X-ray bursts from dense clouds

Abstract

X-RAY heating of matter accreting on to a compact object can create unstable flows leading to irregular X-ray emission1–4. Previous discussions of this instability have concentrated on the case of spherically symmetrical infall onto a stationary object. The rise-time of X-ray variations is then related to the free-fall time-scale from the choking or accretion radius, whichever is relevant. Phenomena due to the presence of a magnetosphere4 may, however, reduce this estimate. It is not clear that bursts with duty cycles as short as 10−3 can arise from the heatine instability. Such bursts are observed, and here I investigate the effect of the motion of a compact object through a relatively dense medium. The passage of the compact object through a denser shell, which builds up outside the X ray-heated gas, produces a short burst of X rays. This creates yet another hot region leading to the repetition of bursts on a crossing time. Such behaviour may be relevant to the weak bursts possibly associated with the Cometary Globule NGC 5367 (ref. 5), and the much shorter timescale luminous X-ray bursts6, some of which are associated with globular clusters.