X-Ray Transients

The universe is a harsh place, and that becomes obvious when you start detecting all the forms of radiation that comes from various sources. Space, in itself, is not very conducive to life, but when you have huge amounts of X-rays randomly traveling through the solar systems, you do sometimes wonder how life developed anywhere. X-rays transients are just one reason why space travel is a lot harder than many people think.

What Are X-rays Transients?

They are X-rays that periodically pass through the solar system, and then they just disappear.

Detecting these has only recently been possible, thanks to space travel. The atmosphere absorbs X-rays, so we can’t detect them on the surface of the Earth.

Fortunately, the use of essentially ICBMs with an X-ray detector strapped to them in the 1960s became a cheap method of trying to detect X-rays and getting a baseline reading of the background X-ray readings. This method detected the first X-ray transient source: Cen X-2.

Nobody knew what it was, and because there was a four-year gap between all three readings, the source remains undiscovered. What this did do was pave the way for the discovery of other X-ray transients, including Cen X-4.

These ICBM launches eventually gave way to Vela satellites, which were ostensibly for nuclear test monitoring. However, they also carried X-ray detectors, which meant that they could be used for X-ray transient detection.

Eventually, sufficient X-ray transient sources were discovered that they were put in their own class. But what were they?

Binary Stars Hold the Answer

As the data was released and declassified — albeit very slowly, as the satellites held militarily sensitive information — astrophysicists were able to identify the areas where X-ray transients occurred: They all appeared to be binary stars.

It was clear that the binary stars were producing these X-rays, but why?

It turns out that these binary systems typically involve a functioning star, such as a main-sequence star or a red giant, and either a neutron star or a black hole. In some cases, it can involve a nova and a black hole.

It seems likely that the two stars have elliptical orbits, where one star passes through the accretion disc of the other at regular intervals. The denser star gobbles up hydrogen and helium, creating a burst of X-rays and gamma rays from the sudden increase in fusible elements. The other star passes through and resumes its journey orbiting around the denser star, albeit with significantly less mass.

In this system, you get low-mass and high-mass X-ray bursters, depending on the mass of the star that is being scavenged by the neutron star or black hole. You also get soft X-ray transients, which exhibit less violent X-ray outputs compared to the bursters. There are also supergiant X-ray transients, which exhibit their own particular set of characteristics.

Other Sources of X-ray Transients

Our own sun can produce X-rays periodically, and this can lead to X-ray transients. The sun’s overall output is factored into the background radiation level, so when it produces a major mass ejection, the X-rays within the solar system suddenly increases. Because it is so close, it can have a significant impact on the earth, creating geomagnetic storms.

Jupiter can also create X-ray transients, and these can be detected quite easily from Earth. However, these are a particular sort of X-rays, so they can be differentiated from ones produced by binary stars.

X-ray transients are a particular space hazard, but they do not generally affect intra-solar-system travel. Should we wish to go further, it’s essential that we understand how these X-rays are produced to ensure we can avoid or compensate for them.