Decay of X-ray source A0535+26

THE All-Sky Monitor (ASM) aboard Ariel V is a pinhole camera sensitive to X rays in the energy range 3–6 keV. It has an effective area of 0.6 cm2 with a duty cycle of ∼ 1% for a given source, and monitors more than 3π of the celestial sphere each orbit1. It is important to note that the regions of sky inaccessible to the ASM include the spacecraft spin axis and a band about the spacecraft equator, so that its field of view and those of the other five Ariel V experiments are mutually exclusive.