X-ray pulses from globular clusters

Abstract

GIANT X-ray pulses have been observed recently by several groups^1–7 from sources located in a number of celestial positions. Two of these sources may plausibly be identified with the globular cluster X-ray sources: NGC6624 and NGC1851. In several cases^2,7 the pulses seem to occur in approximately periodic sequences; in other sources the intervals are irregular, but in one case there is a relation between pulse strength and interpulse interval. The non-burst behaviour of globular cluster X-ray sources has been interpreted^8,9 in terms of massive (≳ 10³M⊙) black holes powered by accretion from a disk supplied by cluster gas. We suggest here that compact stars bound to the central massive black hole and orbiting through the accretion disk may produce, by several possible mechanisms, giant pulses. These pulses might be expected to exhibit a variety of types of X-ray behaviour corresponding to the uncertain characteristics (for example, density and spatial extent) of the individual accretion disks at the different points at which the orbits of the bound compact stars intersect the disks. We note that there may be more than one class of sources of giant X-ray bursts and that our speculation refers only to those sources in globular clusters.