Modeling the Light Curve of an X-ray Binary

Artist's conception of an X-ray binary, showing the normal star and disk of gas surrounding the neutron star. X-rays are emitted in the region nearest the neutron star.

X-ray binaries are two-star systems composed of one normal star and a "compact star" (usually a neutron star or a black hole). X-ray binaries emit x-rays because gas from the normal star flows toward the compact star . As it gets near the compact star, the gas density and pressure increase, causing a rise in temperature of the gas. The gas gets hot enough (a few million degrees) to glow in x-rays. This leads us to a fundamental concept.

Fundamental Concept: The more gas there is, the brighter an x-ray binary will shine in x-rays

By brightness, we also mean the intensity of the x-rays. Since most x-ray satellites count the number of x-rays from a source, we would also say that more gas leads to more x-rays.

The brightness of some x-ray sources is constant. But the brightness of others can vary.

Exercise 2 In this exercise, you will explore why the brightness of an x-ray binary might vary. Remember the Fundamental Concept given above. In the above light curve, at which point must there be more gas around the neutron star: a or b? Why would there be more gas at that point? What would cause the x-ray intensity to be faint sometimes and bright at other times? Which of the following pictures best illustrates the motion of one of the stars near the other to account for the behavior shown in the above light curve?

You now have an idea for why the x-ray intensity from an x-ray binary might vary. Our next step is to become familiar with the Hera software and a light curve data set for GX301-2. After that, we will look for a period in the data.