an upper limit to the X-ray polarisation of Sco X-1 was produced, more sensitive than previous results had managed to achieve, and for which constraints could be placed upon models for the X-ray emission mechanism. Analysis of the data provided strong empirical confirmation of the desirability for future polarimeters to incorporate some previously proposed design features. These include utilisation of focusing methods to allow a smaller detector, and hence a reduced background level; increased area of collection, so that much weaker X-ray sources may be usefully observed; and also sufficiently accurate pointing, narrow f.o.v. and a capability for simultaneous measurement of more than one component of linear polarisation, all to suppress possible spurious modulations. It would be expected that the advantages obtained by following the advocated design features would result in a polarimeter sufficiently sensitive to be able to measure X-ray polarization from weak X-ray sources, and to achieve accuracies that have not yet been remotely approached. Characteristics of a Bragg crystal instrument capable of a sufficient degree of broad band X-ray polarimetry to test for the existence of a fast rotating black hole in the Cyg X-1 system, are also described. Finally, the need for X-ray polarisation measurements throughout the field of current X-ray astronomy is illuminated.