They are caused by the Doppler effect as a pulsar wobbles back and forth under the effects of its co-orbital motion with another massive body.
They are accompanied by sudden releases of X-ray energy from the buildup of hydrogen around neutron stars near the central regions of our galaxy and the centers of some globular star clusters.
They are brief, very intense, non-repeating sudden releases of energy that appear to come from very large distances far outside our galaxy.
They occur in a repeating pattern at very high frequencies and are probably due to energy released along the axes of rapidly rotating neutron stars.
They are caused by the core collapse of a massive star at least eight times heavier than our Sun when it exhausts the processes that release energy by nuclear fusion in its core.