Be stars are massive stars surrounded by an equatorial disk of material (a circumstellar disk). Roughly 2/3 of all high-mass x-ray binary systems consist of a Be star and a compact object in orbit around each other and, not surprisingly, these systems are known as Be X-ray binaries.
Be X-ray binaries are generally quiescent, exhibiting low-luminosity (in a few cases) or no X-ray emission. The transient X-ray outbursts are thought to be due to interactions between the compact object and the circumstellar disk surrounding the Be star, and two different types of outburst have been identified:
Type I outbursts tend to be short and periodic, occurring around periastron when the two objects are at their closest approach. This would suggest that the orbit of the compact object around the Be star is wide and highly eccentric, limiting interactions with the circumstellar disk to when the objects are closest. Type I outbursts typically last only a few days and are the result of an increase in the amount of material impacting the compact object as it passes through, or close to the circumstellar disk.
Type II outbursts are much larger than Type I outbursts and can last for several weeks. They are not linked to the orbital period of the system but tend to be preceeded by increased activity in the Be star. This can change the density, size and other characteristics of the circumstellar disk, which in turn will affect the X-ray emission as the compact object interacts with the disk material.
The projected semi-major axis, x, of a binary system is the projection of the semi-major axis onto the line of sight:
where i is the inclination angle of the binary orbit (with respect to the plane of the sky). This implies that orbits which are viewed face-on have x = 0, while for edge-on systems x equals the actual semi-major axis of the binary system.
This parameter is mostly important because of its direct relation to the Roemer delay, which enables its measurement through pulsar timing.