It may also emit radiation pulses at other wavelengths. They usually last no longer than 10 seconds, but pack as much energy as the Sun emits in its corona in 3000 years. The bursts are due to sudden burning of hydrogen which has accumulated on the surface of a neutron star. Sometimes stars will let out a large amount of energy, which may be visible at more than one wavelength, and then seem to disappear or become quiet, energetically. We call these objects transients because their energy outburst is not permanent, but short-lived.
Some transients have outbursts which occur at regular intervals or periodically.
Transient outbursts are due to a change in rate of material falling onto a neutron star or a black hole. The rate of the infalling material can change if there is an instability in a companion star or an instability in the accretion disk surrounding the neutron star or black hole. A good analogy for transient events is a fireworks display. Your X-ray detector may see a bright flash where there was none before, and then like a burst of fireworks, it fades away.