The most obscured celestial objects of our Galaxy: a neutron star orbiting around a supergiant star, inside a cocoon of dust created by the powerful wind of the supergiant star.
In the first case, a neutron star orbits around a supergiant along a roughly circular orbit. Since the cocoon of gas/dust totally blankets the whole system, the neutron star stays permanently inside the cocoon, so there is a persistent source of X-rays.
In the second case, the orbit is more eccentric and the neutron star crosses only periodically into the cocoon of gas/dust covering the supergiant. Therefore there is intermittent emission of X-rays during that time.